indian ocean and monsoon coupled interactions in … - indian...south asian monsoon trough and the...
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Indian Ocean and monsoon coupled interactions in a warmingenvironment
Panickal Swapna • R. Krishnan • J. M. Wallace
Received: 23 December 2012 / Accepted: 22 April 2013 / Published online: 8 May 2013
� Springer-Verlag Berlin Heidelberg 2013
Abstract Several studies have drawn attention to the
steady warming of the equatorial and tropical Indian Ocean
(IO) sea surface temperature (SST) observed during recent
decades. An intriguing aspect of the IO SST warming trend
is that it is has been accompanied by a pronounced
weakening of the large-scale boreal summer monsoon
circulation. Based on a detailed diagnostic analysis of
observed datasets, reanalysis products and IPCC AR4
coupled model output, this study examines how the
observed changes in the summer monsoon circulation
could have contributed to this SST warming trend. The
present results reveal that the weakening trend of the
summer monsoon cross-equatorial flow has favored a
reorientation of surface westerlies towards the equatorial
IO during recent decades, relative to summer monsoons of
earlier decades, which were dominated by stronger cross-
equatorial flow. Our analysis suggests that the weakening
of the summer monsoon cross-equatorial flow has in turn
significantly accelerated the SST warming in the central
equatorial IO. While the trend in the equatorial westerlies
has promoted downwelling and thermocline deepening in
the eastern equatorial IO, the central equatorial IO warm-
ing is attributed to reduced upwelling in response to a
weakening trend of the wind-stress curl. The observed
trends in Indian monsoon rainfall and the near-equatorial
SST warming are shown to be closely related to variations
in the meridional gradient of the monsoon zonal winds. An
examination of the twentieth century simulations from 22
IPCC AR4 models, suggests that some models capture the
recent equatorial IO warming associated with the weak-
ened summer monsoon circulation reasonably well. The
individual member models, however, exhibit significant
inter-model variations in representing the observed
response of the IO and monsoon coupled system.
Keywords Indian Ocean warming � Equatorial
westerlies � Weakening of boreal summer monsoon winds
1 Introduction
Unlike the tropical Pacific and Atlantic ocean basins where
the mean surface winds are dominated by easterly trades,
the annual mean surface winds prevailing over the tropical
Indian Ocean are characterized by a westerly flow associ-
ated with the strong boreal summer monsoon (also known
as the Southwest monsoon) circulation (Schott and
McCreary 2001). The sea surface temperature (SST) vari-
ations in the tropical IO are known to be strongly influ-
enced by the seasonal cycle and variability of the
monsoonal winds on interannual and intraseasonal time-
scales (e.g., McCreary et al. 1993; Rao and Sivakumar
2000; Lee et al. 2000; Sengupta et al. 2001; Weller et al.
2002; Waliser et al. 2004; Ramesh and Krishnan 2005;
Duncan and Han 2009; Vialard et al. 2011). Anomalous
SST variations in the tropical IO have been linked to
produce global climatic impacts ranging from droughts
over the Sahel region (Giannini et al. 2003), to variations in
monsoon precipitation over regions of Africa, Asia and
Australia (e.g., Saji et al. 1999; Webster et al. 1999; Behera
et al. 1999; Yamagata et al. 2004; Swapna and Krishnan
2008; Krishnan and Swapna 2009 and others); as well as
P. Swapna (&) � R. Krishnan
Centre for Climate Change Research, Indian Institute of Tropical
Meteorology, Pune 411008, India
e-mail: [email protected]; [email protected]
J. M. Wallace
Atmospheric Sciences, University of Washington,
Seattle, WA 98195, USA
123
Clim Dyn (2014) 42:2439–2454
DOI 10.1007/s00382-013-1787-8
the forcing of extratropical teleconnection patterns such as
the North Atlantic Oscillation (Hoerling et al. 2004).
One of the scientific issues that has drawn considerable
attention in recent years is the steady SST warming in the
tropical IO, which has proceeded at the rate of about 0.5�–
1 �C during the last 5 decades (e.g., Alory et al. 2007;
Alory and Meyers 2009; Yu et al. 2007; Du and Xie 2008;
Ihara et al. 2008 and others). The map of observed SST
trend shown in Fig. 1a indicates an overall warming of the
IO basin during 62-year period, with maximum warming in
the central equatorial IO. The trend at each grid point has
been computed for the summer monsoon season (June–
September) after removal of the global mean SST, so that
the IO SST warming in Fig. 1a can be interpreted as a SST
increase in excess of global warming. The statistical sig-
nificance of the SST trends has been computed using the
Student’s t test (see Balling et al. 1998). It can be seen that
the SST trends, enclosed by the solid contour in Fig. 1a, in
a large region of the near-equatorial Indian Ocean exceed
the 99 % confidence level.
By analyzing heat flux products during recent periods
(1988–2000), Yu et al. (2007) noted the absence of an
increase in the net heat flux in the tropical Indian Ocean.
They argued that the recent surface warming in the region
is unlikely to be related to net heat flux variations. Studies
have also suggested that ocean dynamics might play a
larger role in the Indian Ocean warming as compared to
forcing from surface heat fluxes (e.g., Lee 2004;
Schoenefeldt and Schott 2006; Alory et al. 2007). The
objective of the present work is to understand the observed
IO SST warming trend in the context of changes in the
summer monsoon circulation during recent decades. The
motivation for this study is partly based on evidence fur-
nished by several studies that indicate a possible weaken-
ing of the large-scale summer monsoon circulation in
recent decades (e.g., Joseph and Simon 2005; Rao et al.
