Role of Indian Ocean SST variability on the recent global warming hiatus

    Role of Indian Ocean SST Variability on the Recent Global Warming Hiatus Anika Arora, Suryachandra A. Rao, R. Chattopadhyay, Tanmoy Goswami, Gibies George, C.T. Sabeerali PII: DOI: Reference:

S0921-8181(15)30041-2 doi: 10.1016/j.gloplacha.2016.05.009 GLOBAL 2426

To appear in:

Global and Planetary Change

Received date: Revised date: Accepted date:

16 September 2015 26 February 2016 23 May 2016

Please cite this article as: Arora, Anika, Rao, Suryachandra A., Chattopadhyay, R., Goswami, Tanmoy, George, Gibies, Sabeerali, C.T., Role of Indian Ocean SST Variability on the Recent Global Warming Hiatus, Global and Planetary Change (2016), doi: 10.1016/j.gloplacha.2016.05.009

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Role of Indian Ocean SST Variability on the Recent Global Warming Hiatus

T

Anika Arora1, Suryachandra A. Rao1*, R. Chattopadhyay1, Tanmoy Goswami1, Gibies George1 and

IP

C. T. Sabeerali2 Indian Institute of Tropical Meteorology, Pune, 411008, India

2

Center for Prototype Climate Modeling, New York University, Abu Dhabi, UAE

CE P

TE

D

MA

NU

SC R

1

AC

*Corresponding author: Suryachandra A. Rao

Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India. E-mail: [email protected] Phone +91-020-25904245 Fax + 91- 020-25865142.

Page | 1

ACCEPTED MANUSCRIPT Abstract Previous studies have shown a slowdown in the warming rate of the annual mean global surface temperature in the recent decade and it is referred to as the hiatus in global warming. Some recent studies have suggested that the hiatus in global warming is possibly due to strong cooling in the tropical Pacific. This study

IP

T

investigates the possible role of the Indian Ocean warming on the tropical Pacific cooling. Despite the

SC R

continued rise in sea surface temperature (SST) over the tropical Indian Ocean, SST over the tropical Pacific has shown a cooling trend in the recent decade (2002-2012). It is well known fact that the Indian Ocean and the Pacific Ocean are strongly coupled to each other and the Indian Ocean basin wide warming is triggered

NU

by El Niño on interannual time scale. However, in the recent decade, this relationship is weakening. The recent Indian Ocean warming is triggering a Matsuno-Gill type response in the atmosphere by generating

MA

anomalous cyclonic circulations on either side of equator over the tropical Indian Ocean and anomalous easterlies along the tropical Pacific Ocean. These anomalous easterlies result in Ekman divergence in the

D

equatorial Pacific and produce upwelling Kelvin waves, cools the tropical Pacific and therefore indirectly

TE

contributes to the hiatus in global warming.

CE P

Keywords

Global warming; large-scale teleconnections; air-sea coupling 1 Introduction

AC

The continued rise in anthropogenic greenhouse gases in the atmosphere are expected to provide a long term warming trend in the global mean surface temperature. Although the long term global warming trend seems to be persistently evident in surface temperature, there is also evidence of slowing down of warming trend (hiatus) in the recent decade (2002-2012) as reported by the previous studies. At the same time, top of the atmosphere radiative flux still showing an imbalance raises concern about the extra heat added to the earth climate system but the same not being reflected as rise in global mean surface temperature. Extra heat added to the climate system could be responsible for the rise in temperature of the atmosphere or the ocean or both (Kosaka and Xie 2013, England et al. 2014, Easterling and Wehner 2009). Previous studies have suggested different theories of the recent hiatus in global mean surface temperature and can mainly attributed to decrease in stratospheric water vapor (Rosenlof and Reid 2008, Solomon et al. 2010) and increase in stratospheric aerosol effect (Solomon et al. 2011, Xie et al. 2013). This hiatus in global mean surface Page | 2

ACCEPTED MANUSCRIPT temperature has posed a challenge to the prevailing view of the role of the increasing trend in anthropogenic greenhouse forcing related to the global warming (Easterling and Wehner 2009, Foster and Rahmstorf 2011). Although the regional variation, seasonality of global warming signatures and stratospheric link are the commonly described mechanisms in literature (Solomon et al. 2010, 2011, Meehl et al. 2011, England et

IP

T

al. 2014), the role of dynamical feedback has started evolving in recent only. Kosaka and Xie (2013) have

SC R

reported strong association between the eastern tropical Pacific Ocean cooling and the recent hiatus seen in global warming but whether cooling is a consequence of internal dynamics or externally forced remains unclear. Intensification of trade winds in the tropical Pacific Ocean is partly attributed to observed cooling

NU

trend in SST (Ding et al. 2013, England et al. 2014). In hiatus period, amount of heat missing from the atmosphere is trapped in the form of global ocean heat content and out of this about 70% of this heat is

MA

stored in the Indian Ocean (Lee et al. 2015). They also show that increase in heat content of the Indian Ocean cannot be explained through surface fluxes. Discharge of warm water from the Pacific Ocean into the

D

Indian Ocean through Indonesian Passage is responsible for an abrupt change in heat content of the Indian

TE

Ocean. Indian Ocean seems to work as a giant reservoir in maintaining global mean temperature in the

CE P

recent decades. This study advocates the role of wind anomaly associated with La-Nina like conditions in intensification of Indonesian Through Flow (Lee et al. 2015) but atmospheric response generated by increased heat content in the Indian Ocean and its role in modulating global mean temperature through the

AC

Pacific Ocean SST remains a puzzle. Roemmich et al. (2015) also claimed that imbalance in heat in the climate system can be explained by heat uptake by subsurface ocean. Using coupled model experiment with similar energy imbalance at the top of the atmosphere as observed, Meehl et al. (2011) showed that warming in subsurface ocean below 300m is accelerated and upper ocean seems to warm at much reduced rate during hiatus period. Balmaseda et al. (2013) also supports similar hypothesis to explain the recent hiatus. The recent hiatus has created a significant interest in dynamically explaining the cause associated with it and its implications on the future trend of global warming. In the recent decade, strong cooling trend is observed in the tropical Pacific in contrary to the warming trend in the Indian Ocean (Figure 1(b)). The rapid Indian Ocean warming (Levitus et al. 2000, Hoerling et al. 2004, Du and Xie 2008, Rao et al. 2012, Roxy et al. 2014, Sabeerali et al. 2014, Swapna et al. 2014 and references there in) and the recent tropical Pacific cooling is quite interesting as this can have Page | 3

ACCEPTED MANUSCRIPT several implications in the modulation of teleconnection between these two oceans (Trenberth et al. 1998). It is now well known that the feedback between the Pacific and the Indian Ocean determines the variability of coupled modes in these two basins. El Niño southern oscillations (ENSO) can have important implications in determining SST patterns over the Indian Ocean (Bjerknes 1969, Rasmusson and Carpenter 1983, Venzke

IP

T

et al. 2000, Alexander et al 2002, Wu and Kirtman 2004). Similarly, it is shown by various studies that the

SC R

Indian Ocean variability may have significant role in ENSO transitions and the tropical Indian Ocean can contribute significantly to ENSO variability (Meehl 1987, Saji et al. 1999, Annamalai et al. 2005, Xie et al. 2009). Thus, the change in basic state of SST and variability over both the basins can have a significant

NU

impact on teleconnection relationship between the Pacific and the Indian Ocean as well as the Pacific and the Indian summer monsoon (Krishna Kumar et al. 1999, Kumar et al. 2000, Kinter et al. 2002, Turner et al.

