I.
Observational analysis of three oceanic Kelvin waves coupled to atmospheric
convection
Abstract of
a thesis presented to the Faculty
of the University at Albany, State University of New York
in partial fulfillment of the requirements
for the degree of
Master of Science
College of Arts and Sciences
Department of Atmospheric and Environmental Sciences
Lynn Michele Gribble Verhagen
May 1st, 2010
II.
Abstract
This study analyzes the apparent coupling between intraseasonal oceanic Kelvin
waves in the Pacific Ocean and atmospheric moist deep convection in a particularly high
amplitude event during the winter of 1986/-1987 and two events during the winter of
2009-2010. These waves were triggered initially by westerly wind bursts that developed
in association with the active convective phase of the Madden Julian Oscillation (MJO).
After initiation of the Kelvin waves by the MJO, the convective anomaly generally
slowed to roughly 1.5 m s-1, suggesting that the events became distinct from the MJO,
which usually propagates at roughly 5-7 m s-1. This study demonstrates how surface
winds, currents, SST anomalies, fluxes of sensible and latent heat across the sea surface,
and atmospheric convection evolved throughout the events. Results suggest that the
convective envelope and oceanic Kelvin wave are mutually beneficial and serve to
prolong and enhance each other. The impact that this slowly moving, enhanced
convection has on both tropical and extra-tropical weather is also discussed.
III.
Observational analysis of three oceanic Kelvin waves coupled to atmospheric
convection
A thesis presented to the Faculty
of the University at Albany, State University of New York
in Partial fulfillment of the requirements
for the degree of
Master of Science
College of Arts and Sciences
Department of Atmospheric and Environmental Sciences
Lynn Michele Gribble Verhagen
May 1st, 2010
IV.
Acknowledgments
The NOAA/OAR/ESRL PSD provided the NCEP SST OI data set. The
Environmental Systems Science Centre (ESSC), of the British Atmospheric Data Centre,
(Robinson, G.J., 2002) provided Cloud Archive User Service (CLAUS) brightness
temperature data. The Woods Hole Oceanic Institute for providing the WHOI OAFlux
data set. Funding was provided by NSF Grant # 0850642 to Paul Roundy.
I would like to thank Dr. Paul Roundy for his constant help and support. I would
also like to thank Carl Schreck, Matthew Janiga, and Dr. Gareth Berry for their technical
and theoretical assistance.
V.
Table of Contents
I.
Title Page of Abstract
1
II.
Abstract
2
III.
Title Page
3
IV.
Acknowledgments
4
V.
Table of Contents
5-6
VI.
List of Figures
VII.
VIII.
Introduction and Significance
7-10
11-
Review of Research
a.Oceanic Kelvin Waves
b.Tropical Convection
c.Coupling Between Oceanic Kelvin waves and Atmospheric Convection
IX.
Aims
X.
Methods
XI.
Results
a.1986-1987 Event
i.Kelvin Wave Initiation
ii.Advection and Modification of SSTs
iii.Surface Fluxes
iv.Atmospheric Convection
v.Additional Notes on Horizontal Wind Structure
b.2009-2010 Event
i.Kelvin Wave Initiation
ii.Advection and Modification of SSTs
iii.Surface Fluxes
iv.Atmospheric Convection
v.Additional Notes on Horizontal Wind Structure
XII.
Discussion
XIII.
Conclusion
XIV. Bibliography
XV.
Figures
VI.
List of Figures
FIG 1. Time/Longitude plot of Outgoing Longwave Radiation (OLR), shaded, and Kelvin
Dynamic Height (contoured in cm, every 2 cm), contoured. Line A (solid) indicates
an approximate phase speed of 7 m s-1. Line B (dot-dashed) indicates an approximate
phase speed of 1 m s-1. Line C (dashed) indicates an approximate phase speed of 1.5
m s-1. All lines are for reference.
FIG 2. 10 m horizontal winds, shaded, in m s-1 averaged from 2.5°N to 2.5°S. Contours,
dot-dashed, and dashed lines are the same as in Figure 1. (a) Total winds and (b) 121day low pass filtered winds.
FIG 3. SST averaged from 5.5°N to 5.5°S in °C, shaded. Contours , dot-dashed, and
dashed lines are the same as in Figure 1. (a) Total SST and (b) 10-120 day band-pass
filtered SST.
FIG 4. SODA zonal current (shaded) and dynamic height (contoured) (a) Data from
October through December of 1991, dynamic height is unfiltered. (b) Data from
October 1986 through February 1987, contours, same as Figure 1.
FIG 5. Time/longitude plots of surface heat fluxes, averaged from 5°N to5°S, with a 5 day
moving average applied. Contours, dot-dashed, and dashed lines are the same as in
Figure 1. (a) Latent heat flux in W m-2 shaded. (b) Sensible heat flux in Wm-2 shaded.
FIG 6. (a) Cloud top brightness temperature (K), averaged from 2.5°N to 2.5°S, shaded.
Contours, dot-dashed, and dashed lines are the same as Figure 2. (b) is the same as
(a), except with a shorter timescale.
FIG 7. (a) Time-longitude plot of 121-day low pass filtered SST (shaded) and 21-day
smoothed brightness temperature (contoured). (b) Time-longitude plot of 121-day low
pass filtered 10m horizontal winds (shaded) and 21-day smoothed brightness
temperature (contoured).
FIG 8. 5-day running averaged wind patterns associated with Kelvin wave propagation
from 11/01/1986 to 12/15/1986. Top panel of each figure: horizontal wind vectors
indicating magnitude and direction of smoothed wind from 30°N to 30°S and 140°E
to 100°W. Bottom panel of each figure: Amplitude of Kelvin dynamic height in cm
from 140°E to 100°W.
FIG 9. A time-longitude plot with CLAUS brightness temperature in K, averaged from
10°S-10°N, and Kelvin filtered dynamic heights in cm, solid-red contours are positive
dynamic height anomalies and dashed-blue contours are negative dynamic height
anomalies. Hurricane symbols indicate genesis date and longitude, defined as the first
time a storm reaches 25 knots in the IBTrACS dataset. Blue symbols are for storms
that formed 15°S-0, and red symbols are for storms that formed 0-15°N.
FIG 10. A time-longitude plot with OLR in W m-2 (shaded) and dynamic height in cm
(black contours). The solid red lines represent the approximate phase speed of the two
active convective phases of the MJO that are visible. The phase speeds are
approximately 4.3 m s-1, for the November MJO, and 6.9 m s-1, for the January MJO.
The dashed lines represent the approximate phase speeds for the oceanic Kelvin
waves. The phase speeds are approximately 2.6 m s-1, for the first Kelvin wave (wave
A), and 2.4 m s-1, for the second Kelvin wave (wave B).The dot-dashed lines
represent the approximate phase speeds of the convective envelopes associated with
the Kelvin waves. The phase speeds are approximately 2.6 m s-1, for wave A, and 1.7
m s-1, for the wave B.
FIG 11. Time-longitude plots with zonal winds in m s-1 (shaded) and dynamic height in
cm (contoured), dashed and dot-dashed lines black lines are reproductions from
Figure 10. (a) Zonal wind averaged from 2.5°N – 2.5°S, (b) Zonal wind averaged
from 2.5°S - 7.5°S.
FIG 12. Time-longitude plots of SST in °C (shaded) and dynamic height in cm
(contoured) dashed and dot-dashed black lines are reproductions from Figure 10. (a)
SST averaged from 2.5°N – 2.5°S, (b) SST averaged from 2.5°S - 7.5°S.
FIG 13. (a) A time-longitude plot of surface sensible heat flux in W m-2 (shaded) and
dynamic height in cm (contoured), dashed line is a reproduction from Figure 10. (b)
Same as (a) except that shading is surface latent heat flux in W m-2.
FIG 14. A time-longitude plot of flux proxy in °m s-1 (shaded) and dynamic height in cm
(contoured), dashed and dot-dashed lines are reproductions from Figure 10.
FIG 15. Time-longitude plots of flux proxy in °m s-1 (shaded) and equatorial OLR in W m2
(contoured), dashed and dot-dated lines are reproductions from Figure 10. (a) Flux
proxy averaged from 2.5°N – 2.5°S, (b) Flux proxy averaged from 2.5°S - 7.5°S.
FIG 16. 5-day running averaged wind patterns associated with Kelvin wave propagation
from 11/29/2009 to 12/153/2009. Top panel of each figure: horizontal wind vectors
indicating magnitude and direction of smoothed wind from 30°N to 30°S and 140°E
to 100°W. Bottom panel of each figure: Amplitude of dynamic height in cm from
140°E to 100°W. Solid line is daily dynamic height values, dashed line is values
averaged over 09/09-02/10.
