Effect of topography on the cold water region in the east entrance area of Qiongzhou Strait

A cold water region will occur in the east entrance area of Qiongzhou Strait when upwelling develops off the northeast coast of Hainan Island. The effect of topography on this cold water region was investigated by combining satellite observations and numerical simulations. Multi-source high-resolution remote-sensed sea surface temperature images revealed that the western periphery of the cold water region frequently occurred at a quasi-fixed location with similar jagged structure. Compared with observations, the numerical model employed in this study could satisfactorily simulate the intense tidal currents in the strait, and reasonably reproduce the jagged western periphery of the cold water region. Under strong westward tidal flow, the cold upwelling water intruded deeply into the interior strait, whereas under sustained eastward tidal currents, much stronger tidal currents occurred where there were deeper waters connecting the inner strait, therefore the cold upwelling water faded towards east faster and formed the concave parts (looking in east-to-west direction) of the western periphery of the cold water region; conversely, convex parts would develop over the adjacent shallower shoals. The eastward movement of the cold water was dominated by topography-modified tidal currents, and only after sustained eastward tidal currents (usually in the second half or near the end of the eastward tidal current phase), the western periphery of the cold water region would occur at a quasi-fixed location with similar jagged shape. Moreover, the development of the upwelling off the northeast coast of Hainan Island, which was the precondition of the cold water region, was found to be mainly driven by sustained SE ~ S winds.