Diagnostic calculations of circulation over the northeastern Gulf of Mexico

Comparison is made of three methods for the calculations of the surface dynamic height using hydrographic station and shipboard acoustic Doppler current profiler (ADCP) data over the northeastern Gulf of Mexico. Hydrographic station and ship board ADCP data were collected on eight cruises of the Northeastern Gulf of Mexico Chemical Oceanography and Hydrography Program (NEGOM-COH), supported by the U.S. Minerals Management Service (MMS). Measurements were taken along 11 cross-shelf lines and the 1000-m isobath in the region between the Mississippi River delta and Tampa, Florida.; The calculations of surface dynamic height and associated geostrophic velocity are referred to as: M0, Montgomery's (1941) method; M1, the method of Sheng and Thompson (1996); and M2, the solution of Poisson's equation using an estimated field of vorticity based on the ADCP measurements (Cho, 1996). Among the three methods, M2 yields the closest mean velocity and vorticity to the ADCP measurements. Furthermore, M2 also has the highest correlation of the calculated velocity (0.82) and vorticity (0.98) to the observed ADCP fields. In contrast, the widely used M0 does not perform well in this region, showing significant differences between the calculated and the observed velocity and vorticity. The overall correlation between the calculated velocity field and the observed velocity field is 0.48 and that between the calculated vorticity field and the observed vorticity field is 0.35.; The sea surface topography mapped by M2 is thus considered the benchmark for comparison with M0 and M1. Roughly 33% of the variance of the observed velocity (the divergent part) has been filtered out by M2. Comparison is further made between the calculated geostrophic, or non-divergent, velocity by M0 and M1 with that by M2. The correlations between the M1 and M2 methods are 0.698 for calculated velocity and 0.539 for calculated vorticity, respectively, while those between the M0 and M2 methods are 0.453 and 0.355, respectively.; It is concluded that the Montgomery (M0) method for estimating dynamic topography and associated geostrophic currents should be restricted to those continental shelves for which the flow is primarily along-shelf with associated sea bed density structure which is primarily a function of the local water depth. Otherwise the method M1 or M2 should be employed, if adequate direct current measurements are available (at least along the open boundaries for M1 or throughout the shelf for M2).