Quantification of Improvements in an Operational Global-Scale Ocean Thermal Analysis System

Abstract

Global-scale analyses of ocean thermal structure produced operationally at the U.S. Navy's Fleet Numerical Oceanography Center are verified, along with an ocean thermal climatology, against unassimilated bathythermograph (bathy), satellite multichannel sea surface temperature (MCSST), and ship sea surface temperature (SST) data. Verification statistics are calculated from the three types of data for February?April of 1988 and February?April of 1990 in nine verification areas covering most of the open ocean in the Northern Hemisphere. The analyzed thermal fields were produced by version 1.0 of the Optimum Thermal Interpolation System (OTIS 1.0) in 1988, but by an upgraded version of this model, referred to as OTIS 1.1, in 1990. OTIS 1.1 employs exactly the same analysis methodology as OTIS 1.0. The principal difference is that OTIS 1.1 has twice the spatial resolution of OTIS 1.0 and consequently uses smaller spatial decorrelation scales and noise-to-signal ratios. As a result, OTIS 1.1 is able to represent more horizontal detail in the ocean thermal fields than its predecessor. Verification statistics for the SST fields derived from bathy and MCSST data are consistent with each other, showing similar trends and error levels. These data indicate that the analyzed SST fields are more accurate in 1990 than in 1988, and generally more accurate than climatology for both years. Verification statistics for the SST fields derived from ship data are inconsistent with those derived from the bathy and MCSST data, and show much higher error levels indicative of observational noise. Verification of the subsurface thermal fields with bathy data clearly show improvements in the accuracy of the analyzed thermal fields between 1988 and 1990, even though the number of hathy observations available for assimilation into the analysis is less in 1990 than in 1988. The analyzed subsurface thermal structure is also generally more accurate than climatology, particularly in 1990, indicating that the OTIS model makes effective use of the bathy data. Errors are much larger in the western halves of the midiatitude ocean basins than in the eastern halves, primarily as a result of the strong and unresolved fronts and eddies associated with the western boundary currents. Prominent subsurface maxima in the error profiles for both the analysis and climatology, probably a result of unresolved thermocline variability, are present in all five tropical verification areas.