Effects of Tidal Mixing on the MJO-SST Coupling in the Maritime Continent

Abstract:

The Maritime Continent (MC) is a region with pronounced tidally-induced vertical mixing, most strongly manifested as the enhanced subsurface cooling over sills, channels, and seamounts. The vertical mixing and subsurface cooling from tides subsequently interact with the wind-induced mixing under westerly winds during the active phases of the Madden-Julian Oscillation (MJO), potentially affecting the deep convection associated with the MJO.

This study hypothesizes that the tidally-modified sea surface temperatures (SSTs) in the MC constitute a significant oceanic forcing of the MJO. Two sets of coupled, ensemble model simulations are conducted using the Scripps Coupled Ocean-Atmosphere Regional (SCOAR) model (WRF-ROMS), in which the ocean component of the coupled model is forced with and without tides. The tidally-forced oceans create a cascade of time-dependent coupled ocean-atmosphere responses in the MC. Basin-averaged SSTs are reduced by 0.27°C in the eastern MC with tides, and localized cooling exceeds 1°C near the Lesser Sunda Islands, Sulawesi, the Banda Arc, and the Sulu sill. Importantly, the magnitude of SST cooling due to tides is comparable to the intraseasonal changes in SST associated with the MJO. Furthermore, the magnitude of tide-induced SST cooling strongly depends on MJO phases. The SST cooling is more enhanced during the MJO active phases than the suppressed phases. This indicates a significant impact of the tide-induced vertical mixing on the wind-driven vertical mixing, resulting in changes to intraseasonal SST variability. Convective precipitation during active MJO phases is also reduced due to the colder SSTs with tides. Given that the tidal effect is of oceanic origin, the MJO phase-dependent tidal cooling and the reduced MJO-related precipitation suggest that tides have important implications for the oceanic forcing of atmospheric deep convection.