Interannual to Multidecadal Forcing of Mesoscale Eddy Kinetic Energy in the Subtropical Southern Indian Ocean

Abstract

A region of elevated mesoscale eddy activity spans the subtropical southern Indian Ocean (SSIO) between Madagascar and Australia. The interannual and decadal changes in eddy activity in the SSIO eddy band, as represented by the variability of eddy kinetic energy (EKE), have implications for the large‐scale circulation, mixed‐layer budgets, and biological activity. An analysis of nearly two and a half decades of sea level anomaly (SLA) data from merged satellite altimetry shows that, in the southeast Indian Ocean east of 90°E, the variations of EKE and SLA are positively correlated on interannual and decadal time scales. Moreover, EKE exhibits a multidecadal increasing linear trend that corresponds to an increasing trend of SLA in the region. The EKE‐SLA covariability in the southeast Indian Ocean does not appear to be associated with a preference for anticyclonic over cyclonic eddy activity; rather, it can be attributed to the common remote forcing from the tropical Pacific associated with the El Niño–Southern Oscillation and the Pacific Decadal Oscillation. In the west central SSIO, wind stress curl just south of the eddy band forces potential vorticity anomalies that affect conditions for instability in the west central SSIO; potential density and potential vorticity gradient anomalies also suggest a remote forcing mechanism originating in the region southwest of Australia. The interannual to multidecadal variability of EKE in the SSIO and its relationship with large‐scale SLA has implications for mixed‐layer dynamics and biogeochemistry and provides a basis for assessment of model simulations of eddy activity in the region.

Lagrangian eddy kinetic energy of ocean mesoscale eddies and its application to the Northwestern Pacific

“…Therefore, EKE is not an appropriate metric to characterize the KE of coherent mesoscale eddies. Generally, the signals of coherent mesoscale eddies could be obtained by applying spatial and/or temporal filters onto the SSHA or EKE field 23 – 25 . To properly do this, the typical spatial–temporal scales of mesoscale eddies (i.e., mean eddy size and lifetime) need to be identified in advance 25 .…”

“…Generally, the signals of coherent mesoscale eddies could be obtained by applying spatial and/or temporal filters onto the SSHA or EKE field 23 – 25 . To properly do this, the typical spatial–temporal scales of mesoscale eddies (i.e., mean eddy size and lifetime) need to be identified in advance 25 . Since these scales are subject to specific geographical domain 25 , it would be difficult to apply a universal scale in the analysis focusing on coherent mesoscale eddies in the basin-wide or global ocean.…”

“…To properly do this, the typical spatial–temporal scales of mesoscale eddies (i.e., mean eddy size and lifetime) need to be identified in advance 25 . Since these scales are subject to specific geographical domain 25 , it would be difficult to apply a universal scale in the analysis focusing on coherent mesoscale eddies in the basin-wide or global ocean. Moreover, coherent mesoscale eddies are tracked in Lagrangian framework while the EKE is commonly calculated on the fixed grid boxes in Eulerian framework.…”

“…However, the usage of the gridded EKE is incapable of directly distinguishing the KE of eddies with different polarities. Although some studies attempted to isolate the KE of CEs from that of AEs based on the signs of corresponding SSHA 25 , the problem behind this attempt is that a negative (positive) SSHA does not always represent a CE (an AE) and cannot provide the factual information about the mesoscale coherent structures. Therefore, to probe into the differences between CEs and AEs, it is important to take the eddy polarity into account when defining the KE of coherent mesoscale eddies.…”

Spatiotemporal Variations of Mesoscale Eddies in the Southeast Indian Ocean

“…On the interannual scale, by examining long‐lived anticyclonic eddies, Fang and Morrow (2003) suggest that more warm eddies tend to be shed during anomalously warm conditions such as 1996 and 1999, with fewer in anomalously cold years such as 1997. By using long‐term satellite observations, it has been suggested that the EKE in the LC system is closely related to ENSO, with high values in La Niña years (Delman et al, 2018; Zheng et al, 2018).…”