NATURE|Vol 447|24 May 2007 ESSAY

OCEANOGRAPHY Power of pull As the complex interplay of forces in the ocean responds to climate change, the dynamics of global ocean circulation are shifting.

Martin Visbeck hours, along with the first high-resolu- in basements so that the hot, less dense tion computer models (basically upside- water flows upwards into a house’s radia- Global climate change is edging the down cloud convection models) that could tors.) To keep a density-driven ocean cir- oceans towards a daunting future of sea- simulate this process in vivid detail. We culation going, you need vertical mixing level rise, an increase in dissolved carbon learned that this deep convective mixing of warmer waters into the ocean depths, dioxide and in acidity, reduced subsurface occurs sporadically, when high surface which makes deeper water less dense, oxygen, and the possible loss of marine salinities come together with enough cold prompting upwelling — that is, pull. biodiversity and ecosystem functions. In and windy winter days to make the surface Much remains to be discovered about all of this, large-scale ocean circulation water dense enough to let it plunge to the ocean mixing. We do know it is largely plays a central role. Aside from affect- driven by the wind and tide. 200 ing and to some extent being affected by Wind, for instance, creates sur- 100 these changes, the oceans are pivotal in face waves that stir the upper 90 global climate regulation. Any big shifts layers of the ocean directly, and

80 Centimetres per second in this complex system present a consid- 70 also induces surface friction erable challenge to science and society. 60 that generates large-scale, deep- In large-scale ocean circulation, often 50 reaching currents. In their turn, compared to a global conveyor belt, 40 these currents can create turbu- warm surface waters flow northwards 30 lent mesoscale eddies — ring- from the equatorial Atlantic, giving up 20 like flows with a radius of 10 to

C. BÖNING & A. BIASTOCH (IFM-GEOMAR) C. BÖNING & A. BIASTOCH heat to the atmosphere. The cold, dense 15 100 kilometres. When wind- or water sinks in the Greenland and Lab- 10 tide-driven currents interact rador Seas and returns to the South- 5 with undersea topography, steep ern, Indian and Pacific Oceans as deep 0 subsurface internal waves are currents. This global overturning circu- Simulation of near-surface ocean current speeds. generated, some of which break lation is key to climatic stability because just like waves on a beach to help it contributes to more than half of the glo- depths of the ocean. And it turned out that mix density layers at depth. Tropical hur- bal ocean heat transport and allows many convection is also confined to a few key ricanes and nekton — the huge schools of

gigatonnes of CO2 to be stored in vast regions in the centre of the Greenland and krill and small fish that create ‘biological amounts of deep ocean water, out of con- Labrador Seas in late winter. stirring’ as they swim — are other possible tact with the atmosphere. Global-scale dif- Push is also a key player in carbon sources of ocean mixing. ferences in water density — built at the sea sequestration. When surface and subsur- What does the future hold? By century’s surface through solar heating, clouds and face layers in those seas become cold and end, warming will cause more rainfall in rain, and eroded by internal mixing caused dense enough to cause intense vertical polar regions, diluting the oceans along by winds and tides — drive the movement mixing, this water, replenished from the with meltwater from glaciers and the of water. But what determines the vigour atmosphere with high levels of oxygen, Greenland ice sheet. All this will lower

of global circulation: the ‘push’ of dense CO2 and other substances, is transferred ocean salinities and, together with warmer waters to depth in the polar regions, or to the depths. From there, a complex set of winter temperatures, reduce deep convec- the ‘pull’ of mixing those waters upwards deep currents, most of which hug the west- tion: the push will weaken. Current climate throughout the ocean basins? ern sides of the ocean basins, spreads the models predict that the Atlantic overturn- At Kiel University, Germany, in the late newly ‘ventilated’ water around the world. ing will slow by 30%. 1980s, we were focused on push — the Where does that leave pull? It took until But over the longer term, shifts in pull sinking of cold, salty, dense surface waters 1994 for me to see its true importance. will matter. Those same models suggest in the North Atlantic, known as deep-water Confronted with simulations of one of increased winds over the Southern Ocean formation. We felt we were making exciting the first global ocean circulation mod- and the possibility of stronger hurricanes. progress in understanding what drives the els, devised by Robbie Toggweiler at the Both would boost ocean mixing and could, ocean conveyer belt. What we didn’t real- Geophysical Fluid Dynamics Laboratory crucially, offset the trend towards slower ize then was that pull — the slow upwelling in Princeton, New Jersey, I was shocked. circulation. More speculative is the role of that arises from ocean mixing — was a cru- The model showed that artificially increas- any change in nutrient availability, which cial piece of this giant marine puzzle. ing wind speeds over the Southern Ocean would affect the abundance of nekton Push plays an obvious part in deep enhanced deep upwelling, and the North and thus biological stirring. Global ocean ocean circulation: if a deep, dense water Atlantic responded with an increase in observations, improved ocean models, reservoir grows in volume, its spread into overturning circulation. smart brains and a multidisciplinary all deep ocean basins will accelerate. But Then I remembered Johan Sandström. approach will all be needed to advance this how does the water get from the surface In 1908, the Swedish oceanographer frontier of science. ■ to the depths? We began to see intriguing elegantly demonstrated how heating at Martin Visbeck is at the Leibniz Institute new direct measurements of vertical flows, depth is vital for maintaining a global- of Marine Sciences at the University of capable of mixing fluid ‘parcels’ down to scale circulation in tank experiments. Kiel, Düsternbrooker Weg 20, 24105 Kiel, depths of 1 to 2 kilometres within a few (After all, boilers are generally installed Germany. CONCEPTS 383