FIG 17. A time-longitude plot with OLR in W m-2 (shaded) and dynamic height in cm
(contoured). Red dots indicate the genesis time and longitude of tropical storms that
formed between 0-15°S, defined as the first time a storm reached 25kts in the
UNISYS best track data set.
FIG 18. A schematic time/longitude plot with a 3 month timescale, and a zonal extent of
300° of longitude. Black contours represent Kelvin dynamic height. Black, dashed
contour represents the convective envelope. Blue shading represents the location of
the combination of both the surface sensible and latent heat fluxes. Red shading
represents the location of the strongest westerly wind anomalies. Green shading
represents the location of the warm SST anomalies, with the darker shading being the
strongest anomalies.
VII.
Introduction and Significance
Recent research has suggested that intraseasonal Kelvin waves in the Pacific
Ocean, which were originally forced by westerly wind bursts associated with the active
convective phase of the Madden Julian Oscillation (MJO), occasionally become coupled
to atmospheric convection (Roundy and Kiladis 2006). Although these events have not
received much attention by either the academic or operational communities, they are not
infrequent, with at least twenty events occurring in the Northern Hemisphere winter
(October-March) between 1974 and 2009. (Include a statement here about the amplitude
of their convective signals relative to other features, to help emphasize their overall
relevance).These events behave differently from the MJO by itself, and stand to challenge
the traditional understanding of the interaction of the MJO and El Nino Southern
Oscillation (ENSO) along with several other tropical and extra-tropical interactions.
Ultimately, the goal of this research is to help to improve our ability to predict these
events and their associated changes in background climate and weather both in the
tropical Pacific and beyond.
VIII.
Review of Research
a. Oceanic Kelvin Waves
Intraseasonal oceanic Kelvin waves are a dominant mode of variability within the
equatorial Pacific Ocean (Knox and Halpern 1982; Johnson and McPhaden 1993;
Cravatte at al. 2003). These waves are frequently initiated by westerly wind bursts
embedded within the active convective phase of the MJO over the western equatorial
Pacific (McPhaden and Taft 1988; Hendon et al. 1998; Zhang 2005; Roundy and Kiladis
2006). These Kelvin waves travel at approximately 2-3 m s-1 (Johnson and McPhaden
1993) with the slowest waves during adjustment towards El Nino (Roundy and Kiladis
2006). They are characterized by periods of roughly 20-120 days, with their leading
spectral peak at 70-days in the equatorial east Pacific (Johnson and McPhaden 1993). Sea
surface height anomalies associated with Kelvin waves are clearly discernable in the
Pacific basin using dynamic height data from the Tropical Atmosphere Ocean (TAO)
buoy array (Roundy and Kiladis 2006). McPhaden and Taft (1988) and Johnson and
Mcphaden (1993) found that temperature variations, zonal velocity, and dynamic height
associated with Kelvin waves are nearly in phase and coherent from the surface to at least
250 meters depth and that these waves are occasionally so large that they completely
obscure the seasonal cycle.
b. Tropical Convection
Variations in atmospheric convection have been shown to be associated with
development of oceanic Kelvin waves (Roundy and Kiladis 2006, Roundy and Verhagen
In Press).
Not all MJO events and subsequent atmospheric Kelvin waves evolve
similarly. Roundy and Kiladis (2006) showed that anomalies of atmospheric convection
associated with some active convective MJO phases slow down, reduce in zonal scale,
and propagate eastward together with the developing oceanic Kelvin wave. This pattern
suggests coupling between the oceanic wave and atmospheric convection. The associated
convective signal appears to be distinct from the MJO because it propagates much more
slowly than the MJO, less than 2 m s-1 (Roundy and Kiladis 2006) in the central Pacific,
where the MJO is usually observed to accelerate eastward (Hendon and Salby 1994).
These apparent coupled events have been observed to substantially impact tropical
weather as their associated convective anomalies frequently become the dominant signal
across the Pacific tropics when the occur, organizing zonal and vertical wind anomalies
around the globe (Roundy and Kiladis 2006, Roundy and Verhagen In Press).
In general, diagnosing the forcing mechanisms of convection in the tropics has
been challenging. It is well known that low-level convergence, strong upward surface
fluxes of sensible and latent heat, and variations in column integrated precipitable water
are often correlated with deep convection, but it is difficult to tease out which factors that
are incidental, consequential, or causal (Raymond et al. 2003). There is wide consensus
that convection is ultimately forced by factors that control the release of conditional
instability (Raymond et al. 2003). Lifting mechanisms are limited over the open ocean
due to lack of orography and zonal differential heating; instead lifting occurs mainly due
to small-scale turbulence and gust fronts associated with decaying convection as well as
through large scale vertical motions associated with atmospheric wave dynamics. With
general lack of all but weak lifting mechanisms, thermodynamic processes such as
differential air and sea temperatures and moisture advection and diabatic, or turbulent
processes associated with convectively coupled atmospheric equatorial waves and
intraseasonal oscillations become increasingly important (Raymond et al. 2003).
c. Coupling between Oceanic Kelvin Waves and Atmospheric Convection
The dynamics of the apparent coupling process between oceanic Kelvin waves
and atmospheric convection have not been well established although it is evident that the
oceanic wave and atmospheric patterns interact to modulate each other in a mutually
beneficial way. The eastward migrating westerly wind anomalies move at similar phase
speeds to the Kelvin waves, allowing the winds to enhance the wave amplitude
continuously (Roundy and Kiladis 2006; Shinoda et al. 2009; Roundy and Kravitz 2009).
Where the MJO is traditionally associated with moderate oceanic Kelvin waves, the
coupling process seems to encourage the development of higher amplitude waves
(Roundy and Kiladis 2006).
In an analysis of a simple model, Lau and Shen (1988) showed that upwelling and
zonal advection induced by Kelvin waves can facilitate coupling between the waves and
envelopes of enhanced atmospheric convection organized into synoptic and planetary
scale waves. Lau and Shen (1988) concluded that there exist two modes of coupling
between the atmosphere and the ocean associated with Kelvin waves. The advective
mode was diagnosed by ignoring upwelling due to increased wind stress and by assuming
a sea surface temperature (SST) gradient equivalent to a decrease of 4°C from west to
east over 10000 km and a mean SST of 28°C. Together, the atmospheric convection and
oceanic signals propagate progressively more slowly as background SST increases above
28°C. The second, or upwelling, mode arises from unstable growth of the oceanic Kelvin
wave itself. Unstable growth in this mode favors long waves, but shorter wavelengths
become unstable with increasing SST (above 28°C SSTs). The advective mode arises
from destabilization of the atmospheric Kelvin wave by zonal advection of warm water
and the upwelling mode represents a destabilization of a neutral oceanic Kelvin wave by
air-sea interaction. Lau and Shen (1988) suggest that coupled events of the observed
ocean-atmosphere system would likely develop as combinations of the two modes acting
to destabilize both the ocean and atmosphere. They noted that for SST near or above
28°C, the atmospheric portion of the coupled mode propagates at speeds in the same
range as the Kelvin wave in the ocean, allowing the coupled system to interact with the
same time scale.
The eastward-moving MJO has been identified as one mode of atmospheric
convection that induces variation of zonal wind stress on the ocean surface that amplifies
these oceanic Kelvin waves (Zhang 2005). Most anomalies of atmospheric convection
that were initiated by the active convective phase of the MJO travel across the West
Pacific at roughly 5-7 m s-1 then accelerate eastward across the dateline and the western
hemisphere, where they reach phase speeds near 15 m s-1 (Hendon and Salby 1994). The
amplitudes attained by oceanic Kelvin waves increase with the zonal extent of the 5 m s -1
portion of the MJO-associated winds over the Pacific (Hendon at al. 1998). Previous
works have suggested that the MJO both triggers and interacts with equatorial Kelvin
waves (Seiki and Takayabu 2007; Roundy and Kiladis 2006; Roundy and Kravitz 2009),
which as discussed previously have phase speeds that vary between about 2 and 3 m s-1.
Shinoda et al. (2008) showed with an ocean model that observed phase speed variations
in oceanic Kelvin waves are linked to changes in wind stress patterns rather than to
changes in the thermal structure of the ocean. Their results indicate that different portions
of the wave are preferentially attenuated or amplified by the wind stress patterns
associated with convection, which would result in bulk phase speeds of sea surface height
anomalies that differ from free oceanic Kelvin waves. These waves with different phase
speeds are therefore not pure Kelvin waves from a theoretical perspective, although they
are similar in structure.
If the passage of a downwelling Kelvin wave modulates SST, as McPhaden and
Taft (1988) and Johnson and McPhaden (1993) suggest, and the modulation is
significantly large, it is reasonable to assume that this change might produce the
necessary instability to influence the development of atmospheric convection. Since only
a subset of Kelvin waves exhibit apparent coupling as suggested by coherent propagation
of intraseasonal dynamic height and brightness temperature (or outgoing longwave
radiation (OLR)) anomalies, it is possible that such waves are associated with different
evolutions of SST anomalies than the average Kelvin wave. A coherent SST signal
combined with enhanced westerly wind anomalies, and/or a large difference between the
SST and surface air temperature (SAT), especially at the leading edge of the Kelvin
wave, could lead to enhanced surface heat fluxes, thereby allowing the wave to modulate
atmospheric convection.
