Bo Qiu

The Kuroshio south of Japan is known to vacillate between a straight and a large meander (LM) path. Since 1950, eight LM events have been observed with different durations. The most recent/on‐going LM started in August 2017 and has become the longest event in record. By analyzing eddy‐resolving sea surface height data and by adopting a wind‐forced linear vorticity model, we demonstrate that the on‐going LM is maintained by an exceptionally stable dynamic state of the Kuroshio Extension (KE) forced by wind stresses across the Pacific basin. The highly‐stable KE system not only minimizes westward eddy perturbations from disrupting the upstream Kuroshio path, its strengthened southern recirculation gyre further helps to anchor the Kuroshio across the Izu Ridge. By contrasting the on‐going event to the 2004–2005 event, we argue that the LM duration is more sensitive to the wind‐forced KE dynamic state than the eastward Kuroshio transport south of Japan.

Oceanic submesoscale processes (submesoscales) with O (1–10) km horizontal scale can generate strong vertical buoyancy flux (VBF) that significantly modulate upper‐ocean stratification. Because submesoscales cannot be resolved by the prevailing ocean models, their VBFs have to be properly parameterized in order to improve model performance. Here, based on theoretical scaling analysis, we propose a new parameterization of submesoscale VBF by simultaneously considering mixed‐layer baroclinic instability (MLI) and strain‐induced frontogenesis, which are two leading generation mechanisms of submesoscales that typically co‐occur in open ocean. Compared with the parameterization of Fox‐Kemper et al. (2008, https://doi.org/10.1175/2007jpo3792.1 ; F08) that only considers the MLI, the new parameterization includes mesoscale strain rate and improves vertical structure function. Diagnostic validations based on submesoscale permitting simulation outputs suggest that the newly parameterized VBFs are more realistic than F08 in regard to three‐dimension distributions. How to incorporate this new parameterization into coarser‐grid ocean models, however, needs further studies.