Observational study of the monsoonal dynamics and eddy
shedding phenomena
by
Jelena M. Popovic
B.S., University of Belgrade (1994)
Submitted to the Department of Earth, Atmospheric and Planetary Sciences
in partial fulfillment of the requirements for the degree of
Master of Science
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May 1999 r
@
Massachusetts Institute of Technology 1999. All rights reserved.
Author.............
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Department of harth, Atmospheric and Planetary Sciences
May 17, 1999
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Certified by ...........
R. Alan Plumb
Professor of Meteorology
Thesis Supervisor
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.........................
Ronald G. Prinn
Sciences
Planetary
and
Atmospheric
of
Earth,
Chairman, Department
Accepted by..................................
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-21
Observational study of the monsoonal dynamics and
eddy shedding phenomena
by
Jelena M. Popovic
Submitted to the Department of Earth, Atmospheric and Planetary Sciences
on May 17, 1999, in partial fulfillment of the
requirements for the degree of
Master of Science
Abstract
We analyze atmospheric data in order to study the upper tropospheric dynamics in the
area of the Asian summer monsoon. We look at summers of four years from 1987 to 1990.
The divergent upper tropospheric circulation of the monsoon often adopts the form of a
unified large-scale vortex. However, when the vortex becomes elongated enough, so that the
flow in most of the area is close to zonal, the vortex usually sheds secondary, or daughter,
vortices that then propagate away. Eddy shedding occurs in all four summers that we
examined.
We use the potential vorticity and Montgomery streamfunction fields to document the
characteristics of eddy shedding events. In most cases, the daughter cell propagates westward. In the vertical, eddy shedding is confined to the upper troposphere, between 300 and
100 mb. The relationship between lower convergent and upper divergent levels is strong.
However, we have not found any simple relationship between eddy shedding and lower-level
dynamics.
Thesis Supervisor: R. Alan Plumb
Title: Professor of Meteorology
Acknowledgments
First, I would like to thank my advisor, Prof. R. Alan Plumb, for supporting me and encouraging
me to work independently. I am thankful to him for providing me with the opportunity to work
on this interesting topic at a crucial point in my studies. I thank my former advisor, Prof. John
Marshall, for taking me on as a student and for his overwhelming enthusiasm.
During three years of my graduate studies at MIT, I enjoyed working in the friendly atmosphere
at EAPS department. There are a number of people who helped make my studies pleasant and,
when it was necessary, "possible to survive". I am thankful to all of them. Here, I mention just
a few individually. My classmates Pablo Zurita, Tieh-Yong Koh, and Caixia Wang were always
there for me to discuss the topics related to our classes and research. My devoted friend Caixia,
shared with me all the good and bad moments that we encountered during our studies. Francois
Primeau gave me much scientific advice at the beginning of my studies and research here. Natalia
Beliakova was a great friend, full of useful advice and optimism. The results of Juno Hsu's thesis
were an important motivating factor for the research presented in this thesis. I am thankful for her
advice and encouragement to continue in this direction. Rebecca Morss and Constantine Giannitsis
helped greatly with their comments and suggestions on the first drafts of this thesis. I am indebted
to Vince Larson, who helped a great deal in the proofreading of the final version of this thesis.
Thanks to Linda Meinke for maintaining the computer networks on all three floors where I
worked. Stacey Frangos helped answer all possible questions about the administrative aspects of
graduate school since the time when I was still in Belgrade applying for graduates studies here.
Finally, I am grateful to my family. I dedicate this thesis to my parents Mirjana and Marko
Popovic. Their love, support, and encouragement has benefited me without measure. I am indebted
to my grandmother Ana Popovic for her wisdom and love. Thanks to Djordje Popovic for being
the best brother in the world. In the five years since I met Marko Popovic (Junior), he has been a
tremendous source of love and companionship. I thank him for bearing and sharing all things with
me, positive and negative, big and small. Without his help and encouragement I would not have
been able to finish this thesis.
Contents
11
1 Introduction
2
1.1
Monsoon dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
1.2
Theoretical aspects of eddy shedding . . . . . . . . . . . . . . . . . . . . . .
14
1.3
O utline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
18
Data
2.1
NCAR/NCEP Reanalysis
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
2.2
Data set from 1987 to 1990. . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
21
3 Results of data analysis
3.1
Isentropic coordinates and the time mean fields . . . . . . . . . . . . . . . .
21
3.2
Eddy shedding in the atmospheric data . . . . . . . . . . . . . . . . . . . . .
26
3.3
Other cases of eddy shedding
. . . . . . . . . . . . . . . . . . . . . . . . . .
39
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
. . . . . . . . . . . . .
45
3.4
4
3.3.1
July 1987
3.3.2
July 1988.........
3.3.3
July 1989
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
3.3.4
July 1990
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
3.3.5
A ugust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
Relation to the lower levels. . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
Conclusion
....
. . . . . . .
67
List of Figures
1-1
July 1987-90 mean precipitation rate and wind vectors at 850 mb. . . . . . .
12
1-2
January 1987-90 mean precipitation rate and wind vectors at 850 mb. . . . .
12
3-1
July 1987-90 mean geopotential height field at 200 mb. . . . . . . . . . . . .
22
3-2
July 1987-90 mean potential temperature zonally averaged from 40'E to 100'E. 23
3-3
July 1987-90 mean IPV field at 370 K. . . . . . . . . . . . . . . . . . . . . .
24
3-4
July 1987-90 mean wind field at 370 K. . . . . . . . . . . . . . . . . . . . . .
25
3-5
July 1987-90 mean Montgomery streamfunction field at 370 K. . . . . . . . .
25
3-6
1987-90 mean IPV field at 370 K for December to February (top three plots)
and June to August (bottom three plots).
3-7
. . . . . . . . . . . . . . . . . . .
Hovm6ller diagram of IPV for July 1990 at 370 K, averaged over the 100
latitudinal band between 25'N and 35oN. . . . . . . . . . . . . . . . . . . . .
3-8
30
Hovm6ller diagram of Montgomery streamfunction for July 1990 at 370 K
averaged over the 100 latitudinal band between 25'N and 35'N.
3-9
27
. . . . . . .
31
Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13,
1990. Minimum value of displayed contour lines is 356000 m 2 S-2. Contour
line interval is 250 m 2 s2.
. . . . . . . . . . . . . . . . . . . . . . . . . . . ..
32
3-10 Time sequence of IPV (color shading) and M (line contours) fields at 360K
given in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13,
1990. Minimum value of displayed contour lines is 350500 m 2 S-2. Contour
line interval is 250 m
2
S-2.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
3-11 Time sequence of IPV (color shading) and M (line contours) fields at 380K
given in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13,
1990. Minimum value of displayed contour lines is 361500 m2 s2. Contour
line interval is 250 m 2 S-2.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
3-12 Time sequence of IPV (color shading) and M (line contours) fields at 350K
given in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13,
1990. Minimum value of displayed contour lines is 343500 m2 s2. Contour
.
line interval is 250 m 2 s2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
3-13 Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 27.5'N and 37.5'N, given in
successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13, 1990.
The values smaller than -0.5 PVU are shaded. . . . . . . . . . . . . . . . . .
36
3-14 Time sequence of vertical cross sections of the perturbation from the zonal
mean of M, latitudinally averaged between 27.5'N and 37.5'N, given in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13, 1990. The
values greater than 500 m 2 s- 2 are shaded.
. . . . . . . . . . . . . . . . . . .
37
3-15 Hovm6ller diagrams of IPV at 370 K averaged over the 100 latitudinal band
between 25'N and 35'N, for June (top), July (middle), and August (bottom)
of 1987 (left) and 1988 (right). . . . . . . . . . . . . . . . . . . . . . . . . . .
40
3-16 Hovm6ller diagrams of IPV at 370 K averaged over the 10' latitudinal band
between 25'N and 35'N, for June (top), July (middle), and August (bottom)
of 1989 (left) and 1990 (right). . . . . . . . . . . . . . . . . . . . . . . . . . .
41
3-17 Hovm6ller diagrams of M at 370 K averaged over the 10' latitudinal band
between 250 N and 35 0 N, for June (top), July (middle), and August (bottom)
of 1987 (left) and 1988 (right). . . . . . . . . . . . . . . . . . . . . . . . . . .
42
3-18 Hovm6ller diagrams of M at 370 K averaged over the 100 latitudinal band
between 250 N and 35 0 N, for June (top), July (middle), and August (bottom)
of 1989 (left) and 1990 (right). . . . . . . . . . . . . . . . . . . . . . . . . . .
43
3-19 Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 36-hour intervals from 06 UTC July 5 to 18 UTC July 9,
1987. Minimum value of displayed contour lines is 356000 m2 s2. Contour
line interval is 250 m 2 s2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .46
.
3-20 Time sequence of vertical cross sections of the perturbation from the zonal
mean of M, latitudinally averaged between 25'N and 35 N, given in successive
36-hour intervals from 06 UTC July 5 to 18 UTC July 9, 1987. The values
greater than 500 m 2 s- 2 are shaded. . . . . . . . . . . . . . . . . . . . . . . .
47
3-21 Time sequence of IPV (color shading) and M (line contours) fields at 370K
given in successive 24-hour intervals from 12 UTC July 25 to 12 UTC July 28,
1987. Minimum value of displayed contour lines is 356000 m2 s-2. Contour
line interval is 250 m2 s. . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
48
3-22 Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 25'N and 35'N, given in successive 24-hour intervals from 12 UTC July 25 to 12 UTC July 28, 1987. The
values smaller than -0.5 PVU are shaded. . . . . . . . . . . . . . . . . . . . .
49
3-23 Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 24-hour intervals from 18 UTC July 9 to 18 UTC July 12,
1988. Minimum value of displayed contour lines is 356000 m2 s2. Contour
line interval is 250 m 2 s
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
3-24 Time sequence of vertical cross sections of the perturbation from the zonal
mean of M, latitudinally averaged between 25'N and 35'N, given in successive
24-hour intervals from 18 UTC July 9 to 18 UTC July 12, 1988. The values
greater than 500 m2 s-2 are shaded. . . . . . . . . . . . . . . . . . . . . . . .
3-25 Time sequence of IPV (color shading) and M (line contours) fields at 370K
given in successive 18-hour intervals from 12 UTC July 26 to 18 UTC July 28,
1988. Minimum value of displayed contour lines is 356500 m2 s2. Contour
line interval is 250 m 2 s2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .53
.
