Cumulative Impact of Tropical Cyclones on Ocean Heat Content & Transport

Michael Bueti,
Isaac Ginis,
Lew Rothstein,
Stephen Griffies

This is project is funded by WeatherPredict Consulting

In collaboration with: NOAA/Geophysical Fluid Dynamics Laboratory (GFDL)

With the help of GFDL, we have developed a modeling framework for studying the impact that tropical cyclones have on the global ocean on interseasonal to interannual timescales. Strong surface winds of a hurricane locally cool the surface and warm the subsurface waters via turbulent mixing processes. While surface cool anomalies generally decay in roughly a month, subsurface anomalies can persist over a seasonal cycle, providing a net ocean heat uptake over the course of one to several years.

Figure 1. Schematic of the physical mechanisms responsible for the formation of tropical cyclone mixing-induced upper thermocline warm anomalies.

We use GFDL's MOM4, a global mesoscale eddy permitting ocean circulation model, forced by tropical cyclones blended with the daily CORE atmosphere, to examine questions related to the magnitude and cumulative footprint of subsurface warm anomalies forced by tropical cyclones during the 2004-2005 global tropical cyclone season and beyond.

Figure 2. CORE 10 m wind speed magnitude, with and without embedding of synthetic winds for 2004's Hurricane Frances.

We also consider the time scales over which subsurface heat anomalies decay. Physical characteristics of cyclone-forced surface and subsurface anomalies are elucidated using this model tool. In particular, we examine the spatial extent and magnitude of storm forced subsurface warm anomalies over the entire season.

Figure 3. Mean upper thermocline temperature anomaly, averaged from June 1, 2004 through May 31, 2005. Strong, persistent heating seen throughout the tropical cyclone forcing regions.

We estimate the contribution of cyclone-induced anomalies to the ocean heat content and sea surface temperature, and analyze the meridional redistribution of the anomalous heat by ocean circulation, finding that the global ocean has an accumulated heat uptake of approximately 4·1021 J over the course of the year. TC heating was most significant in the North Atlantic and West Pacific, reaching nearly 8% and 7% of the local seasonal upper ocean heat content signal in those respective regions.

Figure 4. Globally integrated heat loss, heat uptake, and accumulated downwelling surface heat fluxes induced by tropical cyclone forcing.

Further, the results indicate a convergence of anomalous heating in the equatorial Pacific, leading us to explore the key regions of meridional transport, and hypothesize the possible climate implications of this heat input.

Figure 5. Globally integrated anomalous heat export into the deep tropics. Negative values indicate a southward heat transport. Over our five year record, there is a clear equatorial convergence of tropical cyclone induced heating.

Selected References