Sea level and its horizontal gradient are an expression of oceanic volume, heat content, and currents. Large-scale currents have historically been viewed as mostly “baroclinic,” and tides as “barotropic,” respectively, in the loose sense of being strongly related to the oceanic density distribution or not. In particular, the evolution of the barotropic velocity is influenced by a horizontal pressure-gradient force that depends on the gradient of a particular depth-weighting of the density field as well as on the gradient of the sea-surface elevation ζ; hence, even the tides must be viewed as a product of the coupled interaction of barotropic and baroclinic fields. The purpose of this note is to give dynamical precision to the distinction between barotropic and baroclinic contributions to ζ and the surface pressure-gradient force −g∇ζ, and, in the particular case of the tides, demonstrate their combined barotropic-baroclinic interactions with a realistically forced, high-resolution simulation of the Pacific Ocean circulation. While the different tidal sea-level contributions manifest a horizontal scale separation (e.g., more barotropic at larger scales; more baroclinic surface pressure-gradient force at smaller scales), there are cross-mode corrections in both at the level of tens of percent. The proposed barotropic-baroclinic decomposition is generally relevant to the sea-level expression of oceanic currents.

中文翻译：

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斜压海平面

海平面及其水平梯度是海洋体积、热含量和洋流的表达。历史上，大尺度洋流大多被视为“斜压”，而潮汐则分别被视为“正压”，无论是否与海洋密度分布密切相关。特别是，正压速度的演变受到水平压力梯度力的影响，该压力梯度力取决于密度场的特定深度加权的梯度以及海面高程ζ的梯度；因此，甚至潮汐也必须被视为正压场和斜压场耦合相互作用的产物。本文的目的是从动力学上精确区分正压和斜压对ze和表面压力梯度力 − g ∇ ze的贡献，并在潮汐的特殊情况下，证明它们与正压-斜压的综合相互作用对太平洋环流进行真实强制的高分辨率模拟。虽然不同的潮汐海平面贡献表现出水平尺度分离（例如，较大尺度上更具正压性；较小尺度上更多斜压表面压力梯度力），但两者都存在百分之几十水平的交叉模式校正。所提出的正压-斜压分解通常与洋流的海平面表达相关。