Researchers have identified a see-saw relationship between the frequency of hurricanes in the western North Atlantic and the eastern North Pacific that might help refine seasonal forecasts in the years ahead.

With average damages in the US alone estimated at nearly $100 billion a year, and rising, a lot is riding on the ability of forecasters to improve their seasonal predictions.

The researchers say their finding of this "out-of-phase" link between the adjacent ocean basins "implies that the seasonal hurricane outlook may be improved by considering the North Atlantic and the eastern North Pacific together."

The idea of a link between two ocean basins close to one another does not surprise researchers familiar with the large scale of atmospheric circulation, they note, but still, "How are they related?  What physical mechanisms drive this relation?"

The study isolates an atmospheric feature known as "vertical wind shear" which measures the difference in wind speeds between 12,000 meters (about 7.5 miles) up and 1,500 meters (roughly a mile) off the ground.  If the difference is big, hurricanes have a hard time forming.

The image shows how the atmosphere bridges the oceans.  On average, they write, "the wind shear in the main region where hurricanes develop in the North Atlantic tends to be opposite the wind shear in the eastern North Pacific for summer and fall." The study is the work of two Miami, Florida researchers — Chunzai Wang and Sang-Ki Lee of NOAA's Atlantic Oceanographic and Meteorological Laboratory.  It was published this month in the American Geophysical Union journal Eos.

Because prevailing winds differ between the two hurricane-formation regions, changes in large-scale atmospheric circulation result in opposite wind-shear patterns, they write.

The see-saw pattern works like this:  When vertical shear is strong in one ocean basin, powerful high-altitude winds ventilate the rising heat down-wind of the surface disturbance, preventing its development into a hurricane. At the same time, in the other basin, where prevailing winds are blowing the other way, the opposite is the case, because the same large-scale winds dampen the wind shear, encouraging hurricane development.

IMAGE: American Geophysical Union