Synthetic temperature profiles derived from Geosat altimetry: Comparison with air‐dropped expendable bathythermograph profiles

Abstract

Synthetic temperature profiles are computed from altimeter‐derived sea surface heights in the Gulf Stream region. The required relationships between surface height (dynamic height at the surface relative to 1000 dbar) and subsurface temperature are provided from regression relationships between dynamic height and amplitudes of empirical orthogonal functions (EOFs) of the vertical structure of temperature derived by de Witt (1987). Relationships were derived for each month of the year from historical temperature and salinity profiles from the region surrounding the Gulf Stream northeast of Cape Hatteras. Sea surface heights are derived using two different geoid estimates, the feature‐modeled geoid and the air‐dropped expendable bathythermograph (AXBT) geoid, both described by Carnes et al. (1990). The accuracy of the synthetic profiles is assessed by comparison to 21 AXBT profile sections which were taken during three surveys along 12 Geosat ERM ground tracks nearly contemporaneously with Geosat overflights. The primary error statistic considered is the root‐mean‐square (rms) difference between AXBT and synthetic isotherm depths. The two sources of error are the EOF relationship and the altimeter‐derived surface heights. EOF‐related and surface height‐related errors in synthetic temperature isotherm depth are of comparable magnitude; each translates into about a 60‐m rms isotherm depth error, or a combined 80 m to 90 m error for isotherms in the permanent thermocline. EOF‐related errors are responsible for the absence of the near‐surface warm core of the Gulf Stream and for the reduced volume of Eighteen Degree Water in the upper few hundred meters of (apparently older) cold‐core rings in the synthetic profiles. The overall rms difference between surface heights derived from the altimeter and those computed from AXBT profiles is 0.15 dyn m when the feature‐modeled geoid is used and 0.19 dyn m when the AXBT geoid is used; the portion attributable to altimeter‐derived surface height errors alone is 0.03 dyn m less for each. In most cases, the deeper structure of the Gulf Stream and eddies is reproduced well by vertical sections of synthetic temperature, with largest errors typically in regions of high horizontal gradient such as across rings and the Gulf Stream front.