ScienceWeek

CLIMATOLOGY: ON THE THERMOHALINE CIRCULATION

The following points are made by B. Hansen et al (Science 2004 305:953):

1) The thermohaline circulation (THC) of the ocean has become a public theme, and not without reason. The THC helps drive the ocean currents around the globe and is important to the world's climate. There is a possibility that the North Atlantic THC may weaken substantially during this century, and this would have unpleasant effects on our climate -- not a disaster-movie ice age, but perhaps a cooling over parts of northern Europe.

2) The THC is a driving mechanism for ocean currents. Cooling and ice formation at high latitudes increase the density of surface waters sufficiently to cause them to sink. Several different processes are involved, which collectively are termed "ventilation". When active, ventilation maintains a persistent supply of dense waters to the deep high-latitude oceans. At low latitudes, in contrast, vertical mixing heats the deep water and reduces its density. Together, high-latitude ventilation and low-latitude mixing build up horizontal density differences in the deep ocean, which generate forces. In the North Atlantic, these forces help drive the North Atlantic Deep Water (NADW) that supplies a large part of the deep waters of the world ocean. Not everybody agrees that the THC is an important driving mechanism for the NADW flow. The north-south density differences observed at depth might be generated by the flow rather than driving it (1). This argument is tempting, but it neglects some salient features of the real ocean that are at odds with many conceptual, analytical, and even some numerical models.

3) The Greenland-Scotland Ridge splits the North Atlantic into two basins. Most of the ventilation occurs in the northern basin, and the cold dense waters pass southward as deep overflows across the Ridge. According to measurements (2-4), the total volume transport across the Ridge attributable to these overflows is only about one-third of the total NADW production, but the volume transported approximately doubles by entrainment of ambient water within just a few hundreds of kilometers after passing the Ridge.

4) On their way toward the Ridge, the overflow waters accelerate to current speeds of more than 1 m/s, which is clear evidence of THC forcing. After crossing the Ridge, the flows descend to great depths in bottom currents, which again are density-driven. In the present-day ocean, THC drives the overflows, which together with the entrained water feed most of the NADW. This is the reason why people worry about a possible weakening of the THC. In the coming decades, global change via atmospheric pathways is expected to increase the freshwater supply to the Arctic. This will reduce the salinity and hence the density of surface waters, and thereby may reduce ventilation. Even if the ventilation comes to a total halt, this will not stop the overflows immediately, because the reservoir of dense water north of the Ridge stabilizes the overflow. Instead, the supply of NADW would diminish in a matter of decades. In contrast, large changes in low-latitude mixing --even if conceivable -- require a much longer time before affecting the THC (5).

References (abridged):

1. C. Wunsch, Science 298, 1179 (2002)

2. R. R. Dickson, J. Brown, J. Geophys. Res. 99, 12,319 (1994)

3. B. Hansen, S. Østerhus, Prog. Oceanogr. 45, 109 (2000)

4. A. Ganachaud, C. Wunsch, Nature 408, 453 (2000)

5. W. Munk, C. Wunsch, Deep-Sea Res. 45, 1976 (1998)

Science http://www.sciencemag.org

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GEOSCIENCE: ON THE ATLANTIC THERMOHALINE CIRCULATION

Notes by ScienceWeek:

In general, a "thermohaline circulation" is a vertical circulation induced by the cooling of surface waters in a large water body, this cooling causing convective overturning and consequent mixing of waters. In the oceans, this circulation usually involves temperature and salinity variations acting together.

In this context, a "geostrophic current" is a current controlled by a balance between a pressure-gradient force and the Coriolis effect. The Coriolis effect is an apparent force that arises because of the Earth's spin around its axis. Freely moving objects are deflected to the right of their direction of motion in the Northern Hemisphere and to the left of their direction of motion in the Southern Hemisphere. The force is proportional to the speed and latitude of the moving object, and thus varies from zero at the equator to a maximum at the poles.

The following points are made by P.U. Clark et al (Nature 2002 415:863):

1) The ocean affects climate through its high heat capacity relative to the surrounding land, thereby moderating daily, seasonal and interannual temperature fluctuations, and through its ability to transport heat from one location to another. In the North Atlantic, differential solar heating between high and low latitudes tends to accelerate surface waters polewards, whereas freshwater input to high latitudes together with low-latitude evaporation tend to brake this flow. Today, the former thermal forcing dominates the latter haline (freshwater) forcing and the meridional overturning in the Atlantic drives surface waters northward, while deep water that forms in the Nordic Seas flows southward as North Atlantic Deep Water. This thermohaline circulation is responsible for much of the total oceanic poleward heat transport in the Atlantic, peaking at about 1.2 +- 0.3 PW (1 PW equals 10^(15) watts) at 24 degrees N (1).

2) No such deep overturning occurs in the North Pacific, where surface waters are too fresh to sink (2). The lack of a meridional land barrier in the Southern Ocean precludes the existence of strong east-west pressure gradients needed to balance a southward geostrophic surface flow, so that poleward heat transport associated with the thermohaline circulation is small there. Deep-water formation in the Southern Ocean occurs along the Antarctic continental shelf in the Weddell and Ross Seas either through intense evaporation or, more typically, through brine rejection that produces dense water that sinks down and along the slope (3). In addition, supercooled water may be formed at the base of the thick floating ice shelves during freezing or melting and this dense water may in turn flow downslope (4).