2004; Sathiyamoorthy 2005; Ramesh Kumar et al. 2009;
Turner and Hannachi 2010; Fan et al. 2010; Krishnan et al.
2012; Mishra et al. 2012). The weakening of the summer
monsoon winds can be noted in spatial maps of long-term
trends in the summer monsoon surface winds based on the
ERA reanalysis shown in Fig. 1a. The anticyclonic trend
over the Indian landmass and the Arabian Sea seen in
Fig. 1a implies a reduction in the intensity of the south-
westerly low-level monsoon flow. In the region of maxi-
mum SST trend in the equatorial Indian Ocean, one notices
westerly surface wind anomalies over the western and
eastern equatorial IO (Fig. 1a), which are characteristic of
weak summer monsoons over India. The spatial map of
long term trends in surface winds from NCEP reanalysis
(Auxiliary Fig. 15) also shows weaker summer monsoon
flow. During weak monsoon conditions, the southwesterly
flow tends to weaken over the Arabian Sea, the Indian
landmass and the Bay of Bengal and the westerlies tend to
be oriented along the equatorial Indian Ocean (see Rodwell
1997; Krishnan et al. 2006). This occurs downstream of an
(a) (b)
(c) (d)
Fig. 1 Upper panels show
trends in sea surface
temperature (SST in �C per
62 years; the departure from the
global mean SST) and ERA
surface winds (m s-1 per
54 years) in the tropical Indian
Ocean (IO) for the summer
monsoon season. a June–
September; b the remaining
calendar months. Color shading
indicates the magnitude of SST
trends and the contour
corresponds to 99 % confidence
level based on the Student’s
t test (see Balling et al. 1998).
The lower panels show time-
series of SST (�C) bars and
ERA zonal wind anomalies
(m s-1, red lines) averaged over
the equatorial IO (50�E–100�E,
5�S–5�N). c June–September
and d the remaining calendar
months. The trends of the linear
regression best-fit lines exceed
the 95 % confidence level
2440 P. Swapna et al.
123
anomalous southward curvature of the monsoonal flow that
reorients the westerly wind belt along the equatorial Indian
Ocean rather than to the north over the Indian subcontinent.
We do realize that trends inferred from long term
reanalysis wind products may be subject to artificial shifts
due to introduction of widespread satellite data in the late-
1970s (see Kinter et al. 2004). In this connection, Krishnan
et al. (2012) verified the validity of the weakening trend of
the summer monsoon circulation by examining the
meridional gradient of sea level pressure (SLP) variations
based on an independent dataset HadSLP2 (Allan and
Ansell 2006). Their analysis clearly showed the time-series
of meridional gradient of SLP difference between the
South Asian monsoon trough and the subtropical high over
the Southern Indian Ocean exhibited a weakening trend
during the period (1948–2009).
The time-series of the interannual variations of SST
anomaly in the equatorial IO (50�E–100�E, 5�S–5�N)
indicates a warming of the SST starting in the late 1950’s,
with peak values in the most recent decade (Fig. 1c). The
SST warming is consistent with the time series of equa-
torial zonal wind variations (Fig. 1c), which shows a
gradual intensification of westerly wind anomalies along
the equator during summer until 2000 followed by a slight
decrease in the most recent decade. A decrease of
upwelling-related oceanic cooling has been argued to be
one of main causes of the recent surface warming trend in
the equatorial IO (Alory and Meyers 2009). That the
magnitude of the IO SST warming is stronger during the
summer monsoon months (June through September JJAS)
than in the other seasons (see Fig. 1c, d) supports this
interpretation. However, it remains to be determined how
much the weakening of the South Asian monsoon circu-
lation during recent decades has contributed to the tropical
IO SST warming trend. In light of these considerations, the
present study seeks to elucidate the possible role of the
weakening of the South Asian monsoon circulation on the
warming of the tropical IO during recent decades. This
work is based on diagnostic analyses of observed and
reanalysis datasets, and coupled model output from the
IPCC AR4 simulations for the twentieth century. A brief
description of datasets used in the study is given in Sect. 2
and the results are presented in Sect. 3. Section 4 contains a
summary and discussion of our findings.
2 Datasets
The data diagnostics include winds from European Centre
for Medium Range Weather Forecasting 40-year reanalysis
(ERA-40, Uppala et al. 2005) for the period 1958–2001
and ERA Interim (Dee et al. 2011) for the period
2002–2011. Following the approach of Mishra et al.
(2012), the continuous time series of ERA data has been
constructed by merging the ERA-40 for the period
1958–2001 and ERA Interim for the period 2002–2011,
after interpolating ERA Interim data onto the ERA-40 grid.
The winds from National Center for Environmental Pre-
diction (NCEP) reanalysis (Kalnay et al. 1996; Kistler et al.
2001) are used for preparing a set of Auxiliary figures. SST
data from the HadISST1.1 dataset (Rayner et al. 2003) and
Simple Ocean Data Assimilation (SODA; Carton et al.