IX.
Aims
We hope that this work can be a starting point for much more in-depth research
into the topic of coupling between oceanic Kelvin waves and atmospheric convection.
With this project, specifically, we aim to better understand the mechanisms that maintain
coupling between atmospheric convection and oceanic Kelvin waves. We also aim to
understand the evolution of the coupling process and how the coupling changes
throughout the lifecycle of the wave. Similarly, we aim to understand how the coupling
is the same and/or different during different stages of the seasonal cycle. From this
research we hope to develop a basis for pattern recognition of these events that could be
tested in future modeling studies, as well as helpful for real time forecasting.
We chose an apparent atmosphere-ocean coupling event that took place between
October of 1986 and February of 1987 in order to study the evolution and impact of a
particularly high amplitude event, with special attention being paid to the time period
between late November through early January. The 1986 event is examined with respect
to several parameters to better understand the processes that maintain coupling between
convection and Kelvin waves. We first examine the relationship between the Kelvin
wave and zonal wind direction and amplitude. We then analyze SST anomalies and warm
water advection with respect to the development of the Kelvin wave. Next, we analyze,
as best is possible given the lack of observational data, patterns of surface heat fluxes
associated with the wave and propose a possible connection with convection. We also
briefly discuss broad, off-equatorial changes in the horizontal wind field that might also
be associated with the Kelvin wave, including gyres and a preferential location for
tropical cyclone genesis during this events. We then follow the same pattern of analysis
for two waves that appear to have been coupled to atmospheric convection during the
Northern Hemisphere winter of 2009-2010. We use the two analyses to begin to make
generalizations about these events, as well as to address the issue of the impact of
seasonal and background state differences.
X.
Methods
Daily dynamic height, SST, surface air temperature (SAT), zonal winds and fixed
depth and Acoustic Doppler Current Profiler (ADCP) data were obtained from the
Tropical Atmosphere Ocean (TAO) project office of NOAA/ PMEL buoys. The dynamic
height data were augmented through multiple linear regression based on observations at
neighboring buoys and sea level gauge data from island and coastal sites, following
Roundy and Kiladis (2007). Where observational current data was unavailable, the
University of Maryland’s Simple Ocean Data Assimilation (SODA) data was used.
Outgoing Longwave Radiation (OLR) from NOAA was used in the 2009-2010 case as a
proxy for convection. As a high-resolution proxy for convection in 1986-1987, 8 times
daily and half-degree resolution brightness temperature data were obtained from the
Cloud Archive User Service (CLAUS) was used in the 1986-1987 case. Temperature,
sensible heat flux, latent heat flux, relative humidity, and winds were obtained from the
European Center for Medium Range Weather Forecasting (ECMWF) 40 year Reanalysis
(ERA-40) for 1986-1987 and from the ERA-Interim project for 2009. All of the fields are
four times daily with a 2.5-degree resolution for the ERA-40 data, and 1.5-degree
resolution for the ERA-Interim data. Surface heat fluxes were compared to the Woods
Hole Oceanic Institute (WHOI) Objectively Analyzed (OA) air-sea fluxes to check for
consistency. While surface heat fluxes are not directly measurable over the open ocean on
a broad temporal or spatial scale, even rough estimates or temporal trends in these
quantities might help explain this event. Although these data undoubtedly include errors,
we use their relative orders of magnitude and temporal and spatial evolution during the
event with caution.
SSTs were analyzed by using both the one-degree weekly means from the
National Centers for Environmental Prediction (NCEP) Reynolds Optimum Interpolation
(OI) data, and from the TAO array. The Reynolds OI interpolates in situ and satellite
SSTs. These data are adjusted for biases using the method of Reynolds et al. (2002) and
interpolated from weekly to daily to simplify the analysis. The Reynolds method that
adjusts for biases might obscure the amplitude of the small changes in SSTs associated
with the intraseasonal Kelvin waves. The OI approach might also smooth over rapid
changes such as those associated with these Kelvin waves, leading us to compare the
TAO to the OI data averaged from 5°N to 5°S before using either exclusively. Large
inconsistencies occur between the two data sets, especially with respect to local extrema
associated with higher frequency disturbances. We thus have used the OI SST data to
resolve the spatial structures associated with background patterns, and have chosen to use
the TAO data to resolve local temporal changes associated with the Kelvin waves.
Anomalies were calculated for each data set in several ways. Intraseasonal anomalies
were calculated by applying a 20-120 day band pass filter, and a 121-day lowpass filter
was applied to analyze interannual anomalies. Some smoothing was done to wind fields,
and is mentioned when applicable.
The data fields are generally analyzed between 140°E and 110°W, along the
equator, or averaged from 2.5°N to 2.5°S, unless otherwise stated. The 1986-1987 Kelvin
wave was tracked by using dynamic height anomalies that were filtered for Kelvin waves
by means of a Fourier transform to retain data between 20 and 120 days (Roundy and
Kiladis 2006,2007; Roundy and Kravitz 2009). The 2009-2010 Kelvin waves were
tracked using unfiltered TAO dynamic height data.
XI.
Results
a. 1986-1987 Event
Figure 1 is a time-longitude plot showing OLR (shaded) and dynamic height
(contoured). Cool colors of OLR indicate enhanced convection. Figure 1 shows an
envelope of enhanced convection associated with the active convective phase of the MJO
progressing from about 60°E to 140°E between 25 October 1986 and 15 November 1986.
The solid line labeled “A” has a slope of about 7 m s-1 and is included as a reference to
indicate that the convective envelop propagates within the expected MJO range. As the
envelope approaches 140°E, it begins to slow to just over 1 m s-1, and reduce in zonal
scale (line “B”, dot-dashed). At that time, the envelope becomes coincident with positive
Kelvin dynamic height anomalies (red contours) moving eastward at approximately 1.5 m
s-1 (line “C”, dashed). This pattern is strongest from 140°E to just past the dateline, but it
exhibits coherence until about 150°W from mid-November to early January. The
remainder of this portion of the paper (section XI. a.) is devoted to analyzing
observations and reanalysis data to shed light on the mechanisms and patterns involved
during this period of time and comparing this result with idealized coupling processes
known to occur on similar temporal and spatial scales that have been identified from
previous works (Lau and Shen 1988).
i. Kelvin Wave Initiation
Figure 2a shows the total equatorial 10 m zonal winds during the Kelvin wave
initiation and throughout the event, and figure 2b follows the same conventions except
that the winds have had a 10-120 day band-pass filter applied. The dot-dashed and dashed
lines represent the same phase speeds as in Figure 1. The warm colors indicate westerly
wind anomalies, and cool colors indicate easterly anomalies. In early November, westerly
winds picked up around 140°E in association with the passing of the active convective
phase of the MJO (Figure 2a). Between 140°E and 165°E, the winds appear to be
strongest right at the crest of the wave, building in intensity ahead, and waning behind.
Westerly winds persisted with a magnitude of between 6 and 10 ms-1 at that location until
the beginning of January (1987). The winds were not longer actually westerly east of the
dateline; however they were anomalously westerly from 140°E to about 120°W (Figure
2b). Collocation of westerly wind anomalies with the Kelvin wave for such a long period
suggests the possibility that there was interaction between the Kelvin wave and the
winds. The simple model analysis of Shinoda at el. (2009) indicates that such wind
patterns would amplify the Kelvin wave. This wind amplification could have served to
enhance the surface fluxes especially at the initiation of the wave, which will be
discussed further in section XI. a. iii.
ii. Advection and Modification of SST
Basin-wide changes were occurring in the Pacific SST distribution during the
winter of 1986 and 1987 (Figure 3) in association with the Kelvin wave. Figure 3(a)
shows NOAA OI SST (shaded) and Kelvin dynamic heights (contoured) in the longitude
time domain, while Figure 3(b) follows the same conventions except for 121-day low
pass filtered data. The dot-dashed and dashed lines are reproduced from Figure 1. The
figure shows that the Pacific is initially close to the climatic average of Levitus (1984),
with a shift throughout the winter from that climatology towards an El Nino-like pattern,
with warm water spread across the Pacific. Previous to this shift, there are two
intraseasonal periods during which the warm pool extends towards the east. The phase
speeds of these bulges vary between 2 and 5 m s-1 which indicates that these eastward
expansions could be the local response to propagating wind stress as well as Kelvin
waves; while a portion of the signal might be due to ocean mixing and radiative
exchanges as atmospheric disturbances pass overhead. The last of the three bulges is
coincident with the leading edge of the Kelvin wave event of interest. An envelope of
positive SST anomalies coincident with the Kelvin dynamic height anomalies for the
duration of the wave, however it is strongest on the leading edge (Figure 3b). The
relatively cooler SSTs associated with the crest of the Kelvin wave could be associated
with cooling from reduced incoming radiation and mixing of rainwater due to enhanced
convection along the crest.
