Baroclinic Sea Level.

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. Plain Language Summary: Sea‐level variations at tidal frequencies occur because of the "astronomical" gravitational force acting nearly uniformly over the oceanic depth. Thus, they are commonly associated with the depth‐averaged velocity, that is, the barotropic current. However, due to oceanic density stratification and variable bathymetry, the resulting barotropic currents also generate vertically varying (baroclinic) tidal currents that also have an expression in the sea level and surface pressure‐gradient force. A prescription is given for how to decompose sea level and its gradient into its barotropic and baroclinic parts for both tides and currents, and the answers are illustrated using a tide‐resolving simulation of the Pacific Ocean. Key Points: Sea level, measured relative to a geopotential iso‐surface, is the surface dynamic pressure for the oceanic momentum balanceBarotropic and baroclinic dynamics combine in determining the sea levelA decomposition of tidal sea level and pressure‐gradient force in a Pacific Ocean simulation shows barotropic‐baroclinic coupling [ABSTRACT FROM AUTHOR]