2000) are also used in the study. These data sets are for the
period January 1950 to December 2011 except for SODA
which is available for the period January 1958 to December
2007. Observed monthly mean zonal winds from radio-
sonde observations at 1000 hPa and 850 hPa for Bombay
(72.8�E, 19.1�N) and Goa (Panaji, 73.8�E, 15.4�N) from
India Meteorological Department and Colombo (79.8�E,
6.9�N) from (http://www1.ncdc.noaa.gov/pub/data/igra/
monthly-upd) are also presented in the analysis. In addi-
tion, the analysis involves the gridded (1� 9 1�) rainfall
data (Rajeevan et al. 2006) over India for 60 years
(1950–2009) and the Asian Precipitation Highly Resolved
Observational Data integration Towards Evaluation of
Water Resources (APHRODITE) gridded (0.5� 9 0.5�)
rainfall dataset for the period 1951–2007 (Yatagai et al.
2009) and the Global Precipitation Climatology Project
(GPCP) dataset (McNab et al. 1997) for the period
1979–2000. The merged satellite altimeter sea level
anomaly data from AVISO (http://www.aviso.oceanobs.
com/duacs/) for the period 1993–2009 is also used in the
analysis. Furthermore, the study includes analysis of SST
proxy records based on coral delta oxygen 18 isotope
values (d18O) from Seychelles (55.4�E; 4.35�S, Charles
et al. 1997) and Chagos (71.3�E; 6�S, Pfeiffer et al. 2004).
In addition to the observational datasets, we have also
presented analyses of the coupled model outputs of the
twentieth century experiment (20C3M) from the World
Climate Research Program’s (WCRP) Third Coupled
Model Intercomparison (CMIP3) multi-model datasets
(Meehl et al. 2007). In the 20C3M simulations, the
greenhouse gas concentration was varied in accordance
with the observed values during twentieth century. The
analysis of CMIP3 outputs covers the period from January
1950 to December 1999.
3 Results
3.1 SST warming trend in the tropical Indian Ocean
In the earlier discussion, it was seen that the SST warming
trend was most pronounced in the central EIO. In partic-
ular, the magnitude of SST warming trend turns out to be
larger during the boreal summer season (Fig. 1a), than in
Indian Ocean and monsoon coupled interactions 2441
123
the non-monsoon months (Fig. 1b). The slopes of the SST
trends are approximately 0.15 �C (10 years)-1 and 0.06 �C
(10 years)-1 for the summer and non-summer monsoon
seasons respectively (see Fig. 1c, d). The coral skeletal
geochemistry provides continuous records that can be used
for reconstructing the climate trends over decades to cen-
turies. We have examined the time-series of coral delta
oxygen 18 isotope values (d18O) in the equatorial IO from
Seychelles (blue line, Fig. 2a) and Chagos (blue line,
Fig. 2b) for the summer monsoon season. The coral records
are inverted and overlaid on the SST data. The coral d18O
can be considered as a simple proxy for changes in SST.
Seychelles has a long period of observations and we have
taken data for the period 1870–1995 and Chagos has a
comparatively shorter record from 1961 to 1995. During
the length of the coral record from Seychelles, d18O has
decreased by 15 per mil which is indicative of warming of
SST of 0.8 �C (Charles et al. 1997). The JJAS SST
anomalies from the HadISST data (red line) in the location
of coral records (blue line) for Seychelles (Fig. 2a) shows
an overall increase of SST by about 0.8 �C during the last
century. Further, it is interesting to note a positive trend in
the time-series of zonal wind anomalies over the equatorial
IO which indicates enhancement of westerly zonal winds
over the equator. The raw time series of SST and coral
isotope (with inverted sign) variations show high correla-
tion coefficients of 0.62 for Seychelles and 0.57 for Chagos
respectively. The values of the detrended linear correlation
between the two time-series are found to be 0.44 and 0.45
for Seychelles and Chagos respectively.
Figure 2c shows time-series of monthly mean zonal
winds from radiosonde observations for the stations
Bombay and Goa, which are located along the west coast
of India; and for the near-equatorial station Colombo in Sri
Lanka (Fig. 2d). It can be seen that the low-level westerly
zonal winds over Bombay and Goa show a weakening
trend while the time-series of near-equatorial winds over
Colombo shows a slight intensification of the equatorial
westerlies (Fig. 2d). From Fig. 2c and d, one can notice a
weakening of the southwesterly summer monsoon flow
during recent decades. This feature is accompanied by
enhanced westerlies along the equator due to the anoma-
lous southward curvature of the low-level winds, a con-
figuration that is typically observed during weak monsoon
conditions (see Rodwell 1997; Krishnan et al. 2006).
The pattern of zonal winds associated with the boreal
summer monsoon low-level circulation over the tropical
Indian Ocean is characterized by easterlies to the south
of the equator and westerlies to the north. In the fol-
lowing analysis, we shall examine the variations of the
monsoon zonal flow by considering a meridional cross
section of zonal winds averaged over the longitude
band 70�E–90�E for the JJAS monsoon season. An
empirical orthogonal function (EOF) analysis is per-
formed on the time-series along a meridional cross
section of surface zonal winds from the merged ERA
data set (1958–2011). The spatial structure of the
dominant mode (EOF1) of zonal wind variation shows
an easterly (negative) anomaly to the north of the
equator and a peak westerly (positive) anomaly to the
south of the equator (Fig. 3a). The positive polarity of
EOF1 pattern represents a weakened boreal summer
monsoon cross-equatorial flow. The corresponding first
principal component (PC1) time-series is shown in
Fig. 3b. The PC1 time-series shows an upward trend.