51
3-26 Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 250 N and 35'N, given in successive 18-hour intervals from 12 UTC July 26 to 18 UTC July 28, 1988. The
values smaller than -0.5 PVU are shaded. . . . . . . . . . . . . . . . . . . . .
54
3-27 Time sequence of IPV (color shading) and M (line contours) fields at 370K
given in successive 48-hour intervals from 18 UTC June 28 to 18 UTC July 4,
1989. Minimum value of displayed contour lines is 356000 m 2 S-2. Contour
line interval is 250
m 2 S~2.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
3-28 Time sequence of vertical cross sections of the perturbation from the zonal
mean of M, latitudinally averaged between 25'N and 35 N, given in successive
48-hour intervals from 18 UTC June 28 to 18 UTC July 4, 1989. The values
greater than 500 m 2 s-2 are shaded. . . . . . . . . . . . . . . . . . . . . . . .
57
3-29 IPV (color shading) and M (line contours) fields at 370 K given at 18 UTC
July 11, 1988 (first plot), 18 UTC July 16, 1988 (second plot), 06 UTC, July
23, 1988 (third plot), and 06 UTC, July 27, 1988 (fourth plot). Minimum
value of displayed contour lines is 356000 m2 s2. Contour line interval is 250
m 22 s ---2 .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
3-30 Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 24-hour intervals from 12 UTC July 18 to 12 UTC July 21,
1990. Minimum value of displayed contour lines is 356000 m2 s2. Contour
.
line interval is 250 m 2 s 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .59
3-31 Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 25'N and 35 N, given in successive 24-hour intervals from 12 UTC July 18 to 12 UTC July 21, 1990. The
values smaller than -0.5 PVU are shaded. . . . . . . . . . . . . . . . . . . . .
3-32 Time sequence of IPV (color shading) and M (line contours) fields at 370
K given in successive 30-hour intervals from 06 UTC August 25 to 00 UTC
August 29, 1988. Minimum value of displayed contour lines is 356000 m2 sContour line interval is 250 m 2 s
2.
.
.
. . . . . . . . . . . . . . . . . . . . . .61
60
3-33 Time sequence of IPV (color shading) and M (line contours) fields at 370
K given in successive 48-hour intervals from 12 UTC August 7 to 12 UTC
August 13, 1987. Minimum value of displayed contour lines is 356000 m2 s-2
Contour line interval is 250 m 2 s2. .
. . . . . . . . . . . . . . . . . . . . . ..
62
3-34 Time sequence of geopotential height at 850 mb given in successive 24-hour
intervals from 12 UTC July 20 to 12 UTC July 27, 1989. Contour line interval
is 10 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
3-35 Time sequence of geopotential height at 850 mb given in successive 12-hour
intervals from 00 UTC July 16 to 12 UTC July 19, 1988. Contour line interval
is 10 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
Chapter 1
Introduction
1.1
Monsoon dynamics
Monsoonal circulations are found over tropical regions of Asia, Australia, Africa and to
some extent North America. A circulation is considered monsoonal if the wind reverses
direction between summer and winter. In summer, monsoons are characterized by intense
precipitation. The accumulated rainfall in monsoonal areas is generally much higher than
in other parts of the world. The important driver of monsoonal circulation is the landsea contrast, i.e. the different thermal characteristics of land and sea surfaces. Therefore,
monsoons are found where a continent borders an ocean, and where wind blows inland from
the cooler oceans toward the warm continents in summer, and from the cold continents
toward the warm oceans in winter.
Figures 1-1 and 1-2 show the precipitation and lower level winds (850 mb), for July
and January of 1987-1990. In northern summer, enhanced precipitation is found in central
America, western tropical Africa, and southern Asia. In southern summer, enhanced precipitation is found in tropical South America, in southern tropical Africa, and in Indonesia and
northern Australia. The African monsoon seems to be the weakest in terms of precipitation,
but a wind reversal is present there. Intense rainfall in the tropical area of the American
continent suggests a monsoonal circulation. However, a wind reversal is not seen there. The
best example of monsoonal circulation is found in the Asian-Australian area. Here we see
both a drastic wind reversal and extremely abundant rainfall.
July
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The Asian summer monsoon alone is so strong that it appears to be one of the main
contributors to the zonally averaged fields of the Hadley cell. The reason for such a strong
monsoonal circulation, beside the strong land-sea contrast, is the presence of the characteristic topographic feature in this region - the Tibetan plateau. This massive feature, with
average height of 4000 m, influences the monsoonal circulation by acting as an additional
heat source as well as a mechanical barrier for the winds.
During the Asian summer monsoon, the lower troposphere is characterized by strong
cross-equatorial flow (Figure 1-1).
Southwesterly flow, off the Eastern African coast and
over the Arabian Sea, brings moisture to the Indian subcontinent. Warming of the continent
and near-surface air results in strong convection, and hence latent heat release. This latent
heating, enhanced by sensible heating from the Tibetan plateau, helps warm the whole
troposphere in this area. The low-level convergence and ascending motion in this area of
strong convection are associated with the strong divergent flow in the upper troposphere.
Large scale anticyclonic circulation is seen in the area of the Asian summer monsoon during
the whole summer.
The Asian summer monsoon lasts for four months. The onset of the monsoon, which is
marked by a sudden change of wind direction and a spectacular onset of rainfall, is usually
very rapid. The onset of the monsoon over southern tip of the Indian subcontinent normally
occurs between May 25 and June 1. Then the monsoon gradually progresses onto land. It
is normally established over the entire Indian subcontinent by July 1.
During the established phase, the monsoon exhibits substantial variability. Monsoon
depressions are synoptic systems, of roughly a few days duration, which occur several times
during the established phase. These are accompanied by a large amount of rainfall. The
active/break cycle happens on time scales of one to two weeks. Active and break monsoon
spells exhibit contrasting features with respect to rainfall and flow patterns (Desai 1987, Das
1987). These phases of the monsoon are associated with the intensity and position of the
monsoon trough. The monsoon trough represents the zone of low pressure over the plains
of northern India. The axis of this trough is normally oriented in a northwest-southeast
direction and it normally extends up to the 500 mb level, tilting southwards with height.
The active phase of the monsoon occurs when the trough axis is to the south of its normal
position with one or two cyclonic vortices embedded in it. It is characterized by a large
amount of well-distributed rainfall over the plains of Northern India, the central parts of
the country, and along the west coast. The break in monsoon activity happens when the
trough axis moves to the north and lies close to the Himalayas. In this phase, the heavy and
well-distributed rainfall is only found along the foothills of the Himalayas, northeast India,
and southeast Peninsular India.
The retreat of the monsoon starts around September 1 in the northwestern parts of India.
The monsoon normally withdraws from northern and central India by October 15.
The amount of rainfall received during the summer monsoonal season as well as its
timing clearly establish the great importance and need (economic and otherwise) for better
understanding of monsoonal dynamics. Intensive interest is especially expressed in India and
other countries directly influenced by monsoons. Because of this, the published literature on
the theory and prediction of monsoons is extensive. However, the role and importance of the
Asian summer monsoon, although greatest in the areas of abundant rainfall, is not limited
only to these regions. It is also part of the global scale circulation in the tropics, and it is
important to understand it in this broader aspect as well (recent literature includes Hoskins
and Rodwell 1995, Sardeshmukh and Held 1984).
This thesis focuses mainly on the large scale upper tropospheric divergent circulation in
the area of the Asian summer monsoon and the eddy shedding phenomenon which is found
to appear in this region. The relation between this upper tropospheric phenomenon and
dynamics of the lower tropospheric levels is addressed as well.
1.2
Theoretical aspects of eddy shedding
The dynamically interesting phenomenon of "eddy shedding", found both in numerical
experiments and observations (Hsu, 1998), inspired this work. The term "eddy shedding"
implies the existence of a "mother" vortex from which secondary (or "daughter") vortices
detach (shed) and propagate away. In the numerical model of Hsu (1998), for certain ranges
of parameters, eddy shedding occurs periodically, and a daughter vortex is seen to propagate
either westward or eastward depending on the type of the asymmetry imposed. Looking at
atmospheric data and concentrating on the month of July 1990, Hsu (1998) found two eddy
shedding events in the monsoonal anticyclone, characterized by westward propagation of the
secondary vortex. However, does eddy shedding happen every summer? How often does it
happen? Is there any periodicity in eddy shedding occurrence? What are the circumstances
responsible for its occurrence? What are its time and space scales? How does this upper
tropospheric phenomenon interact with the lower level dynamics of the monsoon? To address
these and similar questions we analyze atmospheric data and present the results of our
analysis in Chapter 3.
The current theoretical understanding of eddy shedding dynamics developed to some
extent along the line of the studies related to Hadley cell dynamics. The Hadley cell, an
important part of the general circulation of the atmosphere has been successfully modeled
using simple balances for angular momentum and thermal energy. One of the first in a series
of papers dealing with the Hadley circulation was by Held and Hou (1980). They considered
the axially symmetric circulation forced by the annually averaged thermal forcing centered on
the equator. For a (nearly) inviscid atmosphere they obtained a solution with two overturning
cells symmetric around the Equator and confined to the region of the angular momentum
conservation (AMC). Outside that region, the solution was in thermal equilibrium (TE).
In a following paper by Lindzen and Hou (1988), a more realistic solution was obtained by
applying a thermal forcing centered off the Equator. They obtained one strong (winter)
Hadley cell and a weak secondary cell in the summer hemisphere, similar to what one sees
by looking at the zonally averaged observational data.
The interesting conversion from the spherical geometry and axially symmetric solutions
of Held and Hou (1980) (the axis of symmetry being the Earth's axis of rotation) to the
f
plane geometry with axially symmetric thermal forcing (the axis of symmetry being vertical
axis in the center of the domain) was performed by Hsu and Plumb (1998).
Hsu and Plumb (1998) derived the equivalent of Held and Hou's solution in a shallowwater model on an
f
plane, consisting of AMC and TE solutions matched at a location
which is determined by requiring that no mass is lost within the region that the angular
momentum solution holds. The model simulates the thermally driven nonlinear, inviscid,
divergent circulation. The thermal forcing is included through the mass relaxation and is
locally axisymmetric. The goal of their study was to look at the behavior of the circulation
when non-axisymmetric elements are applied to the steady state axisymmetric circulation.
