How Ocean Vertical Mixing and Accumulation of Warm Surface Water Influence the “Sharpness” of the Equatorial Thermocline

Abstract

This work utilizes an equatorial Pacific domain ocean general circulation model that achieves a sharp equatorial thermocline. The model has been developed for simulation of the El Nino-Southern Oscillation phenomenon and employs an integer power (IP) vertical mixing scheme formulation that is based on the observations of Peters, Gregg, and Toole. Model experiments are used to investigate the influence of warm surface water accumulation upon vertical mixing scheme performance. Straightforward techniques, applicable to any model, are used to diagnose the warm surface water accumulation. It is shown that if warm surface water is allowed to accumulate it can prevent the IP vertical mixing scheme from maintaining a sharp thermocline. Experiments reveal that, in the present model, the accumulation of the warm surface water occurs as a consequence of the model's zonal walls and their sponge layers. A parameterized wall heat transport scheme is then used to prevent the accumulation of warm surface water and this allows the IP scheme to maintain a sharp thermocline, but not the Pacanowski and Philander vertical mixing scheme. Results from a literature survey suggest that the problem of warm surface water accumulation may not be uncommon in other tropical ocean models with zonal walls and sponge layers, and that this may be one of the causes of the long-standing diffuse thermocline problem in these models. It is concluded that to achieve a sharp equatorial thermocline it is necessary for an ocean model to maintain realistic meridional heat transports and so correctly regulate the amount of warm surface water in the tropical ocean, and it is demonstrated that problems with vertical mixing and warm surface water accumulation can look confusingly similar. The ability of warm surface water accumulation to prevent a vertical mixing scheme from achieving a sharp thermocline may have influenced previous assessments of vertical mixing schemes.