Some BOBMEX Results



Heat Flux, Wind Speed and Salinity Changes over the Bay


The atmospheric and oceanographic observations under monsoon conditions have been very successively carried out capturing certain unique features of the Bay during the BOBMEX. The panel (a) in the accompanying figure shows the variation of the latent heat flux with the wind speed. The dark line shows the average latent heat flux observed during the TOGA-COARE carried out over the West Pacific. The dots are the individual values of latent heat fluxes estimated using the bulk algorithm for the BOBMEX data set and the line represents the mean, calculated in 1 m/s wind speed intervals. A notable feature of the monsoon conditions is that the latent heat flux is 25% to 50% lower as compared to that over the tropical West Pacific at a given wind speed. The panel (b) shows the temporal variation in the sea surface salinity at 17.5°N and 89°E in the North Bay of Bengal. The low salinity water makes the top layer (10-15 m deep) very stable inhibiting the vertical mixing that helps in maintaining a warm SST despite strong monsoon winds.


Intraseasonal Oscillations over the Bay as revealed by Buoy Data


The moored buoy data from the Bay of Bengal have revealed dominant intraseasonal oscillations (ISO) consisting of alternating periods of cloudy-windy and clear-calm conditions. In the convective phase, clouds reduce insolation at the ocean surface while stronger winds lead to large evaporative cooling. Consequently, the ocean loses heat and the sea surface temperature (SST) falls. During periods of suppressed monsoon convection, the ocean gains heat and SST increases. This figure shows the June-October 1998 evolution of satellite outgoing longwave radiation (OLR) and wind speed and SST from a moored buoy installed by DOD's National Data Buoy Programme at 18°N;88°E in the northern Bay of Bengal. Low (high) values of OLR indicate the presence (absence) of deep clouds. SST rises and falls on time scales of weeks in response to surface heat flux changes associated with monsoon ISO. The amplitude of SST ISO is very high in the north Bay of Bengal because the ocean's upper mixed layer is shallow.

BOBMEX Participants

Indian Institute of Sciences (CAOS), Bangalore • National Institute of Oceanography, Goa

National Institute of Ocean Technology, Chennai • Naval Physical Oceanographic Laboratory, Cochin

India Meteorological Department, New Delhi • Indian Navy, Naval Headquarters, New Delhi

National Centre for Medium Range Weather Forecasts, New Delhi • Indian Institute of Technology, New Delhi

Andhra University, Visakhapatnam • Space Applications Centre, Ahmedabad

Indian Institute of Tropical Meteorology, Pune

ICRP Bulletin                                              Vol.1 • No.1                                               Page 4
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