Seasonal variations of eddy kinetic energy flux in the South Indian Countercurrent region
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Seasonal variations of eddy kinetic energy flux in the South Indian Countercurrent region* CHEN Zhongqian1, 2, WANG Faming1, 2, 3, 4, **, ZHENG Jian1, 3, 4, YANG Yuxing1, 3, 4 1
Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Function Laboratory for Ocean Dynamics and Climate, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China
4
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
Received Mar. 6, 2020; accepted in principle Apr. 20, 2020; accepted for publication Jun. 15, 2020 © Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The inverse cascade flux of kinetic energy (KE) and its length scale in the South Indian Countercurrent (SICC) region were investigated using 24-year satellite altimeter daily data from 1993– 2016. The evolution of the eddy life cycle in the SICC was presented systemically. It was found that the arrest and inject scales of inverse cascade were within the baroclinic most unstable scale and observed energy-containing eddy scale. The seasonal cycle of the arrest scale, inject scale, and amplitude of the inverse cascade were compared with the eddy kinetic energy seasonal cycle, which revealed a 1.5-month lag of the eddy kinetic energy to the vertical shear of zonal velocity, implying the existence of nonlinear processes during the eddy growth phase. Meanwhile, the anisotropy of the inverse cascade indicated that kinetic energy might be transferred from meridional motions to zonal motions, which is probably caused by β effect. These results would be beneficial for a better understanding of the KE transfer processes in the SICC region. Keyword: mesoscale eddies; spectral kinetic energy flux; inverse cascade; South Indian Countercurrent (SICC)
1 INTRODUCTION Accumulated satellite altimeter data can be used to examine ocean mesoscale eddy evolution process and the kinetic energy route (Ferrari and Wunsch, 2010; Morrow and Le Traon, 2012). The sea surface height (SSH) variability in the altimeter shows the mesoscale eddy signals at a spatial scale of 50–200 km and temporal scale of 50–200 days (Robinson, 1983), which mainly reflects the movement of the first baroclinic mode (Wunsch, 1997). Particularly, high eddy activities represented by high SSH root mean square (RMS) variability band in Fig.1, are found in the South Indian Countercurrent (SICC) region between 18°S–30°S, which is also referred to as the Subtropical Southern Indian Ocean (SSIO) (Delman et al., 2018). The region shown in Fig.1 is shared by the SICCSouth Equatorial Current (SEC) system, which is baroclinically unstable due to strong vertical velocity
shear (Palastanga et al., 2007). Hence, eddy kinetic energy (EKE) and its seasonal to interannual variability are largely governed by the baroclinic instability of the region (Jia et al., 201
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