Indian Ocean‐monsoon coupled interactions and impending monsoon droughts

Abstract

[1] Monsoon droughts over the Indian subcontinent emanate from failures in the seasonal (June -September) monsoon rains. While prolonged dry-spells (''monsoonbreaks'') pervade on sub-seasonal/intra-seasonal time-scales, the underlying causes for these long-lasting anomalies remain elusive. Based on analyses of a suite of observed data sets, we report an ocean-atmosphere dynamical coupling on intra-seasonal time-scales, in the tropical Indian Ocean, which is pivotal in forcing extended monsoon-breaks and causing droughts over the subcontinent. This coupling involves a feedback between the monsoonal flow and thermocline depth in the Equatorial Eastern Indian Ocean (EEIO), in which an anomaly of the summer monsoon circulation induces downwelling and maintains a higherthan-normal heat-content. The near-equatorial anomalies induce strong and sustained suppression of monsoon rainfall over the subcontinent. It is concluded that the intraseasonal evolution of the ocean-monsoon coupled system is a vital key to unlocking the dynamics of monsoon droughts.

Cloud‐radiative impacts on the tropical Indian Ocean associated with the evolution of ‘monsoon breaks’

“…Anomalously warm SST in the SETIO associated with strong east-west SST gradients favour westerly winds along the equator, which enhance the moisture convergence and convective activity over the SETIO region (Krishnan et al, 2006). GCM simulation experiments suggest that the enhancement of convective activity over the warm SETIO can induce anomalous subsidence over the Indian subcontinent and weaken the monsoon Hadley cell, thereby leading to deficient rainfall over India (Krishnan et al, 2003).…”

“…Rajeevan (2001) suggested a similar mechanism for anomalous SST warming in the tropical Indian Ocean associated with the inter-annual variability of high clouds. In addition, Krishnan et al (2006) have recently reported the possibility of a dynamical coupling between the southwest monsoon circulation and the thermocline depth variations in the eastern equatorial Indian Ocean on intra-seasonal timescales. In this dynamical feedback, anomalous westerly winds along the equator push warm water eastward, so as to deepen and warm the surface mixed layer in the eastern equatorial Indian Ocean and also maintain a strong east-west gradient of SST in the equatorial Indian Ocean.…”

“…While monsoon breaks are associated with suppression of rains over the Indian landmass, it has been known that convection generally intensifies over the near-equatorial Indian Ocean during break periods (e.g. Yasunari, 1979;Sikka and Gadgil, 1980;Lau and Chan, 1986;Krishnan et al, 2000;Annamalai andSlingo, 2001 andKrishnan et al, 2006).…”

“…The 2002 monsoon drought has been explored by several investigators from different viewpoints (e.g. Fasullo, 2005;Gadgil et al, 2005;Kumar and Krishnan, 2005;Krishnan et al, 2006;Saith and Slingo, 2006). The mean climatological conditions for July and the anomalous conditions during July 2002 are depicted in Figure 1.…”

Indian Ocean Dipole Modulates the Number of Extreme Rainfall Events over India in a Warming Environment

“…The seasonal mean (JJAS) SST anomalies show significant cooling in southeast Indian Ocean in many IOD years and hence IOD has the potential to influence monsoon. Influence of IOD on monsoon are discussed in detail by previous studies (e.g., Ashok et al 2001Ashok et al , 2004Gadgil et al 2004;Krishnan et al 2006). Here, we consider the cool SST anomalies in the southeastern equatorial Indian Ocean to represent IOD.…”