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THE OCEAN is in perpetual motion: that’sobvious to anyone who looks out over the sea.But the tides and waves that we see are just atiny part of the movement in the ocean as awhole. Most movement comprises large-scale,unseen shifts to equal out water densitydifferences caused by variations in temperatureand salinity (saltiness) from the Poles to theTropics. Dense water (i.e., cold and salty) tendsto sink and then flow sideways at its appropriatedensity level, generating currents that flowfrom basin to basin, redistributing heat and saltas they go. This is called thermohalinecirculation.

Cold, salty deep water is produced in the seasaround Antarctica and the high North Atlantic.Freezing winds cool the surface water andincrease its density. The formation of salt-freesea ice causes the surrounding water to becomesaltier and even more dense. The dense waterplummets to abyssal depths and then flowsalong the seabed, hugging thewestern margins of ocean basinsdue to the “steering” effect of theearth’s rotation (the Coriolis effect).The currents created by this processare called Deep Western BoundaryCurrents.

LESSONS OF THE PAST

Ocean circulation:the planet’s great heat engineBarbaraManighetti

Giant currents inthe deep oceantransport so muchheat around theglobe that theyplay a critical rolein shaping theearth’s climate.

left:Beach or abyss?Sand ripples onthe seabed at over4 km depth lookremarkably likethose from a tidalestuary or beachand demonstratethat there is watermovement at thatdepth as well asnear the oceansurface.

The Pacific gatewayToday, 40% of the densest deep water entersthe world’s ocean via the southwest Pacific,and New Zealand is at the gateway of this hugeflow.The Deep Western Boundary Current of thesouthwest Pacific travels 2–5 km below thesurface along the east side of the CampbellPlateau, around Chatham Rise and aroundHikurangi Plateau (see inset map, below right).On its journey past New Zealand, the boundarycurrent helps to redistribute heat, salt andnutrients. To compensate for this northwardflow of deep water, there is a southwardmovement of shallower water in the upperlayers of the ocean. Similarly, in the NorthAtlantic, deep water is replenished by anorthward flow of shallow water in the GulfStream. These deep thermohaline currents,with their accompanying shallow flows, are akey factor in the Earth’s climate system.

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For explanations ofwords in bold in thetext, refer toGlossary on page 31.

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inset above: A detailed map of the seafloor around NewZealand, showing cold deep water entering the Pacific throughthe southwest Pacific gateway. Some of this water flows northpast New Zealand in the Deep Western Boundary Current(DWBC) while the rest continues its eastward flow in theAntarctic Circumpolar Current (ACC). Deepening colourindicates increasing intensity of flow towards the westernboundary.

left: The Great Global Conveyor. This map shows the generaloutline of sinking in the Antarctic and North Atlantic, andentrainment of these deep waters into the Antarctic CircumpolarCurrent (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.

From the moment of sinking, deep water cantake up to 1,500 years to complete its journeyaround the globe and re-emerge at the surface.During this time, the deep water collectsnutrients from material that rains down fromthe upper ocean. When the deep water nears thesurface again in upwelling regions, it suppliesfood for huge blooms of plankton.

Thus, oceanographers think of global oceancirculation as a gigantic conveyor belt. Deepwater from both the Arctic and Antarctic joinsthe huge ocean current that encircles theSouthern Ocean. This current – the AntarcticCircumpolar Current – links the Atlantic,Pacific and Indian oceans (see pages 15–17, thisissue). Little or no deep water forms in thePacific and Indian oceans but differences inwater temperature, rainfall and evaporation stilllead to large flows in and out of their basinsvia the Southern Ocean.

Thermohaline circulation is finely balanced,especially in relation to saltiness. A 3% boostin salinity increases the density of seawater asmuch as a 5°C cooling! The vigour and extentof the deep and shallow flows depend upon abalance between evaporation and fresh watersupply, temperature distribution through theocean, and wind patterns. Any or all of thesefactors may change as global warmingcontinues.

There have certainly beenchanges in the past, and NIWAscientists and the Ocean Drilling Program(ODP) have documented fluctuations in thestrength of the New Zealand sector of theconveyor belt over ice ages (glacials) and warmperiods (interglacials). (For example, see Water& Atmosphere 7(1): 14–16; 8(3): 24–26.)

