National Oceanic and Atmospheric Administration

Atlantic Oceanographic and Meteorological Laboratory

Physical Oceanography Division

Relationship Identified Between Atlantic "Warm Pool" and U.S. Landfalling Hurricanes

In this study highlighted in the Editors' Choice of Science Magazine (issue of Oct. 21, 2011), NOAA scientists have identified a relationship between large–scale climate factors, the Atlantic warm pool, and hurricanes making landfall in the U.S. This relationship indicates that a large warm pool is an unfavorable condition for hurricanes to landfall on the United States coast.

In a paper recently published in the American Geophysical Union journal Geophysical Research Letters, scientists used more than 40 years of oceanic, atmospheric, and hurricane data, as well as a computer model, to study the role that the Atlantic warm pool played in keeping storms from landfall.

The "warm pool" is a large body of warm water in the tropical North Atlantic that exceeds 83.3°F (28.5°C). The warm pool in 2010 was double its average size, likely contributing to the environmental conditions that steered storms away from the U.S. by influencing wind patterns and storm formation sites.

The 2010 hurricane season was one of the most active on record with 19 named storms. A dozen of them strengthened to hurricanes and five reached category 3 to 5 (winds of at least 110 mph) on the Saffir-Simpson hurricane scale. None made landfall in the United States.

"The Atlantic warm pool of 2010 was much larger than its normal size and extended from the east coast of Mexico to the west coast of Africa. The large warm pool caused a change in atmospheric circulation, which steered hurricanes toward the northeast as they approached the U.S.," said Chunzai Wang, lead author and a physical oceanographer and climate scientist with AOML's Physical Oceanography Division.

Both a large warm pool in the tropical North Atlantic and a La Niña event in the tropical Pacific increase the number of storms in the Atlantic; however, their influences on the hurricane track are opposite. While a large Atlantic warm pool influences storms to stay over the open ocean east of the United States, a La Niña event tends to enhance the possibility for a hurricane to make landfall in Central America, Caribbean Islands, and the southeastern United States.

"Our study indicates that the effects of the 2010 La Niña on hurricane tracks were offset by the influences of an extremely large Atlantic warm pool, which may partially explain why no hurricanes made landfall in the United States," Wang said.

The scientists also found that when the Atlantic warm pool is large and expands eastward, the nursery where tropical storms are born moves closer to the west coast of Africa. The eastward shift of this tropical storm birthplace contributes to the decreases in the possibility for a hurricane to make landfall by allowing more time for the storm to turn north before impacting the U.S. east coast. The combination of the warm water-induced steering winds and birthplace shift reduces the storms' ability to make landfall in the United States.

As part of their overall analysis, the team noted that in years when the Atlantic warm pool was small, the associated steering winds contributed to favorable conditions for landfall in the U.S. In years when the warm pool was at its normal size, the changes in the steering winds were very small compared with the years when there was a large or small warm pool.

"Most hurricane-related costs and damages occur when hurricanes make landfall." said Robert Atlas, AOML director and a co-author of the paper. "Our study suggests that a better prediction of climate variability in conjunction with further improvements in the numerical models can help us to make seasonal projections for landfalling hurricanes in the future." AOML is currently researching other large-scale climate phenomena, using numerical modeling, to understand and predict additional factors that influence seasonal hurricane tracks.

In addition to Wang and Atlas, other authors of the study include Hailong Liu and Sang-Ki Lee, both researchers at AOML and NOAA's Cooperative Institute for Marine and Atmospheric Studies at the University of Miami.

Figure. Shown are the TC genesis location (dots) and sea surface temperature (color shading) for (a) large and (b) small Atlantic warm pool years and the tracks of TCs that formed in the main development region for (c) large and (d) small Atlantic warm pools.

Wang, C., H. Liu, S.-K. Lee, and R. Atlas, 2011. Impact of the Atlantic warm pool on United States landfalling hurricanes. Geophys. Res. Lett., 38, L19702, doi:10.1029/2011GL049265

The current and past evolution of the AWP index since 1985 can be found here: Warm Pool Monitoring at NOAA/AOML.