Air-Sea Coupling Over The Equatorial Indian Ocean

Abstract

The present work aims at understanding the upper ocean variability of the EIO on a seasonal scale and then to explore the atmospheric forcing responsible for it.To achieve the objective, the Equatorial Indian Ocean between latitude 10oS-10oN and longitude between 40oE-110oE was selected for the present study. For the study, both in situ data and remote sensing data were used. The oceanographic data on temperature, salinity were obtained from 3 sources – World Ocean Database 2005 (WOD05) which contains CTD data during the period 1970-2000, Responsible National Oceanographic Data Centre (RNODC) which contains CTD data collected by Indian research vessels during the period 1983-2004, and ARGO data during 2000 to 2006. In addition to this, the monthly mean ocean surface current (Ocean Surface Current Analysis- Real time - OSCAR) data during 1992 to 2006 was also used. The atmospheric data on air temperature, wind speed and direction, net heat flux, wind stress, and fresh water flux used for this study was derived from Southampton Oceanography Center. The dataset is a modified version of the Comprehensive Ocean - Atmosphere Dataset (COADS) of marine meteorological reports spanning the period 1980 - 1993, which has been corrected for biases arising from variations in observing procedure.The total number of CTD data after quality control was 3782 while that from ARGO was 13069.his quality-controlled data was binned into a 1-degree longitude by 1-degree latitude grid (1°1°) within the study domain and were used for further calculations.The study of the seasonal cycle of the ocean and atmospheric parameters showed very strong coupling. The seasonal cycle of mixed layer depth and barrier layer thickness were tightly coupled to the variability in the momentum and fresh water fluxes. In fact heat flux had little role in regulating the mixed layer. On the contrary, ocean dynamics played a major role in regulating the mixed layer in the equatorial wave-guide during spring and fall inter-monsoons. Though, the heat flux during both these periods were positive which coincided with the primary and secondary heating, the mixed layer were expected to be thinner, deep MLDs were encountered in the equatorial wave guide. It is quite natural to expect that these seasonal changes in the upper ocean physics will have great impact in the biogeochemical cycles, and there is ample scope for addressing the coupling between the physical forcing and biogeochemical variability in the equatorial Indian Ocean. This would need special targeted co-located measurements.