Remote Sensing of Environment 91 (2004) 153 – 159
www.elsevier.com/locate/rse
Observing the coupling effect between warm pool and ‘‘rain pool’’ in the
Pacific Ocean
Ge Chen *, Chaoyang Fang, Caiyun Zhang, Yong Chen
Ocean Remote Sensing Institute, Ocean University of China, 5 Yushan Road, Qingdao 266003, China
Received 1 July 2003; received in revised form 19 February 2004; accepted 21 February 2004
Abstract
Traditionally, the tropical zone is known as the ‘‘heat reservoir’’ of the ocean and the ‘‘firebox’’ of the atmosphere. The western equatorial
Pacific has been identified as both the warmest portion of the heat reservoir, named ‘‘warm pool’’ (WP), and the hottest portion of the firebox
where a huge amount of precipitation-induced latent-heat release is accumulated. The latter mirrors a fact that the western tropical Pacific is
also the wettest area on the globe, termed ‘‘rain pool’’ (RP), where the maximum annual precipitation is observed. The accumulation of
continuous satellite data has reached a point that decade-long simultaneous observations of many important geophysical parameters have
become available in recent years. One such example is the availability of a concurrent dataset of sea surface temperature, oceanic
precipitation, and sea surface wind field for 1993 – 2002 derived from NOAA/AVHRR (Advanced Very High Resolution Radiometer),
TOPEX/TMR (TOPEX Microwave Radiometer), and ERS-1,2/QuikSCAT, respectively. In the present study, this dataset is used to
demonstrate and investigate the coupling and covarying effects of the Pacific WP and RP, leading to a number of interesting findings on their
structural similarity, locational shift, phase lag, and evolutional coherency in association with the development of and the vacillation between
El Niño and La Niña events.
D 2004 Elsevier Inc. All rights reserved.
Keywords: Warm pool; Rain pool; Pacific Ocean
1. Introduction
In recent decades, tropical Pacific has become one of the
focused regions for scientific studies of climate variability
and global change. It has been identified as the originating
area of the El Niño-La Niña phenomenon, which affects
more than half of the world’s population. The geophysical
background behind this is perhaps the fact that tropical
Pacific is an area of most intensive air –sea interaction on
the globe. It is understood that in the tropics, the ocean and
the atmosphere are closely coupled, with the precipitation
system providing a major linking mechanism. The atmosphere’s impact on the ocean is exerted through winds, which
get most of their energy from the release of latent heat by
precipitation. An estimated two-thirds of this precipitation
fall in the tropics. The atmosphere, in turn, responds
strongly to the distribution of sea surface temperature
* Corresponding author. Tel.: +86-532-2033196; fax: +86-5322032424.
E-mail address: [email protected] (G. Chen).
0034-4257/$ - see front matter D 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.rse.2004.02.010
(SST) anomalies. This happens because the tall, heavily
raining cloud systems grow rapidly over regions of warm
SST anomalies and are considerably suppressed where the
SSTs are colder. In addition, rainfall also acts as a significant
buoyancy input to the sea surface which stabilizes the mixed
layer, and feeds back to the SST via lensing effect.
The western Pacific warm pool (WP), defined as the
oceanic areas with SSTs>28 jC (e.g., Ho et al., 1995) or
29 jC (e.g., McPhaden & Picaut, 1990), holds the warmest
seawaters in the world. It spans the western areas of the
equatorial Pacific to the eastern Indian Ocean, and is in fact
the largest heat reservoir of the global ocean. On the other
hand, of all the various atmospheric phenomena that characterize the tropical region, the most vivid feature is perhaps
the intertropical convergence zone (ITCZ), or the meteorological equator, which is also referred to as the atmosphere’s
‘‘firebox’’. Over three-fourths of the firebox energy come
from precipitation-related latent heat release as a result of
dramatic convective cloud activities along the ITCZ. A
particularly important and active part of the firebox is again
located in the western Pacific region (including the maritime
continent), where the maximum annual precipitation on the
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G. Chen et al. / Remote Sensing of Environment 91 (2004) 153–159
globe is observed. This ‘‘wettest’’ oceanic area, which is
called ‘‘rain pool’’ (RP) for simplicity, acts as the strongest
heat engine of the global atmosphere. The warm pool, along
with the rain pool, is believed to play a critical part in
shaping the global climate. Moreover, they have taken on a
special significance in El Niño-Southern Oscillation (ENSO)
studies since 1990s, following the recognition of their close
links to the onset and termination of El Niño and La Niña
events (e.g., McPhaden & Picaut, 1990; Spencer, 1993).
