Internal Waves due to Intrusion of Sub-surface High-saline waters in the Central Bay of Bengal

Abstract:

The Bay of Bengal is one of the most peculiar regions in the tropical oceans as it experiences the seasonally reversing monsoon winds along with a large amount of freshwater flux from the river discharge during Indian summer monsoon. The in-situ observations from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoy at (15° N, 90° E) along with output from Ocean ReAnalysis System 5 (ORAS5) are used to study the seasonal variability of density stratification and associated internal waves activity. Highly stratified waters are present below 80 m during summer and winter, whereas they are found at relatively shallow depths of 40-60 m during autumn. In spring, stratification is maximum in two separate layers – one near the surface around 40 m and the other in the subsurface at around 100 m. The analysis of relative importance of temperature and salinity to density stratification shows that salinity plays a dominant role up to 60 m, and the temperature is more important below that depth in all seasons, except during spring. In spring, both temperature and salinity play an equivalent role in controlling the stratification. This is due to the development of secondary or seasonal thermocline developed near the surface because of solar heating. Morlet wavelet analysis of density showed strong internal waves at 40 m depth during autumn of 2013, 2014, and 2015. These are caused by the advection of high saline waters below the surface and low-saline waters near the surface due to heavy precipitation during Indian summer monsoon, thereby forming a strong salinity gradient. Analysis of ORAS5 salinity and current fields show that the high saline waters below the surface came from the Arabian Sea with the help of West India Coastal Current and Southeast Monsoon Current. These advected waters reached the central Bay of Bengal through meanders and eddies, thereby causing strong internal waves at 40 m depth.