Diurnal variability of upper ocean temperature and heat budget in the southern Bay of Bengal during October-November, 1998 (BOBMEX-Pilot)

Time-series data on upper-ocean temperature, Vessel-Mounted Acoustic Dopple r Current Profiler (VM-ADCP) measured currents and surface meteorological p arameters have been obtained for the first time in the southern Bay of Beng al at 7 degrees N, 10 degrees N, and 13 degrees N locations along 87 degree s E during October November, 1998 under BOBMEX-Pilot programme. These data have been analysed to examine the diurnal variability of upper oceanic heat budget and to estimate the eddy diffusivity coefficient of heat in the upp er layer. Diurnal variation of near-surface temperature is typical at north ern location (13 degrees N) with a range of 0.5 degrees C while the diurnal range of temperature is enhanced to 0.8 degrees C at the central location (10 degrees N) due to intense solar radiation (1050W/m(2)), clear skies and low wind speeds. At the southern location (7 degrees N), the diurnal varia tion of temperature is atypical with the minimum temperature occurring at 2 000 hrs instead of at early morning hours. In general, the diurnal curve of temperature penetrated up to 15 to 20 m with decreasing diurnal range with depth. The VM-ADCP measured horizontal currents in the upper ocean were pr edominantly easterly/northeasterly at southern location, north/northerly at central location and northwesterly at northern location, thus describing a large-scale cyclonic gyre with the northward meridional flow along 87 degr ees E. The magnitudes of heat loss at the surface due to air-sea heat excha nges and in the upper 50 m layer due to vertical diffusion of heat are high est at the southern location where intense convective activity followed by overcast skies and synoptic disturbance prevailed in the lower atmosphere. This and the estimated higher value (0.0235 m(2)/s) of eddy diffusivity coe fficient of heat in the upper ocean (0-50 m depth) suggest that 1-D process es controlled the upper layer heat budget at the southern location. On the other hand, during the fair weather conditions, at the central and northern locations, the upper layer gained heat energy, while the sea surface lost (gained) heat energy at northern (central) location. This and lower values of eddy diffusivity coefficient of heat (0.0045 and 0.0150 m(2)/s) and the northward intensification of horizontal currents at these locations suggest the greater role of horizontal heat advection over the 1-D processes in th e upper ocean heat budget at these two locations.