In Hsu and Plumb, the asymmetries were imposed in the problem in two ways - by
including a uniform zonal flow (uniform flow experiments) or by adding the 3 effect, i.e.
adding the planetary vorticity gradient (3 plane experiments). The control parameters for
the runs are expressed as the ratio of the velocity induced by the asymmetric element to the
magnitude of the localized divergent flow induced by the axisymmetric thermal forcing. If
the control parameter is smaller than some critical value, the model flow is in a "subcritical"
regime: the central vortex remains united but with the center shifted south-eastward (in the
uniform flow experiments) or south-westward (in the
#
plane experiments). For the larger
values of the control parameters, the model flow is in a "supercritical" regime: the vortex
structure is distorted such that the main anticyclone periodically sheds eddies eastward (in
the uniform flow experiments) or westward (in the
/3 plane experiments).
The model in Hsu and Plumb may correspond to the flow in the upper troposphere in
the Asian summer monsoon because the flow in this area is anticyclonic, i.e. divergent. In
addition, studies of the vorticity budget show that the vorticity balance is essentially nonlinear and nearly inviscid in this area (Sardeshmukh and Held 1984, Sardeshmukh and Hoskins
1985). The primary balance is between the divergence term and advection of absolute vorticity by the time mean horizontal flow. These conclusions are drawn from the vorticity
budget derived both from the ECMWF (European Center for Medium Range Forecast) data
analysis (Sardeshmukh and Hoskins 1985) and from a general circulation model (Sardeshmukh and Held 1984) which produces a reasonable simulation of the large-scale features of
the northern summer tropical upper tropospheric flow without the inclusion of sub-grid scale
processes that strongly damp the upper tropospheric vorticity.
1.3
Outline
The upper tropospheric dynamics in the area of the Asian summer monsoon is the main
focus of this observational study. We analyzed summers of 1987 to 1990, looking for the
evidence of eddy shedding, previously found by Hsu and Plumb (1998) for summer 1990.
Chapter 2 contains the description of data used in the study. Chapter 3 presents the
results of the observational study. It starts with a presentation of the summer climatology.
This is followed by a sample case of eddy shedding from July 1990. The main characteristics
of the upper tropospheric circulation in the Asian monsoonal region, and other eddy shedding
events, are presented next. At the end of Chapter 3, the relation to the lower tropospheric
dynamics is discussed. Chapter 4 summarizes the most important conclusions of this thesis.
Chapter 2
Data
2.1
NCAR/NCEP Reanalysis
The data used in this study are the NCEP/NCAR (National Center for Environmental
Prediction/National Center for Atmospheric Research) Reanalysis data. This data set is
global with horizontal resolution 2.50 latitude by 2.5' longitude (73 by 144 grid points).
There are 17 standard pressure levels used: 1000, 925, 850, 700, 600, 500, 400, 300, 250,
200, 150, 100, 70, 50, 30, 20, and 10 mb. Data output is available every 6 hours at 0000,
0600, 1200 and 1800 UTC. We use pressure level data of zonal and meridional wind components (u and v), air temperature (T), and geopotential height (h), from 1987 to 1990. The
NCEP/NCAR reanalysis also provides data of relative and specific humidity and the vertical
velocity component (w) at 8 lower levels from 1000 to 300 mb. These data are available on
the tapes stored in netCDF files.
Other fields that are available at this same horizontal resolution include surface data
and data at the tropopause level (temperature and pressure). Set of data including fluxes,
precipitation, clouds, and similar are given on a Gaussian grid, with a resolution of 94 by
192 grid points. From this set of data we looked at the precipitation rate. Daily means of
these data fields are available to us from the NCEP/NCAR web page.
The main point of the reanalysis project is to have a unified way of constructing a gridded
data set throughout a long period of time. Therefore, in the NCEP/NCAR Reanalysis, a
frozen state-of-the-art analysis/forecast system was used to perform data assimilation using
the past data from 1957 to 1997 (Kalnay et al., 1996). The same frozen analysis/forecast
system continued to be used from 1997 to the present. The observing system has been
changing with time (since 1957). The Reanalysis project makes use of the most data available
at any time.
In the Tropics, one of the greatest improvements of analyses and forecasts was made by
the replacement of 01 (optimal interpolation) analysis by SSI (spectral statistical interpolation) analysis (Kalnay et al. 1996). Variables that were most sensitive to the changes in
the analysis-forecast system were upper-level divergent flow, precipitation, and stratospheric
winds.
In general, there are four categories of data in the Reanalysis, depending on how much
information comes directly from the observations and how much is determined from the
model. The most reliable class of data (class A), in which the analysis variable is strongly
influenced by observed data, includes temperature, wind and geopotential height. Class B
consists of those analysis variables whose values are directly affected by the observational
data, but which are also strongly influenced by the model. Some examples are humidity,
vertical velocity, surface temperature.
The diagnostic fields mostly fall into class C: no
observations directly affect the variable, and the values are determined solely from the model
fields, forced by the data assimilation to remain close to the atmosphere (e. g. precipitation,
clouds, surface fluxes).
And the forth class D represents fields which are obtained from
climatological values and which do not depend on the model (e.g. surface roughness, landsea mask).
2.2
Data set from 1987 to 1990
In the set of four years that we analyzed, Asian summer monsoon rainfall was below normal
in 1987 and above normal in 1988 to 1990. The contrast in the amount of monsoonal rainfall
between 1987 and 1988 was studied by a number of authors (e.g. Krishnamurti 1989, 1990,
Inoue 1997, Kalnay et al. 1996, Schaack and Johnson 1994). In some of these studies, the
relation between the El Nino-Southern Oscillation (ENSO) "state" and the amount of the
monsoonal rainfall is drawn. This is because 1987 was a warm ENSO episode (El Nino) and
1988 was a cold ENSO episode (La Nina). However, although weak monsoons (monsoons
with sub-normal rainfall) are frequently associated with El Nino, the correspondence between
the strength of the rainfall anomaly and the strength of El Nino is not always strong (Slingo
1997).
Chapter 3
Results of data analysis
3.1
Isentropic coordinates and the time mean fields
For the visualization of the dynamics of the upper tropospheric monsoon circulation, we
will mainly use isentropic (or 0) coordinates. This vertical coordinate is chosen because it
offers the best way of representing Ertel's potential vorticity (P). "PV-thinking," in turn,
is a powerful way of viewing dynamics (Hoskins et al. 1985). Ertel's potential vorticity is
in general defined as P = p-(, -VO, where p is density, (a is the absolute vorticity and 0 is
potential temperature (Andrews et al. 1987). In adiabatic, frictionless flow Ertel's potential
vorticity is conserved following the motion.
Therefore, it represents an excellent tracer
for motions on time scales smaller than the time scales for friction and diabatic heating.
Potential vorticity is very useful in one more sense. Once the potential vorticity field is
known, we can use the invertibility principle to calculate the streamfunction. Thence, we
are able to deduce the wind, pressure and temperature fields.
The NCEP/NCAR data are given on pressure levels. In order to work in isentropic coordinates, we had to interpolate the data to isentropic surfaces. Specifically, we first calculated
potential temperature 0 on pressure levels, then calculated the pressure for desired isentropic
surfaces, and finally linearly interpolated all the needed variables onto those isentropic surfaces.
SON
2200
- 1
12300
5g0
1
40N
_______________
1
0>
12250
400
12300
12450
12450
30N
12300
12350
12400
12350
20N
12400
12400
1ON
12450
EQ
10s
16 0E
180
160W
140W
120W
100W
80W
12400
60W
40W
20W
Figure 3-1: July 1987-90 mean geopotential height field at 200 mb.
Potential vorticity in isentropic spherical coordinates is given by
P
=aO
or
(3.1)
where
1 Op
=
(3.2)
-
is the effective "density" and
1 Ov
a cos # aA
1 u
ao#
u tan#
a
(3.3)
is the vertical component of absolute isentropic vorticity. The other symbols have their usual
meaning: g is gravitational acceleration, p is pressure, u and v are the horizontal components
of the wind, a is the radius of the Earth, and
#
and A represent the latitude and longitude
of the Earth.
Let us first look at the main characteristics of the upper tropospheric summer monsoon
'"""EQ
5N
10N
15N
20N
25N
30N
40N
35N
4NN
5ON
55N
60N
65N
Figure 3-2: July 1987-90 mean potential temperature zonally averaged from 40'E to 100'E.
circulation. In this study, we analyzed four years of data, from 1987 to 1990. July monthly
mean fields averaged over these four years are assumed to represent the July (summer)
climatology. The dominant feature of the 200 mb (upper troposphere) geopotential height
field is anticyclonic circulation centered over south Asia and extending over almost half of
the globe (Figure 3-1).
The large scale zonal asymmetry in the summer global mean circulation in the Asian
monsoon region is caused by diabatic heating associated with the monsoon. One of the
principal heat sources during northern summer is located in the region of the Asian summer
monsoon. The maximum heating exceeds 300 W m-
2
over the northern Bay of Bengal
(Yanai and Tomita 1998). From the analysis of the horizontal and vertical profiles of the
heat sources and moisture sinks, Yanai and Tomita showed that the major component of
the heat source during northern summer in the tropics is latent heat released by cumulus
convection. Over land areas at higher latitudes, including the Tibetan Plateau, their analysis
implies that sensible heating from the ground is also significant.
For our area of interest (-
30'N), the isentropic surfaces that correspond to the upper
5ON
40N -4
34
30N2ON1ONEQ-
--
1-
00
10S20W
0
20E
40E
60E
80E
12OE
1OE
140E
160E
50N
7
40N-
7
6
-
-6
5
6
5
30N20N-
4
4
4
5
-4
-3
1
EQ-
O1
0
0
lo s
.-
160E
--
180
160W
14bW
- - - - - - - - - - - - -
12OW
100W
----------------------1---
80W
60W
-
40W
-----20W
Figure 3-3: July 1987-90 mean IPV field at 370 K.
troposphere (between roughly 300 and 100 mb) are 350 K to 390 K. This may be seen from
Figure 3-2, where the time mean 0 field for July is zonally averaged between 40'E and 100'E.
Therefore, we chose the 370 K isentropic surface as the representative isentropic level of the
upper troposphere dynamics for most of the figures that follow.
The mean July isentropic potential vorticity (IPV) field at 370 K is shown in Figure 3-3.
Potential vorticity is given in PV units where 1 PVU
= 10-6
K kg
1
m 2 s-.
Tropospheric
values of PV are generally below about 1.5 PVU. Typically, there is a sharp jump of PV
at the tropopause to stratospheric values of about 4 PVU, and there is a further steep rise
of PV with height in the stratosphere. From Figure 3-3 we see that the upper tropospheric
anticyclone is characterized by low PV, considerably lower than at other places at the same
latitude on this isentropic surface. Elsewhere, such low PV values are found only within a
few degrees of the equator. This can be viewed as a signature of the recurrent convective
heating below.
