Simulation of large anti-cyclonic eddies generated by westward propagating Rossby waves in the southern Bay of Bengal during the summer Monsoon season 2019

Tommy G Jensen1, Hemantha W Wijesekera2, Adam Rydbeck1, Ewa Jarosz1, David W Wang3, Conrad A Luecke4, Maria K. Flatau5 and Jerome Schmidt6, (1)U.S. Naval Research Laboratory, Ocean Sciences, Stennis Space Center, MS, United States, (2)U.S. Naval Research Laboratory, Ocean Sciences, Stennis Space Center, United States, (3)U.S. Naval Research Laboratory, Stennis Space Center, United States, (4)NRC Postdoctoral Fellow at Naval Research Laboratory, Stennis Space Center, MS, United States, (5)US Naval Research Laboratory, Marine Meteorology Division, Monterey, CA, United States, (6)U.S. Naval Research Laboratory, Marine Meteorology, Monterey, CA, United States
Abstract:
In support of the field experiments of the Monsoon Intra- Seasonal Oscillations in the Bay of Bengal (MISOBOB) we have performed daily high resolution runs with the coupled atmosphere-ocean-wave modelling system COAMPS developed at the U. S. Naval Research Laboratory. These simulations have been done since May 2015 and are still being run. The model ocean has a resolution of 2 km, fully resolves mesoscale eddies and includes 3D-var data assimilation of altimetry, sea surface temperature and available hydrographic temperature and salinity profiles.

During the monsoon season in 2019 the westward propagating and downwelling Rossby waves were formed along the eastern boundary of the Bay of Bengal from about 5-15N starting in the beginning of June after the onset of the Southwest Monsoon. The southern part of the wave interacted with the Southwest Monsoon Current to form anti-cyclonic eddies. During July a large anti-cyclonic eddy propagated westward to the Sri Lanka coast and spanned the latitude range 4-8N by the end of the month. A relative weak cyclonic Sri Lanka dome was found to the north, squeezed in between the southern anti-cyclone and a similarly intense anti-cyclone to the north between 12-16N and centered along 82E. The circulation near the east coast of Sri Lanka was dominated by the southern gyre that resulted in a strong northward flow into the Bay of Bengal. Currents reached to 800 m with a northward flow of 0.6 m/s at 400 m in the model and were found to have higher speeds and reach greater depths in observations.

During the second half of August, the southern anti-cyclone moved northward while shedding a cyclonic eddy and merged with the northern anticyclone.

The two anti-cyclonic eddies in 2019 dominated the summer monsoon circulation near Sri Lanka causing a predominant northward flow. In contrast, in 2018, a stronger Sri Lanka Dome prevented the migration of the southern anti-cyclone to reach the Sri Lanka coast and the subsequent northward migration and the flow near Sri Lanka was southward.

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