Many previous studies have addressed climate-related issues
in such a context. Among them, Bjerknes (1969) is perhaps
the first who demonstrates that the basin-scale SST difference between the huge WP in the western Pacific and
upwelling cold water of the eastern equatorial Pacific is a
fundamental determinant of the circulation and precipitation
regimes of the tropical Pacific. Rasmusson and Arkin (1993)
present an idealized representation of the time/space hierarchal relationship among several modes of tropical precipitation variability in the equatorial belt of the eastern Indian
Ocean– western Pacific sector (see their Fig. 5), and discuss
their connection with underlying SST pattern. As they point
out, for example, the changes in equatorial SSTs in the
eastern and central equatorial Pacific during the ENSO cycle
exercise strong control on the annual cycle of precipitation.
Also, they suggest that the primary precipitation pattern of
the Madden –Julian intraseasonal oscillation can be broadly
described as an east –west oscillation around the region of
high SST associated with the west Pacific– Indian Ocean
monsoon. More recently, scientists are able to track and
analyze the entire evolution of the 1997 – 1999 El Niño/La
Niña with unprecedented details using both field (e.g.,
McPhaden, 1999) and satellite data (e.g., Quartly et al.,
2000). A major advancement of these studies is the complete
recording of a full cycle of the ENSO oscillation with at least
100-year return extremes via a ‘‘troika’’ of SST, wind field,
and rainfall. The purpose of the present study is to further
illustrate and investigate the coupling effect between the
oceanic warm pool and the atmospheric rain pool in terms of
spatial similarity and temporal coherency. These are thought
to be essential for the understanding of ENSO mechanism in
particular, and air – sea interaction in general.
2. Satellite data
A decade-long simultaneous dataset of NOAA/AVHRR
(Advanced Very High Resolution Radiometer), TOPEX/
TMR (TOPEX Microwave Radiometer), and ERS-1,2/
QuikSCAT spanning January 1993 through December
2002 and covering 50jS 50jN, 100jE 290jE has been
compiled, on the basis of which 120 collocated fields of
1 1j monthly SST, rain rate, and wind velocity are
obtained and described, respectively. (1) The SST data are
extracted from a NOAA/NASA Oceans Pathfinder Product
called ‘‘Equal Angle Best SST’’, which has a spatial
resolution of 9 km and a temporal resolution of 8 days
(Vazquez et al., 1998). In the equal-angle projection there is
an equal number of pixels in both the longitude and latitude
directions. The 9-km dataset consists of data with 4096
pixels in the east – west direction (longitudinal) and 2048
pixels in the north –south direction (latitudinal). This product retains, after a series of statistical tests, only high quality
pixels. All versions of the Pathfinder SST algorithm are
based on the NOAA/National Environmental Satellite Data
and Information Service (NESDIS) nonlinear SST operational algorithm (Kilpatrick et al., 2001). An optimal interpolation is performed to reconstruct the SST dataset with a
1 1j spatial grid and a 1-month time interval. (2) The
TOPEX/Poseidon mission offers the first opportunity to
observe rain cells over the ocean simultaneously by a
dual-frequency radar altimeter (TOPEX) and a three-frequency radiometer (TMR). A joint index is proposed to
quantify the effects of rain on TOPEX altimeter and TMR
radiometer (Chen et al., 1997). The oceanic precipitation is
derived using a semi-analytical algorithm based on coincident TOPEX/TMR GDR (Geophysical Data Record) data
provided by AVISO (AVISO, 1996). Technical aspects of
the TOPEX/TMR-based rain algorithms as well as comparisons between TOPEX/TMR and other precipitation climatologies, such as GPCP (Global Precipitation Climatology
Project, Huffman et al., 1997), can be found in Chen et al.
