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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |