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

Abstract

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.