(a)
(b)
(c)
(d)
Fig. 2 Time series of yearly coral d18O anomalies in per mil (blue
line) and JJAS SST anomalies (�C) from HadISST (red line) (a) at
Seychelles (b) and at Chagos. The coral d18O anomalies has been
negated and overlaid with SST anomalies c Time series of monthly
mean zonal wind (m s-1) at 1000 hPa at station Bombay (Mumbai,
72.8�E, 19.1�N) during the summer season d Time series of monthly
mean 850 hPa zonal wind (m s-1) at Colombo (79.8�E, 6.9�N). The
trends of the linear regression best-fit lines in (a, b, c) exceed the
95 % confidence level
2442 P. Swapna et al.
123
The positive trend in PC1 indicates that while the
monsoon westerly flow to the north-of-equator has
weakened during recent decades, the westerly winds
along the equator and to the south of the equator have
strengthened. To ensure that spurious jumps are not
introduced by merging the ERA40 and ERA-Interim
products, we have verified the EOF patterns of winds
averaged between 70�E and 90�E separately for the
ERA-40 and ERA-Interim datasets for the common
period (1979–2001). The EOF1 pattern is found to be
similar in both datasets (see Auxiliary Fig. 14) and the
PC1 time-series in the two datasets show an upward
trend. The two PC1 time-series are strongly correlated
(r = 0.94). The patterns obtained by regressing the SST
field upon the PC1 time-series of zonal winds from the
merged ERA data are shown in Fig. 3c. The SST pat-
terns show positive values (warming) in the tropical
Indian Ocean, especially in the central and eastern EIO.
Previous studies of Robinson (1966), Gill (1975),
Miyama et al. (2003) have shown that Ekman drift induced
by a patch of equatorial westerly winds generates equa-
torward currents in both hemispheres, leading to equatorial
convergence, downwelling and positive SST anomalies in
the equatorial region. The pattern of warm anomalies on
the eastern side (Fig. 3c) favors enhanced precipitation
over the eastern equatorial IO and the positive zonal gra-
dient of SST can, in turn, act to strengthen the equatorial
westerly wind anomalies similar to a Bjerknes-like feed-
back (Krishnan et al. 2006). To further confirm the link
between the equatorial zonal winds and the eastern IO SST
warming, we repeated the EOF and regression analysis,
replacing the latitudinal section of zonal winds at the sur-
face with the latitudinal section of zonal winds at the
850 hPa level. EOF1 and PC1 of the latitudinal section of
850 hPa zonal winds; as well as the corresponding SST
regression maps are shown in Fig. 3d–f. It can be seen that
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3 Upper panels show the
leading EOF of the meridional
profile of ERA zonal wind
anomalies averaged from 70�E
to 90�E along a section
extending from 30�S to 30�N
for the JJAS season a surface,
d 850 hPa. Middle panels show
time series of the corresponding
PC. Lower panels show the
anomaly patterns obtained by
regressing SST upon the PC1 of
winds for the summer monsoon
season for 1958–2011. The
trends of the linear regression
best-fit lines in (b, e) exceed the
95 % confidence level. Units of
regression pattern are (�C)
(ms-1)-1
Indian Ocean and monsoon coupled interactions 2443
123
the regression patterns in Fig. 3f bear a close resemblance
to those in Fig. 3c suggesting that the trends in the zonal
winds and SST in the tropical IO consistently indicate a
weakening of the westerly monsoon flow to the north of the
equator, the strengthening of the equatorial westerly winds
and enhanced SST warming (over and above the global
warming trend) in the equatorial IO.
To understand whether these changes are reflected in the
summer monsoon rainfall, we regressed the gridded rainfall
(APHRODITE and IMD) data upon PC1 of the meridional
cross section of zonal winds. The anomaly patterns gen-
erated by regressing the APRHODITE rainfall upon PC1 of
the 850 hPa zonal wind section are shown in Fig. 4a and
the corresponding regression maps for the IMD rainfall
dataset are shown in Fig. 4b. The regression maps show
significant negative anomalies over the western Ghats and
central-north India, which is consistent with a decreasing
trend in monsoon rainfall over these regions. Recently,
Krishnan et al. (2012) pointed out that a weakening trend
of the large-scale Southwest monsoon flow can result in a
significant decrease of orographic precipitation over the
Western Ghats. The positive anomalies along the foothills
of Himalayas over northeastern India and over southeastern
India are reminiscent of the signature of ‘‘monsoon-
breaks’’ over India (see Ramamurthy 1969; Krishnan and
Sugi 2000; Mishra et al. 2012).
3.2 Response of the equatorial Indian Ocean to changes
in the wind pattern
The response of the IO to the equatorial westerly wind
forcing is also corroborated by trends in the sea level
anomalies (SLA) from the SODA reanalysis (Fig. 5a).
Consistent with the warming trend in SST, the trends in
(a)
(b)
Fig. 4 Anomaly patterns generated by regressing the rainfall upon
the first PC of the 850 hPa zonal wind profile averaged from 70�E to
90�E from ERA a APHRODITE rainfall b IMD gridded rainfall.
