Eastern Pacific Ocean Heat Content Estimates for SHIPS Forecasting
Lynn K. Shay and Jodi Brewster
Division of Meteorology and Physical Oceanography Rosenstiel School of Marine and Atmospheric Science University of Miami, Miami, FL 33149
Using a hurricane season (May through October) ocean climatology derived from the U.S. Navy’s Generalized Digital Environmental Model, isotherm depths, reduced gravities and oceanic heat content (OHC) estimates are compared to profile data acquired from moored measurements from the Tropical Atmosphere and Ocean (TAO) array in the Eastern Pacific Ocean and during the Eastern Pacific Investigation of Climate (EPIC) in Sept and Oct 2001 sponsored by NSF and NOAA. During EPIC, mesoscale oceanic current, temperature and salinity variability was mapped by deploying oceanic airborne expendable current profilers and expendable conductivity, temperature and depth profilers (AXCPs, AXCTDs) from NOAA WP-3D research aircraft.
Comparisons of SSTs and isotherm depths have been made at the TAO buoys where hourly thermal structure was measured as well as salinity at selected depths. SSTs from the TRMM microwave imager (TMI) provided a better estimate than the Reynolds SST product (RMS differences where 0.5oC compared to 0.6oC). Regression slopes indicate that the TAO-TMI data have a slope of 0.94 compared to TAO-Reynolds data of 0.63, leading to an OHC underestimation using Reynolds 7-day analysis. By blending several radar altimetry products from NASA TOPEX/Jason-1, U.S. Navy Geosat-Follow-On mission, and European Research Satellite (and/or Envisat), surface height anomaly (SHA) fields are objectively mapped to 0.5o grid to estimate isotherm thickness of 26oC water, and OHC. One particular focus is to map an observed warm eddy observed during EPIC to improve the satellite-retrievals. These warm eddies, with surface elevations of 12 to 15 cm, propagate west-southwest at a speed of 13 cm s-1 in accord with in situ measurements with 26oC isotherm depths of about 50 to 55 m (OHC ~ 50 kJ cm-2).
In the Eastern Pacific Ocean, the seasonal thermocline shoals and tightens the stratification (thermal structure), forcing large buoyancy frequencies (vertical density changes) across the oceanic mixed layer base. Given the strength of the stratification (i.e. shape of the thermal profile), the area underneath the curve relative to the SST and 26oC isotherm depth is estimated and compared to in situ profiles and TAO time series. At 10oN and 95oW, for example, satellite-inferred isotherm depths and OHC estimates are within a few meters and kJ cm-2, respectively suggestive of accurate retrievals from radar altimetry and TMI. The approach is being applied to several years of measurements to assess uncertainties and errors in the retrievals to build a reliable climatology for use with SHIPS forecasting in the Eastern Pacific Ocean.