Formation mechanism of Anticyclonic Eddies South East of Sri Lanka during Summer Monsoons

Annunziata Pirro, University of Notre Dame, Department of Civil & Environmental Engineering & Earth Sciences

Hopeman 224

The ocean circulation in the Bay of Bengal (BoB) is still not well studied specifically based on detailed field measurements. In the summers of 2015 and 2018, two cruises were conducted in the BoB directed at understanding air-sea processes and their role on the dynamics of Indian Summer Monsoon, a land–atmosphere–ocean coupled system that affects the life of more than one billion people in the Indian subcontinent. In-situ observations from the two cruises show that the regional circulation in the southern BoB consists of the summer monsoon current (SMC) and two large mesoscale eddies: a cyclonic eddy (the Sri Lanka Dome) to the east of Sri Lanka, and an anticyclonic eddy to the southeast of Sri Lanka. In August 2015, an anticyclonic mesoscale feature, found ~110 km southeast of Sri Lanka had surface velocities of about 1 m/s. Satellite Sea Surface Height Anomalies, Sea Surface Temperature and COAMPS outputs model indicate that the eddy is part of a Rossby wave trains present during summer monsoon season. It has been suggested that the formation of the anticyclonic eddy is a result of the interaction between the SMC and a northwestward propagating Rossby waves originating in the eastern equatorial Indian Ocean. Here we propose an alternative hypothesis for possible generation mechanism of the anticyclonic eddy in the Rossby wave framework based on a set of laboratory experiments. A laboratory experiment was performed in a circular rotating tank with beta effect mimicked by a sloping bottom and SMC by a jet. The laboratory Rossby number was maintained at 0.23 and 0.35. Sensitivity of the results to (Sri Lanka) coastal shape was also considered. Under control parameter space, laboratory results reproduced basic features of the oceanic observations, indicating that Rossby waves triggered by the SMC can be a leading contender for anticyclonic eddy-formation.