3) The idea that the Atlantic thermohaline circulation may have many speeds is now a century old (2), but not until the 1960s did a quantitative, albeit idealized, framework emerge to explain the physics behind the potential existence of these multiple equilibria (5). Subsequently, ocean and coupled atmosphere-ocean general circulation models were shown to support multiple equilibria. Such studies have revealed that multiple equilibria exist because the atmosphere responds to anomalies of sea surface temperature, but not salinity. They have further shown that transitions between different states are often abrupt and can be induced through small perturbations to the hydrological cycle.

4) In summary: The possibility of a reduced Atlantic thermohaline circulation in response to increases in greenhouse-gas concentrations has been demonstrated in a number of simulations with general circulation models of the coupled ocean-atmosphere system. But it remains difficult to assess the likelihood of future changes in the thermohaline circulation, mainly owing to poorly constrained model parameterizations and uncertainties in the response of the climate system to greenhouse warming. Analyses of past abrupt climate changes help to solve these problems. Data and models both suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic thermohaline circulation in response to small changes in the hydrological cycle. Atmospheric and oceanic responses to these changes were then transmitted globally through a number of feedbacks. The paleoclimate data and the model results also indicate that the stability of the thermohaline circulation depends on the mean climate state.

References (abridged):

1. Ganachaud, A. & Wunsch, C. Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data. Nature 408, 453-457 (2000).

2. Weaver, A. J., Bitz, C. M., Fanning, A. F. & Holland, M. M. Thermohaline circulation: High latitude phenomena and the difference between the Pacific and Atlantic. Annu. Rev. Earth Planet. Sci. 27, 231-285 (1999).

3. Killworth, P. D. Deep convection in the world ocean. Rev. Geophys. Space Phys. 21, 1-26 (1983).

4. Grumbine, R. W. A model of the formation of high-salinity shelf water on polar continental shelves. J. Geophys. Res. 96, 22049-22062 (1991).

5. Stommel, H. Thermohaline convection with two stable regimes of flow. Tellus 13, 224-230 (1961).

Nature http://www.nature.com/nature

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WARMING OF THE SOUTHERN OCEAN SINCE THE 1950S

Notes by ScienceWeek:

The term "isopycnal" refers to a line joining points of equal density within a water mass. A 3-dimensional surface of equal density is called an "isopycnal surface".

The following points are made by Sarah T. Gille (Science 2002 295:1275):

1) The Southern Ocean plays a critical role in global climate. With no continental barriers, it serves as a conduit to transmit climatic signals between the Pacific, Atlantic, and Indian Oceans. The predominant current of the Southern Ocean, the fast-flowing Antarctic Circumpolar Current, is characterized by strongly tilting isopycnals. Because water mixes preferentially along constant density surfaces, tilting isopycnals bring mid-depth water into contact with the ocean surface and serve as a barrier to southward transport, possibly helping to isolate the Antarctic continent from mid-latitude climate variability (1).

2) Recent examinations of global ocean temperature changes have shown substantial warming in the upper 1000 meters, averaging approximately 0.1 degrees celsius between 1955 and 1995 (2). Southern Hemisphere ocean warming also averages approximately 0.1 degrees celsius over the same time period, but detailed study of the Southern Ocean has been hampered by the limited number of shipboard observations available south of 30 degrees S (3-5). The author reports a study that makes use of the large number of mid-depth temperature observations collected during the 1990s by Autonomous Lagrangian Circulation Explorer floats to characterize modern-day temperatures in the Southern Ocean, the study comparing these temperature measurements with historic shipboard measurements.

3) In summary: Autonomous Lagrangian Circulation Explorer floats recorded temperatures in depths between 700 and 1100 meters in the Southern Ocean throughout the 1990s. The author reports these temperature records are systematically warmer than earlier hydrographic temperature measurements from the region, suggesting that mid-depth Southern Ocean temperatures have risen 0.17 degrees celsius between the 1950s and the 1980s. This warming is faster than that of the global ocean and is concentrated within the Antarctic Circumpolar Current, where temperature rates of change are comparable to Southern Ocean atmospheric temperature increases. The subsurface Southern Ocean has warmed during the past 50 years, and these changes may have broader implications, since the water that is ventilated in the region around Antarctica spreads gradually around the globe.

References (abridged):

1. S. R. Rintoul, C. W. Hughes, D. Olbers, in Ocean Circulation and Climate, G. Siedler, J. Church, J. Gould, Eds. (Academic Press, San Diego, 2001), pp. 271-302.

2. S. Levitus, J. L. Antonov, T. P. Boyer, C. Stephens, Science 287, 2225 (2000)

3. T. P. Boyer et al., World Ocean Database 1998, vol. 5, Temporal Distribution of Ocean Station Data Temperature Profiles, NOAA Atlas NESDIS 22, (U.S. Government Printing Office, Washington, DC, 1998).

4. P. S. Wong, N. L. Bindoff, J. A. Church, J. Clim. 14, 1613 (2001)

5. T. P. Barnett, D. W. Pierce, R. Schnur, Science 292, 270 (2001)

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