(1997, 2003) and Quartly et al. (1999). (3) The gridded
mean wind fields are retrieved from satellite scatterometers
onboard ERS-1 (01/1993-05/1996), ERS-2 (06/1996-12/
2000), and QuikSCAT (01/2001-12/2002) (CERSAT,
2002a,b). In order to reconstruct gap-filled and averaged
synoptic wind fields from discrete observations over a given
time period, a statistical interpolation is performed using an
objective analysis (Bentamy et al., 1999). The method is
based on a minimum variance scheme related to the kriging
technique which is widely used in geophysical studies.
3. Spatial similarity of the Pacific WP and RP
Previous studies have revealed that the interannual variabilities of the western Pacific WP (e.g., Ho et al., 1995;
McPhaden & Picaut, 1990) and RP (e.g., Joyce & Arkin,
1997; Spencer, 1993) are both characterized by a zonal
oscillation of their central positions, with the eastern and
western extrema being reached during El Niños and La
Niñas, respectively. It can therefore be anticipated that
geographical correlations between the WP and RP may be
better illustrated by their behaviors during El Niño and La
Niña years. For the period of our data duration (1993 –
2002), the years 1993, 1994, 1997, and 2002 are dominated
by El Niños, whereas La Niñas prevail during 1999 and
2000. The annual average rain rate and SST of the Pacific
Ocean for the El Niño and La Niña years are shown in Fig.
1a– d, respectively. Several striking characteristics can be
identified from the four sub-plots. First, both the WP and the
RP exhibit a two-core structure in the El Niño mode (as
G. Chen et al. / Remote Sensing of Environment 91 (2004) 153–159
155
Fig. 1. Averaged oceanic precipitation (in mm/day) of the Pacific for the composite (a) El Niño (1993, 1994, 1997 and 2002), and (b) La Niña (1999 and 2000)
years during 1993 – 2002. (c) and (d) are similar maps but for SST (in jC). The two-core structures of RP and WP are marked with white arrows in (a) and (c).
The boundaries of the WP in (c) and (d) are depicted by thick contours corresponding to 28 jC.
indicated with the white arrows in Fig. 1a and c, respectively), in contrast to a well-defined single-core pool in the
La Niña mode (Fig. 1b and d). Second, the WP and the RP
share a double-tongue pattern at their eastern edges in the La
Niña mode (Fig. 1b and d), but it becomes less obvious in
the El Niño mode, especially for the WP (Fig. 1a and c).
Third, a systematic zonal migration of approximately 30j is
observed for the centers of both the WP and the RP from El
Niño (in the east) to La Niña (in the west). Fourth, a
westward shift of about 20j is found for the RP with respect
to the WP, and this displacement seems to be of little change
from El Niño to La Niña. In summary, it is clear that the WP
and the RP are well correlated, an overall similarity exists in
their spatial patterns, and these patterns shift apart in space
but vary coherently with time. Naturally, several interesting
questions arise: (1) Why do the WP and the RP bear a
similar spatial pattern? (2) Why is the RP shifted westward
with respect to the WP? (3) Why does the WP and hence the
RP oscillate zonally on interannual time scales?
The answer to the first question is straightforward in
principle: The WP and the RP are dynamically and thermodynamically coupled. The WP waters are warm enough to
drive heat and moisture high into the atmosphere, and the
ability of the atmosphere to hold water vapor increases
nonlinearly with temperature. The atmospheres laden with
heat and moisture from sensible heating and surface evaporation over the WP converge to form a zone of increased
mean convection, cloudiness, and hence precipitation.
Therefore, the existence of an atmospheric counterpart of
the oceanic WP with correlated spatial structure can be
expected. Specifically, the split of the WP and hence the RP
into two cores during El Niños is likely to be a surface
manifestation of the formation of a cooling zone in the
subsurface upper ocean near 160jE (see, e.g., the bottom
panel of Fig. 2 in Webster & Palmer, 1997).
