A cross-shelf penetrating front and its associated biogeochemical-ecological responses: a case study on the East China Sea shelf in spring.

Front is among the most important mesoscale/submesoscale processes in the global oceans. The East China Sea (ECS) shelf is well known as the hotspot of a typical cross-shelf penetrating front (CPF). However, there is quite a limitation in studies on the biogeochemical-ecological responses to CPFs based on interdisciplinary perspectives and field observations due to the sparse data. By combining an in-situ investigation conducted during April 2021 and related satellite-derived data, this study identified a spring CPF on the ECS shelf, and explored its associated biogeochemical-ecological responses. A noticeable tongue-shaped CPF with low salinity and temperature inversion occurring in the upper layers penetrated southeastward long distance from the Zhejiang coast to the middle ECS shelf. It was formed mainly by the combined effects of convergence/collision of the Zhe-Min Coastal Current and Taiwan Warm Current, along-shelf and cross-shelf halocline/pycnocline undulations, the upwelling associated with the onshore intruded Kuroshio Subsurface Water, and the anticyclonic-like structure. The CPF could act as an important channel for the seaward transport of nutrients from the eutrophic coastal area. The CPF-driven nutrient transport played a vital role in nourishing primary production, leading to an offshore isolated phytoplankton bloom event; meanwhile, the stratification and anticyclonic-like structure within the offshore CPF-dominated region might also contribute to phytoplankton bloom. Our findings highlight the potential importance of CPF for regional marine ecosystem sustainability, carbon cycling and bottom oxygen depletion on the ECS shelf, and will greatly contribute to an understanding of the physical-biogeochemical-ecological interactions in the CPF-dominated region.