Lionel Carter's Antarctica Report
From Icebergs to Pongas: Antarctica, the Southern Ocean and New ZealandProf Lionel Carter
Antarctic Research Centre
Victoria University of Wellington
The Southern Ocean – the link with the ice
New Zealand has a special place in the world ocean. North of about Kaikoura Peninsula, the surface ocean is influenced by subtropical waters whereas to the south, subantarctic and polar waters prevail at the surface. At depth waters formed around Antarctic and as far north as the North Atlantic Ocean, dominate the abyss along the entire eastern NZ margin.(Figs 1 to 2).
(Fig. 1) The surface ocean with the warm subtropical water (red) meeting northward migrating sub-Antarctic waters (blue) from the Southern Ocean. Warm meets cold along an oceanic front positioned west to east along Chatham Rise. There, ocean salt content and temperature change rapidly to form the Subtropical Front (STF), which is often marked by clouds because of the warm and cool conditions. Image from Carter et al. (2002) and NIWA.
New Zealand is surrounded by vast submarine plateaux, ridges and rises that are fragments of the ancient super-continent, Gondwana. This topography forms the SW Pacific Gateway and guides two of the world’s great current systems.
 The Antarctic Circumpolar Current [ACC] flows around Antarctica from west to east. It is the largest current and only one to connect the three major oceans; the Pacific, Indian and Atlantic (Fig. 2). The current is driven mainly by the westerly winds of the Roaring Forties and Furious Fifties. However, it is not a simple surface current but a complex of flows that extend from the ocean surface to seabed at over 4000 m depth. On its globe encircling journey, the ACC encounters two choke points; the Drake Passage between South America and Antarctica, and off New Zealand’s Campbell Plateau. At both sites, the current accelerates but is also disrupted by large eddies as it intercepts the submarine topography. It is also forced well north of its normal pathway.
(Fig. 2) The broad meandering pathway of the Antarctic Circumpolar Current (ACC) with the two main constrictions off NZ and South America. The ACC is not a single flow, but a complex of currents and fronts that include the Sub-Antarctic Front (SAF and north boundary of the ACC) and the Southern Boundary Front (SB and south boundary). The 3 main Deep Western Boundary Current (DWBC) offshoots into the Atlantic, Indian and Pacific sectors of the Southern Ocean are also shown. Image modified from Orsi et al. (1999).
(2) As the ACC passes clockwise around Antarctica, large deep currents are tapped off into the Atlantic, Indian and Pacific Oceans. The Deep Western Boundary Currents are typically found deeper than ~2000 m water depth (Fig. 3, 4). And in contrast to the wind-driven ACC, they are forced by their density differences with the surrounding ocean. Density is controlled mainly by ocean temperature and salt content or salinity. As water becomes more saline or colder or both, it reaches a point where it sinks to produce a thermohaline current (thermo = temperature; haline = salt). Collectively, the Deep Western Boundary Currents contribute to the global thermohaline circulation. This loosely connected group of currents is a major distributor of heat around the planet and therefore has a strong influence on climate (Fig. 3).
(Fig. 3) The global thermohaline circulation is a loosely connected group of currents driven by sinking of cold and or saline water around Antarctica and the North Atlantic. Modified from Manighetti (2002) and NIWA.
The thermohaline circulation works by virtue of waters sinking off Antarctica and in the North Atlantic. At key sites off Antarctica - the Weddell Sea, Adelie coast, Ross Sea – water becomes super cold (down to -2oC) as it passes under the ice, especially beneath the extensive Ross and Filchner-Ronne ice shelves (Fig. 5). Density is increased further by mixing with saline deep waters that have risen to near the ocean surface at the Antarctic margin. Finally there is sea ice. As the ocean undergoes its annual freeze, brine is expelled from the ice (a salty ice cube has yet to be invented) to further enhance the salt content of the underlying waters. This combination of extreme cold and additional salt increases water density, to a point where it sinks and spread towards the equator at depths down to 4000 m and deeper. This is the famous Antarctic Bottom Water – the main cause of the extreme cold in the marine abyss. However, some water that has descended to lesser depths is captured by the ACC to be redirected into the main ocean basins as Deep Western Boundary Currents (Fig. 3). In the case of the Pacific, New Zealand hosts the largest of the currents (Fig.4). East of Chatham Rise the total volume of water transported is 20 Sv (1 Sv or Sverdup = 1 million cubic metres of water per second).(Fig. 4) The Big Two. Off New Zealand, the Antarctic Circumpolar Current (ACC) is constricted by Campbell Plateau and forced north of its normal path. Under the ACC, is the Deep Western Boundary Current, so-called because it passes along the western sides of the major ocean basins. Both currents carry waters of Antarctic, North Atlantic and other origins. Image modified from Carter and Wilkin (1999) and NIWA.
