Post-tsunami oceanographic conditions in southern Arabian Sea and Bay of Bengal

Physical oceanographic conditions along the east and west coasts of India immediately after the recent devastating tsunami are presented here. The thermocline in the southeast coast exhibited trivial downward tilt towards north. A mixed layer deepening (>50 m) associated with low-salinity ( 36.0 psu) was identified as the Arabian Sea high salinity water mass along the west coast of India around 100 m depth up to 10°N. The heat content changes closely followed similar changes in the depth of the 20°C isotherm. Turbidity measurements using light scattering sensor showed the existence of two layers of high-scattering, one around 40 m and the other around 250 m. The shallow high-scattering layer is associated with high chlorophyll a concentration, but the deeper high-scattering layer noticed at shallow stations off the west and east coasts of India may be due to the resuspension of the sea-floor sediments due to turbulence generated by the tsunami

Hydrography and circulation of the Bay of Bengal during withdrawal phase of the southwest monsoon

Hydrographic data were collected from 3 to 10 September 1996 along two transects; one at 18 degrees N and the other at 90 degrees E. The data were used to examine the thermohaline, circulation and chemical properties of the Bay of Bengal during the withdrawal phase of the southwest monsoon. The surface salinity exhibited wide spatial variability with values as low as 25.78 at 18 degrees N / 87 degrees E and as high as 34.79 at 8 degrees N / 90 degrees E. Two high salinity cells (S > 35.2) were noticed around 100 m depth along the 90 degrees E transect. The wide scatter in T-S values between 100 and 200 m depth was attributed to the presence of the Arabian Sea High Salinity (ASHS) water mass. Though the warm and low salinity conditions at the sea surface were conducive to a rise in the sea surface topography at 18 degrees N / 87 degrees E, the dynamic height showed a reduction of 0.2 dyn.m. This fall was attributed to thermocline upwelling at this location. The geostrophic currents showed alternating flows across both the transects. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18 degrees N transect with velocity slightly in excess of 30 cm s(-1). Similar high velocity (> 40 cm s(-1)) pockets were also noticed to extend up to 30 m depths in the southern region of the 90 degrees E transect. However, the currents below 250 m were weak and in general < 5 cm s(-1). The net geostrophic volume transports were found to be of the order of 1.5 x 10(6) m(3) s(-1) towards the north and of 6 x 10(6) m(3) s(-1) towards west across the 18 degrees N and 90 degrees E transects respectively. The surface circulation-patterns were also investigated using the trajectories of drifting buoys deployed in the eastern Indian Ocean around the same observation period. Poleward movement of the drifting buoy with the arrival of the Indian Monsoon Current (IMC) at about 12 degrees N along the eastern rim of the Bay of Bengal has been noticed to occur around the beginning of October. The presence of an eddy off the southeast coast of India and the IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data.La circulation thermohaline et les propriétés chimiques du golfe du Bengale sont étudiées pendant la phase de renversement de la mousson du sud-ouest ; les données hydrologiques ont été collectées du 3 au 10 septembre 1996 sur deux radiales, l'une à 18° N, l'autre suivant 90° E. La salinité de surface présente une grande variabilité spatiale, de 25,78 (par 18° N / 87° E) jusqu'à 34,79 (par 8° N / 90° E), avec deux maxima (plus de 35,2) vers 100 m de profondeur sur la radiale 90° E. La forte dispersion des températures et des salinités, observée entre 100 et 200 m de profondeur, est attribuée à l'eau très salée en provenance de la mer d'Arabie (ASHS). Bien que les eaux superficielles chaudes et peu salées élèvent la topographie de la surface par 18° N / 87° E, la hauteur dynamique présente une baisse de 0,2 m dyn, attribuée ici à la remontée de la thermocline. Les flux géostrophiques sont alternés à travers les deux radiales. Des courants relativement plus forts (dépassant 30 cm s−1) et opposés sont observés vers 25 m de profondeur à travers la radiale 18° N. Des poches similaires de fort courant (plus de 40 cm s−1) dépassent 30 m de profondeur dans la partie sud de la radiale 90° E. Cependant, au-dessous de 250 m de profondeur, les courants sont faibles (moins de 5 cm s−1). Les flux géostrophiques nets sont respectivement de l'ordre de 1,5 × 106 m3 s−1 vers le nord et 6 × 106 m3 s−1 vers l'ouest à travers les radiales 18° N et 90° E. Les schémas de la circulation superficielle sont établis à partir des trajectoires de bouées dérivantes déployées dans l'est de l'océan Indien pendant la même période. Le mouvement est dirigé vers le pôle au début du mois d'octobre, à l'arrivée du Courant Indien de Mousson (IMC) le long du bord oriental du golfe du Bengale, vers 12° N. La dérive des bouées met en évidence la présence d'un tourbillon au large de la côte sud-est de l'Inde et le Courant Indien de Mousson à la périphérie du golfe du Bengal

Influence of the monsoon trough on air-sea interaction in the head of the bay of bengal during the southwest monsoon of 1990 (monsoon trough boundary layer experiment-90)

The analysis of 3-hourly time-series data on surface meteorological parameters collected at 20° N, 89° E in the head of the Bay of Bengal during the southwest monsoon period (18 August–19 September) of 1990 under the MONTBLEX-90 programme reveals considerable temporal variability in sea-level pressure, sea-surface temperature (SST) and the fluxes of heat and momentum at the air-sea interface. This variability is related closely to the north-south movement of the monsoon trough and the formation and development of synoptic weather systems during this period. A rapid increase in wind speed, cloudiness, instability, momentum flux, sensible heat flux and moisture flux (by 80 Wm-2), and a decrease of SST (by 0.3 °C) and net surface heat flux by 80 Wm-2, was associated with the development of a depression when the monsoon trough moved southwards. At the peak of the depression, values of the latent heat flux and evaporation reached up to 270 Wm-2 and 1.0 cm day-1 respectively. During the depression period the heat loss across the air-sea interface matched well with the heat loss in the upper (≈100 m) ocean. With the northward movement of the monsoon trough, the momentum and surface heat fluxes decreased rapidly while the sea surface gained heat energy at rates up to 195 Wm-