Fresh Water Feeds Hurricanes’ Fury: Climate Change, the Hydrological Cycle, and Tropical Cyclone Activity
Improving our ability to forecast tropical cyclones and to mitigate their destructive potential requires knowledge of various environmental factors that influence a cyclone’s path and intensity. U.S. Department of Energy scientists at Pacific Northwest National Laboratory found that tropical cyclones intensify considerably when passing over ocean regions with a barrier layer. (A barrier layer in ocean environments, or mixed layer depth, is defined as the depth where the density increases from the surface value due to a prescribed temperature decrease of some value (e.g., 0.2°C) from the surface value while maintaining constant surface salinity value.) Using a combination of observations and model simulations, the team demonstrated that barrier layers, formed through high fresh water input reducing the salinity in the upper tropical oceans, significantly increase the intensity of tropical cyclones. When tropical cyclones pass over these regions, the increased stratification and stability within the layer reduce storm-induced vertical mixing and sea surface temperature cooling. Their findings underscore the importance of observing salinity structure in deep tropical barrier layer regions. As the hydrological cycle responds to global warming, any associated changes in the barrier layer distribution must be considered in projecting future tropical cyclone activity.
Balaguru, K., P. Chang, R. Saravanan, L. R. Leung, Z. Xu, M. Li, and J. S. Hsieh. 2012. “Ocean Barrier Layers’ Effect on Tropical Cyclone Intensification,” Proceedings of the National Academy of Sciences of the USA, DOI: 10.1073/pnas.1201364109.