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.

Key Points

First attempt to successfully simulate the wavenumber-4 (W4) pattern of Southern Ocean sea surface temperature (SST) using a coupled model, uncovering the underlying physical processes

Southwestern subtropical Pacific SST plays a crucial role in generating SST W4 pattern via circumpolar atmospheric variability

The ocean mixed layer and upper ocean processes are found to be important for the growth and decay of the SST pattern

Plain Language Summary

In the subtropical-midlatitude Southern Hemisphere, we often observe year-to-year fluctuations in sea surface temperature (SST) linked to a specific pattern known as wavenumber-4 (W4). This study represents the first successful attempt to simulate this temperature pattern using a climate emulator called SINTEX-F2, allowing us to uncover its physical processes. Our research reveals that SST variations in the southwestern subtropical Pacific (SWSP) play a pivotal role in generating the W4 pattern in the atmosphere, subsequently influencing SST during austral summer. Interestingly, this pattern is almost independent of tropical SST variability. The process starts with heating in the SWSP, causing atmospheric disturbances. This leads to an undulation in mid-latitude atmospheric flow, evolving into a well-established global Rossby wave with four positive (negative) loading centers, forming a W4 pattern. This atmospheric wave interacts with the ocean’s surface, leading to heat exchange between the atmosphere and the upper ocean. In turn, it influences the depth of the mixed layer in the upper ocean, which receives solar energy. When solar energy penetrates into a shallower (deeper) mixed layer, it warms (cools) the mixed layer effectively, resulting in higher (lower) SSTs. Afterwards, the energy exchange between the mixed layer and the deep ocean contributes to the decay of the SST pattern.