Simulation of Deep Cycle Turbulence by a Global Ocean General Circulation Model

Deep cycle turbulence (DCT) is a diurnally oscillating turbulence that penetrates into a stratified shear layer below the surface mixed layer, which is often observed in the eastern Pacific and Atlantic above the Equatorial Undercurrent (EUC). Here we present the simulation of DCT by a global ocean general circulation model (OGCM) for the first time. As the k-epsilon vertical mixing scheme is used in the OGCM, the simulation of observed DCT structure based on in situ microstructure measurements can be explicitly demonstrated. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Zonal and meridional variations of DCT in the entire equatorial Pacific and Atlantic are described through constructing the composite diurnal cycle. In the central Pacific where the maximum shear associated with EUC is deep, the separation of DCT from the surface mixed layer is much more prominent than other areas. Plain Language Summary Deep cycle turbulence (DCT) is a nighttime intensified turbulence that develops in the stratified layer below the base of the surface mixed layer. It is often observed below the equatorial Pacific and Atlantic cold tongue regions above the Equatorial Undercurrent (EUC). Mixing caused by DCT is essential in modulating sea surface temperature (SST), which could have a large impact on air-sea interaction and thus global climate variability. However, simulations of DCT in global ocean models have not been demonstrated so far, and the spatial variation of DCT characteristics in the entire equatorial oceans is not well known. This study presents the first global ocean general circulation model simulation of DCT, demonstrated by the comparison of simulated turbulence with that derived from in situ observations. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Large-scale spatial variability of DCT in the equatorial Pacific and Atlantic is described through the analysis of model output. The DCT layer completely separated from surface mixed layer is found at locations where the EUC is deep, such as the central equatorial Pacific near the dateline.