Friday, March 14, 2008

Ocean circulation in a warming climate

Climate models predict that the ocean's circulation will weaken in response to global warming, but the warming at the end of the last ice age suggests a different outcome.

There is an old truism in climate circles that the cold climate at the Last Glacial Maximum (LGM), which occurred 21,000 years ago, had stronger winds. This idea fits with the common observation that it is windier in the winter than in the summer because there is greater thermal contrast within the atmosphere in the winter hemisphere. Temperature reconstructions from the LGM show that Equator-to-pole gradients in sea surface temperature were indeed larger — that is, the polar oceans were colder than the tropical ocean at the LGM in comparison with the temperature differences today.

It is now becoming clear that the winds in the atmosphere drive most of the circulation in the ocean. If the LGM climate really did have stronger winds, it would thus be expected that the circulation in the ocean was more vigorous. The oceans seem to tell a different story, however. The deep water in the ocean's interior is continuously being replaced ('overturned') by surface waters from the poles. This overturning circulation in the Atlantic Ocean seems to have been weaker at the LGM1. The water in the deep ocean was also very 'old' in relation to the atmosphere — in terms of having a low radiocarbon content — indicating that the ocean's interior was poorly mixed and poorly ventilated2. The overturning circulation then seems to have strengthened as Earth began to warm about 18,000 years ago. The increased overturning vented the radiocarbon-depleted carbon dioxide (CO2) to the atmosphere, as seen in a pair of big dips in the radiocarbon activity of the atmosphere and upper ocean3. This addition of CO2 to the atmosphere helped to warm the climate and bring the last ice age to an end.

These findings present a conundrum. If the winds were stronger in the cold glacial state and became weaker going into the warm interglacial state, then why was the ocean's circulation weaker during the cold glacial period? And how did it increase in strength during the transition to the warm interglacial period, causing the ocean's interior to become better mixed and better ventilated? Are researchers missing something about the factors that affect ocean circulation, or is it the old truism about the strength of the winds during the cold glacial period that is flawed?

During the 1990s, the first generation of coupled climate models predicted that the ocean's overturning circulation would weaken markedly over the next 100–200 years in response to global warming4. The predicted weakening is a response to the warming itself and to a stronger hydrological cycle, both of which make the ocean surface waters in the models less dense and less able to sink in relation to the water below. Thus, the models suggested that circulation would be less vigorous in a warming climate, somewhat like the weakening expected from diminished winds in a warmer climate outlined above. But again, the real ocean became better mixed and better ventilated when Earth began to warm about 18,000 years ago. So what will happen to the ocean's circulation in a warming climate? Are the models getting it wrong?
Winds and the ocean's overturning circulation

Until recently, the circulation of the ocean was thought to comprise two fairly independent parts. The wind-driven circulation drove the surface currents in the ocean gyres, whereas the overturning circulation ventilated the interior with cold and relatively saline water from the poles. The latter was called the 'thermohaline' circulation to emphasize that it was driven by buoyancy forces — warming, cooling, freshening and salinification — rather than the stress on the surface coming from the winds.

The inconsistencies mentioned earlier could be overlooked if this dichotomy holds, because the winds and the wind-driven circulation in the upper ocean could still have been stronger during the LGM while the thermohaline circulation was less vigorous. However, the dichotomy and the use of the term 'thermohaline' have almost disappeared from the oceanographic literature, because the circulation in the interior is now increasingly seen as being driven by turbulent mixing from the winds and tides5, 6 and directly by the winds themselves7.

The westerly winds over the Southern Ocean seem to be crucial in this regard7. The Antarctic Circumpolar Current (ACC) is a wind-driven current that goes around Antarctica through an east–west channel between South America, Australia and Antarctica that is not blocked by land. Because the winds over the channel and the flow of the ACC are aligned for the length of the channel, the ACC is easily the world's strongest current (by volume of water transported). According to Carl Wunsch8, about 70% of the wind energy going into ocean currents globally goes directly into the ACC.

The same dense water found in the interior north of the ACC is also found just below the surface around Antarctica, and the westerly winds driving the ACC draw this dense water directly up to the surface (Fig. 1). In this way, the winds driving the ACC continually remove dense water from the interior. Dense water must sink elsewhere to replace the water drawn up by the winds around Antarctica.