The Ocean DrillingProgram recoveredcores of deep sea mudand rock from the NewZealand region,containing evidence ofpast climates andoceanic conditionsspanning over 60million years. (Photos:courtesy of the OceanDrilling Program)

right: ODP coring rig.

below:The ODP research vessel JOIDESResolution.

right: Examining split cores in thelaboratory.

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Typical kina barren atD’Urville Island.(Photo: Steve Mercer)

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Glacials to interglacials: naturalglobal warming at workThe earth’s climate has seen plenty of changesin the past, with glacials and interglacialsoccurring in a fixed cycle. We are currently ina rather warm interglacial, but a change tocolder conditions is likely over the next fewthousand years.

Knowledge of how the ocean andatmosphere behaved during the coldperiods and, par t icular ly, duringwarming intervals – periods of changefrom maximum glacial to maximuminterglacial – can help us to predict thepossible future of the planet under aCO

2-induced super-warming.

During the last glacial period there wasa large reduction in the southward flowof North Atlantic deep water. At thesame time, the northward flow of warm,shallow water in the Gulf Stream –Europe’s “heater” – also reduced. So theregion quickly cooled down.

In the south, however, we think thatduring glacial periods the Deep WesternBoundary Current increased its speedaround New Zealand but relaxed duringinterglacials, including the presentwarm phase which began about 10,000

years ago. In the last major interglacial, ~130–120,000 years ago, the current may have beenless vigorous than today. This period is oftenused as an analogue for predict ing theconsequences of global warming, s incetemperatures were 1–2°C warmer than now.

It seems likely that the current’s strength isrelated to the rate of production of deep wateraround Antarctica, with higher productionduring glacials and less in interglacials. Thiswas probably because of changes in theformation of sea ice around the continent, and

the resultant fluctuation in the salt content ofsurface waters.

A short-lived event known as the “YoungerDryas cooling” occurred in the NorthernHemisphere just as warming was gettingunderway after the last glacial, about 13,000years ago. Something happened suddenly anddramatically to reduce or cease production ofdeep water in the northern seas. This catapultedthe region back into near-glacial conditions,with cooling in some places estimated to be asmuch as 10°C in a few decades. No suchdramatic event has been demonstrated in theSouthern Hemisphere, but evidence isaccumulating to suggest that a slow-down orreversal in the post-glacial warming processoccurred in Antarctica and in the seas aroundNew Zealand about 1,500–2,000 years beforethe Younger Dryas event in the north. (The coolperiod is named after a daisy-like flower, Dryasoctopetala, that became common at that time inparts of Europe. The plant also occurred in anearlier cool period, known as the Older Dryas.)

Lessons from the past?What can we learn from these events ofthousands of years ago?

It is possible that, as global temperatureincreases, the accompanying changes inrainfall, evaporation and sea-ice cover willslow down the formation of deep water in theNorth Atlantic. The insulating effect of theSouthern Ocean means that Antarctic deep-water production is less likely to be affectedunless or until warming is further advanced.However, all the oceans communicate via theAntarctic Circumpolar Current. This ensuresthat, eventually, the impact of NorthernHemisphere changes will reach the NewZealand region – although it may take 500 orso years to arrive.

First effects may be a greater contrast betweenshallow and deep-water temperatures, leadingto changes in the sinking and upwellingbehaviour critical for chemical and nutrientcycling in the ocean. Through feedback effectsvia the atmosphere, however, changes in theocean can generate abrupt climate events as thesystem moves into a new mode of operation.In future, switching off the “global heat pump”through greenhouse warming could, ironically,help initiate a new ice age. ■

The efficiency of the ocean atredistributing heat could bedelaying and regulatingglobal warming. Will thispersist? There is evidence oflarge and abrupt changes inocean circulation that havehad major impacts on globalclimate in the past.

The sediment in thisdeep ocean core,recovered during theOcean DrillingProgram, representsmaterial accumulatedover many thousandsof years, includingorganic and inorganicremains that canprovide clues to oceanenvironmentalconditions long ago.(Photo: courtesy of theOcean DrillingProgram)

Barbara Manighetti is based at NIWA inWellington.

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