Long-range forecasting is the goal of many an atmospheric scientist, including those in the employ of the National Oceanic and Atmospheric Administration. noaa is trying to develop the capability of predicting climate on longer time scales--a season ahead, or a year ahead, rather than three days ahead, says atmospheric scientist Donald Hansen of the University of Miami. And the generally accepted belief is that the longer-term variations in climate are influenced very strongly not just by the atmosphere but by the temperature structure in the ocean.
In search of clues to help explain long-term climate patterns, Hansen and his noaa colleague Hugo Bezdek recently pored over 45 years’ worth of North Atlantic sea surface temperature records, from 1948 to 1992. Unfortunately, they didn’t find out how to predict the climate a year in advance. But they did uncover some strange ocean phenomena that, so far, defy conventional explanation: huge blobs of warm and cool water slowly roaming around the North Atlantic.
The patches were immense--typically 500 to 600 miles across--but were nevertheless hard to spot. Hansen and Bezdek found them only because faced with so much data, they were forced to simplify their task. We looked particularly at the most extreme things that occurred--the anomalies, only the top 10 and bottom 10 percent of sea surface temperatures, says Hansen. And because they were interested only in long- term variations in temperature, they filtered out any temperature fluctuations that occurred over less than four years.
Hidden in those 45 years of temperature records was the existence of large cold and warm blobs of water--one or two degrees colder or warmer than the surrounding seas--that sprang up in the North Atlantic at various times, and that roughly followed general ocean current patterns-- counterclockwise in the northern Atlantic, clockwise farther south. After slinking about for anywhere from four to ten years, the blobs would disappear. Their depth couldn’t be determined, but they probably extended no more than 1,200 feet.
Among the puzzling aspects of these blobs is their speed: about a mile per day, just one-third or one-quarter the speed of prevailing currents. The water is probably flowing through them and changing its temperature as it goes, says Hansen. He and Bezdek also don’t know how the blobs form or why they last so long, though Hansen says there must be some long-lasting interaction between the atmosphere and the sea that maintains the blobs. Nor do they know if the blobs influence climate, though they note that one warm blob coincided with a prolonged drought in Scandinavia in the late 1950s. While future research might answer these questions, for now much about the blobs remains a mystery.