Since we are analyzing the PV field on isentropic surfaces, it is convenient to interpolate
the wind to those surfaces, as well. Figure 3-4 displays the July mean wind field at 370 K in
Figure 3-4: July 1987-90 mean wind field at 370 K.
50N
40N
30N
20N
1ON
EQ
1Os
2
50N
40N
30N
20N
1ON
EQ
10S
Figure 3-5: July 1987-90 mean Montgomery streamfunction field at 370 K.
the area of the anticyclone. The Montgomery streamfunction (M) is another variable which
is natural to use in the isentropic coordinate system. The transformation of the horizontal
pressure gradient force from z to 0 coordinates is given by !Vzp
=
VOM. Therefore, in the
geostrophic balance in 0 coordinates, wind vectors are parallel to the isolines of M. The
Montgomery streamfunction is defined by
M = cT + D
where c,=1004 J kg-
1
(3.4)
K- 1 is the specific heat at constant pressure, T is temperature, and
4= gz is the geopotential. Mean July Montgomery streamfunction field at 370 K in units
of m2 s-2 is presented in Figure 3-5. Similar to the geopotential height field (Figure 3-1), the
Montgomery streamfunction field is characterized by mid-Pacific and mid-Atlantic troughs,
and most dominantly by the large scale anticyclonic circulation in the eastern half of the
globe. In what follows we analyze the characteristics of this anticyclonic circulation in more
detail.
3.2
Eddy shedding in the atmospheric data
In this and the following section we show evidence of eddy shedding in the atmosphere and
explain why it may occur. In July of four analyzed years, eddy shedding occurred several
times. In this section we present the event in July 1990, which appears to be the "cleanest"
case in these four years.
Before we look at the time evolution of the fields in the upper troposphere, we present
the monthly mean IPV fields at 370 K for winter and summer months of years 1987-90,
thus contrasting these two different seasonal regimes (Figure 3-6). In this and other color
figures that follow, the IPV on the 370 K isentropic surface is plotted such that only the
tropospheric values between 0 and 1.5 PVU are color shaded. Higher values are not shaded
so that white areas on the plots represent high PV stratospheric air. There are also small
white areas of negative PV bordered by red color.
Let us take a closer look at Figure 3-6. For the moment we look separately at the eastern
.....................
I .:I..
. I ................
.......................................
..
...
,
............
30N
20N
iON
EQ
60E
0
50N -
0
60W
120W
18uary180
120E
-
30N
20N
1ON
EQ
60E
10N -0
50N40N-.
30N
20N
iON
EQ
120E
Auus
0
60W
-
120W
180
M
060E
120OE
Jue180
2O6w0
EQ066E
120E
Juy180
2O6w0
666E
12OE
Auut180
066E
120OE
50N30N20N
iONWHM
50N30N20N
iON
EQ
6
10
d
50N
30N20N
EQ
v
0
1801206w0
===OR
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1
1.2 1.3
1.4
1.5
PVU
Figure 3-6: 1987-90 mean IPV field at 3 70 K for December to February (top three plots)
and June to August (bottom three plots).
and western hemispheres. In the western hemisphere, low PV air is completely confined to
the tropics, rarely crossing 10'N in summer months (three lower plots) and migrating bandwise more northwards (up to 20'N) in winter months (three upper plots). This also indicates
that the tropical 370 K isentropic surface lies at lower altitudes in winter than in summer.
The extension of low PV further north in winter than in summer is even more evident in the
eastern hemisphere. However, one other feature, as we already noted in Figure 3-3, appears
to be dominant in summer months, especially in July - the "blob" of low PV over south and
southwest Asia. This low PV tropospheric air, associated with the anticyclonic divergent
flow in the upper troposphere, is almost completely "isolated" in the surrounding high PV
stratospheric air.
The next step is to analyze the time evolution of the IPV and streamfunction fields
during summer. One practical way of looking at time evolution is to make time-longitude
plots, also called Hovmller diagrams. Hovm6ller diagrams are a useful way to easily identify
stationary features, propagation of features, oscillatory nature of the field, etc. Here, these
diagrams are plotted such that time is on the ordinate, increasing from the top of the page
downwards, and longitude, from 20'W to 180'E, is on the abscissa.
Figures 3-7 and 3-8 show the Hovmller diagrams at 370 K for IPV and Montgomery
streamfunction, respectively, averaged over the 10' latitudinal band between 25'N and 35'N.
We consider every low PV feature in Figure 3-7 that separates from the main low PV body
(which itself remains quasi-stationary) and propagates westward, as a possible candidate for
eddy shedding. Two obvious features that fall into this category started around July 10 and
July 17, 1990. The first of these two was a particularly clear example of eddy shedding.
From now on, we shall refer to that event as the "sample case".
In Figures 3-9 to 3-14 we plot the time sequences of horizontal and vertical fields of the
sample case eddy shedding event in July 1990. Figure 3-9 represents the time sequence plotted every 24 hours, from July 10 at 00 UTC to July 13 at 00 UTC, on the 370 K isentropic
surface. The IPV field is color shaded as before. The contour lines are Montgomery streamfunction isolines with a minimum plotted (outer) isoline of 356000 m 2 S-2 and a contour
interval of 250 m 2 s2. In this event, what appears as one unified elongated anticyclonic vortex at the beginning of the shown time period, gradually becomes divided into two separate
vortices. We notice from Figure 3-9 that low IPV pulled away from the mother vortex (main
low IPV body on the eastern side) and started to move southwestwards. After the filament
reached 40'E at 00 UTC on July 11 (second plot), the secondary anticyclonic vortex was
shed on the western side and propagated westwards, with its center reaching 30'E (southeast
Mediterranean) by July 13 (fourth plot). After July 13, the secondary vortex existed for two
more days. During that time it intensified and moved northward to the area of the strong
westerly jet. Then, it was advected eastward by the jet and recaptured by the mother vortex.
The divergent circulation in the upper troposphere is maintained by strong diabatic heating
in the monsoonal circulation. The daughter vortex which was shed and moved westward,
moved away from the source of its maintenance, as well. Its life is therefore not expected to
be very long.
It is important to notice that in the days preceding the shedding, the anticyclone was
stronger than usual. This can be seen from the Hovm6ller diagram of M in Figure 3-8
and the horizontal sections of M and IPV (not shown). In addition, the anticyclone was
becoming more and more elongated (the eccentricity of the isolines of M in the central
region especially increased). The flow became almost parallel, i.e. almost purely westerly in
the northern part and almost purely easterly in the southern part of anticyclone. The almost
zonal flow suggests one possible explanation for why the eddy shedding occurred. In these
circumstances we may apply the criteria for instability in the parallel (shear) flow locally the change of sign in the potential vorticity gradient implies that the necessary condition
for instability has been met. As a consequence, eddy shedding occurs, and by July 13 we
notice two separate anticyclones in Figures 3-9 to 3-11. The existence of this dynamically
unstable condition, which is necessary to generate the transient eddies in the observations,
agrees well with the results that Hsu (1998) obtained in her numerical model.
The eddy shedding phenomenon is relatively shallow. Figures 3-13 and 3-14 show the
vertical structure of the deviation from the zonal mean of the IPV and M fields in our sample
case. In these figures, vertical cross sections are latitudinally averaged between 27.5'N and
37.5'N. The negative values of the deviation from the zonal mean, in Figure 3-13 arise
because the potential vorticity field has much lower values in the region of interest compared
to other longitudes. To stress the important areas, the values smaller than -0.5 PVU are
IPV 25-35N
370 K
1JUL1990
6JUL 1990-
11JUL 1990 -
16JUL 1990-
21JUL 1990 -
26JUL 1990-
1AUG1990 I
20W
0
0
20E
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1 1.1 1.2 1.3 1.4 1.5 PVU
Figure 3-7: Hovm6ller diagram of IPV for July 1990 at 370 K, averaged over the 100 latitudinal band between 25'N and 35'N.
MSF 25-35N
370 K
IJUL 1990 -
6JUL 1990-
11JUL1990 -
16JUL 1990-
21JUL 1990 -
26JUL 1990-
IAUG1990
20W
3 .5625
0
3.565
2
26E
3.5675
140E
3.57
.5725 3.55 -3-.5775 5
160E
180
.508
Figure 3-8: Hovm5ller diagram of Montgomery streamfunction for July 1990 at 370 K averaged over the 100 latitudinal band between 25'N and 35 0 N.
....
..
....
50N
45N
40N
35N
30N
25N
20N
15N
iON
5N
140E
45N40N35N30N25N20N 15NiON5NEQ 20W
0
20E
40E
60E
80E
10E
120E
50N -45N40N35N30N25N20N 15NION
5N
EQ
20W
140E
1
160E
OOZ12JUL1990
50N --
45N40N35N30N25N20N15NiON
5N
EQ
20W
160E
OOZ1 1JUL1990
50N -_
0
20E
40E
60E
80E
100E
120E
140E
160E
0OZ1 3JUL1 990
0
20E
40E
60E
80E
100E
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
120E
140E
160E
1 1.1 1.2 1.3 1.4 1.5
180
PVU
Figure 3-9: Time sequence of IPV (color shading) and M (line contours) fields at 370 K given
in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13, 1990. Minimum
value of displayed contour lines is 356000 m 2 S2. Contour line interval is 250 m 2 S-2
50N
45N
40N
35N
30N
25N
20N
15N
1ON
5N
EQ
20W
OOZ10OJUL1 990
360K irv&msf
0Z
ipv&msf
360K
0
20E
40E
0
20E
40E
60E
80E
10E
120E
100E
120E
J~
9
140E
160E
140E
160E
OOZ1 JUL1990
50N
45N
40N
35N
30N
25N
20N
15N
1ON
5N
EQ
20W
OOZ12JUL1990
50N
45N
40N
35N
30N
25N
20N
15N
1ON
5N
EQ
20W
1
160E
140E
OOZ1 3JUL1990
50N
45N
40N
+
35N
30N
25N
20N
15N
6
1
r
'
?-1f1'
EQ2
~cc
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1
1.2 1.3 1.4 1.b
PVU
Figure 3-10: Time sequence of IPV (color shading) and M (line contours) fields at 360K given
in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13, 1990. Minimum
value of displayed contour lines is 350500 m 2 s-2. Contour line interval is 250 m 2 s-2
50N -
45N
40N
35N
30N
25N
20N
15N
1ON
20W
0
20E
40E
60E
80E
100E
120E
140E
50N --
45N40N35N30N
25N
20N15N
1ON
5N
EQ
20W
50N
45N
40N
35N
30N
25N
20N
15N
1ON
160E
180
OOZ11JUL1990
0
40E
20E
60E
80E
100E
120E
140E
160E
OOZ12JUL1990
.
