Effects of explicit tidal forcing in an OGCM on the water-mass structure and circulation in the Indonesian throughflow region

Abstract

Tidal and internal gravity waves significantly contribute to ocean mixing processes, yet these small-scale processes have been parameterized traditionally by sub-grid scale mixing schemes. With free surface methods growing in popularity in z-coordinate models it offers the option to explicitly resolve tides in such models (although sub-grid scale processes associated with tides, such as internal wave breaking, even in eddy-resolving OGCMs still have to be parameterized). The eight most important constituents of the semi-diurnal and diurnal tides have been implemented in a global version of the MOM3 GFDL ocean model. Tidal currents play an essential role in transport and mixing processes in the Indonesian Seas. The simulated long-term mean impact of tidal mixing on water-mass structures in the Indonesian Archipelago is compared with observations. Tidal rectification and associated mixing in the Indonesian Archipelago improve the agreement with observed long-term mean and seasonal pattern of sea-surface temperature (ΔSST≈0.3 °C) and heat fluxes ( Δheat flux ≈20 W/m 2), sea-surface salinity (ΔSSS≈0.15 psu) and sub-surface properties including throughflow transports. However, regional differences exist and seem to be related to details of the bottom topography. Furthermore, as tides influence ocean properties via a model’s mixing parameterization, rectification of the ocean state by tides strongly depends on the choice of sub-grid scale parameterizations. In the present model, this leads to excessive thermocline warming in the open ocean. Two examples with different mixing parameterizations plus tidal forcing are discussed.