A three-dimensional mixed finite-difference Galerkin function model for the oceanic circulation in the Yellow Sea and the East China Sea

Abstract

A three-dimensional mixed-type model which uses a finite-difference approximation in the horizontal plane and function expansions in the vertical direction is developed for the simulation of oceanic flows in the Yellow Sea (YS) and the East China Sea (ECS). The model assumes a hydrostatic balance and solves the three-dimensional, non-linear free-surface, primitive equations for homogeneous fluids. To represent the velocity structure of oceanic currents, a set of linear interpolation functions is used from the sea surface to a depth of 300 m, characterizing the thickness of the oceanic flow, and a similarity function of the exponential type underneath. The vertical eddy viscosity takes a flow-related form in which the strong mixing due to the M-2 tide is incorporated as the background eddy viscosity. A radiation condition developed by Flather (1976. Memories de la Societe Royale des Science de Liege 10, 141-164) is employed along the open boundaries. A series of numerical experiments have been carried out using linear and quadratic bottom friction formulae. The coefficient of linear bottom friction was given by Hunter's formula (1975. Estarine and Coastal Marine Science 3, 473-475), taking into account that the oceanic flows in shelf seas are of secondary importance. The quadratic bottom friction coefficient was taken as 0.0025, the same value used in previous numerical experiments of oceanic circulation in the study area (for example, Lee, 1996. Ph.D. Thesis, Kyushu University). Both results are quite similar over the outer shelf region (Okinawa Trough and the shelf break west of Kyushu) in which tidal effects on the bottom friction are relatively small, and are qualitatively in good agreement with recent observations by ARGOS buoy tracking (Lie and Cho, 1997. The Journal of the Korean Society of Oceanography 32, 1-7; Lie et al., 1998. Journal of Geophysical Research 103, 2963-2976). A clear difference was, however, found in the distribution of sea surface elevation and horizontal currents in the YS and shelf sea regions of the ECS where tidal currents constitute the primary circulation. The linear bottom friction gives no evidence of deep penetration of oceanic currents into the YS, while the quadratic bottom friction produces weak but distinct large meanders in the wide shelf of the ECS, and flows entering into the southern YS, seemingly in the form of the Yellow Sea Warm Current (YSWC), which forms a basin-size clockwise gyre in the YS. The calculation with linear bottom friction supports the view based on hydrographic and drift data (Lie, 1985. Journal of the Oceanographical Society of Japan 41, 291-298; Kim et al., 1991. The Journal of the Oceanological Society of Korea 26, 262-277 (in Korean); Beardsley et al., 1992. Lamer 30, 297-314; Lie et al., 1998. Journal of Geophysical Research 103, 2963-2976) that the oceanic currents mostly turn around Cheju Island. Calculation with an inflow of 25.1 Sv through open boundaries at the both sides of Taiwan shows that 1.1 Sv goes through Korea/Tsushima Strait, 20.9 Sv through Tokara Strait and 2.7 Sv through open boundaries along the Ryukyu Islands. (C) 2000 Elsevier Science Ltd. All rights reserved.