Does global warming affect sea temperature?

A North Atlantic Ocean circulation system was weakened considerably in the late 1990s, which could have dramatic effects on climate, maintains a potentially groundbreaking study from NASA. The slowing of this current--which moves water in a counterclockwise pattern from Ireland to eastern Canada--is an indication of changes occurring in the North Atlantic Ocean.

Known as the subpolar gyre, the current has weakened in the past in connection with certain phases of a large-scale atmospheric pressure system known as NAO (North Atlantic Oscillation). NAO has switched phases twice in the 1990s, while the subpolar gyra current has continued to weaken. Whether the trend is part of a natural cycle or the result of other factors related to global warming is unknown.

"It is a signal of large climate variability in the high latitudes," says Sirpa Hakkinen, a researcher at NASA's Goddard Space Flight Center, Greenbelt, Md. "If this trend continues, it could indicate reorganization of the ocean climate system, perhaps with changes in the whole climate system, but we need another good five to 10 years to say something like that is happening."

Adds Peter Rhines, an oceanographer at the University of Washington, Seattle, "The subpolar zone of the Earth is a key site for studying the climate. It's like Grand Central Station there, as many of the major ocean water masses pass through from the Arctic and from warmer latitudes. They are modified in this basin. Computer models have shown the slowing and speeding up of the subpolar gyre can influence the entire ocean circulation system."

Satellite data makes it possible to view the gyre over the entire North Atlantic basin. Measurements from deep in the ocean, using buoys, ships, and new autonomous "robot" Seagliders are important for validating and extending the satellite data. The subpolar gyre can take 20 years to complete its route. Warm water runs northward through the Gulf Stream, past Ireland, before it turns westward near Iceland and the tip of Greenland.

The current loses heat to the atmosphere as it moves north. Westerly winds pick up that lost heat, creating warmer, milder European winters. After frigid Labrador Sea winters, the water in the current becomes cold, salty, and dense, plunges beneath the surface, end heads slowly southward back to the equator. The cycle is sensitive to the paths of winter storms and to the buoyant fresh water from glacial melting and precipitation, all of which are experiencing great change.

While previous studies have proposed winds resulting from NAO have influenced the subpolar gyre's currents, this study found heat exchanges from the ocean to the atmosphere may be playing a larger role in the weakening current. Using available data, the researchers inferred Labrador Sea water in the core of the 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.