INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.3585 Short Communication Mechanism of intraseasonal oceanic signature in the region off southern tip of India during boreal summer A. Jayakumar, a,b * C. Gnanaseelan a and T. P. Sabin a a Indian Institute of Tropical Meteorology, Pune, India b Nansen Environmental Research Center India, Cochin, India ABSTRACT: Sea surface temperature (SST) in the region off southern tip of India (STI, 75–83 ° E, 5–8 ° N) exhibited a prominent variability in the intraseasonal time scale (both 30–90 d and 10–30 d band) during boreal summer. Mechanisms associated with this intraseasonal variability are studied using three-dimensional ocean general circulation model (OGCM) sensitivity experiments, satellite observed outgoing longwave radiation, SST and winds for the period 1998–2007. The background oceanic structure of the STI characterized by a shallow thermocline and moderate mixed layer provided ideal conditions for strong oceanic sub-surface processes. The model mixed layer heat budget reveals that the oceanic processes such as horizontal advection and vertical processes are the dominant mechanisms in the STI region as compared with air-sea flux. Sensitivity experiments with the OGCM reveals that the ocean dynamical processes contribute to most of the intraseasonal SST variability and the wind stress contributes to 85% of the variability whereas surface flux contributes to only 15% for the 30–90 d SST variability. Higher amplitude of surface flux perturbation and its contribution to SST in the 10–30 d as compared with the 30–90 d band are evident in the model experiment and are consistent with the observational analysis. There is year-to-year variability in the relative role of horizontal and vertical processes for different intraseasonal SST events over STI. Copyright  2012 Royal Meteorological Society KEY WORDS ocean general circulation model; Surface Flux; intraseasonal SST; Ekman pumping; sub-monthly variability Received 2 April 2011; Revised 9 May 2012; Accepted 30 July 2012 1. Introduction Winds over the tropical Indian Ocean exhibit a sea- sonal reversal during boreal summer, accompanied by prominent upper ocean variability on different spatial and temporal scales. The seasonal upper ocean circulation in the region off southern tip of India (STI, 75–83 ° E, 5–8 ° N) reflected a complex feature, by linking Bay of Bengal with rest of the Indian Ocean through passage of eastward southwest monsoon current and Rossby wave interaction (Schott et al., 1994). Forcing mechanism of this seasonal monsoon current around Sri Lanka and STI (McCreary et al., 1993; Vinayachandran and Yamagata, 1998; Shankar et al., 2002) and its intraseasonal fluctua- tion (Sengupta et al., 2001) are well documented. Using Tropical Rainfall Measuring Mission Microwave Imager (TMI), Rao et al. (2006) observed sub-monthly (10–30 d band) variability in sea surface temperature (SST) over STI. They attributed the variability in this upwelling region to changes in the surface divergence and wind mixing. SST variability in this region is influenced by the active-break cycle of the summer monsoon over * Correspondence to: A. Jayakumar, Indian Institute of Tropical Mete- orology, Dr. Homi Bhabha Road, Pune 411 008, Maharashtra, India. E-mail: jkumar@tropmet.res.in the Bay of Bengal (Vecchi and Harrison, 2002; Joseph et al., 2005; Joseph and Sabin, 2008; Vialard et al., 2012). Ganer et al. (2009) speculated that the evaporative cooling along with the upwelling associated with the active-break cycle is responsible for this variability. In addition to the local forcing the dynamical remote forcing may have some role in the intraseasonal variability in this upwelling region (Smitha et al., 2008; Webber et al., 2010). But none of these studies looked at the processes associated with STI intraseasonal SST variations in detail. Duncan and Han (2009) carried out model investigations to understand SST variability over areas of maximum intraseasonal variance over Indian Ocean domain, but they have not conducted detailed analysis over STI, as their model performed poorly over this region. Our earlier studies using the ocean general circulation model (OGCM) showed good agreement with the TMI SST over STI in the 30–90 d band (Jayakumar et al., 2011; Jayakumar and Gnanaseelan, 2012; Vialard et al., 2012). Further Vialard et al. (2012) showed that STI is one of the key regions where intraseasonal SST variability is very significant. These help in the model fidelity in understanding the SST variability over STI using Modular Ocean Model Version 4 (MOM4) (Griffies et al., 2004). Copyright  2012 Royal Meteorological Society