ADCP current data were used to estimate advection, while the temporal and
spatial structure of the surface current was analyzed using the Simple Ocean Data
Assimilation (SODA) from the University of Maryland (Carton et al. 2000a.b). Zonal
advection was calculated as follows:
,
(1)
where u is the zonal current at 10m depth, and dSST/dx is the zonal SST gradient.
ADCP current data are not available for the 1986/1987 event prior to January,
thus we were limited to analyzing the flow associated with this wave after January, which
is several days after the passage of the Kelvin dynamic height anomalies through the
region of enhanced convection. Further, for the time that current data were available, data
were not available in the vicinity of the strongest zonal SST gradients (they would ideally
have been available around 165°W to maximize sampling of the strongest SST
gradients). Although we cannot assess the effects of zonal temperature advection during
this event directly, we analyzed similar Kelvin wave events during subsequent years
because these other events might bear similarities with the 1986-1987event. We also
analyzed zonal currents in SODA for the same periods. Unfortunately, analysis of the
observational and SODA data provide opposite perspectives on the nature of the warming
observed in the east central Pacific with this event. We begin with a discussion of the
analysis of the subsequent events, followed by a discussion of the SODA analysis for
1986-1987.
We identified two similar Kelvin wave events during more recent years that
exhibited current values of around 1.2 m s-1 as the maximum Kelvin dynamic height
anomalies crested near 165°E and we examined the subsequent currents collocated with
stronger SST gradients farther east. We included events only if current data were
available at a given location for at least 15 days before and after the passage of the wave
crest. The events occurred during November of 1991 and January of 1992. We found that
for these events advection alone would have increased SST between 0.2 and 1°C over
one month in the vicinity of the strongest zonal SST gradients (generally around 170°W)
assuming near continuity of the current, which is supported by SODA data for both
(Figure 4a).
For the 1986/1987 event, if we assume continuity of the strong current observed
at 165°E through to where the SST gradient was stronger (an assumption supported by
both SODA and TAO data from the comparable event during December of 1991 (Figure
4a)) we estimate that advection could easily have resulted in about 1.8°C of warming in
the area along the equator from 175°W and 165°W over the period of 15 days before to
15 days after the wave crest passage. However, observed net temperature increases were
smaller. The average SST at 170°W for the month leading up to the passage of the Kelvin
wave, November, was 29.3°C, with a minimum daily value of 28.8°C. The peak SST
(according to the OI data since there is no TAO data near 170°W for 1986/87) following
the passage of the Kelvin wave was 29.7°C. The maximum predicted SST estimated by
integrating the advection term over time through the event, 30.6°C, was 0.9°C higher
than the actual peak SST. If zonal advection resulted in warming of this magnitude, then
it implies that local cooling associated with other processes counteracted some of the
warming.
Although some heat might have been lost due to mixing, a substantial portion of
the cooling might be associated with transfer of sensible and latent heat to the
atmosphere. If we assume that the mass of 1 m3 of water is 1035 kg, the specific heat of
salt water is 3993 J, an average temperature difference of 0.9°C, a thermocline depth of
100 m, over a span of 30 days, we can estimate that the difference between the observed
temperature and the predicted temperature based on warm water advection should yield a
surface flux of about 140 W m-2 if all of the heat was lost to the atmosphere and not
mixing; this compares well with combined surface sensible and latent fluxes from
reanalysis which are on the order of 120 W m-2 (Figure 5b). The similarity between the
fluxes and the heat loss supports the perspective that the total current associated with the
wave near 170°W was eastward and characterized by large amplitude, consistent with the
comparable events. The magnitude of the fluxes are also similar to those associated with
the advective mode of Lau and Shen (1988) in which differential advection of warm
water by Kelvin wave leads to convergence of the low level winds and evaporation of the
anomalously warm water, thus increasing the energy in the atmosphere.
According to the SODA data, the eastward current at 10m associated with this
Kelvin wave amplified to about 1m s-1 until the wave reached 170°E where it dropped to
0 m s-1 just past the dateline (Figure 4b). Past 170°E, across the remainder of the basin,
the wave was associated with a reduction of the climatological westward current to near 0
m s-1 (Figure 4b); this calming of the flow represents an anomaly of about 0.2 m s-1 (not
shown). The TAO fixed depth current data from early January also show no current at
140°W, however this location is east of the maximum zonal extent of the active
convection (near 150°W), suggesting that the wave was no longer coupled to atmospheric
convection there. Unfortunately we cannot confirm the SODA current values from
170°W with observational data. If the SODA data are correct that the current near 170°W
was near zero during this event (in contrast with observations of the similar event
discussed above) then the observed surface warming beyond 170°E to near 150°W would
have been caused by processes other than warm water advection, and the high values in
the flux data would also need to be explained. In the event that the SODA data are
correct, we speculate that the observed warming near 170°W might be accounted for by
suppression of cold westward advection by the background current that occurred as the
eastward flow anomaly associated with the Kelvin wave passed. The Kelvin wave might
also have suppressed local upwelling. These mechanisms would cause the radiative
balance observed prior to the wave to be broken, allowing for surface warming even in
the absence of net eastward advection. (how could this account for the large fluxes
necessary to sustain the convection???)
iii. Surface Fluxes
With changes in SSTs on the order of a half of a degree Celsius, it isn’t
immediately clear that any changes should occur to the environment that would make it
any more or less likely to support convection than its original state. However, previous
research has shown that model atmospheres are sensitive to small variations in SST,
especially where SST is high and surface winds are light (Fairall at al. 1996). Johnson et
al. (2007) analyzed the influence of surface heat fluxes on convection by comparing a
model run with an average sensible heat flux of 19 W m-2 and latent heat flux of 131 W
m-2 with another model that did not include any surface fluxes. These flux values were
similar to those estimated from observations during the Tropical Ocean-Global
Atmosphere Coupled Ocean-Atmosphere Research Experiment (TOGA COARE). The
model run without surface fluxes was found to have a 17% reduction in overall
precipitation with a 23% reduction on convective precipitation when compared to the
model run with fluxes included. We are confident then, that, it has been shown that
sensible surface heat flux on the order of as little as approximately 20 W m -2 and latent
heat flux on the order of 130 W m-2 can make the difference between majority stratiform
and convective precipitation (in our case implied by very cold loud tops or significantly
decreased OLR values) (Johnson et al. 2007). Especially during November 1986, the
reanalysis heat fluxes near the Kelvin wave crest appear to support an enhanced
convective environment (Figure 5a,b) as expected, being of similar magnitude as those in
Johnson et al. (2007).
Figure 5 is a time-longitude plot showing both surface heat fluxes with a 5-day
running average applied (shaded) along with the Kelvin dynamic heights (contoured),
dot-dashed and dashed lines are reproductions from Figure 1. Both fluxes are the most
enhanced at the beginning of the coupling in mid-November followed by weaker
enhancements of surface fluxes that track along over the Kelvin wave (Figure 5). While
both fields show ample activity outside of the envelopes, it is evident that they are
enhanced within the Kelvin wave region. Surface latent heat fluxes (Figure 5a) are
continually enhanced over the Kelvin wave region. They are also clearly influenced by a
westward propagating disturbance, which based on the approximate phase lines drawn on
Figure 5a indicating about 1.5m s-1 propagation, could have been a slowly moving
Equatorial Rossby (ER) wave. The strongest enhancements occurred at about 160°E and
165°E, in mid-November and early-December respectively, with values well above 200
W m-2 at those times. Those enhancements are coincident with the strongest zonal wind
anomalies and largest positive SST anomalies (Figures 2b, 3b, 5a). The surface sensible
heat fluxes progressed similarly to the latent heat fluxes initially, with their largest
enhancements being in approximately the same location with values in excess of 20 W m2
. The progressions of the sensible fluxes, however, appear to have been limited by the
warmest water (Figures 3a, 5b). From comparison with previous work (Johnson et al.
2007), it is reasonable to assume that fluxes of these magnitudes would have an
enhancing effect on atmospheric convection.
iv. Atmospheric Convection
Figure 6a shows a hovmoller of unfiltered brightness temperature (shaded) and
Kelvin dynamic height (contoured) from October 1986 to February 1987 while Figure 6b
shows the same fields, but focusing on the months of November and December. Both
figures have the lines from Figure 1 reproduced on them. During the winter of
1986/1987, one large-scale feature predominated the brightness temperature field (Figure
6). This one feature appears as a broad envelope of cooler cloud tops that maintains
continuity in space (from 140°E to 160°W) and time (from November to the end of
February).
