Model Helps Scientists Home In On Tropical Climate Controls

It has long been known that tropical climate – by redistributing vast amounts of solar energy through welling hot air and the formation of towering cumulous clouds – influences weather in other parts of the world.

It remains unclear, however, how much the tropics can be affected by higher latitudes.
Now, with the help of a sophisticated computer model, scientists at the University of Wisconsin-Madison have shown that vast atmospheric “bridges” and oceanic “tunnels,” created by overturning air and water, link the high latitudes to the tropics and can warm ocean temperature near the equator.

The finding, reported in the May 13 issue of the journal Geophysical Research Letters, has implications for better understanding global and regional climate change, and is the first to identify high-latitude phenomena that significantly influence climate in the equatorial regions of the world.

The prevailing wisdom, according to Zhengyu Liu, lead author of the paper and director of UW-Madison’s Center for Climatic Research, was that climate and weather phenomena at higher latitudes tended to be static, with no far-reaching influence.

“That was the conventional thinking,” he says. “But our model shows that these phenomena are equally weighted, that climate and weather at higher latitudes have as much of an influence on the tropics as tropical weather and climate influence the higher latitudes. Both are very important.”

The discovery reveals a hidden climate mechanism that may be of critical importance to studies of past and future global and regional climate change, says Liu.

According to the scenario depicted by the modeling experiments conducted by Liu and colleague Haijun Yang, the heat carried via the atmospheric bridges from the tropics to higher latitudes is reduced as a result of warming climate in the higher latitudes. At the same time, warm extratropical water is funneled into the subsurface oceanic tunnels and is carried to the equator where it upwells and warms the tropical ocean.

The study suggests that even a 2-degree Celsius warming of the ocean in regions beyond the tropics can raise ocean surface and subsurface temperatures in the tropics by as much as 1 degree Celsius as less warm air flows out of the tropics and warm, extratropical water is channeled toward the equator by the oceanic tunnels depicted in the study.

“That is a significant change” in temperature, says Liu. “It is fundamentally important.”

The new study, says Liu, provides a missing piece of the climate puzzle. It will enable scientists to gain more insight into climate and climate change as an unknown mechanism is revealed and added to the mix of variables that researchers must grasp as they wrestle with the hugely complex problem of understanding and forecasting climate change.

“The magnitude of this influence and the relative contributions of the atmospheric bridge and oceanic tunnel have remained uncertain,” Liu says. “But we have found that the extratropics exert a strong control on tropical climate. This is our first estimate of the extratropical influence on the tropics.”

The Center for Climatic Research at the University of Wisconsin-Madison is a leading center of research into world climate. It is a part of the UW-Madison Gaylord Nelson Institute for Environmental Studies.