5N
20W
50N - 45N 40N35N30N
25N
20N
15N
1ON
5N
EQ
20W
0
20E
40E
60E
80E
100E
140E
120E
160E
OOZ1 3JUL1990
6
20E
40E
60E
80E
100E
120E
140E
160E
180
E,
0
0.1 0.2 0.3 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.5
PVU
Figure 3-11: Time sequence of IPV (color shading) and M (line contours) fields at 380K given
in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13, 1990. Minimum
value of displayed contour lines is 361500 m 2 s-2. Contour line interval is 250 m 2 S-2
50N
45N
40N
35N
30N
25N
20N
15N
ION
5N
EQ
2 Ow
0OZ10JUL1990
350K ipv&msf
0
20E
40E
6OE
80E
100E
120E
0
20E
40E
60E
80E
100E
120E
0
2OE
46E
66E
86E
100E
50N
45N
40N
35N
30N
25N
20N
15N
5N
EQ
OW
160E
18(
140E
160E
18,
OOZ12JUL1990
50N
40N
35N
30N
25N
20N
15N
iON
5N
EQ
140E
OOZ1 1JUL1990
120E
50N
45N
40N
35N
30N
25N
20N
15N
iON
5N
EQ
20W
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
140E
160E
OOZ13JUL1990
1
1. 1 1.2 1.3 1.4 1.5
PVU
Figure 3-12: Time sequence of IPV (color shading) and M (line contours) fields at 350K given
in successive 24-hour intervals from 00 UTC July 10 to 00 UTC July 13, 1990. Minimum
value of displayed contour lines is 343500 m 2 s-2. Contour line interval is 250 m2 s-2
zm
27.5-37.5N
dev ipv
0OZ10JUL1990
0OZ1 1JUL1990
390
380370-
-3
360
350
A
340-
.
330320
20W
3 -0.5
2
.0-.5
. . . -0
. . 5. .
.
0
20E
-.
.
.
.
.
40E
.
.
.
.
.
.
.
60E
.
.
80E
120E
100E
140E
160E
1
OOZ 12JUL1990
390--
-2
380370360-,
350-0.5
340
4
330320
20W
0
20E
46E
66E
80E
120E
100E
--
380370-
160E
1 80
0-25
-0.5
-
-
360
2
-~-----
350
0.5
340-
5
- --- -~~
- -~
330
20W
140E
0OZ13JUL1990
390-
320
0
2
6
26E
40E
60E
86E
100E
~
---
- - - -
120E
14OE
160E
180
Figure 3-13: Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 27.5'N and 37.5'N, given in successive 24-hour
intervals from 00 UTC July 10 to 00 UTC July 13, 1990. The values smaller than -0.5 PVU
are shaded.
390380-
37036035034033032021
0
20W
20E
40E
60E
80E
100E
120E
140E
160E
180
OOZ1 3JUL1 990
3901000
-500
380370
15000
1 000
360
350 340V-,
1)0000
0
-
330-
320- -20W
0-
5500
-
0
20E
40E
60E
80E
100E
120E
140E
160E
180
Figure 3-14: Time sequence of vertical cross sections of the perturbation from the zonal
mean of M, latitudinally averaged between 27.5'N and 37.5'N, given in successive 24-hour
2
2
intervals from 00 UTC July 10 to 00 UTC July 13, 1990. The values greater than 500 m sare shaded.
shaded. The strongest deviation from the zonal mean is found close to the 370 K level. The
magnitude of the deviation from the zonal mean decreases downward but at the same time
the absolute value of the PV field itself decreases as well. The eddy shedding in Figure 3-13
is easily recognized by looking at large deviations from the zonal mean. Contour lines of -1.5
and -2 PVU are connected on July 10, and even then -2.5 PVU contour is almost connected.
With time, the -1.5 and -2 PVU contours separate as the western part sheds and moves
westward, causing the -1.5 and -2 PVU contours to separate.
From Figure 3-13, eddy shedding in the IPV field is confined to the layer between the
tropopause (-380 K) and the 360 K surface. We see this also in Figures 3-10 and 3-11, which
show the IPV and M fields at the 360 K and 380 K isentropic surfaces. Note that the range
of shaded PV values in Figure 3-11 extends to 2.5 PVU. At 350 K (Figure 3-12), the low IPV
of the monsoonal anticyclone is not separated from the low IPV in the tropics. However,
the shedding in the M field is seen even at these lower levels (down to 350 K). Although the
shedding is still shallow in the streamfunction field, the fact that it appears deeper in the
streamfunction field than in the IPV field is to be expected from the invertibility principle.
The expected depth (H) is about 7 km or roughly between 100 and 300 mb. (H = LR
LHR,
where the Rossby radius of deformation LR ~ 3000 km, the tropopause height HR ~ 15
km, and the horizontal extent of the eddies L ~ 1500 km.) Therefore, the shedding in the
streamfunction field is observed in Figure 3-12 (350 K), but it is not observed on the levels
below that.
In Figure 3-14, we see the evolution of the vertical structure in the M field. The Montgomery streamfunction has large values in the monsoon anticyclone, so that the deviation
from the zonal mean is positive in this area, exceeding 1500 m 2 S-2 near the 370 K surface.
Figure 3-14 also shows that the shedding in the streamfunction field is deeper than in the
PV field (Figure 3-13).
Eddy shedding in the sample case occurred when the primary vortex achieved a regular
elongated elliptical shape with an almost parallel flow. Therefore, the sample case almost
perfectly corresponds to the modeled flow in Hsu (1998). However, the atmosphere rarely
strictly conforms to any simple theoretical model and there are always variations and slight
differences. All other cases that we examine next have many similarities with the modeled
flow and our sample case from atmospheric data, but some differences as well.
Figures 3-15 to 3-18 represent small versions of Hovm6ller diagrams of IPV and M at 370
K for the 100 latitudinal band between 25'N and 35'N for June (top panels), July (middle
panels) and August (bottom panels) of four analyzed years. Years 1987 and 1988 are shown
in Figures 3-15 and 3-17, while years 1989 and 1990 are shown in Figures 3-16 and 3-18.
These plots show just the overall pattern of IPV and M fields throughout the summer on
the selected isentropic level of 370 K. Detailed structures will be shown in the next section
on the time sequences of selected events.
From Figures 3-15 and 3-16 we may immediately notice the following: from June to
August, at the isentropic level of 370 K, the bulk of the low PV (tropospheric) air in the
25'-35'N latitudinal band is confined between the longitudes of 20'E and 120'E. There
are periods of time when the low PV strip remained stationary, indicating the presence of
the anticyclonic vortex at almost the same position for several days. However, we also see
a number of westward-propagating features and a few eastward-propagating features. We
notice that most of the westward moving features coexisted with the almost stationary low
PV body, and these were actually the eddy shedding events. The eddies that are shed are
most often smaller than the eddies from which they are detached. Although smaller, they
are still large with diameters of 2000 to 3000 km. There were also two westward moving
features which did not have a low PV body counterpart on the east (August 1987 and July
1989). The anticyclone did not break up in these two cases but it propagated westward as
a whole.
3.3
Other cases of eddy shedding
The sample case of eddy shedding in the upper tropospheric monsoonal circulation that
we presented was the clearest example of this phenomenon in the four years we analyzed. In
this section we present all other eddy shedding cases that happened in July 1987 to 1990, as
well as other characteristics in the evolution of the chosen meteorological fields. All these are
depicted using the time sequences of IPV and M fields at 370 K (we showed four different
levels for the sample case) and time sequences of vertical cross sections of 10' latitudinally
June
1JUN 1987
June 1988
1987
6JUN1987
11 JUN 1987
16JUN1987
21JUN1987
26JUN1987
1JUL1987
21
July 1987
6JUL1987-
6JUL1987-
6JUL1988
1 1JUL1987-
IIJUL1988
16JUL1987-
16JUL1988-
21JUL1987
21JUL1988
26JUL 1987-
26JUL1988
1AUG1987
87 W
0
26E 46E 60E 86E 100E 120E 140E 160E I
1988
IJU 1988 -July
AUG 1988
20W
-
-
0
20E
40E 60E
80E 100E 120E 140E 160E I
0 0.1 0.2 0.30.4 0.0OA .7080S9
August 1987
6AUG1987
1AUG1988
6AUG1987-
6AUG1988
11AUG1987-
11AUGI 988 -
16AUG1987-
16AUG1988
21AUG1987-
21AUG1988
26AUG1987
26AUG1988-
1SEP19872
20W
0
20E
120E 140E 160E 180
August
-20W 0
20E
40E 60E
1 1.1 1.2 IJ
4 5
1988
120E 140E 160E 180
Figure 3-15: Hovm6ller diagrams of IPV at 370 K averaged over the 100 latitudinal band
between 25'N and 35 0 N, for June (top), July (middle), and August (bottom) of 1987 (left)
and 1988 (right).
June 1989
June 1990
1JUN1989
1JUN1990
6JUN1989
6JUN1990-
11JUN1989
11JUN1990-
16JUN1989
16JUN1990-
21JUN1989
21JUN1990-
26JUN1989
26JUN1990-
1JUL 1989
0OW
0
20E 40E 60E 80E 100E 120E140E 160E 180
July 1989
1JUL1989
1JUL1I99O
0W
6JUL1990-
11JUL1989
11JUL1990-
16JUL1989
16JUL1990-
21JUL1989-
21JUL1990-
26JUL 1989
26JUL1990-
20E 40E 60E 80E 100E 120E 140E 160E 180
20E
1AUGI99O
0W
46E 66E 86E 100E 120E 140E 160E 180
July 1990
1JUL199O
6JUL1989
1AUG1989
U 0W 0
0
0
20E
40E
60E 80E 100E 120E 140E 160E 180
0 0.1 0.20,3G0.AOA M7 OA 0.9
August 1989
1AUG1989
1AUG1990
~
11AUG1989
11AUG1990
16AUG1989
16AUG1990
21AUG1989
21AUG1990
26AUG1989-
26AUG1990-
1SEP1989
20W
P
6AUG1990 -t
6AUG1989
6
2OE
40E 60E 80E 100E 120E 140E 160E 180
1 1.1 1.2 1.3 1.4 I
August 1990
1SEP199
20W
0
20E
40E
60E 80E 100E 120E 140E 160E 180
Figure 3-16: Hovm6ller diagrams of IPV at 370 K averaged over the 10' latitudinal band
between 250 N and 35'N, for June (top), July (middle), and August (bottom) of 1989 (left)
and 1990 (right).