Units of regression pattern are (mm day-1) (ms-1)-1
(a)
(b)
Fig. 5 a Trends in (a) sea level (m per 50 years) from 1958 to 2007
during the JJAS summer monsoon season based on the SODA
reanalysis. b As in (a) except for thermocline depth (D20 in m per
50 years, shaded) and surface currents (m s-1 per 50 years, vectors)
2444 P. Swapna et al.
123
SLA show a positive sea level trend in the eastern
equatorial IO during past 50 years (1957–2007). Han
et al. (2010) have reported a positive sea level trend in
the equatorial IO caused by enhanced mass convergence
into the equatorial Indian Ocean, induced by anomalous
atmospheric circulation. A more recent study by Luo
et al. (2012) have noted a strengthening of the easterly
trade winds over the tropical Pacific and an associated
rise of sea level in the western tropical Pacific during
the last 20 years. They have shown that the enhanced
tropical Indian Ocean warming in recent decades favors
stronger trade winds in the western Pacific. Krishnan
et al. (2006) suggested that the anomalous equatorial
westerly winds observed during weak-monsoon phases,
tend to deepen the thermocline in the eastern equatorial
IO and thereby maintain warmer than normal SST. To
understand the thermocline response to the equatorial
westerly wind forcing, we examined the long-term
trends in the depth of the 20� isotherm (D20) and sur-
face currents from the SODA reanalysis during the
summer monsoon season. One can notice positive trend
values in the D20 anomalies in the central and eastern
equatorial IO consistent with thermocline deepening in
response to enhanced equatorial westerly winds. Fur-
thermore, the trends in the surface currents (Fig. 5b)
show anomalous eastward flow in the equatorial IO
which is consistent with the thermocline deepening in
the east. The study by Han et al. (2010) found positive
SLA extending from the eastern equatorial IO into the
Bay of Bengal. This feature is consistent with the
deepening of the thermocline in the eastern EIO and
Bay of Bengal shown in Fig. 5b. We have also
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 6 Spatial pattern of the
leading EOF of a SLA (m) from
SODA b heat content anomalies
(J m-2) from SODA, and
(c) SST anomalies from
HadISST and their respective
PC time series, juxtaposed as
indicated in (d, e, f). The data
are for the period 1958–2007.
The trends of the linear
regression best-fit lines exceed
the 95 % confidence level
Indian Ocean and monsoon coupled interactions 2445
123
compared the sea-level variability in the SODA and
AVISO datasets during the common period
(1993–2007). It is noted that the spatial pattern of the
leading EOF of sea-level variability in the eastern
equatorial IO is broadly consistent in both datasets,
although there are some differences in the phase and
amplitude of variations between the two products (not
shown). The variance explained by first PC of sea-level
variability is found to be 30.1 % for AVISO and 26.7 %
for the SODA data.
Furthermore, the sea-level variations in the tropical
Indian Ocean are found to be consistent with the variations
of ocean heat content, as illustrated in Fig. 6. One can
notice that the spatial pattern of the first EOF of sea-level
variations (Fig. 6a) from SODA closely resembles that of
the first EOF of heat content variations (Fig. 6b). The
spatial patterns of EOF1 in Fig. 6a and b bear resemblance
with the patterns of trends shown in Fig. 5a and b
respectively. Likewise, the spatial pattern of the first EOF
of SST (Fig. 6c), with maximum warming in the central-
eastern equatorial Indian Ocean, closely resembles the
spatial map of trend of SST shown in Fig. 1a. It is found
that the time-series of the first principal component (PC) of
sea level, for the period (1958–2007), is strongly correlated
with the first PC of heat-content (r = 0.92). Also the cor-
relation coefficient between the PC1 time-series of SST
and sea-level anomalies is found to be *0.70.
Important evidence for the Central Indian Ocean SST
warming comes from the Seychelles (55.4�E; 4.35�S) coral
isotopic records to the south of equator (Fig. 2a). During
the summer monsoon season, the climatological mean
winds over the Indian Ocean normally cause upwelling
around the Seychelles region, primarily due to the effect
of the curl of zonal wind stress (Yokoi et al. 2008;
Murtugudde and Busalacchi 1999; Xie et al. 2002). The curl
of the zonal wind stress has an equatorially antisymmetric
structure w.r.t the equator (Fig. 7a). Yokoi et al. (2008)
showed that the meridional gradient of zonal wind-stress
term �1qof
osx
oy
� �is the dominant term in the wind stress curl that
contributes to the upwelling over the Seychelles Dome. The
time-series of interannual variations of �1qof
osx
oy
� �averaged
over the Seychelles Dome is shown in Fig. 7c. One can see a
decreasing trend in the wind-stress curl, indicative of a
decline in upwelling in the Seychelles Dome area during the
recent decades. This suggests that the southern flank of the
near-equatorial warming is related to reduced upwelling in
the Seychelles Dome, while the warming confined to the
equatorial region is perhaps due to Ekman convergence by
equatorial westerlies and near-equatorial Rossby waves as
discussed in Rao and Behera (2005).
