Eddy characteristics in the South Indian Ocean as inferred from surface drifters

S Zheng,J Li,Y Du,X Cheng

doi:10.5194/os-11-361-2015

Abstract

Abstract. Using a geometric eddy identification method, cyclonic and anticyclonic eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have been statistically investigated based on 2082 surface drifters from 1979 to 2013. A total of 19 252 eddies are identified, 60% of them anticyclonic eddies. For the submesoscale eddies (radius r<10 km), the ratio of cyclonic eddies (3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, the number of anticyclonic and cyclonic eddies with radius r≥10 km is almost equal. Mesoscale and submesoscale eddies show different spatial distributions. Eddies with radius r≥100 km mainly appear in the Leeuwin Current, a band along 25° S, Mozambique Channel, and Agulhas Current, areas characterized by large eddy kinetic energy. The submesoscale anticyclonic eddies are densely distributed in the subtropical basin in the central SIO. The number of mesoscale eddies shows statistically significant seasonal variability, reaching a maximum in October and minimum in February.

Highlights

The South Indian Ocean (SIO) has unique current systems
The results show that the mean eddy kinetic energy (EKE) of large eddies at the eastern boundary, in a band along 25◦ S and at the western boundary, is larger than for other groups, but this comparison is reversed in the interior ocean
The submesoscale eddies are densely distributed over the entire SIO (Fig. 9d), with a similar pattern to that of total eddies (Fig. 7a)

Summary

Introduction

Mesoscale eddies in the SIO have been studied using satellite data (e.g., Palastanga et al, 2006; Chelton et al, 2011), ocean modeling (e.g., Backeberg et al, 2008) and in situ observations (e.g., de Ruijter et al, 2004; Ridderinkhof et al, 2010). In austral spring, enhanced heat flux forcing of combined meridional Ekman and geostrophic convergence strengthens the upperocean meridional temperature gradient and intensifies the vertical velocity shear This modulation of the vertical velocity shear changes the intensity of baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and underlying SEC system, leading to the seasonal variations of EKE (Jia et al, 2011).

Data and methods

Number and radius

Spatial distribution

Temporal variations

Findings

Discussion and conclusions