June
1JUN1987 -
1987
6JUN1987-
6JUN1988 -
11JUN1987 -
11JUN1988 -
16JUN1987-
16JUN1988-
21JUN1987 -
21JUN1988 -
26JUN1987-
26JUN1988-
1JUL19872IW 0
20E
S140E160E 11
.iii lo
1JUL1988-
6JUL 1987-
6JUL1988 -
11JUL1987-
11JUL1988-
16JUL 1987-
16JUL1988-
21JUL1987-
21JUL1988-
26JUL1987-
26JUL1988-
"
140E 160E 11
0
July 1988
1JUL1988-
0
W
QR7
iJUL1987-
1AUG1987 I
20W
June 1988
1JUN 1988 -
.
.
20E 40E 60E 80E 1OE 120E 140E 160E 11
1AUG1988 I-M
20W 0
3.5525 3.565 3.5675 3.57 3.5725 3.575 3.5775 3.58 3.5825
August 1987
6AUG1987-
1AUG1988
6AUG1988-
988.
11IAUG1
-
16AUG1987
21AUG1987.
21AUG1988.
26AUG1987-
26AUG1988-
1SEP198712 'W
0
--
16AUG1988 -
20E
120E 140E 160E 180
1SCP1981
20W
0
20E 40E
60E 80E 100E 120E 140E 160E 1SO
Figure 3-17: Hovm5ller diagrams of M at 370 K averaged over the 100 latitudinal band
between 25'N and 35 0 N, for June (top), July (middle), and August (bottom) of 1987 (left)
and 1988 (right).
June 1989
1JUN1989
6JUN1990-
6JUN1989 -
1 1JUN1989-
11JUN1990-
16JUN1989-
16JUN1990-
21JUN1989 -
21JUN1990-
26JUN1989-
26JUN1990-
1JUL1989-
1990
June
1JUN1990-
W
0
IJUL19902
)E 66E 86E 1d0E 120E 140E 160E 1
20E
6JUL 1990-
11JUL1989-
11JUL1990-
16JUL 1989-
16JUL 1990-
21JUL1989-
21JUL1990-
26JUL 1989-
26JUL 1990-
I
0
20E
.
1AUG1990
40E 60E 80E 100E 120E 140E 160E 1
August
1AUG1989
1989
6AUG1990
11AUG1989-
11AUG1990
16AUG1989-
16AUG 990
2 1AUG1989 -
21AUGI 990
26AUG1989-
26AUG1 990
2OW
0
20E
-
14Qn
140E 160E 180
Auaust 1990
1AUG1990
6AUG1989-
1SEP1989
140E 160E 180
20E
IJUL1990-
6JUL1989 -
1AUG1989
20W
IW 0
9
imv
JulV 1989
1JUL1989 -
tV-
-
0
140E 160E 180
1SEP 19902W
6
20
Figure 3-18: Hovm6ller diagrams of M at 370 K averaged over the 100 latitudinal band
between 25*N and 35 0 N, for June (top), July (middle), and August (bottom) of 1989 (left)
and 1990 (right).
averaged zonal deviations of IPV or M.
3.3.1
July 1987
July 1987 was marked by two eddy shedding events. The first one is shown in Figures
3-19 and 3-20, and it was not a very clear example. The second one is shown in Figures 3-21
and 3-22.
The upper tropospheric anticyclone at the beginning of July 1987 was rather elongated
and moderately strong. It was centered about 80'E and 25'N. For the first few days of
the month, the winds were almost pure westerlies in the northern part of the anticyclone
and pure easterlies in the southern part. The change of sign in the gradient of potential
vorticity in this parallel flow regime indicates the existence of the necessary condition for
dynamical instability. From what we have seen so far in the sample case, we may expect
to see eddy shedding occurring at some later time of the month. Actually, what started
on July 5 (first plot in Figure 3-19) may be considered as an eddy shedding event since we
observed two cells in the center of the large scale anticyclonic circulation. Figures 3-19 and
3-20 shows the first part of the ten day period when the two cells in the streamfunction field
existed. These figures show time sequences of the horizontal and vertical cross sections, in
time steps of 36 hours. However, this case also differed in several aspects from what we
have described so far as an eddy shedding event. Namely, the two cells remained at almost
the same position during their lifetime, i.e. the western cell did not move westward with
time. Another important difference was related to the IPV field. At first, it looked like
the other clear eddy shedding events. From the horizontal section we see how the low IPV
(yellow area) strip stretched to the west (second plot) and the new low PV cell formed on
the western side (third plot). However, later on the western low PV cell was broken and did
not last as long as the M western cell. From the vertical section of M we also notice the
transition from a one-vortex to a two-vortex structure. The strength and the depth of both
cells varied with time.
The anticyclone regained its unified form by the middle of the month, becoming rounder
(the isolines of the streamfunction achieving elliptical shapes of lower eccentricity). After
that short period the isolines became more elongated once more and the vortex became
stronger, with its center moving westward. As a result, on July 26 another eddy shedding
event started. From the time sequence with a 24 hours interval shown in Figures 3-21 and
3-22, we see that the elongated anticyclonic cell centered around 60'E and 30'N on July 25
(upper plot) started to gradually break into two entities (July 26 - second plot). On July
27 and 28, the mother vortex remained on the eastern side, building up with time while the
daughter vortex moved westward and weakened. This eddy shedding event ended three days
after the last time plot shown (bottom plot). This was one more case in which the low IPV
of the weakened daughter vortex was advected by the westerly jet toward the mother vortex.
3.3.2
July 1988
In summer 1988, the upper tropospheric circulation was much stronger than in the other
three analyzed years. For the first five days of July, the flow had a strong anticyclonic
circulation centered at 60'E and 30'N and also had rather low eccentricity. After July 5,
the anticyclone became weaker and much more elongated. The center itself gradually moved
eastwards to around 80'E and the whole system acquired much larger dimensions than usual.
From the plot on the top of Figure 3-23 we see that the streamlines were almost zonal, and
so was the wind direction in most of the area. However, this was not a typical case of eddy
shedding. In this case, the main anticyclonic cell remained almost at the same position,
losing much of its eccentricity by July 12 (bottom plot) and residing in the area of the
climatological mean position of the anticyclone. At the same time, on the northeastern end
of the large scale anticyclonic circulation, we observe a strong easterly jet and a filament
of low PV (upper plot) which rolled up in the anticyclonic direction.
By July 11 (third
plot), it built up into the secondary anticyclonic vortex which lasted for three days and then
vanished.
After this unusual event, by mid-July the anticyclone regained its (unusual) strength
and large eccentricity and remained like that for the following 10 days. It was particularly
strong around July 20. This rather long period of such a strong and large scale anticyclonic
circulation gradually started to change around July 25, resulting in another eddy shedding
event. At that time, the center of the large scale anticyclone shifted slightly northeastward
and a daughter vortex shed on the western side. An eighteen-hour time sequence of this
50N -
45N
40N
35N
30N
25N
20N
15N
1ON
5N
EQ
20W
0
20E
40E
6
20E
40E
60E
80E
100E
120E
60E
80E
10E
120E
140E
160E
180
18ZO6JUL1987
50N -
45N
40N
35N
30N
25N -
20N
/
15N
1ON
5N
EQ
20W
160E
140E
160E
140E
160E
18ZO9JUL 1987
50N -
45N40N35N30N
25N20N15N1ON
5N
EQi
20W
140E
06ZO8JUL1987
0
Iim
0
20E
40E
60E
80E
own===
100E
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
120E
180
--- a
1 1.1 1.2 1.3 1.4 1.5 PVU
Figure 3-19: Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 36-hour intervals from 06 UTC July 5 to 18 UTC July 9, 1987. Minimum
value of displayed contour lines is 356000 m 2 s-2. Contour line interval is 250 m 2 S-2
dev msf
zm
390-.,
6ZO5JUL1987
25-35N
380370500
360
350
10 0 . . ..
340 330
0
3201
20W
500.
- -- -
.. .
0
.5
0
20E
40E
60E
80E
120E
100E
140E
160E
12
390-38037036035034033032020
06ZO8JUL1987I
390
380370360 1000
150
--500
350-
500
340330-
0
-500
320
20 W
0
20E
40E
60E
86E
100E
120E
140E
160E
120E
140E
160E
390380-
-500-
370-
)00
360-
1500000
350- ,-500
340
0
3350
320
W
0
20E
40E
60E
86E
100E
180
Figure 3-20: Time sequence of vertical cross sections of the perturbation from the zonal mean
of M, latitudinally averaged between 25'N and 35*N, given in successive 36-hour intervals
from 06 UTC July 5 to 18 UTC July 9, 1987. The values greater than 500 m2 S-2 are shaded.
...........
.................
50N
45N
40N
35N
30N
25N
20N
15N
1ON
5N
140E
160E
12Z26JUL1987
20E
40E
60E
80E
100E
120E
140E
1
160E
12Z27JUL1987
45N 40N 35N30N 25N20N15N
1ON
5N
EQ
2DW
0
20E
40E
60E
80E
10E
120E
140E
160E
12Z28JUL1987
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1 1.1 1.2 1.3 1.4 1.5
PVU
Figure 3-21: Time sequence of IPV (color shading) and M (line contours) fields at 370K given
in successive 24-hour intervals from 12 UTC July 25 to 12 UTC July 28, 1987. Min imum
value of displayed contour lines is 356000 m 2 S-2. Contour line interval is 250 m 2 S-2.
zm dev IDv
390-,
380
370360350 -
'-.5
340-
--
- --- - - - - - - -
-- --
-
60E
80E
0.5.
. . . . . . .. . .
330320-
20W
0
20E
40E
100E
160E
140E
120E
180
12Z26JUL1987
390
380370360350-
-
340-)
.
.
..
. .
.
.
.
.
.
.
.
.
.
..
.
.
.-
.
.-..0 5
-.
.
.-
-
330320 1
20W
20E
0
40E
60E
80E
10OE
120E
140E
180
160E
12Z27JUL1987
390380370360350----
340
-
330
-
320
2(
-I
-
0
-U..5
U
.. . . . . . . . . . .
