Southern Hemisphere Circumpolar Wavenumber-4 Pattern Simulated in SINTEX-F2 Coupled Model

Abstract

Interannual sea surface temperature (SST) variations in the subtropical-midlatitude Southern Hemisphere are often associated with a circumpolar wavenumber-4 (W4) pattern. This study is the first attempt to successfully simulate the SST-W4 pattern using a state-of-the-art coupled model called SINTEX-F2 and clarify the underlying physical processes. It is found that the SST variability in the southwestern subtropical Pacific (SWSP) plays a key role in triggering atmospheric variability and generating the SST-W4 pattern during austral summer (December-February). In contrast, the tropical SST variability has a very limited effect. The anomalous convection and associated divergence over the SWSP induce atmospheric Rossby waves confined in the westerly jet. Then, the synoptic disturbances circumnavigate the subtropical Southern Hemisphere, establishing an atmospheric W4 pattern. The atmospheric W4 pattern has an equivalent barotropic structure in the troposphere, and it interacts with the upper ocean, causing variations in mixed layer depth due to latent heat flux (LHF) anomalies. As incoming climatological solar radiation goes into a thinner (thicker) mixed layer, the shallower (deeper) mixed layer promotes surface warming (cooling). This leads to positive (negative) SST anomalies, developing the SST-W4 pattern during austral summer. Subsequently, anomalous entrainment due to the temperature difference between the mixed layer and the water below the mixed layer, anomalous LHF, and disappearance of the overlying atmospheric W4 pattern cause the decay of the SST-W4 pattern during austral autumn. These results indicate that accurate simulation of the atmospheric forcing and the associated atmosphere-ocean interaction is essential to capture the SST-W4 pattern in coupled models.