IES CALIBRATION AND MAPPING PROCEDURES

The procedures used in the Synoptic Ocean Prediction experiment for ca libration of inverted echo sounder (IES) travel time tau data have led to substantially improved accuracy. In previous work, tau was convert ed to main thermocline depth Z(tau) using point measurements of the 12 degrees C depth from coincident expendable bathythermograph (XBT) cas ts. The new method presented in this study vertically integrates the X BT temperature to produce the quantity Q(T), which is very tightly rel ated to tau. The advantage of this method is that the calibration cons tants have roughly half the uncertainty obtained with the traditional method. Seasonal changes in tau determined from hydrographic data in t he Gulf Stream region were found to vary with geographic region; near 68 degrees W tau changed by 1.8 ms from March to September, whereas ne ar Cape Hatteras the annual change was only 1.0 ms. The fully calibrat ed IES Z(T) has an estimated error of 19 m (one standard deviation). An iterative optimal interpolation scheme is described, by which the G ulf Stream thermocline depth was mapped on a two-dimensional grid. Ini tially, daily maps were generated using established mapping techniques . Subsequently, these maps were averaged for 31-day periods and in tur n used as the new ''mean fields'' for the final maps. Verification of the mapped fields against independent XBT data gives rms differences o f 31-46 m, which is only 3%-5% of the 900-m range of the observed Z(T) .