Indian Ocean and monsoon coupled interactions in a warming environment Panickal Swapna • R. Krishnan • J. M. Wallace Received: 23 December 2012 / Accepted: 22 April 2013 / Published online: 8 May 2013 Ó 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