The use of the ERS-2 synthetic aperture radar to study the wind driven ocean circulation in the Gulf of Tehuantepec, Mexico, during Norte events

This thesis examines whether Synthetic Aperture Radar (SAR) images can play a useful role in observing the meso-scale structures characteristic of the Gulf of Tehuantepec during 'Norte' conditions, and whether a combination of nearly coincident SAR and AVHRR derived sea surface temperature images can provide new information about these physical processes occurring in the Gulf.Sets of coincident SAR and AVHRR-derived SST images are shown to illustrate different stages in the response of the Gulf of Tehuantepec to wind forcing during a 'Norte' event. From the close spatial match between the thermal signature of meso-scale structures, and the radar image patterns, we conclude that the SAR could be used to monitor this complex air-sea interaction process even when cloud cover prevents the use of infra-red sensors.Furthermore, the SAR data have revealed much additional detail about the ocean response to wind forcing. Other fronts and current zones which showed no thermal signature have been detected. The SAR also provides a means of determining both wind speed and direction. It has not previously been practicable to measure how sharp the wind front is at the edge of the wind jet. The SAR backscatter shows that it remains extremely tight for a long way offshore, consequently imposing a strong horizontal shear to the sea surface. However, by themselves it would be harder to interpret the SAR images in relation to the Norte processes, and we conclude that the most effective use of remote sensing in to use SAR and SST images in combination.As a case study, a SAR image is selected as expressing a typical thermal front established on the west side of the wind jet during a 'Norte' event to estimate the horizontal currents associated with the front by inverting the wave refraction pattern of the swell crossing the front. The horizontal currents estimated are in very good agreement with values reported in the literature. Furthermore, when these currents are used in a numerical solution of the action balance equation coupled with a radar backscatter model, the normalised radar cross section modulations across the front observed in the SAR image are reproduced well both qualitatively and quantitatively.