On the variability of the Loop Current in the Gulf of Mexico
It is of considerable interest to know to what extent offshore currents may drive flows on the continental shelf. We have used the northernmost position of the Loop Current, from hydrographic data, to piece together a time series 13 years long. This record samples the lowest frequencies well but undersamples the amplitude of variations with periods of ∼8 months by a factor of 2. The "annual" variation of the Loop Current appears to be a relatively broad spectral peak rather than a sharp spectral line. We find as much power at periods near 30 months as at periods near a year; this is a new result. Both bands seem to be, at least in part, wind forced. There are also fluctuations having periods near 8 months, and this may be a beat frequency. As the 3D-month and annual signals drift in and out of phase over ∼5 years, the envelope of the 8-month signal varies from zero to a maximum of ∼2.5 degrees of latitude, peak-to-peak, which is the same as the range of the 3D-month signal.
Our primary finding is that the north-south fluctuations in Loop Current position are correlated with sea level at the coast and presumably with coastal currents. The results are essentially the same using tidal data at either St. Petersburg or Key West. The phase delay is such that the inferred southerly flowing currents on the shelf reach a maximum before Loop Current position reaches its maximum northern position, by 1 to 3 months. If the Loop Current is inherently unstable, as the numerical model of Hurlburt and Thompson (1980) suggests, the wind forcing may merely set the frequency of the variability. Observations at the outer edge of the West Florida Shelf have shown flow to the south of 10 to 20 cm/sec, persistent over many months, which is consistent with this model.
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