Ocean Currents: Is the Global Conveyor Belt Slowing Down?
Ocean waters circulate around the globe in established patterns or currents that derive from the following factors:
• Differences in solar energy received by the equator and the poles
• Topography of the ocean floor and coastal land masses
• Changes in seawater density
• Rotation of Earth around its axis
• Atmospheric winds
One major pattern of ocean currents, the so-called Global Conveyor Belt, or Thermohaline circulation, involves the northward flow of warm surface waters from the Caribbean along the Atlantic coast of the United States. This is known as the Gulf Stream. The flow continues across the Atlantic Ocean toward Great Britain, a current called the North Atlantic drift. These warm currents contribute to the higher average temperatures of the East Coast of the United States, Europe, and Scandinavia, which are about 5°C warmer than other land masses at the same latitude.
In the North Sea, these waters become denser as their temperature drops, and they become more saline from evaporation of surface water and ice formation. The denser waters flow south as the Greenland and Labrador Currents and. eventually they reach Antarctica and veer eastward. This deep, cold flow, the Antarctic Circumpolar Current, splits both around the Horn of Africa and around New Zealand and heads northward. Passing across the equator, these waters warm, rise to the surface, and join the general westward flow of the Equatorial Currents across the Pacific, the Indian, and, finally, the Atlantic oceans.
Global Conveyor Belt of ocean currents through which warm surface currents of the North Atlantic (red line) cool around Greenland and become more salty as water evaporates and icebergs form. Both processes increase the density of the water, which sinks in the Atlantic, forming a cold bottom current (blue line). This current circulates to Antarctica, until it warms in the Indian or Pacific ocean and again rises to the surface. The black circle in the central Labrador Sea denotes where satellite measurements of sea level since 1994 indicate decreased southward current flows. The dashed line between the North American and African continents denotes a transatlantic section where measurements over the last 50 years show slowing southward currents in the mid-ocean.
Polar regions have experienced rapid warming over the last 25 years, averaging an increase of between 1°C and 2°C per decade. This results from changes in the albedo when snow or ice melts, reveals bare ground, and increases the solar radiation absorbed. This increase is small in absolute magnitude but large in relative terms at the poles, where the total energy input is smaller. This warming trend at the poles has not only diminished the amount of fresh water captured in the Arctic Sea Ice but also has added to the volume of fresh water from rivers flowing into the Arctic Ocean. Both processes decrease the salinity of the waters in the North Atlantic and, thereby, their density.
Might these changes interfere with the Global Conveyor Belt? Several lines of evidence indicate that the Global Conveyor Belt in the Northern Atlantic has slowed in recent years. Satellites have monitored sea surface heights all over the world with high precision for over a decade. Because water flows downhill, differences in sea surface heights from place to place indicate the direction and magnitude of currents.
The Labrador Current by this measure has been declining.  Moreover, oceanographers have conducted transects across the Atlantic Ocean at latitude 25° north five times since 1957, measuring water temperatures and current flow and direction at various depths. In particular, the North Atlantic Oscillation refers to the long-term fluctuations (ranging in period from decades to centuries) in the relative strengths and positions of a low atmospheric pressure system sitting near Iceland versus a high-pressure one sitting near the Azores . Large differences in the pressure between these two systems (a condition known as a “high index”), especially during the winter months, bring warm, moist westerly winds into Europe and, consequently, are associated with cool summers and mild winters in the region. By contrast, a small pressure difference (a “low index”) suppresses westerly winds and induces more-severe seasonal temperature swings in Europe.
The North Atlantic Oscillation since 1994 appears roughly correlated with changes in the surface height of the Labrador Sea . The surface height is an indicator of the strength and direction of the Labrador Current. Detailed analyses of these data, however, have lead to divergent conclusions: Some investigators believe the contributions of the North Atlantic Oscillation to changes in the Global Conveyor Belt to be large , some consider it medium , and others believe it to be small . Additional measurements over a longer time should provide more definitive answers about the relationships among currents, temperatures, and pressures in the North Atlantic and other regions.
 Han, G. Q. and C. L. Tang (2001) Interannual variations of volume transport in the western Labrador Sea based on TOPEX/Poseidon and WOCE data. Journal of Physical Oceanography 31:199-211.
 Latif, M., C. Boning, J. Willebrand, A. Biastoch, J. Dengg, N. Keenlyside, U. Schweckendiek, and G. Madec (2006) Is the thermohaline circulation changing? Journal of Climate 19:4631-4637.
 Böning, C. W., M. Scheinert, J. Dengg, A. Biastoch, and A. Funk (2006) Decadal variability of subpolar gyre transport and its reverberation in the North Atlantic overturning. Geophysical Research Letters 33.
 Häkkinen, S. and P. B. Rhines (2004) Decline of subpolar North Atlantic circulation during the 1990s. Science 304:555-559.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents