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December 14, 1998

Major changes in ocean circulation may have accompanied past global warming

By Tim Stephens

Changes in the wind-driven ocean circulation may have been a key factor in sustaining unusually warm polar temperatures during certain periods in Earth's history.

Graduate student Matthew Huber and assistant professor of earth sciences Lisa Sloan, both of UC Santa Cruz, used computer models to explore how surface winds and ocean currents might change under the temperature conditions that prevailed during past periods of global warming. The results were dramatic in some cases, with drastic rearrangements of the large-scale atmospheric and oceanic circulation.

Warm climate intervals documented in the geologic record include the Eocene, about 36 to 55 million years ago, and the Late Paleocene Thermal Maximum (LPTM), about 56 million years ago. The Eocene warming was gradual and long-lasting, whereas the LPTM was a time of rapid warming that lasted less than 100,000 years. During the Eocene, crocodiles lived within the Arctic circle and palm trees grew in what is now Wyoming.

While temperatures at high latitudes were unusually warm during these periods, temperatures in the tropics were much the same as today. As a result, the temperature gradient from warm tropics to cool polar regions was much weaker than it is today, with profound implications for atmospheric and oceanic circulation patterns.

The equator-to-pole temperature gradient is the engine that drives major wind patterns such as the tropical trade winds and the temperate zone westerlies. These surface winds, in turn, drive oceanic currents, creating huge gyres that circulate water throughout the oceans. The oceanic circulation transports not only water, but also heat, salt, and other elements around the globe. The oceanic transport of heat between the tropics and higher latitudes is a major determinant of global climatic conditions.

Huber used a sophisticated climate model called GENESIS to predict the wind-driven currents that would result from various warm climate scenarios. His results suggest that warm currents may have extended farther north during the Eocene and the LPTM than they do today.

"In very specific regions, such as the North Pacific, we find the warm western boundary currents penetrate into the far northern latitudes," Huber says.

The most extreme changes occurred when Huber used the model to simulate conditions during the LPTM. In some cases, winds switched direction and oceanic gyres that now turn clockwise began cycling counterclockwise.

"The model shows a very foreign-looking world, so it's hard to recognize what's going on, but it is really fascinating to study," Huber says.

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