Atmospher Sci & Global Chg
Research Highlights

June 2018

Oceanic Divide Factors Into Tropical Cyclone Strength

Sea surface temperatures have less influence on tropical cyclone intensification in the Atlantic than in the Pacific.

The Science

Tropical cyclones intensify using heat from the ocean surface, so sea surface temperatures (SSTs) under the eye of the storm are critical to their development. However, pre-storm SSTs are poor predictors of tropical cyclone intensification in the Atlantic basin compared to the eastern and northwest Pacific.

Researchers at the U.S. Department of Energy's Pacific Northwest National Laboratory contributed to a study showing that SST does not have as much influence in the Atlantic as in the Pacific. They uncovered a combination of factors explaining why the relationship between SST and tropical cyclone intensification differs among ocean basins.

The Impact

Sea surface temperatures are one of the most important predictors in statistical intensity prediction models, which consistently perform better in the Pacific compared to the Atlantic. The findings of this study could help improve statistical model predictions for the Atlantic.

The schematic shows the strength of the relationship between tropical cyclone intensification rate and pre-storm SST in each ocean basin, based on contributions from: along-track pre-storm SST variance (SST var.); the combination of TC initial intensity and SST wake (Intens.); and the background atmospheric conditions (Atmos.). Across all basins, red indicates an increased correlation and blue indicates a reduced correlation; black notes a negligible difference. Larger words and arrows indicate the most important variable per basin.

Sea surface temperature is one of the most important parameters for tropical cyclone intensification. This study showed that the relationship between SST and tropical cyclone intensification varies considerably among ocean basins, with SST explaining less than 4 percent of the variance in tropical cyclone intensification rates in the Atlantic, 12 percent in the northwest Pacific, and 23 percent in the eastern Pacific.

The study revealed several factors responsible for these interbasin differences. First, SST variability along tropical cyclone tracks is lower in the Atlantic. This is because the SST variability across the domain is relatively small in the Atlantic compared to the eastern Pacific. Also, storm-induced cold wakes in the Atlantic reduce the pre-storm SST contribution to tropical cyclone intensification. The damping occurs because SST tends to vary in phase with tropical cyclone-induced SST cooling. In the Gulf of Mexico and northwestern Atlantic, where SSTs are highest, tropical cyclones tend to be strongest and their translations (the speeds at which they move forward) slowest, resulting in the strongest storm-induced cooling. The tendency for tropical cyclones to be more intense over the warmest SST in the Atlantic also limits the usefulness of SST as a predictor since stronger storms intensify less.

Finally, in the northwest Pacific, SST tends to vary out of phase with vertical wind shear (change of wind speed or direction with height) and outflow temperature (the temperature in the upper troposphere). This strengthens the relationship between SST and tropical cyclone intensification more in the northwest Pacific than in the eastern Pacific or Atlantic.

Acknowledgments

Sponsors: The U.S. Department of Energy (DOE) Office of Science, Biological and Environmental Research supported KB and SMH as part of the Regional and Global Climate Modeling program. GRF was supported by the National Oceanic and Atmospheric Administration (NOAA) Climate Program Office and base funds to NOAA/Atlantic Oceanographic and Meteorological Laboratory.

Research Area: Climate and Earth Systems Science

Research Team: Gregory R. Foltz, NOAA/Atlantic Oceanographic and Meteorological Laboratory; and Karthik Balaguru and Samson Hagos, PNNL

Reference: G.R. Foltz, K. Balaguru, S. Hagos, "Interbasin Differences in the Relationship between SST and Tropical Cyclone Intensification." Monthly Weather Review 146(3), 853-870 (2018). [DOI: 10.1175/MWR-D-17-0155.1]