The Upper Ocean's Thermal Response to Tropical Cyclones

The ocean has long been recognized as a source of energy for tropical cyclones (TCs) and many past studies have investigated the role of the ocean in creating favorable or unfavorable conditions for TC intensification. A few studies have concentrated on the response of the ocean to tropical cyclone passage, and the majority of those employed case studies. In this study we use five years of tropical cyclone best track data and a nearly global ocean (65o N to 65o S) dataset that contains estimates of various measures of upper ocean energy content to study the response of the ocean mixed layer to tropical cyclone passage.

An important part of this work is development of a five-year global ocean dataset. Daily ocean analyses are created using the Navy Coupled Ocean Data Assimilation (NCODA, Cummings 2005) system and datasets contained in the Global Ocean Data Assimilation Experiment (GODAE) data archive (www.usgodae.org). A data fitting approach is used, where the first guess field is provided by the previous analysis, rather than a model forecast. Following the data assimilation, several quantities that relate to the upper ocean heat content were derived including two measures of Oceanic Heat Content or Tropical Cyclone Heat Potential, one using the depth of the 20o isotherm and the other, and the more traditional calculation, which uses the depth of the 26o C isotherm. Other parameters such as average temperatures to 100m (or to the ocean floor), the depth of the 0.5oC temperature inversion, 0.15 kg/m density inversion and the depth of the maximum stability were also computed.

Using the TC best track positions and the ocean data described above, we compiled homogeneous statistics of the response of the ocean to TC passage. Then we constructed composites of various measures of the upper ocean's heat content and based on latitude, intensity, translation speed, initial conditions, and storm kinetic energy. The goal of this effort is to work from this large sample of ocean-based results and answer the following questions about TCs and the ocean: 1. How much energy does a TC typically extract from the ocean? 2. How much does the ocean typically cool at the surface and in the mixed layer? 3. What are the important and observable factors of a TC that are related to oceanic energy extraction /cooling? 4. How long does the TC induced cooling typically last? 5. Are there any obvious differences between the ocean basins with respect to the ocean response? Using the composite results, we will attempt to answers the above questions.

Disclaimer: The views, opinions, and findings contained in this manuscript are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration or U.S. government position, policy, or decision.