Characteristics and Dynamics of the Interannual Eddy Kinetic Energy Variation in the Central Pacific Sector of the Southern Ocean

Abstract

Mesoscale eddies play a crucial role in the dynamical balance of the Southern Ocean (SO) circulation. Yet, it remains unclear why the strength of the SO eddy field has significant variations on interannual time scales, and to what extent these low-frequency variations are attributed to wind forcing changes. Here we use a functional analysis tool, namely, the multiscale window transform (MWT), and the MWT-based theory of canonical transfer and a time-dependent energetics framework to investigate these issues, with a focus on the central Pacific sector of the SO in which both time-mean and interannual variability of eddy kinetic energy (EKE) maximize. It is found that wind stress contributes to the interannual EKE variability in three different ways. The baroclinic pathway, in which the mean kinetic energy (MKE) is generated by the wind, converted to the mean available potential energy (APE), and is further released to eddy APE and finally to EKE through baroclinic instability, is the dominant one. The time lag of the response of EKE to the wind power injection is about 2.4 years. In contrast, the barotropic pathway, in which MKE directly fuels EKE through barotropic instability, is much faster but secondary, and its influence is only concentrated along the mean jet west of 140. The wind stress is also found to directly energize the eddy field, resulting in an almost simultaneous EKE response on interannual time scales. However, this direct pathway is much smaller in magnitude compared to the baroclinic and barotropic pathways.

Strong Eddy Kinetic Energy Anomalies Induced by Baroclinic Instability in the Southwest Region of the Kerguelen Plateau, East Antarctica

“…In the present study, the 1/12° Global Ocean Reanalysis and Simulation Data (GLORYS12V1, https://doi.org/10.48670/moi-00021) acquired from Copernicus Marine Environment Monitoring Service (CMEMS) is employed to investigate the time‐varying energetics, limited by the scarcity of in‐situ observation in polar regions. The GLORYS12V1 is a global ocean eddy‐resolving reanalysis product with a horizontal resolution of 1/12° × 1/12° and 50 vertical levels, spanning the altimetry era from 1993 onward, which has been used to analyze energetics in the Southern Ocean (G. Fu et al., 2023). Utilized three‐dimensional monthly and daily variables including sea surface height (SSH), current velocity, potential temperature, and salinity across all vertical levels from 1993 to 2020.…”

“…The Lorenz energy cycle (Lorenz, 1955) is an effective approach to investigating the variability of a current system and quantitatively describing energy transfer among reservoirs, providing insights into the underlying ocean dynamics (e.g., Dong et al., 2022; Ivchenko et al., 1997; Kang & Curchitser, 2015; Rieck et al., 2015; von Storch et al., 2012). It has been extensively employed in analyzing energetics in the Southern Ocean (e.g., Chen et al., 2014, 2016; G. Fu et al., 2023; Youngs et al., 2017). Kang and Curchitser (2015) developed a systematic framework for energetics analysis to examine the energetics of coupled eddy‐mean flow systems.…”

“…The results demonstrated that the westerlies can locally supply sufficient energy to initiate baroclinic instability in the Indian and Pacific sectors of the ACC. In the central Pacific sector, the interannual variation of EKE in the upper ocean layer has revealed that wind stress impacts the variation through both baroclinic and barotropic pathways (G. Fu et al., 2023). Despite having a high level of eddy activity in the Indian sector, the energetics dynamics and EKE modulations in the confluence region between the ACC and subpolar gyre remain poorly explored, and the understanding of physical processes governing eddy energy in this subpolar region remains rather limited.…”