MA

2005).

With this background, it becomes important to explore how the recent tropical Pacific cooling and

D

the Indian Ocean warming are dynamically related to each other. The cooling trend in the tropical Pacific is

TE

explained through several factors, but the role of inter-basin teleconnection between the Indian Ocean and

CE P

the Pacific Ocean on the hiatus in global temperatures has not been examined so far. The teleconnection between these two basins may be an important link to explain the hiatus since the Indian Ocean and the Pacific Ocean are connected through an atmospheric bridge named Walker Circulation (Julian and Chervin

AC

1978, Webster and Yang 1992). In this study our objective is to explore the possible role of the Indian Ocean warming in maintaining the observed Pacific cooling trend in the recent decade through observational analysis and model experiment. It will also help to establish a mechanism of change in inter-basin teleconnection on decadal scale. Section 2 provides the details of the dataset used for this study and details of the experiments carried out using an Atmospheric General Circulation Model (AGCM) ECHAM5. Detailed discussion on the result is presented in section 3. Finally section 4 concludes the study. 2. Data and Method 2.1 Data In order to study temporal variation and linear trend of SST we have used monthly mean Hadley Center Sea Ice and Sea Surface Temperature dataset (HadISST; Rayner et al. 2003, 2006). For cross Page | 4

ACCEPTED MANUSCRIPT validation of SST trend, TMI AMSR-E fused daily SST for the period June 2002-December 2012 is also analyzed

(Wentz

et

al.

2000,

2001).

The

TMI

AMSR-E

data

is

obtained

via

http://data.remss.com/sst/daily/tmi_amsre. This study also uses NCEP/NCAR Reanalysis 2 zonal and meriodional winds (Kanamitsu et al. 2002), Modern-Era Retrospective Analysis for Research and Applications

IP

T

(MERRA) net surface heat flux (Rienecker et al. 2011) and Met Office Hadley Centre subsurface ocean temperature

SC R

observations (version EN.4.1.1.; Good et al. 2013) for the period January 1979 to December 2012. NOAA

interpolated outgoing longwave radiation (OLR) dataset for the period June 1974-December 2012 (Liebmann and Smith 1996) is analyzed to study atmospheric response to ocean surface temperature. For

NU

most of Figures, we consider the period 2002-2012 as hiatus period and refer to it as the recent decade. For comparison with the past we consider the period 1982-1992 as pre-hiatus period. Reason for choosing these

MA

two particular decades for analysis is discussed in section 3.2. 2.2 Model experiments

D

The analysis of SST dataset reveals that the Indian Ocean shows a strong warming trend during the

TE

period 1950-2002 (Fig. 1b), prior to the emerging of cooling trend in the Pacific Ocean. In order to quantify

CE P

the possible role of the Indian Ocean, we have performed sensitivity experiment with AGCM ECHAM5. Detailed description of ECHAM5 model is provided in Roeckner et al. (2006). Previous studies (Annamalai et al. 2005, Rao et al. 2010, Sabeerali et al. 2012) have used this model to study the impact of Indo-Pacific

AC

SSTs on the Indian summer monsoon and all these studies have shown that this model is reasonable in simulating the large-scale climatological features during the boreal summer season. Here, our objective is to show how the Indian Ocean warming trend may influence the Pacific Ocean region, which may have a potential impact in the form of global warming hiatus. For this purpose, two types of AGCM experiments are performed in this study. We made the control (CTL) run by forcing ECHAM5 with observed monthly SST and sea ice as the lower boundary condition and integrated for the period 1982-2011. The second run is termed as the Indian Ocean non-warming run (IONW run) where we have modified the forcing SST in such a way that the linear trend in the monthly SST is removed from the Indian Ocean (40°E-120°E, 30°S-30°N) and the observed SSTs are kept elsewhere and integrated the model for the same CTL run period (19822011). In order to account for uncertainty in the initial conditions, we have carried out five member ensemble runs with different initial conditions and these different initial conditions are obtained from five Page | 5

ACCEPTED MANUSCRIPT snapshot runs of the CTL run. Hence, the IONW run is an ensemble mean of five different realizations. Similar set of model experiments were carried out previously (see also Sabeerali et al. (2014) for details). The difference between the CTL run and the IONW run (CTL-IONW run) isolate the pattern associated to the Indian Ocean SST warming trend. Results of the experiments are discussed in detail in section 3.5.

SC R

3.1 Interannual variations of global mean surface temperature

IP

T

3. Results

Global surface air temperature and global SST show a consistent warming for the last 60 years (Fig.1a). However, global SST does not exhibit significant warming in the recent decade (-0.032°C/decade)

NU

and slope of global mean surface temperature trend has decreased in the recent decade (0.091°C/decade) as compared to the previous decade (0.115°C/decade). This phenomenon of slowing down of rate of warming

MA

in global mean surface temperature in the recent decade is referred to as hiatus in global warming trend. Since the global mean SST warming shows a slowdown in the recent decade, it is interesting to look at the

D

temperature trends of the Indian and the Pacific Oceans separately, as these two basins together occupy 66%

TE

of the global ocean. SST anomaly averaged over the tropical Indian Ocean (40°E-120°E, 30°S-30°N;

CE P

magenta) and the Pacific Ocean (130°E-90°W, 30°S-30°N; black ) for the period 1950-2012 is shown in Fig. 1b along with the linear trend (dashed line) calculated over two periods (1950-2002 and 2002-2012) separately. SST in both the basins show a warming trend during 1950-2002, with a faster rate of warming

