Trong thực tế, ngoài một số trường hợp như nghiên cứu chuyển động của sóng bề mặt hay chuyển động biến dạng của phần tử chất lỏng, người ta ít quan tâm đến chuyển động riêng biệt của từng phần tử chất lỏng, hơn nữa nó dẫn đến những tính toán phức tạp nên trong thủy lực học, phương pháp Lagrange được dùng ít hơn phương pháp Euler. | 232 Boundary-Layer and Wind-Gyre Dynamics z h zatm. W y alm. r T . x xatm. X atm. T cry av t s 4 lx s y _w__w_y V__y z 0 T Woce. - fX ì T oce. r _ oce. ----------- ----------- z - h oce. Fig. . Sketch of the boundary-layer depths hatm. and hoce horizontal transports Tat . and Toce Ekman pumping watm and woce and interior ageostrophic flows curved arrows for an atmospheric cyclone uatm over the ocean northern hemisphere . The interior and boundary-layer velocity component perpendicular to the plotted cross-sectional plane are indicated by a dot or cross within a circle flow out of or into the plane respectively . shear not the interior geostrophic shear even when the latter is baroclinic in a stratified ocean. Coastal Upwelling and Downwelling Although the climatological winds over the ocean are primarily zonal Sec. there are some lo Planetary Boundary Layer 233 cations where they are more meridional and parallel to the continental coastline. This happens for the dominant extra-tropical marine standing eddies in the atmosphere anticyclonic subtropical highs and cyclonic subpolar lows referring to their surface pressure extrema . On the eastern side of these atmospheric eddies and adjacent to the eastern boundary of the underlying oceanic basins the surface winds are mostly equatorward in the subtropics and poleward in the subpolar zone. The associated surface Ekman transports are offshore and onshore respectively. Since the alongshore scale of the wind is quite large 1000s km and the normal component of current must vanish at the shoreline incompressible mass balance requires that the water come up from below or go downward near the coast within 10s km. This circulation pattern is called coastal upwelling or downwelling. It is a prominent feature in the oceanic general circulation Figs. . It also has the important biogeochemical consequence of fueling high plankton productivity upwelling brings chemical nutrients . nitrate to the .
