Impact of a New Submesoscale Parameterization Scheme on the Simulation of Kuroshio Extension in a High-Resolution OGCM

Impact of a New Submesoscale Parameterization Scheme on the Simulation of Kuroshio Extension in a High-Resolution OGCM

Ziyi Zhang^1,2, Bo An¹, Zhiwei Zhang³ and Yongqiang Yu^1,2

• Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
• University of Chinese Academy of Sciences, Beijing 100049
• Ocean University of China, Qingdao 266100

ABSTRACT

    In this study, we investigate the impact of a new submesoscale parameterization scheme developed by Zhang et al. (2023; Zhang23 parameterization hereafter) on the simulation performance of the regional version of the 1/10° ocean model LICOM3.0 in the North Pacific. Specifically, a set of numerical experiments with and without Zhang23 parameterization scheme are conducted to study changes in thermal and dynamic characteristics in the Kuroshio Extension region resulting from the submesoscale processes. Compared to the control experiment, the deeper bias in the winter mixed layer depth(MLD) is significantly reduced in the sensitivity one with Zhang23 parameterization in the Kuroshio Extension region, by 26.7m (24.0%) on average. The simulated Kuroshio Extension shifts southward by about one degree of latitude from 36.5°N to 35.5 °N, closer to the observations.

    The simulated SST in the sensitivity run is much cooler than in the control run in the KE region. This contradicts the change in net surface heat flux, implying that the internal dynamical mechanism dominates these changes. Further analyses suggest that the vertical heat fluxes caused by parameterized submesocale processes are mainly concentrated at 39-40°N, 140-150°E in winter. This results in significant warming in the upper 100 m and cooling below, contributing to mixed layer restratification. The ocean heat content decreases in the region where the most energic mesoscale eddy exist in KE region, resulting in a weaker meridional thermal gradient at 37°N. This leads to a southward shift of the ocean front in the upper ocean and KE axis, which weakens meridional heat transport, and thus exacerbates the SST cooling north of 37°N. Although the vertical heat flux of the Zhang23 parameterization occurs primarily in winter, the cold SST anomalies in the subsurface are maintained throughout the year via the reemergence mechanism. The parameterization scheme directly affects winter temperatures by promoting upward heat transport, which warms the surface and cools the subsurface, leading to a southward shift and weakening of the WBC. In other seasons, it influences temperatures via current changes, maintaining a cooling anomaly in the north surface and subsurface layers. Overall, this results in a highly variable surface layer and a relatively stable cold subsurface layer. 

    This study demonstrates the good performance of Zhang23 submesoscale parameterization in improving the simulation of Kuroshio extension in the eddy-resolving OGCM. It reveals the key role of vertical heat transport by submesoscale processes and other oceanic dynamical mechanisms in modulating the sea temperature and its seasonal variation in the Kuroshio Extension region.