Pressure variations on the Atlantic equator

Abstract

Records of more than a year's duration from three deep ocean pressure sensors and three island shallow pressure sensors, all close to the equator between 30°W and 7°E, are analyzed spectrally at frequencies below the diurnal tides. The records were taken as integral parts of the FOCAL‐SEQUAL experiments in the equatorial Atlantic, 1983–1984. The low‐frequency gravitational tides at 1, 2, 3 cycles per month formed an important, nearly simultaneous part of the signal and had to be removed. The seasonal variations associated with equatorial upwelling are prominent in the shallow but not the deep pressure signals. Practically all the non tidal deep pressure signals at 0.1–0.7 cycles per day (cpd) are highly coherent between 20°W and 6°E, some 2900 km, and show no sign of zonal propagation. A comparison of the coherent part of the signals between the equator and Ascension Island (8°S) suggests a southward component of propagation, at least in the region of the Mid‐Atlantic Ridge. These motions are evidently barotropic and are not trapped near the equator. A close relationship to the atmospherically forced barotropic waves at 4–6 days period in the Pacific, identified by Luther (1980), is suggested. Special consideration is given to a comparison of a 3.6‐km‐deep pressure record at 6°E with a shallow pressure record from Ilha São Tomé' (supplied by J. M. Verstraete). Their ordinary tidal components are practically identical, as befits purely barotropic motion, but the sub‐tidal continuum shows fairly low (though significant) coherence, with higher energy in the island record. This is attributed to the presence of internal wave modes of several meters internal amplitude. Theoretically, the contribution of internal waves should give a much stronger signal in the surface pressure than in the deep pressure signal. The relationships are demonstrated by calculations based on acoustic return times by two inverted echo sounder recorders placed close to deep pressure sites by E. J. Katz. Deduced coherences agree reasonably with observed values and are consistent with internal waves dominated by mode 1, uncorrelated with the barotropic motion, sensed in both deep and shallow pressure gauges.