20E
40E
60E
80E
-
-.
. . . . . .. . ..
100E
120E
. .
140E
160E
11
1 2Z28J U L19 87
390 -g-0.5
380 370360350-
-0.5
340-
-0.5'
330320-
20W
0
20E
46E
60E
86E
100E
120E
140E
160E
180
Figure 3-22: Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 25'N and 35'N, given in successive 24-hour
intervals from 12 UTC July 25 to 12 UTC July 28, 1987. The values smaller than -0.5 PVU
are shaded.
...............
- ....
......
.....
I..
......
............ .........
..
50N -45N40N35N30N25N20N 15N1ON
5N
--
-----..
......
18ZO9JUL1988
370K ipv&msf
"'--
0
20E
-
40E
-
60E
80E
100E
120E
140E
160E
18Z1 OJUL 1988
50N --
45N 40N
35N 30N
25N -
20N 15N
iON
5N
120E
45N40N35N30N25N20N15N
1ON
5N
EQ
20W
140E
160E
180
18Z1 1JUL1988
50N --
0
20E
40E
60E
wT
80E
1
100E
120E
140E
160E
180
18Z12JUL1988
===us
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1 1.1 1.2 1.3 1.4 1.5
PVU
Figure 3-23: Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 24-hour intervals from 18 UTC July 9 to 18 UTC July 12, 1988. Minimum
value of displayed contour lines is 356000 m 2 S-2. Contour line interval is 250 m 2 S-2
25-35N
dev msf
390 zm
380370-
360. . .. . .. . . . . . .
3 5 0 - . -3 4 0 -. . . . . . . . . . . . . . . . . .
330-
0
320
20W
60E
40E
20E
0
390
380370
360-
500
350
340
330
50
-
0
-
320
20W
0
60E
40E
20E
390
380370360 --
0
1 5(
-
3501
340500
330 -320
20W
0
20E
r
60E
40E
390
380370--- - - --- 360 - - -- --350340 -
2000'
- - - -1500
0
330
320
20W
0
- L~
20E
40E
60E
Figure 3-24: Time sequence of vertical cross sections of the perturbation from the zonal mean
of M, latitudinally averaged between 25*N and 35'N, given in successive 24-hour intervals
2
from 18 UTC July 9 to 18 UTC July 12, 1988. The values greater than 500 m S-2 are
shaded.
event is shown in Figures 3-25 and 3-26. After July 28 at 18 UTC (last plot shown in Figure
3-25), the daughter vortex was recaptured by the mother vortex. This re-unified vortex
continued its life, moving westward for several more days, at first strengthening for a short
time and then weakening again.
3.3.3
July 1989
July 1989 began with an eddy shedding event. Namely, the end of June was marked with
a weak and eccentric anticyclone that was followed by the eddy shedding depicted in Figures
3-27 and 3-28. The time interval between the subsequent plots in these figures is two days.
The whole event lasted one week. In this case the anticyclone was centered around 80'E
before the shedding commenced (plot at the top). When the shedding happened the mother
vortex remained on the eastern side with its center as far east as 100'E, while the daughter
vortex detached on the western side and moved northwestward with its center reaching 40 E.
This was clearly seen in both fields (vertical cross sections of M in Figure 3-28 also clearly
show the shedding). Notice again in Figure 3-27 how the low IPV filament extended from
the mother vortex westward and rolled up in the anticyclonic sense (third plot) to form
the daughter vortex. This time the low IPV centers and high M were very well spatially
correlated (except for the third localized low IPV center which appeared around 75'E and
200 N).
The evolution of both IPV and M fields continued in a somewhat unusual way throughout
July (at least for what we saw to be "usual" for other three analyzed years). This is illustrated
with the four plots of horizontal cross sections at 370 K in Figure 3-29. After regaining its
unified shape by July 7, the anticyclone also achieved a shape such that its western part
became stronger and wider in meridional direction (M field), extending more to the north
than the eastern part. This whole system was moving westward with time. At the same
time lowest values of IPV remained in the eastern part. This whole time period is illustrated
by the first plot in Figure 3-29, which shows the fields at 18 UTC, July 11. On July 14,
a secondary vortex built up on the east side (centered at 90'E). Two anticyclonic centers
continued their lifetime together for ten days. In the first part of this period, two cells were
almost of the same strength in both fields (second plot at 18 UTC, July 16). In the later part
%555
.++++++++++
."++
*.............i..........xxx.......-+++++++
*** **111111111%*****************************************************************************************************************.*****************
12Z26JUL1988
50N --
45N 40N 35N 30N
25N
20N
15N
1ON
5N
6
26E
40E
2E
40E
60E
80E
12iOE
1OOE
140E
160E
180
06Z27JUL1988
50N -45N 40N 35N -30N
25N
20N
15N
1ON
5N
66E
86E
120E
IOGE
140E
180
160E
OOZ28JUL1988
50N -
45N
40N
35N
30N
25N
20N
15N
1ON
5N
EQ
20W
0
0
20E
40E
60E
80E
10E
120E
140E
160E
100
18Z28JUL1988
0.1 0.2 0.3 0.40.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5
HVU
Figure 3-25: Time sequence of IPV (color shading) and M (line contours) fields at 370K given
in successive 18-hour intervals from 12 UTC July 26 to 18 UTC July 28, 1988. Minimum
2
value of displayed contour lines is 356500 m2 s-2. Contour line interval is 250 m s-2
z. m
390
25-35N
i v
dev
12Z26JUL1988
380370-0.5
360
2
3500.5
- ---- -- -
340330
320
20W
I
26E
40E
60E
86E
100E
12E
14E
16E
1
06Z27JUL1988
390
1 05
380 370
2
3600.5
350
340-
--
---
------
-- - - -
-0.5
.
- -.
. ... . .. . ...
330-
320
20W
I
0
--
20E
!
40E
60E
86E
100E
120E
140E
160E
180
390
380-
370
360
1
350
0.5
-
340
330-
320
20W
1B8Z28JUL1 98
390
380
>-
-
370,
360
350-
340
9
.1
15
-1
0. 5
0.5
--0.5
2
o.'5
330
320
-
20W
0
20E
40E
60E
86E
1 O0E
120E
140E
160E
180
Figure 3-26: Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 25 0 N and 35'N, given in successive 18-hour
intervals from 12 UTC July 26 to 18 UTC July 28, 1988. The values smaller than -0.5 PVU
are shaded.
of the period, the western cell remained strong in the M field, but the IPV values, although
still low (less than 1.5 PVU in the center) were higher than in the eastern cell. The western
cell also started to move northward what is shown in third plot at 06 UTC, July 23. Finally,
by July 26, the western cell went much northward and weakened, so that only the eastern
cell remained to dominate the field. This remaining strong anticyclone (shown in fourth plot
at 06 UTC, July 27) moved westward, losing much of its strength by the end of July.
3.3.4
July 1990
The last analyzed year is 1990. We already described the sample case of eddy shedding
in July 1990. Before that event the upper tropospheric anticyclone was rather unified with
increasing eccentricity in the days preceding our sample case. After the eddy shedding event,
the anticyclonic vortex reunited by July 17.
Another shedding event followed in this month (Figures 3-30 and 3-31), starting on July
19 and ending on July 23. In this case, the western cell (daughter vortex) was stronger
than the eastern one. It moved slightly northward, but not much westward. After July 21
(bottom plot) it moved back eastward and it was recaptured by the mother vortex again.
For the rest of the month the anticyclone remained united.
3.3.5
August
The strong anticyclone usually starts to build up in the first half of June. It is strongest
in July and first part of August, and then starts to gradually weaken in the second part
of August. Two interesting events in August are presented here. One was the propagation
of the whole anticyclone westward in August 1987, and the other one was the shedding
event in late August 1988 (starting on August 25, Figure 3-32). Preceding the shedding, the
anticyclone was again strong and elongated. When the shedding started and as the western
cell was moving westward, the whole system weakened and by the end of August, daughter
vortex ceased to exist.
In August 1987 (similar to the case in July 1989) there was a ten day period when the
anticyclone moved westward with its center moving from ~ 90'E on August 6 to
-
50'E on
.. ....
..
........
....................................
..........................................................
r............................................
..........
......
18Z28JUN 1989
50N --
45N 40N 35N 30N
25N 20N 15N ON
180
160E
140E
18Z30JUN1989
50N --
45N40N35N30N
25N
20N 15N10N5N
0
20E
40E
0
20E
40E
60E
80E
10E
120E
80E
100E
120E
140E
180
160E
18ZO2JUL1989
50N --
45N40N35N30N25N
20N
15N
1ON
5N
EQ20W
1-1
0
1
60E
160E
140E
18ZO4JUL1989
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.b 0.9
1
1.1
1.
-1.
T14
.
P U
Figure 3-27: Time sequence of IPV (color shading) and M (line contours) fields at 370K
given in successive 48-hour intervals from 18 UTC June 28 to 18 UTC July 4, 1989. Minimum
2
value of displayed contour lines is 356000 m2 s-2. Contour line interval is 250 m s2.
-A
F-
18Z28JUN1989
18Z30JUN1989
390
380-
5 0
370
360350-
50000
3400
3301
7500
320
2(
0
20E
40E
60E
80E
100E
120E
140E
160E
18ZO2JUL1989
390-380370360350340330
320
20W
Figure 3-28: Time sequence of vertical cross sections of the perturbation from the zonal mean
of M, latitudinally averaged between 25*N and 35*N, given in successive 48-hour intervals
from 18 UTC June 28 to 18 UTC July 4, 1989. The values greater than 500 m2 s- 2 are
shaded.
.......
.............
...
.....
LrL
.
= -
.... ..
...............................
..
...... ......
18Z1 1JUL1989
370K ipv&msf
50N
-
45N40N]
35N-
140E
180
160E
18Z16JUL1989
50N
45N
40N
35N
30N
25N
20N
15N
1ON
5N
14UL
1bUL
06Z23JUL1989
50N
45N40N35N30N -If
25N
20N
15N
1ON
5N
EQ
20W
0
20E
0
20E
46E
60E
80E
100E
12OE
140E
160E
06Z27JUL1989
50N
45N
40N 35N
30N -
25N
20N 15N
iON
5N
E2QW
20W
0
46E
66E
86E
1OE
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
160E
1 1.1 1.2 1.3 1.4 1.5
180
PVU
Figure 3-29: IPV (color shading) and M (line contours) fields at 370 K given at 18 UTC
July 11, 1988 (first plot), 18 UTC July 16, 1988 (second plot), 06 UTC, July 23, 1988 (third
plot), and 06 UTC, July 27, 1988 (fourth plot). Minimum value of displayed contour lines
is 356000 m2 s-2. Contour line interval is 250 m 2 S-2
58
.......