Furthermore, the Ekman drift due to anomalous westerly
winds over the equatorial Indian Ocean tends to produce
(a) (b)
(c)
Fig. 7 a Climatological mean
zonal windstress (10-2 N m-2)
for the summer monsoon season
from ERA data. b Long term
trend in the zonal wind stress
(10-2 N m-2 per 54 years)
from ERA c Time series of the
curl term �1qof
osx
oy
� �(10 -5 ms-1,
Ref. Yokoi et al. 2008) in the
Seychelles Dome region (50�E–
75�E, 3�S–6�S). Positive values
indicate upwelling
2446 P. Swapna et al.
123
anomalous equatorward currents in both hemispheres, so
that the equatorial convergence and downwelling can cause
equatorial SST warming (e.g., Miyama et al. 2003). A map
of linear trend of zonal wind stress for the summer mon-
soon season from the merged ERA data is shown in
Fig. 7b. One can notice westerly anomalies in the equato-
rial region, with anomalous easterlies farther north of the
equator over the Arabian Sea and the Bay of Bengal. The
SODA climatological mean currents in the Indian Ocean
during the summer season show southward Ekman flow in
the upper ocean, as evidenced by the southward vectors in
Fig. 8a. On the other hand, the linear trend of the SODA
ocean currents, zonally averaged across (40�E–100�E),
shows anomalous equatorward currents associated with
convergence and downwelling near the equator (Fig. 8b).
The pattern of anomalous equatorward currents in Fig. 8b
is consistent with the warm SST response induced by a
patch of anomalous westerly winds over the equatorial
Indian Ocean (e.g., Miyama et al. 2003).
3.3 Equatorial SST warming and latitudinal variation
of monsoonal zonal winds
We shall now focus on understanding the equatorial IO
SST warming and its linkage with the monsoon zonal
winds and rainfall over India. Given that the zonal wind-
stress sxð Þ is proportional to the zonal wind (u), the
meridional gradient of the zonal wind-stress � osx
oy
� �is
basically a measure of relative vorticity associated with the
zonal wind component. With this background, the boreal
summer monsoon can be conceptualized as a large-scale
atmospheric circulation characterized by cyclonic vorticity/
low pressure over South Asia and anti-cyclonic vorticity/
high pressure over the subtropical southern Indian Ocean.
This large-scale picture of cyclonic (anti-cyclonic) vortic-
ity over South Asia (subtropical Indian Ocean) is clearly
evidenced in Auxiliary Fig. 16 which shows the climato-
logical mean � ouoy
� �averaged longitudinally between 70�E
and 90�E. It is important to note that � ouoy
� �attains the
lowest negative value on the equator.
In order to gain deeper insight into the link between SST
variations to the south-of-equator and the Seychelles dome,
we have performed an EOF analysis of � ouoy
� �over the
70�E–90�E longitudinal cross-section (Fig. 9). The leading
EOF pattern (Fig. 9a) shows positive values over the
equator, whereas the off-equatorial values are negative in
both hemispheres indicating a weakened low-level cyclo-
nic vorticity over the Indian subcontinent and a weakened
anti-cyclonic vorticity over the subtropical Indian Ocean. It
is important to recognize that the main easterly and
westerly branches of the monsoon to the south and north of
the equator are connected through the large-scale cross-
equatorial monsoon circulation. Therefore, alterations to
the main branches of the monsoonal winds can modulate
the amplitude of the meridional gradient of the zonal wind
over the equatorial belt. This is consistent with the EOF1
pattern of the meridional gradient of zonal wind whose
shows dominant loading is along the equator. It is also
interesting to note that the corresponding PC1 time-series
for the period (1958–2011) shows a prominent increasing
trend (Fig. 9b). The spatial patterns obtained by regressing
the PC1 time-series on SST, winds (850 hPa) and rainfall
are depicted in Fig. 9c–e respectively. One can clearly
notice the equatorial SST warming pattern both in the west-
central and the eastern IO, together with significant
warming in the Bay of Bengal and the south-eastern trop-
ical IO. Studies have shown that the influence of wind-
stress curl anomalies on SST variations in the near-equa-
torial region involves ocean dynamical processes. For
(a)
(b)
Fig. 8 a Climatological zonal mean (50�E–100�E) currents (cm s-1)
from SODA for the summer monsoon season b Trends in zonal mean
currents (cm s-1 per 50 years) from SODA for the summer monsoon
season, 1958–2007. Meridional and vertical currents are shown as
vectors and zonal currents are shaded
Indian Ocean and monsoon coupled interactions 2447
123
example, wind-stress curl anomalies are known to generate
westward propagating long Rossby waves which in turn
produce thermocline variations through Ekman pumping in
the southern tropical Indian Ocean (see Masumoto and
Meyers 1998; Rao and Behera 2005).
The regression pattern of the PC1 time-series on the
850 hPa winds shows pronounced westerly anomalies
along the equator accompanied by easterly anomalies over
central and peninsular India and a ridge-like feature with
anticyclonic anomaly over north and northwest India. It is
further interesting to note the clockwise circulation
anomaly over the southern sub-tropical IO which is
indicative of a weakened Mascarene High (see Krishnan
and Swapna 2009). Overall the anomaly pattern in Fig. 9d
represents a weakening of the large-scale southwest mon-
soon circulation. The regression pattern of rainfall shows
negative anomalies over regions covering north-central and
peninsular India as well as parts of west-coast of India and
Myanmar, thus corroborating the weak pattern of the
southwest monsoon circulation. The above analysis clearly
shows that as the southwest monsoon circulation weakens,
the large-scale pattern of � ouoy
� �anomalies favors
decreased monsoon rainfall over the Indian subcontinent.
