Causal interactions between Southern Ocean polynyas and high-latitude atmosphere-ocean variabilityRESEARCH PAPER on 07 May 2020
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 impact on the ocean and atmosphere is hence critically important for assessing the fidelity of high-resolution climate models. This study uses Granger causality to examine the atmosphere-cryosphere-ocean interactions associated with polynyas in the high-resolution climate model E3SMv0-HR.
Weddell Sea open-ocean polynyas have been observed to occasionally release heat from the deep ocean to the atmosphere, indicating that their sporadic appearances may be an important feature of high-latitude atmosphere-ocean variability. Yet, observations of the phenomenon are sparse and many standard resolution models represent these features poorly, if at all. In a new research paper in Journal of Climate, we use a fully coupled, synoptic-scale preindustrial control simulation of the Energy Exascale Earth System Model (E3SMv0-HR) to effectively simulate open-ocean polynyas and investigate their role in the climate system. Our approach employs statistical tests of Granger causality to diagnose local and remote drivers of, and responses to, polynya heat loss on interannual to decadal time scales. First, we find that polynya heat loss Granger causes a persistent increase in surface air temperature over theWeddell Sea, strengthening the local cyclonic wind circulation. Along with responding to polynyas, atmospheric conditions also facilitate their development. When the Southern Ocean experiences a rapid poleward shift in the circumpolar westerlies following a prolonged negative phase of the Southern Annular Mode (SAM), Weddell Sea salinity increases, promoting density destratification and convection in the water column. Finally, we find that the reduction of surface heat fluxes during periods of full ice cover is not fully compensated by ocean heat transport into the high latitudes. This imbalance leads to a buildup of ocean heat content that supplies polynya heat loss. These results disentangle the complex, coupled climate processes that both enable the polynya’s existence and respond to it, providing insights to improve the representation of these highly episodic sea-ice features in climate models.