To answer the second question, the rotation of the earth is
a principal factor to consider. As discussed earlier, large
amounts of heat and water vapor are transferred to the
atmosphere over the WP region. The rising air moves
somewhat independently of the earth’s surface since there
is little or low friction between the earth’s surface and the
atmosphere. Moreover, as the WP is basically located near
the equator, meridional deflection due to the Coriolis effect
is negligible. The RP is therefore left to the west of the WP
as the earth rotates toward the east. Also recall that although
this region coincides with a minimum wind of the world’s
oceans, the speed is non-zero (see, e.g., Fig. 1 of Chen et al.,
2002) and the prevailing direction is usually toward west
(except for the bursts of westerlies during the initial phase of
an El Niño episode, see Fig. 4a below). This adds to the
earth’s rotational effect in further shifting the RP toward
west. Besides these, the RP may also be ‘‘locked’’ to the
vicinity of the maritime continent via a sort of ‘‘anchoring
effect’’ which has not been entirely understood. A very
recent study by McBride et al. (2003) suggests that ‘‘boomerang-shaped’’ SST anomalies are ‘‘upstream’’ rather than
being collocated with the maritime continent rainfall. A
mechanism has been hypothesized whereby the interannual
variability of maritime continent rainfall is simply a downstream response to the boomerang sea surface temperature
pattern. This might be a new clue for interpreting the WP/
RP shift, although its validity needs to be further explored.
As far as the third question is concerned, it is widely
accepted that the best known mode of SST variability in the
tropical Pacific is associated with the ENSO phenomenon
(Lau, 1997). In fact, the zonal migration of the western
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G. Chen et al. / Remote Sensing of Environment 91 (2004) 153–159
Pacific WP and its overlying convection are fundamental to
ENSO (McPhaden & Picaut, 1990). This migration has
generally been attributed to wind-driven zonal current
advection. To help understand its mechanism, the SST and
precipitation anomalies for the composite El Niño and La
Niña years relative to their respective decadal climatologies
are shown in Fig. 2. Several notable features are easily
identifiable in the four panels of Fig. 2. First, a dipole-like
pattern exists for the El Niño precipitation anomaly (Fig.
2a). This is found to switch to a sign-opposite dipole for the
La Niña mode (Fig. 2b). A close inspection of Fig. 2a and b
reveals that the maximum rain anomaly during the warm
events is located slightly west of the minimum rain anomaly
during the cold events, as is also found by Hoerling et al.
(1997, see their Fig. 5). Second, the SST anomalies between
El Niños and La Niñas are also anticorrelated (Fig. 2c and
d), with extremely large positive values observed along the
west coast of Peru under El Niño mode. Third, despite an
overall spatial similarity with a systematic zonal shift
between the corresponding precipitation and SST anomalies, the locations of their positive maxima are totally
separated under El Niño mode, with the precipitation
anomaly peaked around the dateline, whereas the SST
anomaly peaked along the Peruvian coast. This, according
to Hoerling et al. (1997), is primarily due to the nonlinearity
of the thermodynamic control on deep convection, as also
illustrated by Berg et al. (2002) with satellite observed
differences between east and west Pacific rainfall systems
over relatively cold or warm SSTs. Another interesting
feature to be noted is that the change of sign near 170jE
on the map of SST anomaly (Fig. 2a) coincides with the core
of the precipitation anomaly at the equator (Fig. 2c). This
might be more than just a coincidence, but a satisfactory
explanation is not readily available. Nevertheless, it is clear
that a kind of teleconnection exists between the peak
intensities of SST and rainfall anomalies, although they
are geographically uncorrelated. Accordingly, the WP
movement in the central western Pacific is found to be
largely in phase with the evolution of SST anomaly in the
southeastern Pacific.
Bearing in mind the distribution of and the relationship
between precipitation and SST anomalies, we now return to
the third question raised above. According to McPhaden and
Picaut (1990), warm SST anomalies in central and eastern
tropical Pacific generate anomalous deep atmospheric convection fed by convergence of surface winds. This surface
convergence leads to a considerable weakening of the trade
winds west of the warm SST anomalies. The ocean responds
rapidly to this weakening: Surface currents locally accelerate eastward, and large-scale equatorial Kelvin and Rossby
waves radiate out from the directly forced region to reinforce and spread the warm SST anomaly across the basin.
This positive feedback loop continues to grow until easterly
trade winds return to their full strength. The entire process
may last for 12 – 18 months during which a cycle of zonal
oscillation of the WP and RP is completed.