.........
50N -
45N 40N 35N 30N
25N 20N 15N 1ON 5N
Ei
50N --
45N40N35N30N
25N20N
15N
1ON
5N
EO
20W
0
20E
46E
6OE
80E
100E
120E
0
20E
40E
60E
80E
100E
120E
140E
160E
140E
160E
12Z20JUL1990
50N --
45N40N35N
30N
25N
20N
15N
1ON
5N
EQ
20W
180
12Z21JUL1990
50N -
45N
40N
35N
30N
25N
20N
15N
1ON
5N
E O
0
IN
20E
41E
6EE
86E
120E
100E
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1
140E
1.2 1.3 1.4
160E
MIN
1.5
180
PVU
Figure 3-30: Time sequence of IPV (color shading) and M (line contours) fields at 370
K given in successive 24-hour intervals from 12 UTC July 18 to 12 UTC July 21, 1990.
2
Minimum value of displayed contour lines is 356000 m2 s-2. Contour line interval is 250 m
S-2
zm
390
dev ipv
25-35N
12Z1 8JUL1 990
380370-
025
360-
0
350
5
-
-
-
-
--
1
-05
340-
-0.5
-------- - -- ---- -- --- - - - ---- -- - - - - - - --
330320
20W
6
26E
46E
80E
60E
120E
100E
140E
.
. .
1 80
160E
12Z19JUL1990
390380-
1
1 5-- 5-2
370-
2
/-'
----
360350-
05
1
-05
340-
-0.5
330-
-
320
2 0W
o
20E
40E
60E
80E
100E
120E
140E
160E
1
30
12Z20JUL 1990
390
05
2
380370360-
--- ---
- - -- -
350340-
-
-- 0
-
--- - -
-
-
-
-
-
.
330.
320
20W
6
20E
40E
60E
80E
100E
120E
1 2Z21
390
380-
140E
-
160E
180
JUL1990
1-
- -----
370360' ----
350-
--
..................
340-
- -
5.....
330320
20W
6
20E
40E
60E
80E
100E
120E
140E
160E
180
Figure 3-31: Time sequence of vertical cross sections of the perturbation from the zonal
mean of IPV, latitudinally averaged between 25'N and 35'N, given in successive 24-hour
intervals from 12 UTC July 18 to 12 UTC July 21, 1990. The values smaller than -0.5 PVU
are shaded.
50N -
45N40N35N30N -
25N20N15N1ON5N
EQ20W
0
20E
40E
66E
80E
120E
1OQE
140E
180
160E
12Z26AUG1 988
50N
45N
40N
35N
30N -
25N
20N
15N
1ON
5N
140E
180
160E
18Z27AUG1988
50N --
45N40N35N30N25N
20N
15N
1ON
5N
EQ
20W
0
20E
40E
60E
80E
100E
120E
140E
160E
180
OOZ29AUG1988
50N --
45N40N35N30N25N
20N15N
1ON
5N
EQ
20W
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5
PVU
Figure 3-32: Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 30-hour intervals from 06 UTC August 25 to 00 UTC August 29, 1988.
Minimum value of displayed contour lines is 356000 m 2 S-2. Contour line interval is 250
m2ss-2
50N --
45N
40N
35N
30N
-
25N 20N 15N
iON5N EQ -20W
50N --
45N40N35N30N25N20N15N
1ON
5N
EQ
20W
0
20E
40E
60E
80E
120E
1OOE
50N --
45N40N35N30N
25N20N15N
ION
5N
EQ
20W
140E
180
160E
12Z11AUG1987
0
20E
40E
60E
80E
100E
120E
140E
1
160E
12Z13AUG1987
5ON -45N 40N 35N 30N 25N 20N 15N iON
5N
EQ
20W
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
U
1
1.1 1.2 1.3 1.4 1.5
PVU
Figure 3-33: Time sequence of IPV (color shading) and M (line contours) fields at 370 K
given in successive 48-hour intervals from 12 UTC August 7 to 12 UTC August 13, 1987.
Minimum value of displayed contour lines is 356000 m 2 s-2. Contour line interval is 250
Ins2s-2
August 16 (Figure 3-33). The low values of IPV were confined to the area of the maximum
values of the streamfunction.
3.4
Relation to the lower levels
The source of the persistent divergent circulation in the upper troposphere is diabatic
heating (in the form of latent heating associated with strong convection, and sensible heating
from the Tibetan plateau). We noticed that when the shedding happens, the daughter vortex
propagates westward (except in the case of July 1988). While it moves away from the source
of the diabatic heating, it weakens and finally vanishes.
The establishment of a quantitative relation between the diabatic heating and the state
of the upper tropospheric anticyclone would require involved analysis of each individual
case. In order to address this question without going into too much detail, we looked at the
geopotential height field in the lower levels. We noticed a few interesting connections.
In the lower levels, the cyclonic systems are characterized by smaller scales than the
anticyclonic systems above. For example, the monsoon trough extends for -20' in the zonal
direction and -10'
in the meridional direction. Also, the monsoon depressions that form
in the Bay of Bengal and move westward over Indian subcontinent have a horizontal scale
of 10-20'. The low which is often seen over Pakistan is about that size as well. All these
smaller scale systems contribute to the large area of low pressure between 30'E and 120'E
and from Equator to 350 N.
In general, as the cyclonic systems intensifies, the upper level divergent system builds up.
The movement of the monsoon depressions and cyclonic systems embedded in the monsoon
trough are often accompanied by the movement of the upper tropospheric anticyclonic cells.
However, our analysis does not allow us to draw definite conclusions regarding the relationship of the lower level cyclonic systems and the upper level divergent flow. The well-defined
trough with one or two cyclonic cells embedded in it is found in cases of shedding as well as
when the anticyclone is united. Similarly, the monsoon depressions are found in both cases,
too. Still, there were some examples of a clear relation in the development of the lower and
upper level systems.
12Z20JUL1989
12Z24JUL1989
12Z21JUL1989
12Z25JUL1989
12Z22JUL1989
12Z26JUL1989
12Z23JUL1989
12Z27JUL1989
Figure 3-34: Time sequence of geopotential height at 850 mb given in successive 24-hour
intervals from 12 UTC July 20 to 12 UTC July 27, 1989. Contour line interval is 10 m.
00Z16JUL1988
0OZ18JUL1988
12Z16JUL1988
12Z18JUL1988
OOZ1 7JUL1 988
0OZ19JUL1988
12Z17JUL1988
12Z19JUL1988
Figure 3-35: Time sequence of geopotential height at 850 mb given in successive 12-hour
intervals from 00 UTC July 16 to 12 UTC July 19, 1988. Contour line interval is 10 m.
One example of the clear mutual relation on the large scale between the lower and upper
level systems was the case of a strong depression i.e. cyclonic storm, in July 1989. This
cyclonic storm formed in the Bay of Bengal (July 20) and then moved northwestward into the
continent, with its center reaching 75E', 28'N (Figure 3-34). This cyclonic storm was formed
at a time when the eastern anticyclonic cell became dominant in the upper tropospheric field
(Figure 3-29, last two plots), and started to move westward, too.
A somewhat opposite example was a monsoon depression in July 1988. This depression
lasted for four days, from July 16 to July 19 (Figure 3-35). This four-day period was a part
of the longer ten days period in which the elongated and strong anticyclone was found in
the upper troposphere. The depression was moving westward, but the anticyclone above
remained in the same position and in the same form. On the other hand, the depression
itself may have helped maintain the strong and united character of the anticyclone. This
case is also exceptional because the shedding did not occur just two or three days after the
formation of the elongated and strong anticyclone (which is usually the case), but rather
after the anticyclone persisted for ten days in that state.
In summary, the relationship between lower and upper levels is usually strong, but eddy
shedding does not always have an obvious connection to the lower levels.
32), June 28, 1989 (Figures 3-27 and 3-28), July 10, 1990 (Figures 3-9 to 3-14), and July
18, 1990 (Figures 3-30 and 3-31). In the event that started on July 5, 1987 (Figures 3-19
and 3-20), the western cell did not move westward. In the eddy shedding event in July
1988 (Figures 3-23 and 3-24) the daughter cell was shed on the eastern side. In all cases
of shedding we found the daughter vortex to be smaller but still of comparable dimensions
to the mother vortex. The whole system of two anticyclones has a size equal or larger
than initial monsoonal anticyclone (spreading over about 1000 longitude and 300 latitude).
The shedding appears to be rather shallow, mainly confined between 300 and 100 mb. The
relationship between lower and upper levels is addressed here but it is not studied in a great
detail. We did not find a unique pattern in the behavior in the lower levels when eddy
shedding occurs in the upper troposphere. A detailed analysis for each case would probably
give more definite conclusions.
Chapter 4
Conclusion
The upper tropospheric divergent flow in the area of the Asian summer monsoon is present
during the whole summer, but it varies greatly in form and strength. The most dramatic
changes are seen during eddy shedding events, and during the westward propagation of the
anticyclone.
Eddy shedding seems to be a regular phenomenon. It is found every summer (at least
in the 1987-1990 data studied here as well as in year 1996 not discussed here). We do not
find periodicity in its occurrence. The duration of the shedding varies but it is usually four
to eight days. In most cases when the anticyclone strengthens and elongates the shedding
happens after two or three days.
Synoptic conditions just prior to eddy shedding were quite compatible with those in
the model of Hsu (1998) and Hsu and Plumb (1998). Both in the modeled flow and in the
atmosphere, when the strip of low PV embedded in the high PV surrounding becomes rather
narrow and long enough in zonal direction, it becomes unstable and sheds eddies. Namely,
in the quite elongated eddy, flow is close to parallel and the flow is dynamically unstable due
to the change of sign in PV gradient. However, the atmosphere as a complex system often
exhibits different behavior from the ideal modeled cases.
In the four analyzed years we found eight cases of eddy shedding. In almost all of these
cases, the daughter cell was shed on the western side. The usual evolution after the shedding
starts is the westward propagation of the daughter cell. These events started in July 25, 1987
(Figures 3-21 and 3-22), July 26, 1988 (Figures 3-25 and 3-26), August 25, 1988 (Figure 3-
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70