Ekman layers and two‐dimensional frontogenesis in the upper ocean
Ekman layers and two‐dimensional frontogenesis in the upper ocean
复制标题
上层海洋的埃克曼层和二维锋生
DOI：
10.1029/1999jc900336
复制
发表时间：
2000
影响因子：
--
通讯作者：
L. Thompson
中科院分区：
文献类型：
--
作者：
L. Thompson
本文用一个二维改进的半地转模式研究了垂直混合作用下海洋锋的演变。用于混合的参数化是一个升高的垂直粘度和摩擦的恒定值，因此Ekman层作为一个替代混合层，然而，垂直剪切允许无处不在。初始条件代表的观测场在锋面海气相互作用实验（FASINEX）中使用，并在单独的垂直混合的存在下，其修改进行了研究。在没有外部强迫的情况下，由于地面存在垂直的地转切变，因此会产生埃克曼层。最大密度梯度在这种流动的驱动下向前缘的稠密侧移动。埃克曼流动的收敛导致锋面表面下的等密度线向下弯曲，这一特征让人想起FASINEX期间拍摄的密度剖面。当正压辐合场在没有混合的情况下或在斜压不稳定锋演变的完全非线性模拟中应用时，这一特征并不明显。一个分析理论表明，最大密度梯度将随着时间的推移，当垂直混合单独存在。在与地面地转流方向相反的负均匀风应力的存在下，锋向密度较大的水移动，急流强度均匀减小。在地面地转流方向上存在正的均匀风时，随着风驱动的Ekman流与非风驱动的Ekman流的对抗以及横锋密度梯度的增加，地面急流开始减弱，然后再次加强。
A two-dimensional modified semi geostrophic model is used to study the evolution of oceanic fronts in the presence of vertical mixing. The parameterization used for mixing is an elevated constant value of vertical viscosity and friction; hence the Ekman layer acts as a surrogate mixed layer, with, however, vertical shear allowed everywhere. An initial condition representative of the observed fields in Frontal Air-Sea Interaction Experiment (FASINEX) is used, and its modification in the presence of vertical mixing alone is investigated. Without external forcing an Ekman layer results because of the presence of vertical geostrophic shear at the surface. The maximum density gradient moves toward the dense side of the front driven by this flow. Convergence of Ekman flow results in downward bowing of the isopycnals beneath the surface expression of the front, a feature reminiscent of density sections taken during FASINEX. This feature is not evident when a barotropic convergence field is applied in the absence of mixing or in fully nonlinear simulations of the evolution of fronts with baroclinic instability. An analytic theory suggests that the maximum density gradient will increase over time when vertical mixing alone is present. In the presence of negative uniform wind stress in the direction opposite to the surface geostrophic flow the front moves toward the denser water, and the jet uniformly decreases in strength. In the presence of a positive uniform wind in the direction of the surface geostrophic flow the surface jet initially weakens but then strengthens again as the wind-driven Ekman flow opposes the frictionally driven Ekman flow and the cross-front density gradient increases.