tailieunhanh - .Ocean Modelling for Beginners Phần 9

Chẳng hạn nước của Địa Trung Hải bị bốc hơi rất mạnh, ít sông suối đổ vào, do đó nước có độ mặn cao và có tỉ trọng lớn. Nước ở dưới sâu chảy từ Địa Trung Hải ra Đại Tây Dương tạo ra sự thiếu hụt, vì thế một hải lưu bề mặt lại chảy từ Đại Tây Dương vào Địa Trung Hải để bù vào chỗ thiếu hụt đó. | The Wind-Driven Circulation of the Ocean 141 The Sverdrup Balance On the p plane the divergence of lateral volume transport inherent with geostrophic fl w is given by d Qgeo d Qg d g dy - Qgeo On the large scale this divergence or convergence of the geostrophic f ow balances the convergence or divergence of the drift in the surface Ekman layer. Effects associated with bottom friction are irrelevant here. Under the assumption of purely zonal wind r d 0 the steady-state balance leading to an equilibrium sea-level distribution is given by p Q r 1 9rgvlnd Po dy This balance is called the Sverdrup relation Sverdrup 1947 and allows for calculation of the meridional geostrophic volume transport also called Sverdrup transport from knowledge of the average zonal wind-stress distribution. The corresponding zonal volume transport can be estimated from d Qgeo . d QS -Q 0 d g dy where the p effect can be ignored since this equation is used for diagnostic purposes only. It is important to note that the Sverdrup balance can only establish with existence of a meridional boundary. The dynamics of unbounded fl ws such as the Antarctic Circumpolar Current is more complex. Interpretation of the Sverdrup Relation First and foremost the Sverdrup relation implies that latitudes of vanishing windstress curl 9rgvind dy 0 coincide with regions of vanishing meridional geostrophic fow. Hence these regions form natural boundaries that for instance separate subtropical from subpolar gyres in the ocean. In the midlatitude ocean of the northern hemisphere the main wind pattern consists of trades to the south and westerlies to the north. This wind pattern provides 9rgvind 9y 0 and produces a convergence of volume transport in the surface Ekman layer. Hence equatorward Sverdrup transport is required to balance this convergence. Since no geostrophic f ow is possible across the natural boundaries marked by the maximum trade winds and the maximum westerlies this .