4. Covariation of the Pacific WP and RP: an
extraordinary case
The 1997 – 1998 El Niño event, which is by far the
strongest in recorded history (McPhaden, 1999), provides
an excellent opportunity to examine the synchronized zonal
migration of the WP and RP. In doing so, time-longitude
diagrams of rain rate and SST at the equator are plotted for
1993– 2002, as shown in Fig. 3a and b, respectively. It can be
seen that the dateline serves roughly as a dividing line
between two characteristic regimes. East to the dateline,
cold features of SST and dry features of precipitation appear
Fig. 2. Oceanic precipitation and SST anomalies corresponding to Fig. 1 with respect to a 10-year climatology from 1993 to 2002.
G. Chen et al. / Remote Sensing of Environment 91 (2004) 153–159
157
Fig. 3. Time/longitude diagram of (a) rain rate (in mm/day) and (b) SST (in jC) along the equator (averaged between 5jS and 5jN) derived from TOPEX/TMR
and NOAA/AVHRR, respectively. The blue and white arrows in (a) and (b) depict eastward propagations of the RP and the WP, respectively. The yellow arrow
in (a) depicts westward propagation of a dry zone associated with the South Pacific marine desert. The pink arrow in (b) depicts westward propagation of the
South Equatorial Current. The dashed curves are duplicates of those with the same color at the corresponding location in the other panel. The vertical dashed
lines in (a) and (b) indicate the dateline.
to propagate westward consistently with a regular seasonal
fluctuation in intensity (see the pink and yellow arrows in
Fig. 3a and b). The cold SST propagation can be traced to the
South Equatorial Current which is driven by the southeast
trade winds and flows westward along the immediate south
of the equator, and further back to the Peru Current which
transports the upwelled cold water from south to north along
the Peruvian coast. On the other hand, the propagation of a
low precipitation zone is a manifestation of the annual
migration of the South Pacific marine desert (see also Fig.
1a and b). The two propagations, though somewhat coherent
in time, are based on different mechanisms. As a result, the
centers of the dry and cold features are well separated
(230jE versus 270jE), with the former propagating at a
speed twice that of the latter. Note that the annual migration
of the dry zone (and hence the South Pacific marine desert) is
more irregular than the cold tongue in terms of propagation
speed and direction. In fact, the dry zone is relatively stable
in its location during 1998, while moving westward in 1999
and eastward in 2001. West to the dateline, however, the
evolution of SST and precipitation is dominated by largescale interannual variabilities. The warm and wet cores of the
WP and RP are found to wander zonally from year to year
with a weak annual cycle in their intensities. Noticeably,
coherent trans-Pacific propagations are observed for both the
WP and the RP from early 1996 to early 1998 (see the white
and blue arrows in Fig. 3a and b). These energetic motions
progress eastward overwhelmingly and suppress, to a large
extent, the inherent seasonality in the central and eastern
equatorial Pacific during 1997 (see the blue box in Fig. 3b).
Meanwhile it can be recognized that the RP is characterized
by a series of local maxima on its way to the east (Fig. 3a),
owing to the intermittent nature of rainfall and the sampling
scheme of TOPEX altimetry. In contrast, the WP migrates
eastward continuously as a whole with a declining strength
(Fig. 3b). The speed of the cross-basin translation increases
gradually from west ( f 0.12 m/s) to east ( f 0.36 m/s) with
an average value of approximately 0.23 m/s, which is
roughly comparable to the advection speed of the geostrophic zonal current (0.35 –0.4 m/s) produced by the equatorial
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G. Chen et al. / Remote Sensing of Environment 91 (2004) 153–159
Kelvin waves (Boulanger & Durand, 2001). These Kelvin
waves are excited by the bursts of westerlies in the western
Pacific, and may potentially lead to the onset of an El Niño
event. Again, the zonal displacement between the two
parallel arrow curves (blue and white) in Fig. 3 is worth
noting. This implies a phase lag of about 50 – 60 days
between the ENSO warm event and the corresponding
rainfall peak during 1996 – 1998. Interestingly, and perhaps
also surprisingly, a return journey of the WP and RP related
features is not seen. Instead, a sudden termination of the
propagation in the eastern Pacific is followed by an immediate restoration of the climate conditions in the western
Pacific, suggesting that the onset and demise of the El Niños
are probably of different nature, rather than being two
subsequent phases of a single process governed by a sole
mechanism.
