Scaling wind stirring effects in an oceanic bulk mixed layer model with application to an OGCM of the tropical Pacific

Abstract

The mixed layer depth (MLD) plays an important role in the climate system through its influences on sea surface temperature (SST). The Kraus–Turner–Niiler (hereafter referred to as KTN) bulk mixed layer (ML) model is designed for describing the MLD and has been adopted widely by many ocean modeling. However, large biases exist in the MLD simulation using the original KTN model in the tropical Pacific. This is partly due to the uncertainties in representing wind stirring effect in the model, which is scaled by a parameter (m0). Traditionally, m0 is taken as a constant uniformly in space. In this study, the m0 is estimated as spatially and seasonally varying through its inverse calculation from a balance equation describing the turbulent kinetic energy (TKE) budget of the ML. It is illustrated that the m0 is spatially varying over the tropical Pacific. The derived m0 fields are then embedded into an ocean general circulation model (OGCM). Compared with the observations and the global ocean data assimilation system (GODAS) analyses, the MLD simulations in the OGCM with varying m0 are substantially improved in the tropical Pacific Ocean on seasonal and interannual time scales. Additionally, the Pacific subtropical cells (STCs) become intensified, accompanied with the strengthening of upwelling in the eastern equatorial Pacific; thus, more realistic simulations are obtained in spatially and seasonally varying m0 case compared with the constant m0 case. As the related cooling effect from the upwelling is enhanced, the simulated SST is slightly cooled down in the eastern equatorial Pacific. Further applications and implications are also discussed.