An anomalous cooling event observed in the Bay of Bengal during June 2009

Abstract

Sea surface temperature (SST) variability over the Bay of Bengal (BoB) has the potential to trigger deep moist convection thereby affecting the active-break cycle of the monsoons. Normally, during the summer monsoon season, SST over the BoB is observed to be greater than 28°C which is a pre-requisite for convection. During June 2009, satellite observations revealed an anomalous basin-wide cooling and the month is noted for reduced rainfall over the Indian subcontinent. In this study, we analyze the likely mechanisms of this cooling event using both satellite and moored buoy observations. Observations showed deepened mixed layer, stronger surface currents, and enhanced heat loss at the surface in the BoB. Mixed layer heat balance analysis is carried out to resolve the relative importance of various processes involved. We show that the cooling event is primarily induced by the heat losses at the surface resulting from the strong wind anomalies, and advection and vertical entrainment playing secondary roles.

What Controls Seasonal Evolution of Sea Surface Temperature in the Bay of Bengal? Mixed Layer Heat Budget Analysis Using Moored Buoy Observations Along 90°E

“…• Temperature: 1,10,15,20,30,50,75,100,200,and 500 m depths • Salinity: 1,10,15,20,30,50,75,100,200, and 500 m depths • 150 kHz acoustic Doppler current profiler (downward facing) measuring current profiles at an interval of 5 m from the near surface to 105 m depth • Meteorological: Downwelling shortwave radiation, air pressure, air temperature, humidity, wind speed, and direction (at 3 m) of MLD in this study (Kara et al, 2000;Du et al, 2005;Neethu et al, 2012 (Figures 1 and 3; Thadathil et al, 2002;Girishkumar et al, 2013).…”

“…Many attempts have been made to understand the seasonal variability of SST in the BoB by estimating the ML heat budget using observations (Rao and Sivakumar 2000;Sengupta and Ravichandran, 2001;Sengupta et al, , 2002Shenoi, 2002;Parampil et al, 2010;Neethu et al, 2012;Girishkumar et al, 2013) and models (Prasad, 2004;Boyer Montégut et al, 2007;Chowdary et al, 2015). All of these studies highlight the importance of net surface heat flux and near-surface salinity stratification for the evolution of SST variability in the BoB (Rao and Sivakumar, 2000;Sengupta et al, , 2002Sengupta and Ravichandran, 2001;Shenoi et al, 2002;Boyer Montégut et al, 2007;Parampil et al, 2010).…”

“…Few studies have analyzed the ML heat budget using moored buoy observations in the BoB (Neethu et al, 2012;Girishkumar et al, 2013), and all of these analyses were restricted to a single mooring for a shorter duration (approximately four months). The primary objective of this study is to understand the meridional variation of relative contributions of air-sea fluxes, ocean dynamics, and seasonal warming and cooling and how it affects the evolution of SST in the BoB.…”

Importance of vertical mixing and barrier layer variation on seasonal mixed layer heat balance in the Bay of Bengal

“…= − (Sprintall and Tomczak, 1992;Girishkumar et al, 2013). Chacko et al (2012) ponited out that the estimation of MLD and ILD by 0.8°C criterion is reasonably good for the BoB compared to 0.5°C or 1.0°C criterion. Air-sea fluxes at the mooring locations are computed from the daily winds extrapolated to the 10-m height, SST, air temperature and relative humidity.…”

“…Thus, stratification helps to maintain a warm surface layer, inhibiting SST cooling, and contribution from vertical mixing remains minimum during this period (Figures 8a, 9b). Chacko et al (2012) suggested the siginificance of vertical mixing over surface forcing in inducing mixed-layer 300 Ocean Sci. Discuss., https://doi.org/10.5194/os-2017-67 Manuscript under review for journal Ocean Sci.…”