Formation and erosion of the seasonal thermocline in the Kuroshio Extension Recirculation Gyre

Abstract

a b s t r a c tData from the Kuroshio Extension Observatory (KEO) surface mooring are used to analyze the balance of processes affecting the upper ocean heat content and surface mixed layer temperature variations in the Recirculation Gyre (RG) south of the Kuroshio Extension (KE). Cold and dry air blowing across the KE and its warm RG during winter cause very large heat fluxes out of the ocean that result in the erosion of the seasonal thermocline in the RG. Some of this heat is replenished through horizontal heat advection, which may enable the seasonal thermocline to begin restratifying while the net surface heat flux is still acting to cool the upper ocean. Once the surface heat flux begins warming the ocean, restratification occurs rapidly due to the low thermal inertia of the shallow mixed layer depth. Enhanced diffusive mixing below the mixed layer tends to transfer some of the mixed layer heat downward, eroding and potentially modifying sequestered subtropical mode water and even the deeper waters of the main thermocline during winter. Diffusivity at the base of the mixed layer, estimated from the residual of the mixed layer temperature balance, is roughly 3 Â 10 À 4 m 2 /s during the summer and up to two orders of magnitude larger during winter. The enhanced diffusivities appear to be due to large inertial shear generated by wind events associated with winter storms and summer tropical cyclones. The diffusivity's seasonality is likely due to seasonal variations in stratification just below the mixed layer depth, which is large during the summer when the seasonal thermocline is fully developed and low during the winter when the mixed layer extends to the top of the thermocline.Published by Elsevier Ltd.

Estimating diffusivity from the mixed layer heat and salt balances in the North Pacific

“…The KEO mooring has been described by Cronin et al [2013] and so here we focus on details of the Station Papa mooring. Unlike the KEO mooring whose line length to water depth scope is 1.4, Papa was a taut line mooring with scope < 1.…”

“…As in Cronin et al [2013], horizontal diffusion and sheared-stratified flow convergence of heat within the mixed layer are assumed to be small and are neglected. We expect that these are much better assumptions at Station P than at KEO, where horizontal gradients can at times be large.…”

“…Following Cronin et al [2013] and Stevenson and Niiler [1983], the mixed layer heat budget can be expressed as:…”

“…Their sampling strategies are described in Cronin et al [2013] and errors are described in Kubota et al [2008]. These data are used to compute both the turbulent and radiative components of the net surface heat flux, Q 0 :…”

“…It is shown here that this provides a more direct relation to the diffusive flux. As with Cronin et al [2013], satellite-based sea surface temperature (SST) fields are used to estimate the effects of heat advection. In addition, at Station Papa where advection is generally weak, mixed layer salinity gradients could be estimated from glider butterfly patterns and Argo floats.…”