148 / 2023-08-20 10:37:56

Seasonality of submesoscale processes-induced vertical heat transport modulated by oceanic mesoscale eddies in the Kuroshio Extension

submesoscale,vertical heat transport,mesoscale eddy

Oceanic submesoscale processes mainly arise from phenomena such as jets, density fronts, and mesoscale eddies. Previous studies suggest that the overall submesoscale processes transport heat vertically upgradient, i.e. from cold to warm. However, it is not clear whether the submesoscale processes-induced vertical heat transport (VHT) in mesoscale eddies remains upgradient. Therefore, the present study focuses on submesoscale-induced VHT modulated by mesoscale eddies. A high-resolution oceanic numerical model product is applied to examine the seasonal variations of the VHT induced by submesoscale processes associated with four cyclonic and nine anticyclonic mesoscale eddies in the Kuroshio Extension (KE) region. The frequency-wavenumber spectra and Rossby numbers in the eddies reveal the existence of submesoscale motions surrounding mesoscale eddies. The variables are decomposed into monthly mean mesoscale and submesoscale components to calculate submesoscale VHT at low-frequency (LF) and high-frequency (HF). The results indicate that submesoscale VHT is modulated by mesoscale eddies mainly along the eddy boundaries. The LF submesoscale VHT (upward in both cyclonic and anticyclonic eddies) dominates the seasonally-averaged submesoscale VHT. The magnitude of the LF submesoscale VHT in winter is stronger than in other seasons. The magnitude of the hourly HF submesoscale VHT is larger than the LF component, while its monthly average becomes negligible because it is caused by periodical internal waves. The energy analysis of LF submesoscale processes demonstrates a seasonal variation in submesoscale kinetic energy. Specifically, the submesoscale kinetic energy increases during winter, primarily due to the contribution of the baroclinic term. Conversely, in summer, the submesoscale kinetic energy decreases, primarily due to the contribution of the barotropic term. The baroclinic term make positive contributions to the seasonally-averaged energy budget in all seasons , and its vertical distribution is consistent with the LF submesoscale VHT, indicating that baroclinic processes are the primary mechanism for the submesoscale phenomena. This phenomenon can affect the deeper interior of the ocean beyond the mixed layer depth. The submesoscale VHT along the eddy edges is influenced by the mixed layer baroclinic instability, strain-induced frontogenesis and turbulent thermal-wind-induced frontogenesis within the upper mixed layer, while it is associated with the strain-induced frontogenesis in the ocean interior.