On the other hand, the anomalies of the wind-stress curl
tend to favor SST warming in the near-equatorial region
through ocean dynamical processes (e.g., Masumoto and
Meyers 1998; Murtugudde and Busalacchi 1999; Xie et al.
2002; Rao and Behera 2005).
3.4 The 20C3M simulation from CMIP3
Here we shall examine to what extent the 20C3M simu-
lations by the CMIP3 coupled models are able to capture
(a) (b)
(c) (d)
(e)
Fig. 9 a The leading EOF of
the meridional profile of (-du/
dy) anomalies from ERA
reanalysis averaged from 70�E
to 90�E along a section
extending from 35�S to 35�N
for the summer season. b The
time series of the corresponding
PC. The trend of the linear
regression best-fit line exceeds
the 95 % confidence level
c Patterns of SST anomalies
obtained by regressing SST
upon PC1 of (-du/dy) for the
summer monsoon season for
1958–2011. Units are (�C) (s).
d Same as (c) except for
850 hPa winds. Units are
(ms-1) (s). e Same as (c) except
for rainfall. Units are
(mm day-1) (s)
2448 P. Swapna et al.
123
the SST warming trends in the tropical IO and the
accompanying changes in the summer monsoon flow.
Since observed wind data are not available for an extended
period of record in the TIO, we have used the NCEP
reanalysis as well as ERA winds as a basis for the evalu-
ation of winds and we have used HadISST for SST com-
parison. Since the 20C3M coupled model simulations
exhibit wide inter-model variability in their representation
of the South Asian monsoon rainfall and atmosphere–ocean
interactions, particularly in the Indian Ocean environment
(Lin 2007; Kripalani et al. 2007; Rajeevan and Ravi 2009;
Fan et al. 2010), it is important to identify a sub-set of the
CMIP3 models that compare reasonably well with the
observed SST warming pattern and with the weakening
trend of the monsoon circulation. Figure 10 shows a scat-
ter-plot of trends in the zonal winds versus SST in the
equatorial IO (50�E–100�E, 5�S–5�N) during 1950–2000
for the different CMIP3 models and observations (NCEP &
ERA winds and HadISST). It can be seen that the trends in
SST and zonal winds simulated by the CMIP3 models
show wide variations. Given the wide variations in the
CMIP3 model trends, we will focus on the following seven
models: (UKMO-HadCM3, MRI-CGCM2.3.2, IPSL-CM4,
GFDL-CM2.0, ECHO-G, CGCM3.1 (T63), CCSM3) for
which the SST and zonal wind trends are somewhat closer
to the observed trends. It is noted that the trends in the
equatorial IO SST and zonal winds for the above subset of
seven models are all positive.
Figure 11 is a plot comparing the multi-model mean of
the climatological summer monsoon precipitation based on
these seven 20C3M models and the GPCP rainfall dataset.
One can notice that the multi-model mean qualitatively
captures the monsoon precipitation distribution over the
South Asian region, including the Bay of Bengal and the
eastern equatorial IO although there are differences
between the simulated and the GPCP dataset. Figure 12a
shows the spatial map of trends in SST and winds, com-
posited from the seven selected 20C3M models, for the
summer monsoon season during 1950–2000. The corre-
sponding plot for the remaining calendar months is shown
in Fig. 12b. The spatial map of trend in JJAS rainfall
(1950–2000), composited from the seven selected 20C3M
models, is shown in Fig. 12c. The statistical significance of
the trends has been computed using the Student’s t test and
the trend values exceeding the 90 % confidence level are
contoured in Fig. 12. It can be seen that the multi-model
composite qualitatively captures the weakening of the
summer monsoon circulation as indicated by the easterly
pattern over the Arabian Sea and the Bay of Bengal,
together with a trend towards stronger westerlies along the
equator. A prominent SST warming trend in the near-
equatorial central IO can also be noted in the multi-model
composite, particularly for the JJAS season (Fig. 12a). It is
interesting to note that the JJAS rainfall trend based on the
multi-model composite from the selected 20C3M models is
indicative of decreased rainfall to the north over the Indian
Fig. 10 Trends in zonal winds (m s-1 per 50 years) and SST (�C per
50 years) in the equatorial Indian Ocean averaged between 50�E and
100�E, 5�S–5�N from the 22 IPCC models. Trends are also shown for
the HadISST and NCEP and ERA winds (red). Results for the
selected models are indicated in blue
(a)
(b)
Fig. 11 Climatological JJAS summer monsoon rainfall (mm day-1)
a GPCP dataset b Multi-model mean based on the seven selected
20C3M models
Indian Ocean and monsoon coupled interactions 2449
123
region and increased rainfall over the central IO to the
south of the equator. However, the pattern of decreased
monsoon rainfall over the Indian region in Fig. 12c is much
different from that of the observed precipitation decrease
over India as described in Krishnan et al. (2012). Moreover
the spatial pattern of trends in JJAS rainfall, for the period
(1950–2000), from the individual 20C3M models are quite
diverse (Fig. 13). This suggests that there is further scope
for improvement in the representation of coupled interac-
tions involving the monsoon circulation, precipitation and
the Indian Ocean dynamics.
