Satellite Thermal Infrared and Inverted Echo Sounder Determinations of the Gulf Stream Northern Edge

The northern edge of the Gulf Stream off Cape Hatteras, North Carolina was located in 155 AVHRR-derived maps of sea surface temperature (SST) using five different methods. One method was subjective location of the northern edge by an analyst; the other four involved objective location of the edge by computer using various statistics of the SST field. Specifically, the quantities considered were: maximum SST gradient (calculated over a 3 × 3 pixel box), maximum SST (on a pixel-by-pixel basis), maximum variance (calculated over a 7 × 7 pixel box), and change in the skewness of the SST distribution (calculated over a 5 × 5 pixel box). The resulting locations were compared with the location of the 15°C isotherm at 200 m (T15) determined from inverted echo sounders (IESs) moored on the sea floor. The best method, which yielded the smallest rms difference from the IES-derived T15, was the subjective one; the surface front was located 9.0 km shoreward of T15 with a rms difference of 14.3 km. The best objective technique used the skew of the SST distribution: Each pixel in the image was replaced by the skew of the distribution of the twenty-five SST values obtained from a 5 × 5 pixel square centered on that pixel. The skew changes sign when a step in the SST data, such as the Gulf Stream northern edge, is crossed. The Gulf Stream northern edge located in the skew images was found to be 14.0 km shoreward of T15 in the mean with a rms difference of 18.2 km. In general, the more spatial information used, the better the estimate.