As the Pacific deep western boundary current moves north, past Chatham Rise, along the Kermadec/Tonga ridge system and into the North Pacific, the amount of water transported reduces as offshoots feed the central Pacific. Eventually water rises to the surface off South America and moves west as a warm equatorial current (Fig. 3). This warm surface current has only 4-8 Sv of flow, but as it moves towards the Atlantic it is reinforced with other water that has risen to the surface. Thus in the central to north Atlantic the transport has reached a respectable 14 Sv. The warm, saline surface current continues orthwards towards Europe as the Gulf Stream and North Atlantic Drift. All the while it looses heat to the atmosphere - a process that is critical for keeping Europe warm. Without ocean heating Europe could be up to 10oC colder than present, especially in Scandinavia. As the surface ocean looses heat, its density increases causing water to sink and return south at depth to Antarctica where the cycle begins again.
Back to the future with ANZICE
Antarctica’s response to the present phase of rapidly warming climate is mixed. This is not surprising for a landmass of 14 million square kilometres or roughly twice the size of Australia. Off the Antarctic Peninsula both climate and ocean are warming rapidly at +0.56oC per decade – one of the more rapidly warming places on Earth. As a result there has been rapid recession of Peninsula glaciers, collapse of the Larsen ice shelf and a reduction in the extent of winter sea ice. The Ross Sea sector is also warming (+0.29oC per decade), but responses of the ice are more muted. Elsewhere, annual temperatures have become slightly colder. However, of 19 sites for which weather records were available, all but one revealed warmer winters. It could be argued that locally increased melting and reduced sea ice cover could reduce the formation of the dense water needed to force the thermohaline circulation. At present there is no indication of reduced formation, although oceanographic measurements suggest a local lowering in the salinity which is tantamount to a slight reduction in density but not enough to affect sinking.
During peak warm periods of the past few hundred thousand years, the atmosphere and surface ocean were up to ~3oC warmer than now. Such temperature increases are in line with those projected for the next century by the Intergovernmental Panel on Climate Change or IPCC. We believe that by deciphering, detailed environmental records of these previous warm extremes, we can provide a template for the future.
Accordingly, ANZICE proposes to:
To achieve these goals, ANZICE has three, complimentary approaches:
ANZICE introduces new innovations that greatly improve our ability to analyse, interpret and apply records of previous warmer climates.
Ice cores, collected from coastal sites, contain information with annual detail, similar to tree rings (Fig. 6). The cores are expected to recover ice at least 10,000 years old, and so will include the Holocene Climatic Optimum when temperatures were ~ 2oC warmer than now. Contained within the ice is a host of environmental clues:
(Fig. 6) Ice core about to be extruded from a corer for sampling to decipher past changes in Antarctica’s climate. Image Credit: Nancy Bertler, Victoria University Wellington (VUW) and GNS Science.
Sediment cores from the ocean also hold a wealth of information, much of which is tied up in the remains of plankton, in particular foraminifers (Fig. 7, 8). As these animals form their sand-sized shell, they incorporate elements and natural isotopes from the ocean. For example, the ratio of magnesium to calcium (Mg/Ca) relates to ocean temperature and the ratio of barium to aluminium (Ba/Al) gives an insight into marine fertility that involves the production of plant and animal plankton – the foundation of the marine food chain. To evaluate these chemical clues ANZICE will deploy the University of Victoria’s hi tech laser ablation, inductively coupled plasma, mass spectrometer. This new analytical system uses a laser beam to “drill” through minute objects, measuring liberated elements as it goes.
The use of observation-based and computer-based models is another feature of ANZICE. Data from the core research will be compiled to form observational models that help answer basic questions of the type “How did the Antarctic environment appear during the Holocene Climatic Optimum, 7,000-9,000 years ago, and what were its impact on the Southern Ocean and New Zealand?” The answers will guide and verify the development of computer models to give a measure of confidence in their results. Once operational, computer modelling will give us the chance to visualise future changes under various warmer world conditions. By changing the forces that control a model, for instance, ocean temperature and salinity, air temperature and carbon dioxide content, wind strength, sea ice extent and other factors, we open a window into the future all the while checking the results against
(Fig. 9) A modeled view of the SW Pacific sector of the Southern Ocean. Red-pink bands, south of New Zealand, form the Antarctic Circumpolar Current as it intercepts the S Tasman Rise and Campbell Plateau. By changing factors such as wind, water temperature, and salinity, models allow an insight into projected change. Image Credit: FRAM AND National Environment Research Council, U.K.
Carter, L., 2002: Currents of Change: the ocean flow in a changing world in Carter, L. and Kilroy, C., Turning up the Heat –New Zealand’s ocean in a warming world. Water and Atmosphere 9, National Institute of Water and Atmosphere, Christchurch. 32pp.
Carter, L., Wilkin, J., 1999: Abyssal circulation around New Zealand - a comparison between observations and a global circulation model. Marine Geology 159, 221-239.
Manighetti, B., 2002. Ocean Circulation: the planet’s great heat engine. in Turning up the Heat –New Zealand’s ocean in a warming world. Water and Atmosphere 9, National Institute of Water and Atmosphere, Christchurch. 32pp.
Orsi, A.H., Johnson, G.C., Bullister, J.L., 1999. Circulation, mixing and production of Antarctic Bottom Water. Progress in Oceanography 43, 55-109.