Variability Due To Seasonal Cycle, Eddies, and Tides Enhances Water Mass Transformation in the Indonesian Seas

Abstract

The Indonesian Seas are a region of rich variability across time scales, with strong air-sea fluxes, and intense diapycnal mixing, leading to water mass transformation within the region. However, the rates of transformation, its distribution, and processes driving it remain poorly understood. Using a high-resolution regional ocean model explicitly simulating internal tides and associated mixing we quantify the water mass transformation in the Indonesian Seas and explore the role of air-sea fluxes, mixing, and tides. Our results show that up to 3.3 Sv of thermocline water is formed within the region from the surface (2.1 Sv) and intermediate and deep (1.2 Sv) water masses. 1.3 Sv, or 40%, of the thermocline water formation is associated with the monsoon-driven seasonal cycle. Monsoons drive large variability in the Indonesian Throughflow and the thermocline water volume, resulting in a net eddy volume transport of the thermocline water out of the Indonesian Seas to the Indian Ocean. This transport adds to the thermocline water transport by the mean circulation, with the total transport being balanced by the thermocline water formation within the Indonesian Seas. The water mass transformation within the region is greatly enhanced by tides. In addition to generating mixing that transforms water masses directly, tidal motions lead to cross-isopycnal (diapycnal) eddy heat and salt fluxes, which facilitate the exchange of properties between water masses and, consequently, their transformation. Our results show that variability across time scales enhances water mass transformation within the Indonesian Seas and should be accurately represented in global ocean and climate models.