Transformations between Normal and Abnormal Mesoscale Eddies in the Bay of Bengal

Abstract It is conventionally accepted that anticyclonic and cyclonic eddies are associated with anomalous warm and cold sea surface temperatures (SSTs), respectively. Nevertheless, recent studies have revealed a common occurrence of abnormal eddies with anticyclonic cold cores and cyclonic warm cores in global oceans, particularly abundant in highly unstable currents and tropical oceans with shallow surface mixed layers. The Bay of Bengal (BoB), known for its huge freshwater input and strong upper-layer stratification, is favorable for generating abnormal eddies, yet little is known about the characteristics of these eddies. Using two distinct methods—the high-pass filter method and the SST anomaly index method—we analyze the spatial–temporal variations and three-dimensional structures of these eddies. Our findings reveal similar geographical distributions and opposing seasonal variations compared to normal eddies. Furthermore, we observe a tendency for normal eddies to transform into abnormal eddies over their life cycle. To understand this transformation, we employ a first-order autoregressive model, which suggests that the combination of mesoscale air–sea turbulent heat flux feedback and mean mixed layer depth (MLD) plays a crucial role in reversing SST anomalies within the eddy cores. Additionally, we investigate the influence of different eddy types on air–sea fluxes and MLD, revealing distinct coupling patterns during winter and summer monsoon seasons. Notably, haline buoyancy fluxes associated with abnormal eddies modulate MLD in a unique manner. Given their abundance, the role of abnormal eddies in Earth’s climate system must be taken into account in observational data analyses and numerical model simulations, particularly in salinity-stratified oceans. Significance Statement The anomalous sea surface temperature (SST) associated with mesoscale eddies provides critical links between the atmosphere and the ocean. This study challenges the conventional understanding of eddy–SST relationships, revealing the existence of abnormal eddies with contrasting temperature cores in the Bay of Bengal (BoB). Our analysis provides new insights into the transformations between normal and abnormal eddies in the BoB, which are closely linked to mesoscale air–sea turbulent heat flux feedback and mean mixed layer depth. Besides, abnormal eddies modulate the air–sea fluxes and mixed layer depths in a manner distinctly different from normal eddies, particularly in salinity-stratified oceans. These findings could enhance our comprehension of mesoscale dynamics and potentially have far-reaching implications for Earth’s climate system and biogeochemical processes.