Annual cycle of sea surface temperature in the North Indian Ocean

Abstract

Sea surface temperature (SST) plays a pivotal role in determining Earth’s climate and its variability through air-sea interaction. Therefore, understanding the variation of SST on different spatio-temporal scales is one of the most important and fascinating problems in tropical climate. The lack of in situ data over the North Indian Ocean has hampered such studies in this region until recently. The Department of Ocean Development, India, launched a moored buoy programme known as the National Data Buoy Programme, in August–September 1997 to collect real-time data on meteorological and oceanographic parameters in the Bay of Bengal and eastern Arabian Sea, which are the genesis regions of depressions and cyclones.

In this study, an effort has been made to understand the evolution of SST at the buoy locations from September 1997 to December 1998. Apart from 3-hourly observations of SST, air temperature, wind, and ocean current from the moored buoys, climatological shortwave radiation from the Earth Radiation Budget Experiment, outgoing longwave radiation from the National Oceanic and Atmospheric Administration (NOAA), USA, relative humidity from the Comprehensive Ocean Atmosphere Data Set (COADS), and climatological mixed layer depth from the Levitus Atlas are the other main data used. I explain the SST change as a response to surface heat flux (consisting of net shortwave and net longwave radiation, and latent and sensible heat flux) and solar penetration below the mixed layer, using a simple one-layer heat balance model. This is the first study of upper ocean thermodynamics using high-frequency in situ data over the Indian Ocean covering a full annual cycle.

Important results of my study are as follows:

Strong seasonality in the cloudiness leads to an annual cycle of incoming shortwave radiation that does not simply follow the annual march of the sun. The magnitudes of shortwave radiation and latent heat flux are much larger than other factors responsible for SST variation. Net heat flux, including solar penetration below the mixed layer, provides a first-order explanation of the observed variation of SST in all seasons except during the boreal spring (March–April–May), when the predicted rise in SST is larger than the observed value. The SST predictions at all buoy locations in spring show great improvement with a small reduction in relative humidity. In many cases, an abrupt drop in SST is not explainable by the net heat flux. This suggests the importance of episodes of mixed layer deepening and entrainment of cooler water from below. Two case studies have been presented where I show that cooling due to mixed layer deepening and advection are responsible for large observed changes in SST.