Causal Interactions between Southern Ocean Polynyas and High-Latitude Atmosphere–Ocean Variability

Abstract

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. 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 the Weddell 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.

On the Generation of Weddell Sea Polynyas in a High-Resolution Earth System Model

“…It hasn't been until recently that we have been in a position to simulate the sequence of events that lead up to WSPs in HR fully-coupled ESMs. This is especially relevant if we wish to ultimately investigate the impact of open ocean polynyas on AABW formation, meridional overturning circulation (Stössel and Kim 2001;Swingedouw et al 2009;Hirabara et al 2012;Martin et al 2013;Patara and Böning 2014;Zanowski et al 2015;Zanowski and Hallberg 2017;Sohail et al 2020), and the climate system (Kaufman et al 2020).…”

Review of: “Multidecadal Polynya Formation in a Conceptual (Box) Model”, by Boot, van Westen, and Dijkstra.

“…In our recent paper (Kaufman et al 2020) we studied the heat content in E3SMv0-HR (a close clone of CESM1), and also found that heat build-up preceded polynya formation. However, our analysis suggests that this heat build-up is driven by a reduced surface heat loss under ice-covered conditions, and not an enhanced ocean heat import (Fig.…”

Can we reconstruct the formation of large open ocean polynyas in the Southern Ocean using ice core records?

“…Changes in surface winds also influence the horizontal oceanic circulation, potentially inducing upwelling of deep waters and thus a salt input in the surface layers creating conditions more prone to deep convection (Cheon et al, 2015;Campbell et al, 2019;Kaufman et al, 2020). In this framework, it has been argued that a persistent negative phase of the Southern Annular Mode (which is the main mode of atmospheric variability in the Southern Hemisphere extra-tropics) in the preceding decade could have created favorable conditions for the formation of the Weddell polynya in the 1970s (Gordon et al, 2007;Kaufman et al, 2020). Finally, the convection itself provides a preconditioning for subsequent years as it maintains a low stability of the water column, explaining why open ocean convection and polynya formation can be sustained over https://doi.org/10.5194/cp-2020-91 Preprint.…”

“…The enhanced evaporation over the polynya leads to a higher moisture content of the air, more clouds and more precipitation locally (Carsey, 1980;Moore et al, 2002;Weijer et al, 2017). The surface conditions may also induce a decrease of sea level pressure over the polynya and thus influence the atmospheric circulation (Timmermann et al, 1999;Moore et al, 2002;Latif et al, 2013;Weijer et al, 2017;Kaufman et al, 2020).…”

“…Finally, a few models display intermittent open ocean convection that leads to the formation of polynyas covering a wide range of sizes and duration (Martin et al, 2013;Stössel et al, 2015;Zanowski et al, 2015;Weijer et al, 2017;Zhang et al, 2019;Kaufman et al, 2020). The opening of those polynyas seems to be generally related to a common mechanism, implying a slow warming at depth due to the input of warmer water originating from the north when convection is shut down, followed by a rapid cooling at depth and surface warming during years with open ocean convection.…”