Departures from the local inverse barometer model at periods of 5 days in the central South Atlantic

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Abstract
Tide gauge data from Ascension and St. Helena in the central South Atlantic, supplemented by altimeter data from the TOPEX/POSEIDON mission, suggest that the local inverse barometer (LIB) model for the forcing of sea level variability by means of air pressure changes is not applicable within a narrow band of the variability spectrum with periods of approximately 5 days. This is in contrast to other sections of the wider frequency band spanning approximately 0.05 to 0.5 cycles/d, wherein the data are either consistent with approximate LIB response or at least suggest no departures from the LIB model of similar magnitude. These results have similarities to findings from Pacific gauges obtained over a decade ago, but so far, no comparisons have been to data from other ocean areas. They are also qualitatively similar to predictions for the region from a recent global barotropic numerical model. The anomalous part of the spectrum coincides in frequency with the Madden and Julian (1972, 1973) 5‐day pressure wave in the atmosphere. If that global scale forcing is responsible for both the Atlantic and Pacific departures from the LIB model around 5 days, then similar features should be observable in tide gauge data from other tropical areas.

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