EOF Analysis for Wind-driven Along and Cross Shelf Exchange around a Sandbank over the Daesan Inner Shelf (Yellow Sea, Korea)

Abstract

It is essential to understand the shelf circulation dynamics qualitatively and, if possible quantitively, since the inner shelf is an important zone located between the surf zone and the continental shelf in terms of the heat exchange, the transport of terrigenous sediment, and so on. In this study, from the compound results of current velocity profiles measured by the moored ADCP and wind observations obtained by KMA (Korea Meteorological Administration) over the Daesan inner shelf located in Yellow Sea, Korea, we found that the inner shelf circulation was mainly driven by a combination of wind stress, pressure gradient, Coriolis force and submarine topography. The Empirical Orthogonal Function (EOF) analysis was used to comprehend the shelf circulation dynamics induced by the wind stress and the wavelet coherence was used to analyze the time frequency distribution of the coherence between currents, which was reconstructed by EOF method and shelf aligned winds. The thickness of the surface and bottom frictional layers mainly depended on wind forcing and seabed topography. Also, it was found that the subtidal currents were strongly dominated along-shelf due to the irregular seabed topography such as a sandbank. The vertical structure of the along-shelf currents was mostly uniform regardless of water depth change. In mode 2 of EOF analysis, it was shown that the cross-shelf currents were polarized into two layers. Mostly wind in the ocean may cause down-welling or up-welling along the direction, however, in the study area, variation of wind induced current and the Ekman layer are not clearly distinguishable. Additionally, it is resulted that the cross-shelf exchange caused by the wind was not significant because of complicated seabed topography including Jangan sandbank and tidal force even though the Ekman depth was changed by the wind forcing in the Daesan inner shelf. From a result of wavelet coherence analysis, the correlation between wind and currents was also insignificant according to shelf-aligned U, V directions and water depths.