NASA - National Aeronautics and Space Administration

We're all familiar with the phenomenon El Nino and La Nina, where unusual changes in Pacific Ocean temperatures often touch off a series of drastic weather consequences, but the Atlantic Ocean has a set of wild cards that influence our weather and climate too.

Ocean Currents: Suspects in Climate Change

Ocean waters are constantly on the move. How they move influences climate and living conditions for plants and animals in the oceans, and on land. Ocean currents flow in complex patterns affected by wind, the water's salinity and heat content, changes in the ocean surface, and the Earth's rotation.

Image Right: This image shows the change in sea-surface height, the height the sea surface would be if there were no waves, during the 1990s. The large blue region in the northern Atlantic represents a slowing of the cyclonically circulating gyre. Click on image to enlarge. Credit: NASA

One important current in the Atlantic is the sub polar gyre, created when warm, subtropical waters enter the northeastern Atlantic from the eastward extension of the Gulf Stream and circulate northward and westward in a counterclockwise motion near Iceland and the tip of Greenland. The current loses heat to the atmosphere as it moves north. Westerly winds transport that warmth to Europe, bringing the region milder temperatures. After frigid Labrador Sea winters, the water in the current becomes cold, salty and dense, and plunges beneath the surface before heading slowly southward to the equator.

Sirpa Hakkinen of NASA's Goddard Space Flight Center and Peter Rhines of the University of Washington recently found that heat given off from the ocean to the atmosphere may be a major factor in a recent slowing of the current, possibly signaling dramatic changes in North Atlantic Ocean climate.

The North Atlantic Oscillation: A Fading Suspect

Previous research has suggested that the weakening of the sub polar gyre is connected with wind patterns created by certain phases of a large-scale atmospheric pressure system known as the North Atlantic Oscillation (NAO). The positive phase is associated with mild winters in eastern United States and longer growing seasons in western Europe, while the reserve is true during the negative phase. Over the past 20 years, the NAO has been primarily positive, except for two major shifts during the winter of 1995-1996 and again in late 1999 to early 2000.

Image Left: This image of North Atlantic Ocean sea surface temperatures represents an eight-day composite from Sept 6 - Sept 13, 2001 from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on Terra. The red and orange colors represent warmer water, while the blue shades represent cold water in the higher latitudes. The Gulf Stream is evident along the U.S. eastern seaboard. The color bar is in units of degrees Celsius. Click on image to enlarge. Credit: NASA

Despite these phase switches, the sub polar gyre current continued to weaken in 1990s, raising questions about the overall influence of the NAO. Largely because of insufficient data on sea surface heights, it remains unclear whether changes to the current are just a natural trend, or related to climate change.

Hakkinen and Rhines argue that studying factors beyond the NAO, such as ocean circulations in the North Atlantic, including the sub polar gyre, are critical when seeking clues about our climate. Many of the major ocean water masses pass through this region from the Arctic and warmer latitudes. Some computer models have shown that the slowing and speeding of the sub polar gyre can influence the entire ocean circulation system. But, it will likely be another five to 10 years until scientists can begin to draw more concrete conclusions about changes in the ocean climate system.

Satellites Confirm Suspicions

Satellite data makes it possible to view the sub polar gyre over the entire North Atlantic. The joint NASA-CNES (French Space Agency) Topex/Poseidon satellite, released in 1992, has proven vital in ocean current research. Here, its sea-surface height data showed that the Labrador Sea water in the core of gyre warmed during the 1990s. This warming reduces the contrast with water from warmer southern latitudes, which is part of the driving force for ocean circulation.

Image Right: An iceberg glides southward along the east coast of Greenland in the Irminger Sea. The tip of this iceberg is about the size of a 2-story house. Credit: C. A. Linder. For a high resolution version of this image please contact Christopher Linder at http://www.chrislinder.com.


Additional studies examining sea surface height, direct observations of sub-surface oceanic circulations, improved mapping of the Earth's oceans and ice cover analysis will be of great importance in establishing the origin of future climate shifts and changes.

For more information, please visit:

North Atlantic Current Top Story Page

Mike Bettwy
Goddard Space Flight Center