Abstract
In recent decades, the Arctic has been warming and sea ice disappearing. By contrast, the Southern Ocean around Antarctica has been (mainly) cooling and sea-ice extent growing. We argue here that interhemispheric asymmetries in the mean ocean circulation, with sinking in the northern North Atlantic and upwelling around Antarctica, strongly influence the sea-surface temperature (SST) response to anthropogenic greenhouse gas (GHG) forcing, accelerating warming in the Arctic while delaying it in the Antarctic. Furthermore, while the amplitude of GHG forcing has been similar at the poles, significant ozone depletion only occurs over Antarctica. We suggest that the initial response of SST around Antarctica to ozone depletion is one of cooling and only later adds to the GHG-induced warming trend as upwelling of sub-surface warm water associated with stronger surface westerlies impacts surface properties. We organize our discussion around ‘climate response functions’ (CRFs), i.e. the response of the climate to ‘step’ changes in anthropogenic forcing in which GHG and/or ozone-hole forcing is abruptly turned on and the transient response of the climate revealed and studied. Convolutions of known or postulated GHG and ozone-hole forcing functions with their respective CRFs then yield the transient forced SST response (implied by linear response theory), providing a context for discussion of the differing warming/cooling trends in the Arctic and Antarctic. We speculate that the period through which we are now passing may be one in which the delayed warming of SST associated with GHG forcing around Antarctica is largely cancelled by the cooling effects associated with the ozone hole. By mid-century, however, ozone-hole effects may instead be adding to GHG warming around Antarctica but with diminished amplitude as the ozone hole heals. The Arctic, meanwhile, responding to GHG forcing but in a manner amplified by ocean heat transport, may continue to warm at an accelerating rate.
Highlights
Over the last few decades, the two polar regions of our planet have exhibited strikingly different behaviours, as is evident in observed decadal trends in surface air temperature shown in figure 1
What is the essential ‘physics’ behind the amplitude and timing of such differing polar responses to greenhouse gas (GHG) forcing? Are they a consequence of the interaction between GHG forcing and local radiative feedback processes that perturb the energy budget in differing ways over the two poles? Are they driven by different responses in atmospheric circulations and energy transports? Do they reflect different patterns of storage of anthropogenically induced temperature signals in the deep ocean? Here, we suggest that, independent of the above mechanisms, the patterns and timing of warming evident in figures 1–3 can be largely explained in terms of the advection of anthropogenic temperature anomalies by the background ocean circulation
The ozone-hole coefficients are chosen to encompass the ensemble-average spread of the climate response functions’ (CRFs) reported in [39] induced by a repeating annual cycle in ozone forcing of order 100 DU in two coupled models and the CRF from the ocean-only Southern Annular Mode (SAM) experiment described in §3
Summary
Over the last few decades, the two polar regions of our planet have exhibited strikingly different behaviours, as is evident in observed decadal trends in surface air temperature shown in figure 1. Enhanced communication of the interior ocean with the surface could have marked effects on the Earth’s climate through changes in rates of heat and carbon sequestration as well as consequences for ice shelves around Antarctica which may be vulnerable to enhanced upwelling of warm water from depth [52,53,54,55,56]. The advective process shaping the response is largely associated with the upper cell of the ocean’s meridional circulation with sinking in northern polar regions and upwelling in the SO around Antarctica (see the review [67]) This cell is a major interhemispheric asymmetry of the global climate, a consequence of differing hemispheric geometrical constraints on ocean circulation. That the warming signal induced by anthropogenic GHG forcing is shaped by ocean circulation, with sea ice and freshwater effects playing a secondary role (cf., e.g., figure 2a and b)
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