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Ocean Circulation: the Planet’S Great Heat Engine

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Ocean Circulation: the Planet’S Great Heat Engine NIWA WATER & ATMOSPHERE 9(4) 2001 LESSONS OF THE PAST Ocean circulation: the planet’s great heat engine Barbara THE OCEAN is in perpetual motion: that’s The Pacific gateway Manighetti obvious to anyone who looks out over the sea. Today, 40% of the densest deep water enters But the tides and waves that we see are just a the world’s ocean via the southwest Pacific, tiny part of the movement in the ocean as a and New Zealand is at the gateway of this huge Giant currents in whole. Most movement comprises large-scale, flow. unseen shifts to equal out water density the deep ocean The Deep Western Boundary Current of the differences caused by variations in temperature southwest Pacific travels 2–5 km below the transport so much and salinity (saltiness) from the Poles to the surface along the east side of the Campbell Tropics. Dense water (i.e., cold and salty) tends heat around the Plateau, around Chatham Rise and around to sink and then flow sideways at its appropriate globe that they Hikurangi Plateau (see inset map, below right). density level, generating currents that flow On its journey past New Zealand, the boundary play a critical role from basin to basin, redistributing heat and salt current helps to redistribute heat, salt and as they go. This is called thermohaline in shaping the nutrients. To compensate for this northward circulation. earth’s climate. flow of deep water, there is a southward Cold, salty deep water is produced in the seas movement of shallower water in the upper around Antarctica and the high North Atlantic. layers of the ocean. Similarly, in the North Freezing winds cool the surface water and Atlantic, deep water is replenished by a increase its density. The formation of salt-free northward flow of shallow water in the Gulf sea ice causes the surrounding water to become Stream. These deep thermohaline currents, saltier and even more dense. The dense water with their accompanying shallow flows, are a plummets to abyssal depths and then flows key factor in the Earth’s climate system. For explanations of along the seabed, hugging the words in bold in the western margins of ocean basins text, refer to Glossary on page 31. due to the “steering” effect of the earth’s rotation (the Coriolis effect). The currents created by this process are called Deep Western Boundary Currents. left: Beach or abyss? Sand ripples on the seabed at over 4 km depth look remarkably like those from a tidal estuary or beach and demonstrate that there is water movement at that depth as well as near the ocean surface. 12 NIWA WATER & ATMOSPHERE 9(4) 2001 From the moment of sinking, deep water can take up to 1,500 years to complete its journey right: ODP coring rig. around the globe and re-emerge at the surface. below: During this time, the deep water collects The ODP research vessel JOIDES nutrients from material that rains down from Resolution. the upper ocean. When the deep water nears the surface again in upwelling regions, it supplies food for huge blooms of plankton. Thus, oceanographers think of global ocean circulation as a gigantic conveyor belt. Deep water from both the Arctic and Antarctic joins the huge ocean current that encircles the Southern Ocean. This current – the Antarctic Circumpolar Current – links the Atlantic, Pacific and Indian oceans (see pages 15–17, this issue). Little or no deep water forms in the Pacific and Indian oceans but differences in water temperature, rainfall and evaporation still lead to large flows in and out of their basins right: Examining split cores in the via the Southern Ocean. laboratory. Thermohaline circulation is finely balanced, There have certainly been especially in relation to saltiness. A 3% boost changes in the past, and NIWA in salinity increases the density of seawater as scientists and the Ocean Drilling Program much as a 5°C cooling! The vigour and extent The Ocean Drilling (ODP) have documented fluctuations in the Program recovered of the deep and shallow flows depend upon a strength of the New Zealand sector of the cores of deep sea mud balance between evaporation and fresh water conveyor belt over ice ages (glacials) and warm and rock from the New Zealand region, supply, temperature distribution through the periods (interglacials). (For example, see Water ocean, and wind patterns. Any or all of these containing evidence of & Atmosphere 7(1): 14–16; 8(3): 24–26.) past climates and factors may change as global warming oceanic conditions continues. spanning over 60 million years. (Photos: courtesy of the Ocean Drilling Program) inset above: A detailed map of the seafloor around New Zealand, showing cold deep water entering the Pacific through the southwest Pacific gateway. Some of this water flows north past New Zealand in the Deep Western Boundary Current (DWBC) while the rest continues its eastward flow