Shixue Li: Resolution dependence of Southern Hemisphere tropical–extratropical coupling in MESACLIP
High-resolution models show SST trends that are in closer agreement with observations than their low-resolution counterparts, particularly in capturing cooling trends over the Southern Ocean and the Southeast Pacific. However, the processes through which increased resolution affects two-way tropical–extratropical coupling remain unclear. Here we examine the resolution dependence of the linkage among the Southern Annular Mode (SAM), Southern Ocean SST variability, and tropical Pacific SST anomalies using MESACLIP simulations with CESM-HR and CESM-LR, together with corresponding AMIP experiments.
We focus on strong austral-winter (JJA) SAM events. In observations and CESM-HR, strong SAM events are associated with a more organized PSA-like wave train and are followed by Southern Ocean cooling, with La Niña-like tropical Pacific cooling emerging around or after the SAM peak. In CESM-LR, by contrast, La Niña-like tropical Pacific anomalies tend to precede the SAM peak, while the Southern Ocean mixed-layer response is stronger and more zonally uniform. Results from the AMIP experiments suggest that this resolution dependence is not purely oceanic: although the low-resolution atmosphere can respond to prescribed tropical Pacific SST forcing, its SAM variability appears less tightly coupled to the tropical-forced PSA signal.
These results suggest that horizontal resolution can alter the timing and dynamical pathway of tropical–extratropical coupling in the Southern Hemisphere. This potentially offers a process-oriented metric for evaluating the added value of high-resolution climate simulations and diagnosing teleconnection biases in lower-resolution model hierarchies.