Brightness Temperature shown in Figure 6 suggests that enhanced convection
occurred preferentially over west Pacific, in close proximity to the warmest water, albeit
more sparsely in October. November through February exhibit a clear enhancement in
cold cloud tops over the west Pacific, extending eastward towards 160°W. This envelope
has an approximate phase speed of 1.5 m s-1, which is slower than the first baroclinic
mode oceanic Kelvin wave, but consistent with the slower coupled advective mode of
Lau and Shen (1988) given the high background SST. This is a broad feature sometimes
encompassing as many as 40 degrees of longitude, but is narrower than the planetary
scale convective features normally considered consistent with the MJO (eg Zhang 2005).
The envelope includes many smaller scale and more quickly moving features, and with
more resolution in Figure 6b, both eastward and westward features are visible.
Figures 6a,b suggest that the convective envelope moves eastward slightly more
slowly than the Kelvin dynamic height anomalies, consistent with the differing phase
speeds of the dashed and dot-dashed lines. The maximum intensification of convection
begins at the leading edge of the wave throughout November and December, and then
progresses so that by mid January, the convection is associated with the trailing edge of
the Kelvin dynamic height anomaly. The most intense convection occurs to the west of
the maximum Kelvin dynamic height. Figure 7a is a time-longitude plot showing the
smoothed brightness temperatures contoured, and the SST anomalies (121-day low-pass
filtered) shaded with the reference phase speed lines from Figure 1. From this figure, we
can see that the convection really starts to become enhanced after the SST anomalies
increase in the beginning of November, from about 140°E to about 165°E. As the Kelvin
wave progresses, the region of enhanced convection moved eastward in association with
expanding warm water anomalies. The first burst of deep convection occurred just after
the maximum SST anomaly, which happens to be almost coincident with the largest
enhancements in both sensible and latent heat fluxes associated with the Kelvin wave
(Figures 5, 7a). Interestingly, the strongest westerly wind anomalies coincide with the
first burst of deep convection as well (Figure 7b), however, the connection between the
two doesn’t manifest itself later in the lifecycle of the wave.
Convection is organized on many different scales within the broader envelope,
from the small individual westward moving clusters, to larger scale convectively coupled
waves (Figure 6b). There also exists a larger, eastward propagating cluster within the
main envelope with an individual phase speed of about 3ms-1. Some of these features
appear to have existed before they encountered the Kelvin wave and intensified in its
presence, and some appear to have developed in situ. Convection begins firing with the
advance of positive anomalies of Kelvin band dynamic height at any given location along
its track, with enhanced cloudiness continuing almost a month afterwards in some
locations. Clouds subsequently crossed the region, but they don’t generally appear to
have developed in situ. This pattern suggests that the convective environment is different
in the vicinity of the wave crest than regions just to its east and west. As the Kelvin wave
and convective envelope progresses eastward, the envelope of convection advances more
slowly than the dynamic height anomalies, perhaps progressing instead with the Kelvin
SST anomalies (Figure 7a), such that dynamic height anomalies become offset to the east
of the convective anomaly near the end of the event (mid-January).
v. Additional Notes on Horizontal Wind Structure
Throughout the event convective anomalies evolve concurrently with horizontal
wind anomalies. As mentioned in section XI a. i., westerly winds are coincident with the
Kelvin wave near the surface throughout the progression of the wave across the pacific
basin (Figure 2). The winds don’t only evolve differently at the equator (Figure 8) in
association with Kelvin waves coupled to convection. Figure 8 is a collection of
snapshots of the evolution of 5-day smoothed horizontal winds and Kelvin dynamic
height during the event. On November 1st, 1986 (Figure 8a), before the initiation of the
Kelvin wave, the tropical Pacific is dominated by strong trade winds. By November 15 th,
1986 (Figure 8b), strong westerly wind anomalies have appeared in the eastern tropical
Pacific concurrent with the growth of the Kelvin wave. The Kelvin wave continues to
grow in amplitude and to propagate to the west. The anomalous westerly winds have
weakened by December 1st, 1986 (Figure 8c), but even by December 15th, the anomalies
are still tracking along with the Kelvin wave crest (Figure 8d). The westerly anomalies
never quite make it across 150°W, consistent with the time-longitude plot discussed
earlier (Figure 2). An additional response to the propagating convective feature is the
creation of an off equatorial cyclonic gyre (Figure 8d) accompanying the enhanced
equatorial westerlies.
Since apparently coupled events tend to dominate the tropical pacific convective
signature, it makes sense that their presence could also modulate the preferred formation
locations of Pacific tropical cyclones (TC). There is preliminary evidence to support this
concept; Figure 9 is a time-longitude plot of brightness temperature (shaded), Kelvin
dynamic height (contoured), and genesis location for tropical cyclones (TC symbols).
The blue TC symbols formed from 0-15°S and the red TC symbols formed from 0-15°N.
There appears to be a preference for TC genesis within the convective envelope
associated with the Kelvin wave. There is ongoing research into the possible causes of
this phenomenon, as well as if the results are robust across all or most of the coupled
events.
b. 2009-2010 Events
The winter before the preparation of this document (2009-2010) exhibited a few
high amplitude oceanic Kelvin waves (Figure 10), some of which appeared to couple to
atmospheric convection. Figure 10 is a time-longitude plot from October of 2009 to
January of 2010 and from 60°E to 60°W, showing OLR (shaded) and dynamic height
(contoured). The first Kelvin wave was initiated in mid-October, but atmospheric
convection did not amplify and progress eastward with that wave. The second wave
(hereafter referred to as “wave A”) was initiated by the active convective phase of the
MJO, with a phase speed of about 4.3 m s-1 (indicated in Figure 10 by the solid red line),
in the beginning of December, at about 150°E. The wave was coherent with convection
for the entire month of December, and travelled to 150°W at a phase speed of about 2.6
m s-1 (indicated by a red dashed line in Figure 10). The Kelvin wave was associated with
convection throughout its lifespan, however the convection drifted from being associated
with the crest of the wave, to its wake in a short span of time. The convection appears to
be truncated, which will be discussed in depth in the following sections. For the period of
time that intense convection is associated with the wave, its phase speed is approximately
the same as the Kelvin wave, about 2.6 m s-1.
A third Kelvin wave (hereafter referred to as “wave B”) was also initiated by the
passage of the active convective phase of the MJO, with a phase speed of about 6.9 m s-1
(indicated in Figure 10 by a solid red line), in mid-January and slightly further east than
its processors at about 170°E. This wave maintained continuity until the end of February,
and travelled to 150°W as well at a phase speed of about 2.4 m s-1 (indicated by a red,
dashed line in Figure 10). Wave B appears to be “better” coupled to atmospheric
convection than its predecessor, wave A. Associated with wave B, intense convection
extended to about 140W, which it had not done earlier in the winter associated with any
of the predecessor Kelvin waves. Similar to the 1986-1987 event, there is a general
movement of intense convection away from the west Pacific warm pool and towards
convection in the central pacific consistent with progression towards an El Nino
background state. However, it should be noted that the 2009-2010 El Nino was slightly
abnormal in that the water in the central pacific has been warmer than average, but the
eastern Pacific warming didn’t begin until late February. Thus the two cases are similar
in that they occurred during winter seasons classified as being ENSO positive, however,
the actual SST patterns associated with each winter was quite different.
The remainder of this portion of the paper (section XI. b.) is devoted to analyzing
observations and reanalysis data to shed light on the mechanisms and patterns involved
during this period of time and comparing this result with idealized coupling processes
known to occur on similar temporal and spatial scales that have been identified from
previous works (Lau and Shen 1988).
i. Kelvin Wave Initiation
In examining the winter of 2009-2010, it proved necessary to use two sets of
figures to analyze each variable. This is because wave B occurred so late into the winter
(when the warmest waters in the central pacific migrate south of the equator) that the
fields associated with coupling seemed to be dominated by Southern Hemisphere (SH)
rather than equatorial processes. Therefore Figure 11a is a time-longitude plot showing
dynamic height (contoured) in cm and TAO zonal winds (shaded) in m s-1, both averaged
from 2.5°N to 2.5°S. Figure 11b follows the same format except for data averaged from
2.5°S to 7.5°S. The black dashed and dot-dashed lines are reproductions from Figure
New1 for reference of the approximate phase speeds of atmospheric convection and
dynamic height anomalies.
Wave A is associated with zonal westerly winds in excess of 10 m s-1 on the
equator (Figure 11a), especially along the crest and near the wave initiation. Similar to
convection, winds appear to drift, from initially being associated with the crest of the
wave and subsequently with its wake. The westerly winds are present for the entire
lifecycle of the wave, albeit, much more weakly that at the outset. The “drifting” of both
the convection and the westerly winds can also be seen as a truncation of their extension,
almost like they are hitting a wall. The “wall” that the westerly winds hit looks to be a
surge in trade winds in mid-December that extends just west of the dateline.