AC

(0.119 °C/decade) in the Indian Ocean than the Pacific Ocean (0.068 °C/decade). These rates of warming are consistent with the finding of Sabeerali et al. (2014). Interestingly, in the recent decade (2002-2012), the Indian Ocean warming has slowed down and the rate of warming is now 0.084 °C/decade. On the other hand, warming trend over the Pacific Ocean is reversed (-0.268 °C/decade). Surprisingly, the tropical Pacific SSTs shows a strong cooling trend in the recent decade while the Indian Ocean SSTs keep warming albeit at slower rate (Fig 1b). McGregor et al. (2014) has recently discussed the role of the Atlantic SST warming on the Pacific cooling using model experiment, which demonstrates strengthening of Walker circulation. Here, using observations and model experiments, we have shown a different perspective on the relation between the Indian Ocean warming and the Pacific Ocean cooling. In order to understand which parts of the ocean basin contribute to these trends, spatial distribution of SST trend in the recent decade is constructed (Fig. 2), using two different data sets (i.e HadISST and TMI-AMSRE). Most of the tropical Pacific shows a dominant Page | 6

ACCEPTED MANUSCRIPT cooling trend as shown in Fig 2. The eastern and central parts of the Pacific Ocean contribute mainly to this basin wide cooling trend observed in the tropical Pacific Ocean. The warming trend continues in many parts of the Indian Ocean particularly the western equatorial and the southern tropical Indian Ocean. The eastern equatorial Indian Ocean cooling trend found in HadISST data set is not observed in TMI-AMSRE. It might

IP

T

be attributed to the fact that SST retrieval in HadISST takes into account in situ observation and infrared

SC R

(IR) channels radiance. SST over the equatorial eastern Indian Ocean is above the convective threshold most

AC

CE P

TE

D

MA

NU

of the time. Therefore, IR retrieval of SST might be biased over this region because of presence of clouds.

Fig.1. Time series of (a) annual mean global sea surface temperature (°C; magenta) and 2 meter air temperature (°C; black) (b) SST (°C) averaged over the tropical Indian Ocean (40°E-120°E, 30°S-30°N; magenta ) and tropical Pacific Ocean (130°E-90°W, 30°S-30°N; black) for the period 1950 to 2012, along with linear trend (dashed line) calculated for two different periods (1950-2002 and 2002-2012).Slope of each trend line is labeled in respective line color.

Page | 7

ACCEPTED MANUSCRIPT In order to relate the rapid cooling trend in the tropical Pacific Ocean with the Indian Ocean, we have focused our analysis for the period 2002-2012 following Kosaka and Xie (2013). The period 1997 to 2001 have witnessed strong ENSO cycle (1997/98 El Niño; 1999-2001 La Niña) in the Pacific as well as strong dipole event (1997 PIOD) in the Indian Ocean. Hence, the selection of 2002-2012 period for studying the

IP

T

recent cooling of the Pacific Ocean is ideal as it will exclude the period of strong interannual variability. In

SC R

the following sections we have looked at relationship between the Indian Ocean and the Pacific Ocean temperatures to understand whether there exist any significant changes in the Indo-Pacific teleconnection

AC

CE P

TE

D

MA

NU

pattern.

Fig. 2. (a) Linear trend in SST (°C/decade) for the period 2002-2012 using HadISST dataset, (b) same as (a) but for TMI-AMSRE. 3.2 Shift in teleconnection pattern between the Indian Ocean and the Pacific Ocean The SST over the Indian Ocean and the Pacific Ocean has a phase link with each other at different lag/lead times (Alexander et al. 2002, Wu and Kirtman 2004). The twenty one year running correlation between the detrended basin averaged SST anomalies of the Pacific Ocean (130°E-90°W, 30°S-30°N) and the Indian Ocean (40°E-120°E, 30°S-30°N) is shown in Figure 3. The relationship between these two basins is very strong until early 90’s. Since late 90’s onwards the correlation dropped below significant value (0.35) and the correlation value is very close to 0.25-0.4. It clearly highlights that significant changes have Page | 8

ACCEPTED MANUSCRIPT occurred in the teleconnection pattern between these two ocean basins in the recent decade. Kripalani et al. (2003) also suggested that Indian and Pacific basins have de-linked after 1990’s; not due to global warming but as a part of natural climate variability. Previous studies also documented the weakening of ENSOmonsoon relationship in recent decades (Kripalani and Kulkarni, 1997, Krishna Kumar et al. 1999, Ashok et

IP

T

al., 2001, Kripalani et al. 2007). The possible reason for the sudden drop in linear correlation in SST

SC R

anomalies could be related to 1976/77 climate shift. There were three El Niño events but no cold events in the Pacific Ocean between 1977 and 1988. This decadal scale atmosphere-ocean variability in the Pacific Ocean could be responsible for the observed change in teleconnection pattern of the Indian Ocean and the

NU

Pacific Ocean (Trenberth 1990, Miller et al.1994, Trenberth and Hurrell 1994). Also, significant anomalous warming observed in the Indian Ocean particularly after 1950s (Ihara et al. 2008, Rao et al. 2012, Roxy et al.

MA

2014, Swapna et al. 2014) cannot be ignored in the context of shift in teleconnection pattern. The Indian Ocean SST is found to be more conducive for convection in the recent decades and it could impact the

D

Pacific Ocean climate by modulating Walker circulation (McPhaden et al. 2011, Luo et al. 2012, England et

AC

CE P

TE

al. 2014).

Fig. 3. Twenty one year running correlation between the two detrended SST time series described in Figure 1b. Dashed red line in indicates where correlation is 90% significant. X-axis denotes 21 year running window chosen. 3.3 Atmospheric and oceanic response to the climate shift in 2002-2012

Page | 9

ACCEPTED MANUSCRIPT It would be interesting to investigate how the atmosphere has responded to the recent strong shift in the teleconnection between the Indian Ocean and the Pacific Ocean. Two different decades during which the teleconnection were strongest (1982-1992) and weakest (2002-2012) are selected to see whether the mean SST pattern has changed significantly between these two periods. It has been observed that mean SST has

IP

T

significantly changed between these two periods with strong warming in the Indian Ocean and the western

SC R

Pacific Ocean and cooling in the eastern Pacific Ocean (Fig. 4a). To further explore the atmospheric response to changes in SST pattern , the difference in mean state of OLR and atmospheric circulation averaged over 5°S-5°N are shown in Fig. 4b and 4c. OLR exhibits negative (positive) over the regions To demonstrate how these changes in OLR are reflected in

NU

where SST shows warming (cooling).

atmospheric circulation, we have shown mean wind changes from past to recent decade in Fig.4a. Lower

MA

value of OLR in the recent decades over the Indian Ocean and the west Pacific is an indicator of enhanced convective activity and act as a dominant heat source in the equatorial region which might affect global

D

atmospheric circulation. As expected from Matsuno-Gill solutions (Matsuno 1966, Gill 1980), the enhanced

TE

convection over the eastern equatorial Indian Ocean and Maritime continent produces anomalous easterlies

