Dynamics of intraseasonal oscillations in the Bay of Bengal during summer monsoons captured by mooring observations

Abstract

The intraseasonal variabilities of current and temperature in the southern Bay of Bengal (BoB) were examined using in-situ observations collected in the upper 500 m of the water column for a period of 20 months, between December 2013 and August 2015. Data were sampled by six subsurface moorings deployed by the United States Naval Research Laboratory as part of an international effort focused on the dynamics of the northern Indian Ocean. The moored observations captured motions of multiple time-scales, including seasonal circulations, eddies, and planetary waves that underlie intraseasonal variability. This study focuses on 30–60-day intraseasonal oscillations (ISOs). Spectral analysis of currents shows enhanced energy and momentum fluxes for the period 46–52-days. The ISO velocity fluctuations were coherent in the upper 500 m of the water column, strongest near the surface and nearly uniform below 100 m. The ISO kinetic energy appeared at the beginning of the 2014 summer monsoon and lasted throughout the summer, but the associated potential energy dropped rapidly after it peaked at the beginning of the monsoon season.The correlation between the 30–60 day ISOs of local winds and the oceanic variability below 120 m depth was fund to be as small as 25% suggesting thus that the local winds were not the main forcing mechanism for ISOs. The 30–60-day band-passed sea-surface-height anomalies indicate westward propagating anomalies with phase speeds between 0.17 m s−1 at 8°N and 0.43 m s−1 at 4°N, within the phase-speed ranges of mode-1 and mode-2 linear baroclinic Rossby waves, pointing to the role of Rossby waves in 30–60-day ISOs.The background mean flow acceleration resulting from the meridional divergence of wave momentum flux in the thermocline was about 10−8 m s−2. As a result, within a wave period, ISOs can enhance the eastward flow in the thermocline by about 25%. The negative shear production computed for the same period is consistent with this finding, suggesting that the mean flow has gained kinetic energy at the expense of the ISO band at a rate of about 0.5×10−8 m2 s−2. The meridional heat-flux divergence was -10−7 °C s−1, and has a tendency for cooling the thermocline by about 0.5 °C when the ISOs are active. The ISOs appear to play an important role in the ocean dynamics of the southern BoB during the summer monsoon.