Simulation of Deep Cycle Turbulence by an Ocean General Circulation Model

Deep Cycle (DC) turbulence is a diurnally oscillating turbulence that penetrates below the mixed layer, extending to a depth several times deeper than the mixed layer depth. It is generally found in a stratified shear layer above the Equatorial Undercurrent (EUC). Such enhanced turbulence layers have been often observed in the equatorial Pacific and Atlantic cold tongue where they could potentially produce large turbulent heat flux. Previous studies of DC turbulence are based on in-situ observations or Large Eddy Simulations (LES), which are either at a single location or within a limited domain. For the first time, we present here the simulation of DC turbulence by a large-scale ocean general circulation model (OGCM), in which spatial and temporal variability of DC turbulence could be fully described. The OGCM is forced with hourly surface fluxes, and the simulation is performed in October-November 2011. A second-moment turbulence closure scheme completed by means of k-ε model is used to parametrize the vertical mixing in the OGCM. The characteristics of DC turbulence in the OGCM simulation agree very well with observations. Results suggest that the vertical shear associated with the EUC primarily control the spatial distribution and the horizontal and vertical extent of the DC turbulence. The magnitude and depth of DC turbulence are also significantly influenced by the zonal wind stress. Furthermore, it is demonstrated that tropical instability waves impact the structure and strength of DC turbulence.