CE P

along the western and central equatorial Pacific (Fig.4a). The major shift in mean climate of the tropical Indian Ocean and the Pacific Ocean during 2002-2012 period in fact provides strong coupled feedback to the climate system in these two basins. Observed changes described here are reflected in atmospheric Walker

AC

circulation as well. As expected, during the recent decade, a strong anomalous vertical motion is evident over the Maritime Continent whereas a strong subsidence is noticed in the central and eastern Pacific region (Fig. 4c) followed by strong anomalous easterlies at surface over the central and eastern Pacific. All the above results clearly highlight that both the Indian Ocean and the Pacific Ocean have undergone significant changes in SST, OLR and atmospheric circulation in the recent decade. The gradient of SST between the Indian and Pacific Ocean can impose a significant change in convection pattern. Recent studies have also highlighted the role of spatial SST gradients under global warming scenario in modulating monsoon circulation and associated rainfall patterns by generating shifts in convergence zone along the equator (Yun et al. 2014, Ueda et al. 2015). Zonal SST gradients enhance the Walker and Hadley circulation as shown in Klein et al. (1999). There is a decrease in convection over the eastern and central Pacific Ocean, which is demonstrated by OLR, winds (Fig 4b&c) and sea level pressure (not shown) patterns. Zonal winds Page | 10

ACCEPTED MANUSCRIPT have anomalous easterly trend in the equatorial Pacific Ocean and anomalous westerly trend in the equatorial Indian Ocean. The significant positive (negative) trend of OLR over the Pacific Ocean (Indian Ocean) is an indication of suppressed (enhanced) convection. In order to understand whether the cooling pattern observed over the eastern and the central Pacific Ocean is due to change in atmospheric fluxes or it is

IP

T

due to the dynamical response of the Pacific Ocean to change in near surface winds, we have looked into the

SC R

differences 20°C isothermal depth (d20) and net surface heat flux between hiatus and pre-hiatus period (Fig. 4d&e). There is increase in net surface heat flux in the equatorial eastern Pacific Ocean extending to the central Pacific Ocean in the recent decade (Fig. 4e) and 20°C isotherm depth difference shows a signature

NU

similar to SST difference (Fig. 4d). Increase in Qnet indicates that observed cooling in the central and eastern Pacific is not because of the change in heat budget of upper ocean. But increase in net surface heat flux

MA

might be because of colder SST response to latent and sensible heat flux. Reduction in cloud cover might also have an impact on surface radiative fluxes. To further see change in the role of atmospheric processes

D

and ocean dynamics in modulating SST gradients in Indo-Pacific region during pre-hiatus and hiatus period,

TE

simultaneous correlation between tendency in anomalous SST (d(SST)) and anomalous net surface heat flux

CE P

(Qnet) is looked at in Figure 5. Neutral (strong negative) correlation between d(SST) and Qnet in the equatorial Indian (Pacific) Ocean and weak positive elsewhere in the tropics is noticed. A simple linear one dimensional model of change in ocean temperature up to mixed layer depth explains the contribution of Q net

AC

and upper ocean processes (e.g. vertical mixing, advection, short-wave penetration that dissipate heat from the mixed layer). Negative correlation in eastern central equatorial Pacific Ocean shows that ocean temperature there is not dependent primarily on surface fluxes and seems to be driven by ocean dynamics whereas Indian Ocean SSTs are controlled by both ocean dynamics and atmospheric fluxes. Many earlier studies have also shown the importance of net surface heat flux forcing of SST variations during ENSO (Klein et al. 1999, Venzke et al. 2000, Behera et al. 2000, Lau and Nath 2000). Murtugudde and Bussalacchi (1999) and Murtugudde et al. (2000) have indicated a prominent role for ocean dynamics in SST variations of the Indian Ocean. Hendon (2003) using simple one dimensional model has shown the dominant role of variations in net surface heat flux in changing the temperature of mixed layer depth in the eastern Indian Ocean by a lag of 2-3 months. Seasonally varying air–sea interaction in the eastern Indian Ocean in response to ENSO conditions in the Pacific can modulate anomalous zonal SST gradient in the Indian Ocean which Page | 11

ACCEPTED MANUSCRIPT can further modulate ocean dynamics. Chattopadhyay et al (2015) using model and observations has also shown that net heat flux do not have a significant role in driving the SST evolutions in central eastern equatorial Pacific and the central eastern equatorial Indian Ocean due to strong dynamical forcing in the ocean. Negative correlations in the central eastern equatorial Pacific Ocean and the equatorial Indian Ocean

IP

T

are stronger in hiatus period compared to pre-hiatus period. This shows that ocean dynamics is becoming

SC R

major contributor to SST variations in the equatorial Indo-Pacific basin in the recent decade. The Indian Ocean SSTs are largely governed by SSTs in the Pacific Ocean through atmospheric bridge (Venzke et al. 2000, Rao et al. 2002, Wang et al. 2012) and the Pacific Ocean influence on the Indian Ocean SST is

NU

decreasing in the recent decades due to increasing contribution of upper ocean mixed layer processes

AC

CE P

TE

D

MA

(Fig.5).

Fig.4. Difference between hiatus period (2002-12) and pre-hiatus period (1982-92) in (a) mean SST (°C) overlaid by winds (m/s), (b) OLR (W/m2), (c) Walker circulation averaged over 5°S-5°N (omega multiplied by a scale factor =-5*absolute value of (average of u/average of omega)), (d) 20°C Isothermal depth (d20) and (e) Net surface heat flux. Yellow line in (a) indicates where difference in SST is 90% significant.

Page | 12

ACCEPTED MANUSCRIPT In a nut shell it can be said that winds near the surface are triggering cooling at subsurface of ocean and heat fluxes at the surface does not play a major role in cooling of the Pacific Ocean. Also, the influence of the Pacific Ocean on the Indian Ocean SSTs is reducing in the recent decade and Indian Ocean is now

CE P

TE

D

MA

NU

SC R

IP

T

more active to change ocean dynamics of the Pacific Ocean.

