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Seasonal Modulation of Eddy Kinetic Energy and Its Formation Mechanism in the Southeast Indian Ocean

Fan JiaPhysical Oceanography Laboratory, Ocean University of China, Qingdao, China

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Lixin WuPhysical Oceanography Laboratory, Ocean University of China, Qingdao, China

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Bo QiuDepartment of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

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Abstract

Mesoscale eddy activity in the southeast Indian Ocean (15°–30°S, 60°–110°E) is investigated based on available satellite altimetry observations. The observed sea level anomaly data show that this region is the only eastern basin among the global oceans where strong eddy activity exists. Furthermore, the eddy kinetic energy (EKE) level in this region displays a distinct seasonal cycle with the maximum in austral summer and minimum in austral winter. It is found that this seasonal modulation of EKE is mediated by baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equatorial Current (SEC) system. In austral spring and summer the enhanced flux forcing of combined meridional Ekman and geostrophic convergence strengthens the upper-ocean meridional temperature gradient, intensifying the SICC front and its vertical velocity shear. Modulation of the vertical velocity shear results in the seasonal changes in the strength of baroclinic instability, leading to the seasonal EKE variations in the southeast Indian Ocean.

Corresponding author address: Lixin Wu, Physical Oceanography Laboratory, Ocean University of China, 238 Songling Road, Qingdao 266100, China. E-mail: lxwu@ouc.edu.cn

Abstract

Mesoscale eddy activity in the southeast Indian Ocean (15°–30°S, 60°–110°E) is investigated based on available satellite altimetry observations. The observed sea level anomaly data show that this region is the only eastern basin among the global oceans where strong eddy activity exists. Furthermore, the eddy kinetic energy (EKE) level in this region displays a distinct seasonal cycle with the maximum in austral summer and minimum in austral winter. It is found that this seasonal modulation of EKE is mediated by baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equatorial Current (SEC) system. In austral spring and summer the enhanced flux forcing of combined meridional Ekman and geostrophic convergence strengthens the upper-ocean meridional temperature gradient, intensifying the SICC front and its vertical velocity shear. Modulation of the vertical velocity shear results in the seasonal changes in the strength of baroclinic instability, leading to the seasonal EKE variations in the southeast Indian Ocean.

Corresponding author address: Lixin Wu, Physical Oceanography Laboratory, Ocean University of China, 238 Songling Road, Qingdao 266100, China. E-mail: lxwu@ouc.edu.cn
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