Tuesday, August 11, 2009

Beyond the CO2 connection

Rainer Zahn
At times in the past, mobile ocean fronts in the subtropics have exercised an influence on the magnitude of climate change by decoupling temperature from levels of carbon dioxide in the atmosphere.

During the past 800,000 years — the late Pleistocene and subsequent Holocene — Earth’s climate has swung between cool and warm and back again many times. Variations in incoming solar radiation, modulated by Earth’s orbital parameters1, along with recurring ups and downs in atmospheric carbon dioxide concentrations2, are among the prominent features associated with these climate swings. At first glance, then, it looks like climate is intimately coupled with solar radiation and CO2. But on closer inspection, differences become apparent between the amplitudes of climate changes, and CO2 variations in particular, that raise the question of how tight that coupling is.

As they describe on page 380 of this issue3, Bard and Rickaby have addressed this question. They analyse palaeoclimatic records from a marine sediment core in the southwestern Indian Ocean, and show that the ocean front separating the warm subtropical ocean from the cold subantarctic zone may influence global-scale temperature control. Bard and Rickaby use a suite of data records to monitor latitudinal migrations of the subtropical front as climate cycled between ice ages and warm ages during the past 800,000 years. What makes their study particularly persuasive is that the sediment core from which they draw their data comes from the entrance to the ocean gateway that connects the Indian Ocean with the Atlantic Ocean at the southern tip of Africa. At this gateway, warm and salty waters leak from the Indian Ocean into the South Atlantic, forming part of the global ocean thermohaline circulation and compensating for the export of deep water from the Atlantic to the rest of the world ocean. The salt-water transport to the South Atlantic causes a south-to-north density gradient in the Atlantic as a whole that has the power to influence the Atlantic’s meridional overturning circulation and, ultimately, the Gulf Stream, with consequences for heat transport and climate across the North Atlantic region4–6. In theory, the latitudinal position of the subtropical front defines how wide the gate is left open for the salt-water transport from the Indian Ocean to the Atlantic.

Ocean fronts are boundaries between water masses of different temperature and salt content, and so of different density. That they affect ocean circulation and climate is not new. In particular, it has long been recognized7 that, during the late Pleistocene, the North Atlantic polar front exerted control over the Atlantic overturning circulation and thus over climate in the Northern Hemisphere. Gains and losses in seawater density near the centres of deepwater formation were tightly linked with movements of the polar front. Shifting the front to a northerly position during the warm periods between ice ages allowed warm subtropical waters to penetrate far into cold subpolar latitudes, invigorating evaporation that increased the salt concentration and hence the density of sea water, thereby stimulating deep convection. A southerly position of the polar front during ice ages tended to reduce the thermal contrast between surface water and the overlying atmosphere, reducing evaporation and saltiness and causing a less-vigorous deep convection. Although it seems simplistic, the validity of this concept was demonstrated in the early days of palaeoceanography8, and since then the focus has been fixed on the high latitudes.

Bard and Rickaby3 shift attention to the subtropics by postulating that the subtropical front has likewise acted as an agent of change. Two ‘superglacial’ intervals serve as test cases— marine isotope stages 10 and 12 — during which Earth experienced unusually cold climates despite levels of atmospheric CO 2 that were no different from those during the preceding and subsequent ice ages. The suite of temperature-sensitive geochemical and faunal records that Bard and Rickaby present constitute a compelling case that, during both superglacials, the subtropical front was shifted by as much as 7 degrees latitude to its northernmost position of the past 800,000 years.

In this position, the subtropical front potentially would have prevented water transport to the Indian–Atlantic gateway by intercepting the Agulhas current — which runs south along the eastern edge of southern Africa — closing the gate on the inter-ocean transport of water. In consequence, the Atlantic overturning circulation could have been forced into an unprecedentedly slow mode. This then reduced the poleward oceanic transport of heat, so the thinking goes4–6, to the extent that ice sheets in the Northern Hemisphere grew beyond their normal limits and climate cycled into a severe cool state.

So much for the concept. But what might have caused the subtropical front to undergo such an exceptional northward migration? The position of the front is a consequence of the processes that alter the temperature distribution at the southern reaches of the large oceanic subtropical gyres. Today, a central element is the interplay between the easterly trade winds in the subtropics and the westerly winds that, in turn, interact with the ocean to move water (and heat) around its surface. The dynamics that would have caused the wind patterns to change, such that the subtropical front may have shifted that far north during marine isotope stages 10 and 12, are not yet clear. It also remains to be seen if a northwardmigrating subtropical front would, by itself, have been strong enough to block a current as mighty as the Agulhas and so affect water transport to the Atlantic. A different way of altering the scale of the leakage is for the Agulhas current to shift gears. The strengths of currents typically vary in parallel with shifting wind patterns and could act to invigorate or weaken the Agulhas leakage to the Atlantic. The details of this mechanism have not yet been fully explored9,10. But the strength of the Agulhas has been steadily increasing since the 1980s, driven by changed gradients in atmospheric pressure and in wind fields11.

Bard and Rickaby3 make a compelling case that, at times in the past, severely reduced water transport between the Indian and Atlantic oceans may have caused climate to cool beyond typical ice-age conditions. But will the reverse also hold? Will an increased leakage (forced by recent shifts in atmospheric pressure and winds11) compensate for the loss of salt in the North Atlantic12,13 (caused by increased precipitation and changes in the freshwater flux from the Arctic)? Would this be enough to stabilize the Atlantic overturning circulation? Let’s keep an eye on what the leakage does next.

The Agulhas current and the subtropical front. The Agulhas is the largest ocean jet current in the Southern Hemisphere, and carries warm, salty waters from the South Indian Ocean subtropical gyre along the eastern edge of southern Africa. Most of the current feeds back into the subtropical gyre, but some of its waters flow through the ocean gateway off the southern tip of Africa and into the Atlantic. This ‘leakage’ alters the density structure of the South Atlantic, with northward propagation of subsurface pressure waves potentially affecting the meridional overturning circulation in the North Atlantic4–6, which has global effects. Migration of the mobile subtropical front, as described in the work of Bard and Rickaby3, is diagnostic of shifted wind patterns that can alter the Agulhas Current and the southern African ocean gateway and so, potentially, the choreography of ocean–climate interactions.

Source of Information : Nature Magazine July 16 2009

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