2012) period

AC

Fig.5. Simultaneous correlation of Qnet and d(SST) for (a) pre-hiatus (1982-1992) and (b) hiatus (2002-

From the above discussion, it is very clear that the Indian Ocean warming induce strong atmosphere response over the western tropical Pacific which further dynamically cools the subsurface and hence surface of ocean in the eastern Pacific. However, it could be argued that the Pacific Ocean cooling can also trigger similar change in teleconnection pattern then how it can be said that observed changes is a consequence of Indian Ocean Warming. In the following section we try to answer this question by carrying out lag/lead analysis. 3.4 Inter-basin teleconnection The Indian Ocean and the Pacific Ocean interact with each other through the atmospheric bridge (Klein et al. 1999, Venzke et al. 2000, Alexander et al. 2002). As a result of the Indian Ocean SST warming Page | 13

ACCEPTED MANUSCRIPT trend in the recent decade compared to the other basins especially the Pacific basin (Fig. 2a); the convection in the atmosphere has been enhanced over the equatorial Indian Ocean and Maritime continent (Fig. 4b&c). The modulation in the Indo-Pacific Walker cell (and weakening of Atlantic limb) imposes an easterly anomaly in the western Pacific (Fig. 4a). This is evident from observed vertical velocity as well as OLR

IP

T

(Fig. 4b&c) and precipitation (not shown), which can modulate the walker circulation and cause subsidence

SC R

in the central Pacific (Fig. 4c). To further investigate the role of the Indian Ocean warming on the Pacific Ocean cooling, we regress the equatorial Indian Ocean (Fig. 1(b)) area averaged SST with the global SST and winds at different lags on monthly time scale. A comparison of the evolution of monthly lag regression

NU

plots during the hiatus period with that of pre-hiatus period is shown in Fig 6. At zero lag both the Indian Ocean SST and the Pacific Ocean SST are strongly correlated and correlation pattern in the Pacific clearly

MA

shows an El Niño type signature. This suggests that El Niño related SSTs cause strong basin wide SST warming in the tropical Indian Ocean. As we move forward from lag4 to lag2, the Pacific El Niño conditions

D

start warming the western tropical Indian Ocean and the warming spreads from the western Indian Ocean to

TE

the eastern Indian Ocean in the subsequent four months. Also El Niño related wind patterns with low-level

CE P

easterlies in the eastern and central Indian Ocean are observed in both pre hiatus and hiatus periods from lag4 to lead0. The existing mechanism in literature suggests that the Pacific Ocean could drive the SST response over the Indian Ocean through atmospheric fluxes, which is consistent with the earlier proposed

AC

theories (Venzke et al. 2000, Rao et al. 2002, Wang et al. 2012). Venzke et al. (2000), using both model experiments and observations, showed that ENSO signal significantly affects interannual variability of the Indian Ocean SST and large scale SST and net surface heat flux variations in the Indian Ocean follow the eastern tropical Pacific Ocean SST after a time lag of about 4 months. The role of El Niño related SST anomalies in the interannual variability of the Indian Ocean is decreasing in hiatus period compared to prehiatus period. SST warming in the Indian Ocean during hiatus period follows SST in pre-hiatus period by two months. This shows that the Pacific Ocean is taking more time to modulate Indian Ocean SST in the recent decade and Indian Ocean is becoming more active to oppose this modulation (Fig.6). Similar analysis done by correlating Niño 3.4 region SST anomalies with global SST and winds confirms this slowed down response of the Indian Ocean (not shown). However, as we move forward in lead-time the results are different in hiatus and pre-hiatus periods. In the recent decade (hiatus period), in contrast to the pre-hiatus Page | 14

ACCEPTED MANUSCRIPT period, the Indian Ocean SST seems to be influencing the Pacific Ocean SST at advanced (4 months) leadtime. In the hiatus period, a cooling in the eastern equatorial Pacific Ocean is noticed at lead of two months’ time which extends to central Pacific Ocean in four months contrast to the pre-hiatus period. This phase shift in the Pacific SST responses at later lead times indicates that the Pacific Ocean can be dynamically cooled at

AC

CE P

TE

D

MA

NU

SC R

IP

T

this later lead time in the recent decade (hiatus period). The dynamical feedback mechanism through

Fig.6. Lead lag correlation of monthly SST and wind anomalies for the period 1982-1992 and 2002-2012, with respect to a reference time series obtained by averaging the SST anomalies over the Indian Ocean (40°E-100°E, 30°S-30°N).

Page | 15

ACCEPTED MANUSCRIPT which Indian Ocean may adjust the Pacific response is proposed by Annamalai et al. (2005). Figure 4 (d&e) emphasizes on the importance of dynamical processes and nullifies the role of surface heat fluxes in the recent cooling observed over the eastern central Pacific Ocean. To substantiate this hypothesis, we looked at lead-lag relationship between zonal wind anomalies (UWINDA) averaged over the central western Pacific

IP

T

Ocean (Niño 4), 20°C isothermal depth (D20A) and SST anomalies (SSTA) averaged over the eastern

SC R

Pacific Ocean (Niño 3). Figure 7(a) shows time series of SSTA, UWINDA and D20A. Month-to-month variability is removed by smoothing each time series by applying 13-month running mean. Resulting time series is detrended and then normalized by its own standard deviation. There is coherent in-phase

NU

relationship of zonal winds over the western central Pacific Ocean with 20°C isothermal depth and SST over the eastern Pacific Ocean. Figure 7(b) shows the lead- lag correlation of D20A with SSTA (cyan) and D20A

MA

with UWINDA (blue) for pre hiatus (dash) and hiatus (solid) period. Strong easterly zonal wind anomaly over Niño 4 region shoals 20°C isothermal depth anomaly in the eastern Pacific after two months and this

D

shoaling in 20°C isothermal depth cools SST in the eastern Pacific Ocean. This physical mechanism well

TE

supported by many previous studies remains unchanged in recent decades also (Wang and Weisberg 2000,

CE P

Kang and Kug 2002, Zelle et al. 2004, Kug et al. 2009, McPhaden and Zhang 2009 ,Wang et al. 2012 and many references therein). Thus based on this study it may be suggested that the Indian Ocean warming may be partly controlling the Pacific Ocean cooling by enhancing the surface anomalous easterlies over western

AC

central Pacific Ocean (Fig. 4a). During the hiatus period (2002-2012) strong easterly anomalies are noticed along the equatorial Pacific, which can give rise to (a) Ekman divergence along the equator, (b) excite upwelling Kelvin waves and reflected Rossby waves and (c) Enhanced evaporation. All the above mechanisms result in strong cooling pattern in the eastern central tropical Pacific Ocean as noticed in Fig. 2. We have presented evidence from observations that in spite of cooling trend in the Pacific, Indian Ocean is warming. Further we hypothesize that the Indian Ocean warming can trigger Matsuno-Gill response and enhanced anomalous easterlies in the western tropical Pacific (Matsuno 1966, Gill 1980). Anomalous easterlies can in turn trigger an upwelling along the eastern equatorial Pacific Ocean and hence cools the tropical Pacific. However, in real world using observations alone it is difficult to get convinced whether the Indian Ocean warming can explain enhanced easterlies in the tropical Pacific. Therefore, additional evidences are presented by conducting a set of model sensitivity experiments with an AGCM. Page | 16

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Fig.7. (a) Normalised time series of detrended anomalies of zonal wind averaged over Niño 4 region and SST and 20°C isothermal depth (d20) averaged over Niño 3 region, (b) lead-lag correlation of d20 with SST

AC

and d20 with zonal wind. Dashed (Solid) line denotes pre-hiatus (hiatus) period.

