Australia: The Land Where Time Began

A biography of the Australian continent 

Southern Ocean Warming Delayed by Circumpolar Upwelling and Equatorwards Transport

Over recent decades there has been little warming of the Southern Ocean, which contrasts starkly with the observed rapid warming in the Arctic. The upper ocean has warmed substantially along the northern flank of the Antarctic Circumpolar Current. In this paper Armour et al. have presented analyses of oceanic observations, as well as general circulation model simulations, which show that these patterns - of delayed warming to the south of the Antarctic Circumpolar Current (ACC) and to the north of the ACC enhanced warming – are fundamentally shaped by the meridional overturning circulation of the Southern Ocean: warming around Antarctica is damped by upwelling that is driven  by wind of unmodified water from depth; surface heat uptake that that is greenhouse gas (GHG) induced is largely balanced by anomalous northward heat transport that is associated with the equatorial flow of surface waters; and the heat is stored preferentially in locations where surface waters are subducted to the north. Also, these processes are due primarily to passive advection of the anomalous warming signal by climatological ocean currents; ocean circulation changes are secondary. It is suggested by these findings that the Southern Ocean responds to greenhouse gas forcing on the centennial, or longer, timescale over which the deep ocean waters that are upwelled to the surface are themselves warmed. According to Armour et al. the multidecadal temperature trends of the Southern Ocean must be understood against this background of gradual warming.

It is further suggested by these findings that Southern Ocean surface warming is set by the time required for deep ocean waters – that originate in the North Atlantic Ocean and ultimately upwelled to the surface of the Southern Ocean (Marshall & Speer, 2012) to be themselves warmed. It is implied by this that that Southern Ocean requires a timescale of multiple centuries to respond to the greenhouse gas forcing, which is consistent with the slow rate of warming in the Southern Ocean seen in both observations and simulations of the GCM (Stouffer, 2004; Li von Storch & Marotzke, 2013; Masson-Delmotte et al., 2013). It is suggested by these results that this trend, and its driving mechanisms, must be understood against a background of warming that it induced by greenhouse gas, instead of the rapid warming that is seen in the Arctic, though these results do not explain the cooling of the Southern Ocean that has been observed over the most recent few decades.

Sources & Further reading

1. Armour, K. C., J. Marshall, J. R. Scott, A. Donohoe and E. R. Newsom (2016). "Southern Ocean warming delayed by circumpolar upwelling and equatorward transport." Nature Geosci 9(7): 549-554.