Wintertime Submesoscale Offshore Events Overcoming Wind-Driven Onshore Currents in the East China Sea

Coastal offshore currents transport terrestrial materials from coastal areas to continental shelf areas in the upper layer; these currents are regarded as unimportant in winter when wind-driven onshore currents are dominant. In this paper, both long-term observations and high-resolution simulations in the East China Sea reveal unique wintertime offshore currents in the upper layer that are widespread in the 30-70-m isobath area and even overcome the wind-driven onshore currents at the 50-70-m isobaths. The mixed layer instabilities, frontogenesis and turbulent thermal wind, which constitute a mechanism in the generation of submesoscale offshore currents, have a growth rate that is fast enough to compete with the wind-driven dynamics when the winter monsoon is relatively weak (<10 m/s). The alternation of the weak-wind and strong-wind modes during the winter monsoons provides a general condition for the repeated occurrence of remarkable submesoscale cross-shore transport and carbon sequestration in coastal seas.

Automated summary

This study reveals the existence of unique wintertime offshore currents in the upper layer of the East China Sea, which are widespread in the 30-70-m isobath area and even overcome the wind-driven onshore currents at the 50-70-m isobaths. The mechanisms behind the generation of submesoscale offshore currents, such as mixed layer instabilities, frontogenesis and turbulent thermal wind, have a growth rate fast enough to compete with the wind-driven dynamics during relatively weak winter monsoons (<10 m/s).