3.5 AGCM results We have done couple of AGCM experiments with different scenarios of SST forcing to confirm the above mechanism. Details of the experimental set up are explained in section 2. The SST, precipitation, 850 hPa wind and Walker circulation difference between the CTL run and the IONW run are shown in Figure 8. The difference between the CTL and Indian Ocean non warming run (CTL-IONW run) isolates the response of the recent Indian Ocean warming. The Indian Ocean warming trend for the period 1982-2011 shows almost uniform warming in the tropics with maximum heating in the central equatorial Indian Ocean. This warming trend is consistent with the previous studies (Rao et al. 2010, 2012). More details regarding general structure of SST warming in these two experiments are given in Sabeerali et al. (2014). This SST warming Page | 17

ACCEPTED MANUSCRIPT trend alters the convection pattern by increasing precipitation anomaly (shaded in Fig.8b) over the Indian Ocean and decreasing over the Maritime continent. In response to the tropical Indian Ocean warming, an

CE P

TE

D

MA

NU

SC R

IP

T

anomalous twin

AC

Fig.8. The difference between the control run (CTL) and Indian Ocean non-warming (IONW) run of ECHAM 5 AGCM. (a) SST; (b) Wind vector (m/s) at 2m height and precipitation (shaded; units: mm/day) (c) Walker circulation averaged over 5°S-5°N (vertical velocity is shaded).

cyclonic circulation are observed on either side of the equatorial Indian Ocean and anomalous easterlies are observed over the western tropical Pacific in line with the Matsuno-Gill pattern (Fig 8b). The strengthening of easterlies over the western tropical Pacific in response to the Indian Ocean warming is also evident from the Walker circulation (Fig 8c). Shaded anomalous vertical velocity in Figure 8(c) shows strong convection in central equatorial Indian Ocean and subsidence over the Maritime region. Therefore, the SST forced AGCM experiments also confirm the role of the Indian Ocean warming in modulating the atmospheric circulation. The strengthening of equatorial easterly mean flow can cause dynamic and thermodynamical Page | 18

ACCEPTED MANUSCRIPT response over the underlying ocean. Strengthening of mean flow itself can enhance evaporation and latent heat release from the ocean surface (not shown). Moreover the easterly wind field over the equatorial Pacific Ocean can lead to the generation of upwelling Kelvin waves which propagate eastward and cool the ocean surface along the way up to the eastern Pacific. In short, these AGCM experiments confirm the contribution

IP

T

of the Indian Ocean warming in generating anomalous easterlies over the western central Pacific which can

SC R

trigger cooling in the eastern Pacific as discussed in the previous sections. 4. Conclusion

In this paper, the role of the Indian Ocean warming trend on imposing the Pacific cooling is

NU

examined using observed as well as model datasets. Increased convection over the tropical Indian Ocean due to the recent Indian Ocean warming has forced twin anomalous cyclonic circulation patterns on either side

MA

of the equatorial Indian Ocean and strong anomalous easterlies in the equatorial western Pacific through Matsuno-Gill response. The anomalous easterlies along the western equatorial Pacific force Ekman

D

divergence and may lead to generation of upwelling Kelvin waves and hence, impose cooling of the region.

TE

A couple of SST forced model experiments were performed to isolate the role of the Indian Ocean warming

CE P

on modulating the recent Pacific cooling and then indirectly modulating the global warming hiatus. It is quite clear that SST forced experiments confirm the observational evidences presented. Though the present study highlights the dynamical response of the Pacific circulation to the Indian Ocean warming, it does not

AC

neglect the other sources of influence. The dynamical feedback mechanism of the Indian Ocean supplements the existing theories of the global warming hiatus. Acknowledgement The Indian Institute of Tropical Meteorology (IITM) is funded by the Ministry of Earth Sciences (MoES), Government of India, New Delhi. The authors thank NCAR for making available the NCL software. All the data sources are duly acknowledged. AMSR data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project and the NASA AMSR-E Science Team. Data are available at www.remss.com. C. T. Sabeerali acknowledges the monsoon mission project for research fund.

Page | 19

ACCEPTED MANUSCRIPT References Alexander MA, Bladé I, Newman M, Lanzante JR, Lau NC and Scott JD (2002) The atmospheric bridge: The influence of ENSO teleconnections on air-sea interaction over the global oceans. Journal of Climate 15(16): 2205-2231

IP

T

Annamalai H, Xie SP, McCreary JP and Murtugudde R (2005) Impact of Indian Ocean sea surface

SC R

temperature on developing El Niño. Journal of climate 18(2): 302-319

Ashok K, Guan Z and Yamagata T (2001) Impact of Indian Ocean Dipole on the relationship between the Indian monsoon rainfall and ENSO. Geophysical Research Letters 28:4499–

NU

4502

Balmaseda MA, Trenberth KE and Källén E (2013) Distinctive climate signals in reanalysis of global ocean

MA

heat content. Geophysical Research Letters 40(9):1754-1759

Behera SK, Salvekar PS and Yamagata T (2000) Simulation of interannual SST variability in

D

the tropical Indian Ocean. Journal of Climate 13(19):3487-3499

TE

Bjerknes J (1969) Atmospheric teleconnections from the equatorial Pacific 1. Monthly Weather Review

CE P

97(3): 163-172

Chattopadhyay R, Rao SA, Sabeerali CT, George G, Rao DN, Dhakate A, Salunke K (2015) Large‐scale teleconnection patterns of Indian summer monsoon as revealed by CFSv2 retrospective seasonal forecast

AC

runs. International Journal of Climatology Ding H, Greatbatch RJ, Latif M, Park W and Gerdes R (2013) Hindcast of the 1976/77 and 1998/99 climate shifts in the Pacific. Journal of Climate 26(19): 7650-7661 Du Y and Xie SP (2008) Role of atmospheric adjustments in the tropical Indian Ocean warming during the 20th century in climate models. Geophysical research letters 35(8) Easterling DR and Wehner MF (2009) Is the climate warming or cooling? Geophysical Research Letters 36(8) England MH, McGregor S, Spence P, Meehl GA, Timmermann A, Cai W, Gupta AS, McPhaden MJ, Purich A and Santoso A (2014) Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus.Nature Climate Change 4(3):222-227

Page | 20

ACCEPTED MANUSCRIPT Foster G and Rahmstorf S (2011) Global temperature evolution 1979–2010. Environmental Research Letters 6(4): 044022 Gill A (1980) Some simple solutions for heat‐induced tropical circulation. Quarterly Journal of the Royal Meteorological Society 106(449): 447-462

IP

T

Good SA, Martin MJ and Rayner NA (2013) EN4: Quality controlled ocean temperature and

SC R

salinity profiles and monthly objective analyses with uncertainty estimates. Journal of Geophysical Research: Oceans 118(12):6704-6716

Hendon HH (2003) Indonesian rainfall variability: Impacts of ENSO and local air-sea

NU

interaction. Journal of Climate 16(11):1775-1790

Hoerling MP, Hurrell JW, Xu T, Bates GT and Phillips AS (2004) Twentieth century North Atlantic climate

MA

change. Part II: Understanding the effect of Indian Ocean warming. Climate Dynamics 23(3-4): 391-405 Ihara C, Kushnir Y and Cane MA (2008) Warming trend of the Indian Ocean SST and Indian Ocean dipole

D

from 1880 to 2004*. Journal of Climate 21(10):2035-2046

TE

Julian PR and Chervin RM (1978) A study of the Southern Oscillation and Walker Circulation phenomenon.