in the Antarctic Circumpolar Current (ACC). Deepening colour indicates increasing intensity of flow towards the western boundary. left: The Great Global Conveyor. This map shows the general outline of sinking in the Antarctic and North Atlantic, and entrainment of these deep waters into the Antarctic Circumpolar Current (ACC), which flows clockwise around the entire globe. Northward "spinoffs" from the ACC flow into the Indian Ocean, and through the southwest Pacific gateway into the Pacific. Return shallow flows complete the conveyor-belt loop. 13 NIWA WATER & ATMOSPHERE 9(4) 2001 the resultant fluctuation in the salt content of The efficiency of the ocean at surface waters. redistributing heat could be A short-lived event known as the “Younger delaying and regulating Dryas cooling” occurred in the Northern global warming. Will this Hemisphere just as warming was getting persist? There is evidence of underway after the last glacial, about 13,000 large and abrupt changes in years ago. Something happened suddenly and ocean circulation that have dramatically to reduce or cease production of deep water in the northern seas. This catapulted had major impacts on global the region back into near-glacial conditions, climate in the past. with cooling in some places estimated to be as much as 10°C in a few decades. No such dramatic event has been demonstrated in the Glacials to interglacials: natural Southern Hemisphere, but evidence is global warming at work accumulating to suggest that a slow-down or The earth’s climate has seen plenty of changes reversal in the post-glacial warming process in the past, with glacials and interglacials occurred in Antarctica and in the seas around occurring in a fixed cycle. We are currently in New Zealand about 1,500–2,000 years before a rather warm interglacial, but a change to the Younger Dryas event in the north. (The cool colder conditions is likely over the next few period is named after a daisy-like flower, Dryas thousand years. octopetala, that became common at that time in parts of Europe. The plant also occurred in an Knowledge of how the ocean and earlier cool period, known as the Older Dryas.) atmosphere behaved during the cold periods and, particularly, during Lessons from the past? warming intervals – periods of change What can we learn from these events of from maximum glacial to maximum thousands of years ago? interglacial – can help us to predict the possible future of the planet under a It is possible that, as global temperature increases, the accompanying changes in CO2-induced super-warming. rainfall, evaporation and sea-ice cover will During the last glacial period there was slow down the formation of deep water in the a large reduction in the southward flow North Atlantic. The insulating effect of the of North Atlantic deep water. At the Southern Ocean means that Antarctic deep- same time, the northward flow of warm, water production is less likely to be affected shallow water in the Gulf Stream – unless or until warming is further advanced. Europe’s “heater” – also reduced. So the However, all the oceans communicate via the Typical kina barren at region quickly cooled down. D’Urville Island. Antarctic Circumpolar Current. This ensures (Photo: Steve Mercer) In the south, however, we think that that, eventually, the impact of Northern during glacial periods the Deep Western Hemisphere changes will reach the New Boundary Current increased its speed Zealand region – although it may take 500 or around New Zealand but relaxed during so years to arrive. interglacials, including the present First effects may be a greater contrast between warm phase which began about 10,000 The sediment in this shallow and deep-water temperatures, leading years ago. In the last major interglacial, ~130– deep ocean core, to changes in the sinking and upwelling recovered during the 120,000 years ago, the current may have been behaviour critical for chemical and nutrient Ocean Drilling less vigorous than today. This period is often cycling in the ocean. Through feedback effects Program, represents used as an analogue for predicting the material accumulated via the atmosphere, however, changes in the consequences of global warming, since over many thousands ocean can generate abrupt climate events as the of years, including temperatures were 1–2°C warmer than now. organic and inorganic system moves into a new mode of operation. remains that can It seems likely that the current’s strength is In future, switching off the “global heat pump” provide clues to ocean related to the rate of production of deep water through greenhouse warming could, ironically, environmental around Antarctica, with higher production help initiate a new ice age. ■ conditions long ago. during glacials and less in interglacials. This (Photo: courtesy of the Ocean Drilling was probably because of changes in the Barbara Manighetti is based at NIWA in Program) formation of sea ice around the continent, and Wellington.
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