Zonal winds on the equator are relatively weak in association with wave B
(Figure 11a), however, in the SH, they are quite strong (Figure 11b). In the SH, westerly
winds that exceed 10 m s-1 are consistently associated with the crest of the wave. The
westerly winds are strongest just east of the dateline. This feature is different from the
1986-1987 case in that the total winds are strongly westerly east of the dateline, while in
1986-1987 only the anomalies were westerly east of the dateline above the Kelvin
dynamic height anomalies.
ii. Advection and Modification of SST
Figure 12a shows a time-longitude plot of dynamic height (contoured) in cm and
TAO SST (shaded) in °C averaged from 2.5°N to 2.5°S. Figure 12b is the same except
that SST is averaged from 2.5°S to 7.5°S. The dashed and dot-dashed black lines on
Figures 12a,b are reproductions from Figure 10. During the winter of 2009-2010 there
was a progression from nearly climatalogical background state (Levitus 1984), with
warm water concentrated in the west Pacific, to a more El Nino-like background, with
warm water spread across the basin (Figure 12a). As discussed earlier, the warmest water
in the winter of 2009-2010 concentrated in the central Pacific, around 160°W, and does
not fully spread across the basin until late February. Also, during this time, the warmest
water was migrating southward with the seasonal cycle (Figures 12a and 12b).
For both wave A and wave B, it appears that a continuous surge of warm water
lead the Kelvin wave dynamic height rises (Figures 12a and 12b). Between the dateline
and 165°W, the anomalous warm water was able to be maintained even across the
upwelling section of the passing Kelvin waves (Figure 12a). This is not necessarily in
contrast with our findings from the 1986-1987 event, as those dynamic height values
were filtered. The same pattern, of a continuous progression of warm water pushing
eastward at the same approximate phase speed of the Kelvin wave preceding the crest
was evident in both cases. It is also worth noting that increases in SST are not simply
associated with the crests of the Kelvin wave; the extent of the 28°C isotherm east of the
dateline appears to be modulated within the area encompassed by the Kelvin dynamic
height contours, especially on the equator (Figure New3a). Similar to the 1986-1987
event, high SST was associated with the leading edge of the oceanic Kelvin wave. Also
similar to the 1986-1987 case, the crest of the Kelvin wave is associated with relatively
lower SSTs than the leading edge of this wave. This difference is likely due to decreased
radiation and rainwater mixing associated with enhanced convection over the crest of the
wave.
Some TAO zonal current data are available in the west Pacific, however data are
largely missing or not yet collected for this event across the remainder of the basin. At
the onset of wave A, the eastward current reaches values exceeding 1 m s-1 along the
wave crest. At 140°W at the end of December (along the trajectory of the wave crest), the
current is much weaker (about 0.2 m s-1), but still eastward. This information suggests
that the current maintained its eastward propagation along the wave crest. There is no
current data for the majority of wave B, however the data at 140°W along wave B’s
trajectory is also eastward at about 0.2 m s-1. Assuming a zonal SST gradient of about 1C
between 165W and 175W, which seems reasonable based on the TAO data (Figures
11a and 11b), and zonal current between 0.2 and 1 m s-1, advection values for the 15 days
before and after the passage of the wave crest would fall between 0.5 and 2.3 °C
according to an integration of equation (1). Realizing that the SST at 170°W never
increased more than half of a degree during the passage of waves A and B, it is likely that
some of the heat advected in the form of warm water into the central Pacific escaped the
ocean surface in the form of sensible and latent fluxes to the atmosphere.
iii. Surfaces Fluxes
Minimal surface flux data are available from the ERA Interim analysis (which is
only available through the end of 2009, and thus completely missing for wave B).
Further, SODA data are also not yet available for the winter of 2009-2010. We use ERA
Interim data to analyze changes in surface fluxes (sensible and latent) associated with
wave A. Given the equations for sensible and latent heat fluxes (respectively):
(2)
(3),
where ρa is the density of air, Ce is the exchange coefficient, Cp is the specific heat
of the air, V is the wind speed, L is the latent heat of fusion, Ts is the temperature of the
sea surface, Ta is the air temperature at the surface, qs is the specific humidity saturated at
sea temperature, and qa is the specific humidity of the air at the surface, we can make
some inferences about the fluxes associated with wave B.
Figure 13a is a time-longitude plot showing surface sensible heat flux (SSHF) in
W m-2 from ERA Interim (shaded) and dynamic height (contoured) and Figure 13b is the
same except for surface latent heat flux (SLHF) in W m-2, both figures have black dashed
lines that are reproductions from Figure 10. There is an enhancement of sensible heat flux
at the beginning of December coincident with the initiation of wave A (Figure 13a).
Similarly, there is an enhancement coincident with the initiation point of wave A in latent
heat flux (Figure 13b). There is also an enhancement of both fields centered on the
dateline in the second half of December (Figures 13a and 13b). While the highest
sensible heat fluxex appear to be almost completely constrained to the wave envelope
(Figure 13a), there is a good amount of latent heat flux activity outside of the envelope of
the wave (Figure 13b). Enhanced latent heat fluxes actually appear to be associated with
previously mentioned surges in the trade winds in this case (Figure 11a), perhaps
indicating a stronger dependence on winds than temperature difference at that time, since
the difference between SST and SAT that that time is small.
Due to the lack of flux data available after December, we created a simple flux
proxy in order to analyze changes in sensible heat flux, which is zonal wind multiplied by
the difference between SST and SAT, according to equation (2). We did not create a
proxy for latent heat in this case since it is generally correlated with sensible heat flux
within the wave envelope, and uncorrelated outside of the envelope. Figure 14 shows a
time-longitude plot of flux proxy as defined above (shaded) in °m s-1, from 2.5°S to
7.5°S, with positive values indicating flux out of the ocean, dynamic height in cm
(contoured), and black dashed and dot-dashed lines which are reproductions from Figure
10. Within the envelope of wave B there is a high concentration of positive flux proxy
values (Figure 14) with a concentration at the end of January around 165°W, probably as
a result of a strong westerly wind burst with winds in excess of 10 m s-1.
iv. Atmospheric Convection
Convection in the eastern and central Pacific during the winter of 2009-2010 was
dominated by a combination of MJO and ENSO signals (Figure 10). The active
convective phase of the MJO is evident beginning in early October, early November, and
late December. The westerly winds associated with each active convective phase of the
MJO appear to initiate oceanic Kelvin waves. The Kelvin wave initiated by the October
MJO is different than those that follow it; convection does not continue along the wave.
As mentioned earlier, convection appears to continue in vigor more in association with
wave B than wave A, although both waves exhibit collocation of the Kelvin wave and
convection.
The convection associated with wave A is most vigorous at the initiation of the
wave, along the crest. As the wave progresses, the convection weakened slightly in
intensity, while continuing to progress eastward, although at a slower speed than the
wave itself. As mentioned earlier, we find it likely that the coupling of the convective
envelope and the oceanic Kelvin wave was truncated by a surge in the trade winds in
mid-December as will be discussed further below. As for wave B, convection is
concurrent with the Kelvin wave throughout its entire lifespan. Relatively intense
convection occurred within the wave envelope until mid-February and as far east as about
170°W.
As previously mentioned, the convection associated with wave A is most intense
at its initiation. Figure 15a is a time-longitude plot showing 5-day smoothed OLR
(contoured, in color) in W m-2, and flux proxy in °m s-1 averaged from 2.5°N to 2.5°S
(shaded). Figure 15b is the same for flux proxy averaged from 2.5°S to 7.5°S. Both
Figures 15a,b have reproductions of the phase speed lines from Figure 10, in black. The
intense convection at the initiation of wave A (Figure 10) is also coincident with a
strongly enhanced flux proxy (Figure 15a). The flux proxy is initially driven by strong
winds (Figure 11b) and then as winds subside, a large difference in between SST and
SAT (on the order of 2-3°C). Another illustrative point from this figure is the location of
negative flux proxy values at the edge of the intense convection, which is most likely
driven by a surge in the trade winds since the SST-SAT difference at that location is less
than a degree.