CE P

Monthly Weather Review 106(10): 1433-1451 Kanamitsu M, Ebisuzaki W, Woollen J, Yang SK, Hnilo JJ, Fiorino M and Potter GL (2002). Ncep-doe amip-ii reanalysis (r-2). Bulletin of the American Meteorological Society 83(11): 1631-1643.

AC

Kinter III JL, Miyakoda K and Yang S (2002) Recent change in the connection from the Asian monsoon to ENSO. Journal of Climate 15(10): 1203-1215 Klein SA, Soden BJ and Lau NC (1999) Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge. Journal of Climate 12(4): 917-932 Kosaka Y and Xie SP (2013) Recent global-warming hiatus tied to equatorial Pacific surface cooling. Nature 501(7467): 403-407 Kripalani RH and Kulkarni A (1997) Climatic impact of El Nino/La Nina on the Indian monsoon: A new perspective. Weather 52(2):39-46 Kripalani RH, Kulkarni A, Sabade SS and Khandekar ML (2003) Indian monsoon variability in a global warming scenario. Natural Hazards 29(2):189-206 Kripalani RH, Oh JH, Kulkarni A, Sabade SS and Chaudhari HS (2007) South Asian summer Page | 21

ACCEPTED MANUSCRIPT monsoon precipitation variability: coupled climate model simulations and projections under IPCC AR4. Theoretical and Applied Climatology 90(3-4):133-159 Krishna Kumar K, Rajagopalan B and Cane A (1999) On the weakening relationship between the Indian monsoon and ENSO. Science 284:2156–2159

IP

T

Kumar A, Barnston AG, Peng P, Hoerling MP and Goddard L (2000) Changes in the spread of the

SC R

variability of the seasonal mean atmospheric states associated with ENSO. Journal of climate 13(17): 31393151

Kang IS and Kug JS (2002) El Niño and La Niña sea surface temperature anomalies: Asymmetry

NU

characteristics associated with their wind stress anomalies. Journal of Geophysical Research: Atmospheres 107(D19)

MA

Kug JS, Jin FF and An SI (2009) Two types of El Niño events: cold tongue El Niño and warm pool El Niño. Journal of Climate 22(6):1499-1515

D

Lau NC and Nath MJ (2000) Impact of ENSO on the variability of the Asian-Australian monsoons as

TE

simulated in GCM experiments. Journal of Climate 13(24):4287-4309

CE P

Lee SK, Park W, Baringer MO, Gordon AL, Huber B and Liu Y (2015) Pacific origin of the abrupt increase in Indian Ocean heat content during the warming hiatus. Nature Geoscience 8(6):445-449

2225-2229

AC

Levitus S, Antonov JI, Boyer TP and Stephens C (2000) Warming of the world ocean. Science 287(5461):

Liebmann B and Smith CA (1996) Description of a complete (interpolated) outgoing longwave radiation dataset. Bulletin of American Meteorological Society 77:1275-1277 Luo JJ, Sasaki W and Masumoto Y (2012) Indian Ocean warming modulates Pacific climate change. Proceedings of the National Academy of Sciences 109(46):18701-18706 Matsuno T (1966) Quasi-geostrophic motions in the equatorial area. J. Meteor. Soc. Japan 44(1): 25-43 McGregor S, Timmermann A, Stuecker MF, England MH, Merrifield M, Jin FF and Chikamoto Y (2014) Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming. Nature Climate Change 4(10): 888-892 McPhaden MJ, Lee T and McClurg D (2011) El Niño and its relationship to changing background conditions in the tropical Pacific Ocean. Geophysical Research Letters 38(15) Page | 22

ACCEPTED MANUSCRIPT McPhaden MJ and Zhang X (2009) Asymmetry in zonal phase propagation of ENSO sea surface temperature anomalies. Geophysical Research Letters 36(13) Meehl GA (1987) The annual cycle and interannual variability in the tropical Pacific and Indian Ocean regions. Monthly Weather Review 115(1): 27-50

IP

T

Meehl GA, Arblaster JM, Fasullo JT, Hu A and Trenberth KE (2011) Model-based evidence of deep-ocean

SC R

heat uptake during surface-temperature hiatus periods. Nature Climate Change 1(7): 360-364 Miller AJ, Cayan DR, Barnett TP, Graham NE and Oberhuber JM (1994) The 1976–77 climate shift of the Pacific Ocean. Oceanography 7(1):21-26

NU

Murtugudde R and Busalacchi AJ (1999) Interannual variability of the dynamics and thermodynamics of the tropical Indian Ocean. Journal of Climate 12(8):2300-2326

MA

Murtugudde R, McCreary JP and Busalacchi AJ(2000) Oceanic processes associated with anomalous events in the Indian Ocean with relevance to 1997–1998. Journal of Geophysical Research: Oceans 105(C2):3295-

D

3306

TE

Rao SA, Behera SK, Masumoto Y and Yamagata T (2002) Interannual variability in the subsurface tropical

CE P

Indian Ocean with special emphasis on the Indian Ocean Dipole. Deep Sea Research II 49:1549-1572. Rao SA, Chaudhari HS, Pokhrel S and Goswami BN (2010) Unusual Central Indian Drought of Summer Monsoon 2008: Role of Southern Tropical Indian Ocean Warming. Journal of Climate 23:5163-5174

AC

Rao SA, Dhakate AR, Saha SK, Mahapatra S, Chaudhari HS, Pokhrel S and Sahu SK (2012) Why is Indian Ocean warming consistently? Climatic change 110(3-4): 709-719 Rasmusson EM and Carpenter TH (1983) The relationship between eastern equatorial Pacific sea surface temperatures and rainfall over India and Sri Lanka. Monthly Weather Review 111(3): 517-528 Rayner NA, Parker DE, Horton EB, Folland CK, Alexander LV, Rowell DP, Kent EC and Kaplan A (2003) Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research: Atmospheres 108(D14) Rayner NA, Brohan P, Parker DE, Folland CK, Kennedy JJ, Vanicek M, Ansell TJ and Tett SF (2006) Improved analyses of changes and uncertainties in sea surface temperature measured in situ since the midnineteenth century: the HadSST2 dataset. Journal of Climate 19(3): 446-469