To further understand the coupling between SST and
rainfall in the equatorial Pacific, time-longitude diagrams of
zonal and meridional wind velocity are examined (Fig. 4).
The zonal wind along the equator is dominated by westerlies
in the western and eastern Pacific, while by easterlies in the
central Pacific. A violation of this basic pattern is observed
in 1997 during which period the westerly over the warm
pool region intensifies significantly and meanwhile propagates eastward until 200jE (Fig. 4a). This is consistent with
the result of McPhaden (1999) who finds that the strongest
surface westerly winds and deep convection are apparent
only over waters warmer than about 29 jC. As a result,
ocean currents forced by these westerlies advect warm water
eastward near the equator, increasing the areal extent of the
warm pool (Fig. 3b). A positive feedback between westerly
wind forcing and warm pool expansion is maintained
throughout 1997. As far as the meridional component of
the wind field is concerned, the equatorial Pacific is largely
controlled by southerly to the east of the dateline. The only
exception during the 10-year period is found in the first few
months of 1998 when northerly prevails in almost the entire
equatorial Pacific (Fig. 4b). More importantly, it should be
noted that the termination of the enhanced westerlies is
followed immediately by the occurrence of a meridional
wind reversal. It may therefore be argued that the onset of
the 1997 – 1998 El Niño is accompanied by anomalous
intensification and migration of westerlies from west to
central Pacific, while the demise of the event is associated
with a sudden reversal of meridional wind from southerly to
northerly along the equatorial Pacific.
Fig. 4. Time/longitude diagram of (a) zonal and (b) meridional component of sea surface wind speed along the equator (averaged between 5jS and 5jN)
derived from ERS-1, -2 and QuikSCAT, respectively. Positive (negative) winds are westerly (easterly) and southerly (northerly) in (a) and (b), respectively. The
vertical dashed lines in (a) and (b) indicate the dateline.
G. Chen et al. / Remote Sensing of Environment 91 (2004) 153–159
5. Concluding remarks
During the past two decades or so, it has become a
general consensus that the tropical Pacific serves, in many
aspects, as the ‘‘heart’’ of the ocean/atmosphere system
from which the pulse of the globe can be best taken. In
this context, the western Pacific WP and RP may act as the
‘‘ventricle’’ and ‘‘atrium’’ which are inherently coupled
with each other. Consequently, it is obvious that the WP
and the RP are critical to the geophysical ‘‘health’’ of our
planet. In this study, taking the advantage of unprecedented decade-long simultaneous satellite observations, the
coupling effects between the Pacific WP and RP are
illustrated and analyzed on the basis of composite SST
and rain rate scenarios corresponding to the El Niño and
La Niña modes. Our results indicate that the RP mirrors
the WP nicely in terms of spatial structure, at least in a
climatological sense. Their geographical locations, however, are found to shift by some 20j with the RP displaced to
the west. The covarying behavior of the WP and RP is
clearly demonstrated by the coherent trans-Pacific propagations of their central locations with a phase lag of about
50 –60 days during the 1997 –1998 El Niño event. The
atmospheric convection, the oceanic advection, the earth’s
rotation, and the El Niño-Southern Oscillation are thought
to be mainly responsible for the observed characteristics.
Finally, we would like to conclude by arguing that
simultaneous observation and joint analysis of tropical
Pacific SST and precipitation, as well as other geophysical
parameters such as wind, wave and sea level, in a ‘‘pool’’
context might be an effective and efficient approach
toward a better characterization and further understanding
of the global air – sea interaction in general, and the El
Niño-Southern Oscillation in particular.
Acknowledgements
This work is cosponsored by the Natural Science
Foundation of China (Project No.: 40025615), the National
High-Tech Project on Ocean Monitoring Technology
(Project No.: 2001-AA63-3060), and the Teaching and
Research Award Program for Outstanding Young Teachers
in Higher Education Institutions of MOE, PRC. The authors
are grateful to the anonymous reviewers for their helpful
comments and constructive suggestions.
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