monsoons:
a brief introduction
Not quite an ordinary day in monsoon land!
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what is a monsoon
• Monsoon is a climatological feature covering roughly
half the tropics (1/4 of the global surface)
• Strictly, a system where the winds and precipitation
reverses (summer rain, winter dry)
• Sufficiently reproducible to host the most
successful agricultural system (5000 years of
success)
• Host 65% of the world’s population
• Small changes in year-to-year climate can be
catastrophic
Basic Aims
a description of the
south asian
monsoon
Halley (of comet fame) was the first to describe monsoon
and attribute differential heating between
land and ocean as the cause
mean august and january precipitation
from MSU satellite (mm/day)
Monsoon regions are the
largest region where precipitation exceeds evaporation.
This excess water must come from somewhere …
…….which defines the scale of the monsoon.
The monsoon is intrinsically inter-hemispheric with the
winter hemisphere being the source of moisture
monsoon of the americas
There are three major
monsoon systems:
• Asian-Australian
• African
• Americas
Each has its own
characteristics and
differences but each is
characterized by summer
rains and seasonal windreversals
Rainfall
Surface pressure
trough
asian-australian & african monsoon systems
rain
Annual cycle of radiative temperature
“Cold” areas denote
Cloudiness and precipitation. “Warm” areas
show clear regions.
Note that there are
large gradients both
east-west and northsouth indicating that
the monsoon is driven
by heating gradients
in both directions.
annual cycle of surface winds over Indian Ocean and South Asia
Annual cycle of convection (OLR) in the Indian Ocean-South Asia region
Annual cycle of SSTin the Indian Ocean-South Asia region
Climatological precipitation over South-Asia/Indian Ocean
• Three maxima (Bay of Bengal, east coast India, and south of equator)
• Equatorial maximum indicates importance of intraseasonal oscillations
Longitudinal & latitudinal sections through monsoon region
variability of the
south asian monsoon
Time Scales of Monsoon Variability
•
Interannual variability: Variations on the annual cycle of the
monsoon producing anomalously wet or dry years. Generally
influenced by sea-surface temperature variations associated with
ENSO or inherent variability in the Indian Ocean.
•
Monsoon Weather: Weather events such as monsoon lows and
depressions, tropical cyclones (hurricanes) and etc. Produce shortlived local flooding (or drought), erosion, high winds and etc.
•
Intraseasonal Variability: “Envelopes”: or clusters of weather
events leading to 20-40 day droughts or flood periods. Most
difficult to forecast but perhaps the most important of all time
scales for economic and agricultural sectors.
El Nino influences Indian Ocean
Changes SST, sea surface slope and regions of maximum precipitation
However, other factors are also important …..
Indian Ocean Zonal Mode:
Inherent mode of Indian Ocean invoked by outside
perturbations including ENSO
Time series of E-W
SST gradient
frequency
spectra
Sea-level variations associated with IOZM:
Positive phase: warm SST WIO, low SSH EIO
Negative phase: warm SST EIO, high SSH EIO
Intraseasonal variability of the monsoon:
Active and break periods of the Summer South Asian Monsoon
The summer monsoon is divided into distinct periods of wet and dry.
When it is dry on the plains, it is wet in the foothills of the Himalaya, just as
occurred this year. The result was Nepal, Assam and Bangladesh floods, the latter due to
strong Brahmaputra discharge.
Intraseasonal Modes: Impact on Rainfall
Precipitation histograms in
central India
•
Intraseasonal variability imposes a
distinct form to precipitation.
•
Histograms of precipitation, shown for
9 years show distinct wet periods with
lulls in between.
•
Lower diagram shows the distinct
spatial character
Forecasting monsoon variability
on 20-30 day time scales:
Taking advantage of a new
understanding of monsoon dynamics
Peter J. Webster
Georgia Institute of Technology
(a) Ganges Valley pentad GPI rainfall for 1986-2002
40
30
20
10
0
1986
1990
1995
2000
(b) Ganges Valley pentad GPI rainfall for 1999-2002
1999
30
20
20
10
10
0
20
30
40
50
60
2001
30
0
20
20
10
10
30
40
pentad
50
30
40
60
0
20
50
60
2002
30
20
0
20
2000
30
30
40
pentad
50
60
Differences between active and break periods of the monsoon
Note distinct bands of precipitation excess or deficit. During
breaks, there is a 20-40 day drought across plains of India and
excess rain over southern, northern and north-eastern regions
Latitude-time section of OLR along 90E: 1995
Active phases of the monsoon commence near the equator and
propagate northward (and southward) across South Asia.
Active
phase
northward
propagation
commence
active phase
some basic physics
Physical Ingredients for a Monsoon
• Land-Sea differences: land and water have
different heating capacities & water can store
more heat because it is a fluid and can mix heat
down for future release
• Planetary rotation: introduces swirl and much
stronger winds
• Moisture: water vapor, collected over oceans
through evaporation, condenses over land and
marginal seas releasing vast quantities of heat
• Orography: acts as elevated heat source which
intensifies flow & also ducts flow.
Impact of Moisture
(a) lower-troposphere
(i) non-rotating
(ii) rotating
low pressure
PG
30•N
p
0
F
C
PG
PG
p1
F
15•N
C
F
PG
p2
0
F
PG
15•S
C
PG
F
p
PG
F
C
30  S
3
F
p
high pressure
4
(b) upper-troposphere
(i) non-rotating
(ii) rotating
high pressure
p8
30•N
p
7
F
C
F
15•N
PG
C
F
PG
PG
p6
p5
p
0
4
C
F
15•S
F
p3
F
C
PG
PG
PG
p2
p
1
30  S
p
0
low pressure
Impacts of Elevated Heat Sources
Note the dominating effect of the Himalayas
Changes in Tropospheric Temperature in Monsoon Region
Temperature changes much larger over Himalayas than elsewhere.
Impact of East African Highlands
Arguably, without the Himalayas & the East African
Highlands, South Asia would be a desert like North Africa
Impact of Rotation:
increases in intensity and scale
Impact of anomalous monsoon seasons
Strong and weak monsoons invoke different
upwelling effects creating SST gradients. Then …….
Explanation of why there is an IOZM with
approximately biennial period
Atmosphere responds
to SST anomaly associated
with strong/weak
monsoon
Ocean responds to
anomalous winds producing
ocean dynamic modes
Anomalous SST produces
a monsoon of opposite
anomaly which wipes out
IOZM
Monsoon is a fully coupled ocean-atmosphere system
predictability of the
south asian monsoon