IES Calibration and Mapping Procedures

Abstract

The procedures used in the Synoptic Ocean Prediction experiment for calibration of inverted echo sounder (IES) travel time τ data have led to substantially improved accuracy. In previous work, τ was converted to main thermocline depth ZT using point measurements of the 12°C depth from coincident expendable bathythermograph (XBT) casts. The new method presented in this study vertically integrates the XBT temperature to produce the quantity QT, which is very tightly related to τ. The advantage of this method is that the calibration constants have roughly half the uncertainty obtained with the traditional method. Seasonal changes in τ determined from hydrographic data in the Gulf Stream region were found to vary with geographic region; near 68°W τ changed by 1.8 ms from March to September, whereas near Cape Hatteras the annual change was only 1.0 ms. The fully calibrated IES Z*T has an estimated error of 19 m (one standard deviation). An iterative optimal interpolation scheme is described, by which the Gulf Stream thermocline depth was mapped on a two-dimensional grid. Initially, daily maps were generated using established mapping techniques. Subsequently, these maps were averaged for 31-day periods and in turn used as the new ?mean fields? for the final maps. Verification of the mapped fields against independent XBT data gives rms differences of 31?46 m, which is only 3%?5% of the 900-m range of the observed Z*T.