Causal Discovery of Episodic Polynya Heat Loss in the Weddell Sea
Polynyas are important components of the climate system in the high-latitude Southern Ocean. Open-ocean polynyas have been observed only sporadically in the observational record, but are recurring features in high-resolution climate models. Understanding their relationship to the high-latitude energy budget is hence critically important for assessing the fidelity of high-resolution climate models. This study uses Granger causality to examine the air/sea-ice/ocean interactions associated with polynyas in the high-resolution climate model E3SMv0-HR.
The fact that the enormous Weddell Sea polynya was present for several years in the mid-70s, but has not reappeared since, has baffled climatologists. The impact of the polynya on the abyssal ocean stratification was substantial, and the impact on the atmosphere must have been equally significant. The reason for its absence since the mid-70s is unclear, but if it is part of a multi-decadal cycle of internal climate variability, as suggested by some high-resolution climate models, we cannot exclude the possibility that it will reappear in the near future, with significant implications for global climate. Also, high-resolution climate models routinely feature Weddell Sea polynyas, possibly contributing to biases compared to observations. Understanding the processes responsible for polynya formation, and quantifying the impacts of their occurrence on the Southern Hemisphere climate, is therefore a key challenge for climatologists.
Weddell Sea polynyas have been intermittently observed to release heat from the deep ocean to the atmosphere, indicating that they may be an important feature of high-latitude atmosphere-ocean variability. Yet, observations are sparse and many standard resolution models represent these features poorly, if at all. We use the high-resolution Energy Exascale Earth System Model (E3SMv0-HR) to investigate the role of polynyas in the Southern Ocean climate system and apply Granger causality to diagnose local and remote drivers of, and responses to, polynya heat loss. First, we find that polynya heat loss causes a persistent increase in surface air temperature over the Weddell Sea, strengthening the local cyclonic wind circulation. But atmospheric conditions also facilitate the development of polynyas; in particular, a rapid poleward shift in the circumpolar westerlies increases salinity in the Weddell Sea and promotes deep ocean convection. Finally, we find that the reduction of surface heat fluxes during periods of full ice cover recharges the subsurface heat reservoir that is episodically released during polynya events.