Mesoscale eddy variability over the Bay of Bengal in response to the contrasting phases of extreme Indian Ocean Dipole events

Abstract

The present study is aimed to investigate the mesoscale eddy variability in response to the two contrasting phases of extreme Indian Ocean Dipole (IOD) events, the 2016 (negative mode) and 2019 (positive mode). A quantification of the eddy characteristics was done by estimating the number of eddies, eddy radius, eddy amplitude, and eddy rotation speed for each of the contrasting events. It is found from the analysis, that the phase of IOD has a significant impact on the spatiotemporal characteristics of eddies over the bay. The number of eddies increased and a gradual shift from anti-cyclonic to cyclonic eddies was observed during the positive IOD year. Eddies formed during 2016 had an average amplitude of 0.085 m, rotation speed was around 0.345 cm/s and eddy radius, on average was found to be 81.25 km. The presence of a unique eddy feature on the east coast of Sri Lanka was observed which remained as a cyclonic eddy during the negative IOD and as an anti-cyclonic eddy during the positive IOD. The sea surface height anomaly showed a dip of −0.1 m at the core of the cyclonic eddy and a +0.4 m bulge was observed at the core of the anti-cyclonic eddy. A high negative SST anomaly of −0.5 °C was observed at the core of the cyclonic eddy. A drastic increase in the mixed layer depth was seen with the presence of the anti-cyclonic eddy. Analysis of the temperature and salinity vertical profiles showed that the anti-cyclonic eddy had a greater influence on the vertical profile than the cyclonic eddy. Both the eddies contributed significantly to upper ocean primary productivity by different mechanisms. Eddy-induced upwelling was predominant in the cyclonic eddy whereas eddy stirring was predominant in the anti-cyclonic eddy.