4 Discussion and summary
The TIO SST has undergone significant warming during
recent decades in parallel with a weakening of the boreal
summer monsoon circulation. We have shown that the
magnitude of SST warming trend is significantly stronger
during the summer monsoon season than during the other
seasons. In order to understand these regional variations
in the observed trends in the atmosphere–ocean coupled
system, a comprehensive analysis of historical datasets
and coupled model output from twentieth century simu-
lations from 22 IPCC AR4 models was performed. The
results reveal that the weakening trend of the summer
monsoon cross-equatorial flow has favored a reorientation
of the surface winds over the tropical IO during recent
decades, with a weakening of the cross-equatorial flow
into the Northern Hemisphere and a strengthening of the
westerlies along the equator. The enhanced equatorial
westerlies have, in turn, promoted downwelling and
thermocline deepening in the equatorial central IO,
thereby accelerating the SST warming in the region.
Furthermore, it is seen that the weakening of the summer
monsoon cross-equatorial flow has altered the amplitude
of the meridional gradient of the zonal wind in a manner
as to reduce the upwelling in the near-equatorial central
IO around the Seychelles region. An examination of the
twentieth century simulations from 22 IPCC AR4 models,
suggests that some models capture the recent warming of
the equatorial IO associated with the weakened summer
monsoon circulation reasonably well. The individual
models exhibit wide variations in their representations of
the trends in SST and zonal winds and rainfall over the
tropical IO and the monsoon region.
(a) (b)
(c)
Fig. 12 The composite spatial
pattern of trends based on the
seven selected 20C3M models
for the period (1950–2000)
a SST (�C per 50 years) and
surface winds (m s-1 per
50 years) for JJAS season
b Same as (a) except for the
remaining calendar months
c JJAS Rainfall (mm day-1 per
50 years). The contours
correspond to trends that exceed
the 90 % confidence level based
on the Student’s t test (see
Balling et al. 1998)
2450 P. Swapna et al.
123
The strengthening of the equatorial westerly winds
and the accelerated SST warming in the equatorial IO
could have major implications for the monsoon hydro-
logical cycle. For example, the strengthening of the
zonal SST gradient associated with the warming in the
central and eastern IO can, in turn, act to intensify the
equatorial westerly winds and thereby favor increased
precipitation over the eastern equatorial IO through
enhanced moisture convergence. The equatorial anom-
alies, in turn, can suppress Indian summer monsoon
rainfall by inducing subsidence over the subcontinent
and thereby giving rise to extended monsoon breaks and
droughts over India (see Krishnan et al. 2006). In fact,
rainfall observations over India show a significant
increase in the frequency and duration of monsoon
breaks during the recent decades (e.g., Ramesh Kumar
et al. 2009; Dash et al. 2004; Turner and Hannachi
2010). The results presented here suggest that this ten-
dency has been enhanced by atmosphere–ocean feed-
back: i.e., that the weakening of the summer monsoon
circulation has accelerated the warming of the equato-
rial IO and the warming, in turn, has contributed to a
further weakening of the monsoon.
Acknowledgments The authors thank the Director, IITM, for the
encouragement and support to carry out this research. We are also
(a) (b)
(c) (d)
(e) (f)
(g)
Fig. 13 The spatial pattern of
trends in JJAS rainfall for the
period (1950–2000) from the
seven individual 20C3M
models. Units are (mm day-1
per 50 years)
Indian Ocean and monsoon coupled interactions 2451
123
grateful to the three anonymous reviewers and the Editor Prof. Jean-
Claude Duplessy for providing constructive comments and sugges-
tions. We acknowledge the modeling groups, the Program for Climate
Model Diagnosis and Intercomparison (PCMDI) and the WCRP’s
Working Group on Coupled Modeling (WGCM) for their roles in
making available the WCRP CMIP3 multi-model dataset. Support of
this dataset is provided by the Office of Science, US Dept. of Energy.
This work is supported the Grant No. SR/FTP/ES-23/2008 received
from by Department of Science and Technology, Govt. of India.
JMW’s support is from the National Science Foundation’s Climate
Dynamics Program Office under Grant #1122989.
5 Auxiliary Figures
See Figs. 14, 15, and 16.
(a)
(b)
Fig. 14 a The leading EOF of the meridional profile of zonal wind
averaged from 70�E to 90�E along a section extending from 30�S to
30�N for the JJAS summer season. The black line is based on the
ERA-40 dataset. The red line is based on the ERA Interim dataset.
Both datasets cover the common period 1979–2001. b Time series of
the corresponding PC
(a)
(b)
Fig. 15 a Trends in sea surface temperature (SST in �C per 62 years;
the departure from the global mean SST) and NCEP surface winds
(m s-1 per 60 years) in the tropical Indian Ocean (IO) for the summer
monsoon season (June–September). Color shading indicates the
magnitude of SST trends and the contour corresponds to 99 %
confidence level based on the Student’s t test (see Balling et al. 1998).
b Time-series of SST (�C) bars and NCEP zonal wind anomalies
(m s-1, red lines) averaged over the equatorial IO (50�E–100�E, 5�S–
5�N). The trend of the linear regression best-fit lines exceeds the 95 %
confidence level
Fig. 16 The meridional profile of climatological -du/dy (910-3 s-1,
black line) and zonal wind (ms-1,red line) averaged longitudinally
between 70�E and 90�E along a section extending from 40�S to 30�N
for the JJAS summer monsoon season from ERA reanalysis. CV
cyclonic vorticity, ACV anti-cyclonic vorticity
2452 P. Swapna et al.
123
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