Wave B is associated with stronger convection than wave A from the outset
(Figure 10) based on its initiation by an MJO with a stronger active convective signal,
however it also maintains convection well east of the dateline to about 170°W. This
prolonged enhanced convection seems to have benefited from both strongly enhanced
westerly winds, which were on the order of 10 m s-1 (Figure 11b), and a SST-SAT
difference of over 3°C. Thus we postulate that wave A began very similarly to wave B,
but it had the misfortune of being intercepted by a trade wind surge that was likely
induced by atmospheric waves, and its growth and coupling were thus cut short. When
looking for coupling events in real time, the strength of the trade winds (and/or their
probability of strengthening) should be a key feature to watch for.
v. Additional Notes on Horizontal Wind Structure
As has been mentioned, along with coupling to convection, westerly winds are
coincident with the oceanic Kelvin waves. In this section we discuss the off-equatorial
horizontal winds associated with oceanic Kelvin waves. Here, we focus on wave A,
because ERA Interim horizontal wind data was only available until the end of December
2009 as of this preparation. We use a similar analysis to the 1986-1987 case, with a few
differences. The major difference comes from the use of unfiltered Kelvin wave data in
this case. Since the data are unfiltered, the baseline that we use to judge anomalous
dynamic height is the average dynamic height at each buoy for the time period in
question (October 2009 – February 2010). The winds are smoothed with a 5-day moving
average, just as before. Figures 16a-d show those smoothed horizontal winds from 30°N
to 30°S and 140°E to 100°W, and equatorial dynamic height (varying = solid line,
average = dashed line), for the dates 11/29/09, 12/04/09, 12/08/09, and 12/13/09.
On November 29th (Figure 16a) wave A had just been initiated and was beginning
to gain amplitude at about 150°E. At that time, the westerly winds in excess of 10 m s-1
were present along the equator extending through to about 170°E. Those winds were
associated with an off-equatorial gyre centered at about 160°E and about 5°N. By
December 4th (Figure 16b) the equatorial westerly winds had extended to about 170°W,
and the dynamic height anomaly crest had progressed to about 160°E. By this time, the
northern gyre had disappeared, but there was a new gyre growing in the SH associated
with the anomalous westerlies, centered at about the dateline and 5°S. This gyre was also
associated with a surge from the extra tropics. At this time it could be traced as far away
as 120°W and 30°S. There was some signature of the same surge feature from November
29th, but it was much less coherent with weaker winds. On December 8th, the westerly
winds receded back into the east Pacific, to 170°E, and weakened to less than 10 m s-1
(Figure 16c). The SH gyre, however, remained. This gyre drifted westward along with
the eastern-most extent of the westerlies, moved south to about 10°S and more or less
maintained its strength. The gyre still appeared to be associated with an extra-tropical
wind surge at this time, although much weaker and only extending to the dateline. At this
time, the oceanic wave peaked at the dateline. By December 13th, the dynamic height
anomalies associated with the wave peaked at about 165°W (Figure 16d). The equatorial
windsmostly returned to easterlies with the exception of some light westerlies extending
to about 165°E (from where?). The SH gyre strengthened and drifted, then became
centered at about 175E and 15S. The gyre persisted, drifting further sound and east,
until it is obscured by another strong extra-tropical wind surge, this time coming from the
dateline, on December 17th.
In this analysis, only the existence of two gyres was focused on, mostly due to
their relative ease of tracking. However, even a cursory glance at Figures 16a-d leaves the
impression that the coupling between atmospheric convection, zonal winds, and oceanic
Kelvin waves has a large impact on extra-tropical circulation patterns, at least over the
Pacific. As mentioned for the 1986-1987 case, there has been some evidence of a
connection between TC activity and coupled oceanic Kelvin waves. Figure 17 is a timelongitude plot showing OLR (shaded) in W m-2, dynamic height (contoured) in cm and
SH (0-15°S) TC genesis locations. Even more striking than the 1986-1987 case, during
the life span of wave B, there was no TC genesis outside of the convective envelope
associated with the oceanic Kelvin wave. Only SH TC genesis has been plotted for this
event because there was only one NH TC that occurred during the lifespan of wave B,
TD 1, which was first named at 8.2°N and 110.6°E on January 18th.
XII.
Discussion
As has been suggested previously (Roundy and Kiladis 2006, Shinoda et al.
2008), it appears that during cases of apparent coupling between oceanic Kelvin waves
and atmospheric convection, the extent and amplitude of westerly winds in the central
Pacific is much greater than is climatologically normal or even than is expected in
association with the MJO. In both winters studied, the envelope of westerly winds
extended to about 150°W (Figures 2a, 12a, 12b), with rare occasions were westerly
anomalies have spread to 135°W. We propose that these winds are able to be maintained
over the Kelvin wave by the process proposed in Roundy and Kiladis (2006), that there is
a mutually beneficial process occurring whereby the westerly winds strengthen the
Kelvin wave, and the SST anomalies associated with the Kelvin wave strengthen the
westerly winds.
In both winters, there was an observed increase in SST associated with the leading
edge of the Kelvin wave (Figures 3a, 13a, 13b). This warming could be associated with
many external, as well as internal processes, however we have demonstrated that a large
portion of the warming is due to advection. Further warming might have been associated
with suppression of upwelling by the Kelvin wave. The continuous westerly wind stress
probably served to enhance the Kelvin wave and increase advection of warm water to
enable SSTs above what would normally be expected with an uncoupled wave. In the
analysis of coupled events (including 1986-1987, 1991-1992, and 2009-2010), eastward
current exceeding 1 m s-1 was observed, mainly to the west of the dateline (Figures 4a,b).
In 1991 and 2009-2010, the current remained eastward during the entire lifespan of the
coupled waves and probably served to enhance advection of warm water, especially
where the zonal SST gradient was highest. In 1986-1987, the actual observations were
sparse, however SODA data suggest that there may have simply been a weakening of the
climatological westward current. This may have lead to a more complicated advection
relationship, however enhanced SSTs were observed in all cases regardless of the
strength of the associated currents. In both the 1986-1987 and 2009-2010 cases, the
Kelvin waves observed to couple with convection essentially were the observed El Nino
signature (Figures 3a, 13a, 13b), forcing the eastward spread of warm water across the
equatorial Pacific.
As mentioned earlier inSection VIII. b. iii., fluxes of sensible and latent heat
across the air-sea interface are functions of wind speed and temperature or specific
humidity (respectively). All of these quantities are modified by the Kelvin wave, so it is
reasonable to expect a change in fluxes within the wave envelope. During the 1986-1987
event there is a clear enhancement in both the latent and sensible flux fields over the
Kelvin wave (Figures 5a,b). The enhancements are by no means fully confined to the
wave, but they are clearly organized within the vicinity of the wave.
For the winter of 2009-2010, flux data are only available for wave A. Sensible
heat fluxes (Figure 13a) seem to be very responsive to changes associated with the Kelvin
wave, however latent fluxes (Figure 13b) appear to respond to something else, which we
suggest is likely to be a punctuated surge in the trade winds that was induced by
westward-moving atmospheric waves that were not initially associated with the oceanic
Kelvin wave. Because of lack of surface heat flux data for wave B, we createdflux proxy
data for sensible heat based on the product of winds and SST-SAT. This flux proxy field
was clearly enhanced over the entire Kelvin wave (Figure 14). The enhanced surface
fluxes associated with changes in wind speed and direction and increased SST are likely
precursors to enhanced convection. In the maritime tropics, where the environment is
essentially always primed for convection, we believe that these enhanced fluxes are
necessary to destabilize the atmosphere to convection; this concept is supported by
previous works (Fairall et al. 1996, Johnson et al. 2007).
Finally, we come to convection. In each case, the envelope of atmospheric
convection starts out as the active convective phase of the MJO with phase speeds
between 4 and 7 m s-1 (Figures 1, 11). In the event of coupling, we observed in both cases
that we studied a slowing of the convective envelope to around 2 m s-1, which is on the
slow end of expected range of phase speeds for oceanic Kelvin waves (Figures 1, 11). In
the winter of 2009-2010, wave A appeared to begin the coupling process, but then its
eastward progression was truncated due to redirection of convection by enhanced trade
winds (Figure 10, Figure 11a). The relationship between the convective signatures and
the oceanic Kelvin wave is not exactly the same in all of the cases. In 1986-1987, the
convection had a consistently slower phase speed than the Kelvin wave (1.3 vs 1.6 m s -1).
Convection associated with wave B when assessed in bulk is slower than the Kelvin wave
(1.7 vs 2.4 m s-1), however it can also be viewed as travelling at about the same phase
speed as the Kelvin wave with a jump at the beginning of February from the wave crest
to behind it. The differences in the relationships between convection and the Kelvin wave
lend more credence to the idea that the enhanced fluxes are influencing convection, since
the location of the enhanced fluxes along the wave is varied, and depends on factors other
than the location of the maximum dynamic height anomaly; wind speed and SST-SAT
for instance.