Page | 23

ACCEPTED MANUSCRIPT Rienecker MM, Suarez MJ, Gelaro R, Todling R, Bacmeister J, Liu E, Bosilovich MG, Schubert SD, Takacs L, Kim G and Bloom S (2011) MERRA: NASA's modern-era retrospective analysis for research and applications. Journal of Climate 24(14):3624-3648 Roeckner E, Brokopf R, Esch M, Giorgetta M, Hagemann S, Kornblueh L, Manzini E, Schlese U and

IP

T

Schulzweida U (2006) Sensitivity of simulated climate to horizontal and vertical resolution in the ECHAM5

SC R

atmosphere model. Journal of Climate 19(16):3771-3791

Roemmich D, Church J, Gilson J, Monselesan D, Sutton P and Wijffels S (2015) Unabated planetary warming and its ocean structure since 2006. Nature Climate Change

NU

Rosenlof KH and Reid GC (2008) Trends in the temperature and water vapor content of the tropical lower stratosphere: Sea surface connection. Journal of Geophysical Research: Atmospheres 113(D6)

MA

Roxy MK, Ritika K, Terray P and Masson S (2014) The Curious Case of Indian Ocean Warming. Journal of Climate 27(22): 8501-8509

D

Sabeerali CT, Rao SA, Ajayamohan RS and Murtugudde R (2012) On the relationship between Indian

TE

summer monsoon withdrawal and Indo-Pacific SST anomalies before and

CE P

after 1976/1977 climate shift. Climate dynamics 39(3-4): 841-859 Sabeerali CT, Rao SA, George G, Rao DN, Mahapatra S, Kulkarni A and Murtugudde R (2014) Modulation of monsoon intraseasonal oscillations in the recent warming period. Journal of Geophysical Research:

AC

Atmospheres 119(9): 5185-5203

Saji NH, Goswami BN, Vinayachandran PN and Yamagata T (1999) A dipole mode in the tropical Indian Ocean. Nature 401(6751): 360-363 Solomon S, Daniel JS, Neely RR, Vernier JP, Dutton EG and Thomason LW (2011) The persistently variable “background” stratospheric aerosol layer and global climate change. Science, 333(6044): 866-870 Solomon S, Rosenlof KH, Portmann RW, Daniel JS, Davis, SM, Sanford, TJ and Plattner GK (2010) Contributions of stratospheric water vapor to decadal changes in the rate of global warming. Science, 327(5970): 1219-1223 Swapna P, Krishnan R and Wallace JM (2014) Indian Ocean and monsoon coupled interactions in a warming environment. Climate Dynamics 42(9-10):2439-2454

Page | 24

ACCEPTED MANUSCRIPT Trenberth KE (1990) Recent observed interdecadal climate changes in the Northern Hemisphere. Bulletin of the American Meteorological Society 71(7):988-993 Trenberth KE, Branstator GW, Karoly D, Kumar A, Lau NC and Ropelewski C (1998) Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface

IP

T

temperatures. Journal of Geophysical Research: Oceans (1978–2012) 103(C7):14291-14324

SC R

Trenberth KE and Hurrell JW (1994) Decadal atmosphere-ocean variations in the Pacific. Climate Dynamics 9(6):303-319

Turner AG, Inness PM and Slingo JM (2005) The role of the basic state in the ENSO–monsoon relationship

NU

and implications for predictability. Quarterly Journal of the Royal Meteorological Society 131(607):781-804 Ueda H, Kamae Y, Hayasaki M, Kitoh A, Watanabe S, Miki Y and Kumai A (2015) Combined effects of

MA

recent Pacific cooling and Indian Ocean warming on the Asian monsoon. Nature communications 6 Venzke S, Latif M and Villwock A (2000) The coupled GCM ECHO-2. Part II: Indian Ocean response to

D

ENSO. Journal of Climate 13(8):1371-1383

TE

Wang C, Deser C, Yu JY, DiNezio P and Clement A (2012) El Nino and southern oscillation (ENSO): a

CE P

review. Coral Reefs of the Eastern Pacific. 3-19 Wang C and Weisberg RH (2000) The 1997-98 El Niño evolution relative to previous El Niño events. Journal of Climate 13(2):488-501

AC

Webster PJ and Yang S (1992) Monsoon and ENSO: Selectively interactive systems. Quarterly Journal of the Royal Meteorological Society 118(507): 877-926 Wentz FJ, Ashcroft P and Gentemann C (2001) Post-launch calibration of the TRMM microwave imager. Geoscience and Remote Sensing IEEE Transactions on 39(2): 415-422 Wentz FJ, Gentemann C, Smith D and Chelton D (2000) Satellite measurements of sea surface temperature through clouds. Science 288(5467): 847-850 Wu R and Kirtman BP (2004) Understanding the impacts of the Indian Ocean on ENSO variability in a coupled GCM. Journal of climate 17(20): 4019-4031 Xie SP, Hu K, Hafner J, Tokinaga H, Du Y, Huang G and Sampe T (2009) Indian Ocean capacitor effect on Indo-western Pacific climate during the summer following El Niño. Journal of Climate 22(3): 730-747

Page | 25

ACCEPTED MANUSCRIPT Xie SP, Lu B and Xiang B (2013) Similar spatial patterns of climate responses to aerosol and greenhouse gas changes. Nature Geoscience 6(10): 828-832 Yun KS, Lee JY and Ha KJ (2014) Recent intensification of the South and East Asian monsoon contrast associated with an increase in the zonal tropical SST gradient. Journal of Geophysical Research:

IP

T

Atmospheres 119(13):8104-8116

Zelle H, Appeldoorn G, Burgers G and van Oldenborgh GJ (2004) The relationship between sea surface temperature

AC

CE P

TE

D

MA

NU

SC R

and thermocline depth in the eastern equatorial Pacific. Journal of physical oceanography 34(3):643-55

Page | 26

ACCEPTED MANUSCRIPT Highlights

CE P

TE

D

MA

NU

SC R

IP

T

Strong teleconnections between IO and PO basins are weakening in late 90’s Indian Ocean is becoming active in recent decades Indian Ocean is forcing easterlies along the equatorial Pacific Ocean Active Indian Ocean is responsible for the recent hiatus in the Pacific Ocean SST forced AGCM experiments supports the proposed hypothesis

AC

    

Page | 27