Winter counter-wind current in western Taiwan Strait: characteristics and mechanisms

Abstract

A new dataset based on in-situ observations and a numerical model are employed to reveal the spatio-temporal variation and driving mechanism of counter-wind current (CWC) in the western Taiwan Strait (TWS) during the winter monsoon. Our results confirm the existence of the CWC in the western TWS during the months of strong northeasterly wind, which is well supported by a rarely observed through-flow pattern recorded by a Lagrangian drifter, during which the average wind speed was 11 m/s. Moreover, both the synchronous ADCP observations and model results demonstrate that the CWC acted as an episodic local event when the winter monsoon went into transient relaxation. By examining the model results, the momentum budget and the time-dependent dynamics, we further revealed that the wind relaxation mechanism is crucial in explaining the existence of the CWC, which acted as a short-lived local event in the form of a return current that lied in the downstream of a significant along-strait sea-level set-up in the northern Taiwan Bank when the prevailing strong wind experienced a short transient relaxation. Last but not least, the large-scale pressure gradient related to the open-ocean forcing was found to play a predominant role in driving a stable CWC in the frame of geostrophy when the winter-monsoon wind gradually relaxed.