Cross‐shelf penetrating fronts: A response of buoyant coastal water to ambient pycnocline undulation

Offshore‐penetrating tongues of coastal water have been frequently observed during the downwelling‐favorable monsoon season at specific locations in waters off the Min‐Zhe Coast, a region influenced by a buoyant coastal current originating from the Changjiang River. This process plays an important role in cross‐shelf material exchange in the East China Sea (ECS), but the underlying mechanisms are not fully understood. This study suggests that the penetrating fronts are the response of buoyant coastal water to along‐isobath undulation of the ambient pycnocline that is controlled by the temperature stratification in seawater. When the ambient pycnocline descends sharply in the downshelf direction, coastal water is transported offshore due to the joint effect of baroclinicity and relief (JEBAR), and thus generates a penetrating front. Along‐isobath pycnocline undulation in the ECS can arise from nonuniform tidal mixing due to tidal wave divergence off the Min‐Zhe Coast. Onshelf intrusion of cold and dense Kuroshio subsurface water prevents thorough mixing of the pycnocline. Different from the common cross‐shelf transport phenomena induced by winds or frontal instabilities, such a tidal mechanism should produce penetrating fronts at specific locations, in agreement with observations.