Two Key Mechanisms of Large-Scale Cross-Shelf Penetrating Fronts in the East China Sea: Flow Convergence and Thermocline Undulation

Cross-shelf penetrating fronts (CPFs) induce significant cross-shelf exchange of water properties and nutrients, and thus are important to coastal environments. In this study, the characteristics and mechanisms of realistic large-scale CPFs in the East China Sea in summer were investigated based on a data assimilative model. The model reproduced CPFs matched well with satellite observations. Although the cross-shelf currents were predominantly offshore off the Zhe-Min Coast, only three strong large-scale CPFs occurred in the summer of 2014. The three-dimensional structure of CPF in the model was similar with that observed in previous research. Two different mechanisms were responsible for the formation of observed CPFs. Two CPFs formed as a result of the convergence of the Taiwan Warm Current (TWC) and the Zhe-Min Coastal Current (ZMCC), while the other one was caused by the undulation of thermocline. Heat budget analysis suggests that the undulation of thermocline was caused by horizontal and vertical advection. Sensitivity experiments suggest that southerly wind relaxation and tidal forcing are indispensable conditions for CPF formation. Tidal forcing makes the axis of the ZMCC shift offshore by ∼50 km, so that the ZMCC could impinge right against the axis of the TWC. The relaxation of the southerly winds allows the ZMCC to extend southward. Southerly wind relaxation in summer is mostly associated with tropical cyclones. Without winds and synoptic variation of the TWC, CPFs form periodically due to the strengthening of the ZMCC during neap tide period.