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: swapna@tropmet.res.in; swapna.panickal@gmail.com

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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