On the nearshore hydrography of the upper Texas coast

Abstract

Analyses of nearshore current, wind, and hydrographic data collected along the upper Texas coast are presented. Results show that hydrographic variations due to advection have a pronounced effect on the overall hydrography of the area. A surface core of fresher water, most likely from the Mississippi delta region, is advected along the Texas coast due to longshore wind stress and can form a strong frontal system 10-30 km offshore for the majority of the year, including winter. The presence of the fresher water appears to modify the mean current regime as well as modify vertical fluxes of heat and oxygen. Variations of the mean longshore current direction result in bottom waters of the study area alternating between characteristics of the fresher and saltier waters. Time scales of these variations are on the order of 12 hours to weeks. And the magnitude of the oxygen content, salinity, and temperature differences of the two water types can result in large variations in environmental conditions. A frictionless analytical model driven by time-varying wind stresses has been developed. Theoretical relationships from the model are considered in the study of observed wind stress and current power, coherence, and phase spectra at sub-inertial frequencies. These considerations indicate a need for a vertical shear stress term in the longshore momentum balance equation. The addition of such a term is required to balance the windstress at lower frequencies. Power spectra of wind stress and bottom currents during the summer of 1978 show that the model can predict longshore current speed amplitudes relatively well but tends to underestimate cross-shelf current speed amplitudes. This is likely a result of non-local forcing during a period of relatively small wind stressing. During the fall of 1978 and the winter of 1979, the power spectra of the wind stress and bottom currents show that the model overestimates both longshore and cross-shelf current speed amplitudes.