Response of the Upper Ocean Temperature of the Equatorial Pacific Ocean to Vertically Varying Thermocline Mixing

The equatorial thermocline is a critical regime that determines the strength and pattern of the global ocean overturning circulation and the large-scale heat budget. Yet, many numerical models demonstrate large, systematic biases in this regime, particularly in the Pacific ocean. Among many factors that may affect the thermocline processes and therefore the tropical ocean circulations, the vertical mixing has get the least attention. The spatial pattern of the vertical mixing, its physical relation to the tropical ocean states, and its effects on the temperature structure at variable time scales are not well understood. Using results of an adjoint (4D-Var) synthesis that is aiming to optimize the mixing parameters of an OGCM in order for the simulation to best match the observed temperature, salinity, sea surface height, etc., we found an apparent vertical structure of the background vertical diffusivity in the upper equatorial Pacific: two layers of strong diffusivity just above and slightly below the thermocline center respectively, and a thin layer in-between them with weak diffusivity. This “tripole” pattern matches some limited observations in both the eastern and western equatorial Pacific. Using long term mooring observations, we demonstrated that the enhanced mixing in the two layers is related to strong shears and low Richardson numbers. The vertically varying diffusivity substantially impacts the upper ocean temperature structure: it tends to result in local cooling of the upper ocean, shoaling and weakening of the thermocline, and remote upper ocean temperature change (either warming or cooling) in both the western and eastern equatorial Pacific via advection and propagation of planetary waves.