An Upper Ocean Thermal Field Metrics Dataset

The upper ocean is a heat source for the tropical atmosphere, and is particularly important for tropical cyclone (TC) formation and intensification. A widely used metric to represent the ocean thermal fields is the Ocean Heat Content (OHC), a vertical integral of heat content above the 26° C isotherm. The OHC has been used as a predictor in statistical-dynamical intensity models with some success over the last few years. The OHC parameter works well for these statistical-dynamical models in open oceans, but could probably be improved upon for TCs approaching shallow water (where the 26° C isotherm intersects the ocean floor) and cooler water (where the 26° C isotherm outcrops). In this study, we developed a suite of products for the years 2005-2011 that are related to the heat content of the upper ocean and that make use of a high-resolution Sea Surface Temperature (SST) analysis and a state-of-the-art oceanic data analysis.

A twice-daily, three-dimensional ocean analysis using a 12-h update cycle is made using the Navy Coupled Ocean Data Assimilation (NCODA; Cummings 2005). NCODA three-dimensional analysis fields are available on the GODAE data server (http://www.usgodae.org). This multivariate optimum interpolation analysis uses the prior NCODA analysis as a first-guess, and assimilates observations from buoys, expendable bathythermographs, ships, ARGO floats, satellite altimetry and satellite SST to produce a three-dimensional analysis of temperature, salinity, geopotential and velocity. Using the daily 00 UTC NCODA analyses, we generate a suite of two-dimensional ocean heat content products that span the globe between 65° N and 65° S on a cylindrical grid at 0.25° resolution.

The 26° C OHC and an alternate OHC using the 20° C isotherm (OHC20C) are calculated via integration and result in energy units [kJ cm-2]. The purpose of OHC20C is to provide more continuous ocean heat estimates, especially poleward of the 26° C isotherm where TCs sometimes track. Values of OHC20C could also provide information in areas where surface stress-induced mixing might apply to deeper levels.

As alternatives to the integrated approach used to calculate the 26° C OHC, we also calculate a range of layer-averaged temperature products, beginning with the 100 m average (T100). This approach, suggested by Price (2009), is based on the typical depth of vertical mixing by a mature tropical cyclone. T100 is simple to calculate and understand, and provides a continuous measure of upper ocean heat even in relatively shallow water and outside of the tropics. In a similar manner, three additional layer-averaged temperature products have been derived. Each is an attempt to define the average temperature above a local depth below which mixing is not expected to occur. The first is an average temperature down to the level of maximum stability. The second is the average temperature in a mixed layer where the depth is defined as the level where the temperature is 0.5° C less than the surface temperature. Lastly, we define an average temperature to a depth where the water density increases by 0.15 kg/m3 relative to the surface. Topology fields of these derived products are also produced.

These ocean heat metrics are now being used for tropical cyclone diagnostic studies and are being evaluated as possible input parameters for the Statistical Hurricane Intensity Prediction Scheme (SHIPS) and the Logistic Growth Equation Model (LGEM). The existing dataset fields are available for download from the Naval Research Laboratory web site (http://www.nrlmry.navy.mil/atcf_web/nopp_ohc/). Future plans are to produce the new parameters twice daily on the GODAE server.

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.