Mean hydrographic fields and their interannual variability over the Texas-Louisiana continental shelf in spring, summer, and fall

Abstract

New hydrographic data from the Texas-Louisiana continental shelf were combined with data from older cruises covering significant portions of this shelf to produce spatial distributions of surface and bottom temperature and salinity as well as of surface geopotential anomaly relative to 70 dbar. These were used to calculate mean fields with their standard deviations for spring (May), summer (July-August), and fall (November). For each season, histograms were prepared of differences between properties in the individual fields and our seasonal mean values at each grid point in the individual fields. These histograms have highly tuned Gaussian distributions centered on zero differences, proving that a distribution selected randomly will likely be quite similar to the mean for the season in which the sample was made. The individual fields of salinity for summer and geopotential anomaly for spring are included for comparison with the mean fields. The mean fields, produced by adding a large data set to that used by Cochrane and Kelly [1986], substantiate the bimodal annual patterns of circulation and property distributions over the inner shelf region described by them. Essentially, there is downcoast (directed from the Mississippi toward Brownsville) nearshore flow except during the summer months. That flow is driven by downcoast along-shelf wind and enhanced by Mississippi-Atchafalaya River discharge. In July and August the average wind has an upcoast component and the nearshore flow is reversed. Patterns and values of the standard deviations are used to infer causes and magnitudes of interannual variability, respectively. Three examples of anomalous property distributions are presented to illustrate the effects of the principal external forcing mechanisms affecting interannual variability on the Texas-Louisiana shelf. These mechanisms are wind stress, Mississippi-Atchafalaya River discharge, and mesoscale eddies in the offshore circulation near the shelf-slope break. For each cruise examined, residuals of geopotential anomaly and surface salinity relative to the seasonal mean are examined in relation to departures of river discharge from the long-term (64 year) average and an index of along-shelf wind component appropriate to the times of the cruises. The residuals of geopotential anomaly were found to be significantly negatively correlated with those of surface salinity, with an intercept of approximately zero indicating that salinity plays the dominant role relative to temperature in year-to-year variability of the geopotential anomaly. Positive river discharge residuals were correlated with negative surface salinity residuals; enhanced downcoast wind resulted in negative surface salinity residuals; and enhanced upcoast wind resulted in positive surface salinity residuals. Most correlations were significant (different from zero) at the 95% confidence level.