Intra-seasonal oscillations in the central Bay of Bengal during summer monsoon 1999

Intra-seasonal oscillations were observed in the meteorological and oceanographic fields of Bay of Be ngal during the summer monsoon season of 1999. Spectral analysis revealed prominent peaks in the intra-seasonal (10 to 32 days), inertial (2.2 days), diurnal and semi-diurnal bands. Oscillation of intraseasonal periodicity significantly influences the ci rculation pattern. The variations in oceanographic fields were dominated by remote forcing compared to local forcing. ONLY a few studies have been carried out in the Indian Ocean to underst and the influence of seasonally rever sing monsoon winds on the variability of water chara cteristics and circulation. A recent study by Yu et al . 1 suggested that for a large -scale low frequency forcing associated with monsoon wind, only long Rossby and Kelvin waves are generated. These waves act as a r emote forcing, determining the upper layer circulation in the Bay of Bengal. McCreary et al. 2 also suggested that the monsoon winds force the ocean locally and excite propagating signals (Kelvin and Rossby waves) that travel a long distance to affect the ocean remotely. Theoretical studies revealed the pre sence of these waves in the current and temperature fields in the central and western sides of the Bay 3,4 . These studies suggest that the Rossby waves excited by the remotely-forced Kelvin waves play a significant role in the var iability of circulation in the Bay of Bengal. This is contrary to some of the earlier studies that stressed the direct infl uence of meteorological forcing for the o bserved oceanic oscillation, citing the Madden and Julian 5 discovery of 40 to 50-day oscillation in the wind field of lower trop ical troposphere. However, the model of Kindle and Thompson 4 showed that the oscillations are not driven by the atmospheric wind field, but instead ar e energized by bar otropic instability of westerly zonal flow. All these theoretical studies have concluded that these waves contributed significantly to the dynamics of the Bay of Bengal, which needs observational evidence. More recently, Premkumar et al. 6 observed intraseasonal oscillations in sea surface temperature (SST) at different locations in the Bay of Be ngal utilizing data from moored buoys. They suggested that the SST varia