Southern Ocean warming delayed by circumpolar upwelling and equatorward transport

Abstract

The Southern Ocean has shown little warming south of the Antarctic Circumpolar Current (ACC) over recent decades, and Antarctic sea-ice cover has been modestly expanding 1 . Along the northern flank of the ACC, however, the upper ocean has been warming rapidly 2, 3 . Using observations and general circulation model simulations, we show that these patterns -of delayed warming south of the ACC and enhanced warming to the north -are fundamentally shaped by the Southern Ocean's meridional overturning circulation: wind-driven upwelling of unmodified water from depth damps warming around Antarctica; greenhouse gas induced heat uptake is largely balanced by anomalous northward heat transport; and heat is preferentially stored along the northern flank of the ACC, where surface waters are subducted.Further, we find that these processes are primarily due to passive advection of the anomalous warming signal by climatological ocean currents; changes in atmospheric and oceanic circulations play a secondary role. These findings suggest that the Southern Ocean responds to greenhouse gas forcing on the timescale over which the deep ocean waters that are upwelled 1

Meridional atmospheric heat transport constrained by energetics and mediated by large-scale diffusion

“…3c), consistent with the results of Huang and Zhang (2014) and with the large atmospheric heat flux convergence over the subpolar oceans (Fig. 3b) where sea-surface warming is delayed by ocean circulations (Marshall et al 2014b;Armour et al 2016). Results for individual models can be seen in Fig.…”

“…This is a simplification given that feedback pattern is largely set by moist atmospheric processes and likely depends on the patterns of surface warming, ocean heat uptake, and AHT changes (e.g., Graverson and Wang 2009;Rose et al 2014;Yoshimori et al 2017;Feldl et al 2017a;Singh et al 2017;PoChedley et al 2018). Moreover, ocean heat uptake has been prescribed within the EBMs; while its meridional pattern is thought to be set by regional ocean dynamics (Marshall et al 2014b;Armour et al 2016), the degree to which the magnitude of regional ocean heat uptake depends on atmospheric processes is not known and should be explored in future work.…”

The sensitivity of West Antarctica to the submarine melting feedback

“…Three considerations support this approach. First, the anticipated warming of the Southern Ocean may be delayed by upwelling of older waters [Armour et al, 2016], so that persistence of present melt rates on existing ice shelves can be considered a reasonable approximation for the 21st century. Second, ocean models currently show little skill at predicting the timing of wind-driven changes in the Southern Ocean and Amundsen Sea [Hellmer et al, 2012;Heuz et al, 2013;Cornford et al, 2015;DeConto and Pollard, 2016].…”

“…This suggests that retreat is limited by supply of heat from the ocean, not some timescale inherent to ice dynamics. This highlights the importance of simulating accurately the timing of heat delivery from the ocean to the ice sheet over the 21st Century [Hellmer et al, 2012;Armour et al, 2016]. Observations or modeling that constrain melt rates will be extremely valuable in assessing the risk of sea level rise over the 21st century.…”

Century‐scale perspectives on observed and simulated Southern Ocean sea ice trends from proxy reconstructions

“…Further north, much of the ice pack lies over the region of upwelling Circumpolar Deep Water. This means that near surface waters are constantly replenished by Circumpolar Deep Water (which due to its long residence time in the deep ocean has a limited anthropogenic signal), thus delaying any anthropogenic warming at the surface [Ferreira et al, 2015;Marshall et al, 2015;Armour et al, 2016].…”