We found with the 1986-1987 case, that the zonal extent of the intense convection
appeared to be strongly correlated with the extent of SST anomalies (Figure 7a) and that
the most intense convection was collocated with the strongest westerly wind anomalies
(Figure 7b). With wave A in 2009-2010, we found again the intense convection appears
to be bounded by the extent of the warmest water (comparing Figures 12a and 15a). The
flux proxy was strongest just before and during the most intense convection (Figure 15a),
which coincides with the strongest westerly winds (Figure 11a) similar to the 1986-1987
event. Wave B in 2010 followed the same pattern of intense convection being bounded
by the zonal extent of the warmest water (comparing Figures 12b and 15b) and the most
intense convection being collocated with strong westerly winds (Figures 11b, 16b). The
flux data and flux proxy data suggest that a connection can be made between changes in
SST induced by the Kelvin wave and intensification of convection over the wave.
In both the winter of 1986-1987 and 2009-2010, there were changes to horizontal
off-equatorial winds in association with enhanced equatorial westerly winds. Both the
1986-1987 event and wave A from 2009-2010 were associated with off equatorial gyres
in the SH that dissociated from the equatorial westerlies and intensified (Figures 8 and
17). The coupling events also appear to be associated with the modulation of the location
of TC genesis in the Pacific (Figures 9 and 17) however, much more work needs to be
done in this area.
Previous research has shown that these coupled events not only influence the
location of tropical convection, or off-equatorial gyres, but they have ramifications for
global weather patterns (Roundy and Verhagen 2010). Roundy and Verhagen (2010)
showed that there is a strong correlation between the location of the coupled oceanic
Kelvin wave and extra tropical 300 hPa heights and wind patterns. They were even able
to use this information to effectively forecast the general circulation of North America
during the winter of 2009-2010 due to the activity ofwave A and wave B discussed
above, with more skill than simply using patterns based on indices of ENSO or MJO.
Figure 18 is a schematic of the composite observed waves while they exhibited
coupling. The schematic is most representative of the fields as they existed between
140°E and the dateline, as the relationships generally shifted further east. Our analysis
shows that an oceanic Kelvin wave was initiated by the active convective phase of the
MJO (as shown by the envelope of convection between 60 and 140°E). The Kelvin wave
progressed to the East, as shown by the increased Kelvin dynamic height anomalies and
warm water anomalies that follow along the leading edge of the wave. The surface
sensible and latent heat fluxes are also enhanced most strongly at the leading edge of the
Kelvin wave dynamic height anomaly early in the life of the wave. In some cases the
latent heat fluxes are enhanced across the breadth of the Kelvin wave, while the sensible
fluxes progress eastward slowly with the warmest water. In other cases, the reverse is
true. This pattern suggests that the type of heat flux is not specific to the ability of the
wave to couple, but coupling depends on the presence of bulk enhanced fluxes. The
westerly wind anomalies are strongest at the initiation of the Kelvin wave, but remain
enhanced across most of the equatorial Pacific Ocean during this time period.
Atmospheric convection appears to track the boundary between the warm and cool
anomalies.
The schematic suggests that these events evolved similarly in some respects to the
model proposed by Lau and Shen (1988) (See their Figure 5a). Similar to Lau and Shen
(1988), we have observed mean SST increasing westward across the Kelvin dynamic
height anomalies and convective envelope once they have crossed the dateline. The
relative locations of the warm and cool SST anomalies and the enhanced surface fluxes
(labeled “evaporation” in Lau and Shen’s figure) with respect to each other are also very
similar to Lau and Shen’s analysis. Our schematic diverges from Lau in Shen’s in the
location of the westerly wind anomalies and the location of the convection. Their analysis
suggests that the convective envelope develops significantly after the warm water
anomalies, and that the warm anomalies are associated with easterly wind anomalies. Our
analysis suggests that, while warm water does precede the convective envelope by some
time, in most locations they are almost coincident (Figure 7a), also, the convective
envelope is almost exclusively associated with westerly anomalies in our analysis (Figure
7b).
XIII.
Conclusion
As mentioned in section IX, one of the aims for this work was to better
understand the mechanisms that maintain coupling between atmospheric convection and
oceanic Kelvin waves. As we have shown in both 1986-1987 and 2009-2010 the process
of coupling exhibits the following pattern:

Westerly winds associated with the active convective phase of the MJO initiate an
oceanic Kelvin wave

Ahead of the crest of the Kelvin wave, slightly higher SSTs are forced by both
advection and downwelling (Johnson and McPhaden 1993).

The regions of increased SSTs destabilize the atmosphere to convectively coupled
waves resulting in an envelope of enhanced convection over the oceanic wave
crest.

Westerly wind anomalies amplify over the wave crest in response to the organized
convection. These enhanced westerly winds transfer momentum to the ocean,
amplifying the oceanic wave.

Further, these winds might enhance both heat and moisture fluxes from the ocean
surface to the atmosphere.

These enhanced fluxes might enhance convection, and a self maintaining
convective system would develop.
Obviously some variations in the process will exist from case to case, but the general
pattern has been observed consistently across the set of events we have examined.
Another aim of this work was to understand how coupling between atmospheric
convection and oceanic Kelvin waves changes throughout the lifecycle of the wave. In
both years studied, the phase speeds of the convective envelope and the height anomalies
associated with Kelvin waves differed, and therefore eventually drifted outside of each
other’s spheres of influence. From our analysis of 1986-1987, it appears that the zonal
extent of the intense convection was associated with the progression of positive SST
anomalies (Figure 7a), while the most intense convection was associated with strong
westerly wind bursts (Figure 7b). We also found that in both coupled events in 20092010 the strongest convection was also associated with strong westerly winds, but that in
the absence of these strong winds, a large SST-SAT difference could also maintain high
surface heat fluxes (Figures 15a,15b). Thus, it seems that the convective envelope
develops in conjunction with amplified zonal winds and SST-SAT difference, rather than
exactly in phase with the dynamic height height anomalies associated with the Kelvin
wave. Thus it might appear that the relationship between the Kelvin wave and convection
changes with time, but it is probably more accurate to say only that the relationship with
the Kelvin wave height anomalies changes with height.
We have found that within our climatology of over 40 cases (including all
seasons) only one case has a convective envelope that crosses the 28°C isotherm. We
know from Lau and Shen (1988)’s prediction that SSTs below 28°C don’t allow phase
speeds of the oceanic Kelvin wave and atmospheric convection that are sufficiently
similar to maintain coupling. This finding deserves more attention in future studies to
further investigate this phenomenon.
Through our analysis of the 1986-1987 case and the two waves in 2009-2010, we
observed coupling that initiated in early November (1986), late November (2009), and
early January (2010). Each of the waves evolved differently in some ways. Some of the
differences might be associated with interactions with high frequency transient signals
propagating across the region of coupling, but other differences might be associated with
modulation of the coupling process by different background states.. Wave B in 2010
occurred so late into the winter season that the warmest water had progressed south of the
equator. This southward shift of the convection lead to many of the fields associated with
the coupling (zonal winds, currents, and SST changes) being most active in the SH
tropics rather than centered on the equator. Again, this observation will lend itself to
further modeling studies for confirmation and validation.
We focused solely on NH winter events for this study, however they only make
up about half of the events in our dataset. It stands to reason that the coupled events that
occur in other seasons will have different internal structures and impacts on global wind
patterns, due to the very different atmospheric and oceanic conditions that they would
encounter.
Already, this research has been used to accurately forecast NH general circulation
several weeks in advance, following the work of Roundy and Verhagen (2010). The
response pattern to the moving, large heat source that is the convective envelope coupled
to the Kelvin wave is significant and predicable. As mentioned in sections XI a.v. and XI
b.v. there also exists the potential that the prediction and/or observation of a coupled
event could assist in predicting probable TC genesis regions.
Some questions that remain about this event include how the Kelvin wave in this
study became coupled to atmospheric convection in the first place and how it ultimately
became decoupled from the atmosphere. As stated earlier, the envelope of enhanced
surface fluxes which stalls around 150°W in 1986-1987 (Figure 6), could be related to
changes in SST and the decoupling process and should be studied in more depth. Also,
initial conditions necessary for coupling will be examined, including strength and timing
of the initial westerly wind burst and any oceanic preconditioning. The majority of the
events (12 of 20) occurred either during an “El Nino” event or during the transition into
or out of one, while only 2 occurred during a “La Nina” event. This leads to the question
of their relationship, be it simply oceanic preconditioning or something more
complicated. We are presently analyzing composites of 20 events which will clarify
details of the evolutions and identify features that tend to occur consistently across
similar events. We are studying how these events evolve in a composite sense in order to
better understand their characteristic features.
This work will hopefully be the starting point for much more research into the
concept of coupling between atmospheric convection and oceanic Kelvin waves. Through
our research we have gained an understanding of the typical progression of coupled
events, as well as some possible deviations that can be expected in the presence of
various background states. We have proposed a possible mechanism for coupling though
the enhancement of surface heat fluxes by enhanced westerly winds and advection of
warm water. We have begun the work of discovering the tropical and global implications
for coupled events. We thus are confident that the research can be continued from this
point with a solid start.
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