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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.

TOPEX/POSEIDON (T/P) altimeter data from the Drake Passage (DP) area of the Southern Ocean, near to the highest latitudes accessible by the satellite, are found to agree as well, in general, to data from an extensive set of in situ instrumentation as has been observed for other parts of the ocean. Root-mean-square (rms) differences between subsurface pressures (SSP) measured by altimetry and bottom pressures (BP) measured by in situ devices are approximately 2 mbar in the northern DP and 4 mbar in the south, after application of a 20-day low-pass filter. Two mbar accuracy would suggest that altimetry could be capable of providing useful additional information on Antarctic Circumpolar Current (ACC) transport fluctuations through the DP on seasonal and interannual timescales. The larger rms differences on what is considered to be the more important southern side are explainable in terms of baroclinic variability at the southern end of the World Ocean Circulation Experiment (WOCE) SR1 section. The first WOCE conductivity-temperature-depth data sets from SR1 have confirmed the existence of a major baroclinic "eddy feature" in the south, which could account to a great extent for the larger rms values. Better southern DP locations could possibly have been chosen for WOCE. However, further recent in situ measurements from other locations near to the DP have shown that the southern DP ACC-related signals in SSP or BP are only slightly larger than T/P accuracy, which suggests that, as yet, they cannot be adequately monitored by means of altimetry alone, whichever location is chosen. Even if the altimetric and in situ observations were perfect, would they provide information directly relatable to transport variability through the passage? This question has been studied by means of Fine-Resolution Antarctic Model (FRAM) numerical modeling investigations which have demonstrated the complexity of the correlation between the variability in DP transport and SSP (or sea level). Within FRAM, northern SSP (or BP) variations are virtually uncorrelated with transport fluctuations, and the modelling has confirmed the importance of southern measurements, as inferred from in situ measurements over a decade ago. It is evident that, for the foreseeable future, an extensive set of in situ instrumentation will be required, in addition to the altimetry, if further insight into circulation in this part of the Southern Ocean is to be obtained. This paper has two main objectives. The first one is concerned with verifying that high T/P accuracy is also obtained in the DP area by comparison of the altimetry to sea level, or proxy sea level, data from coastal tide gauges, deep-sea bottom pressure recorders (BPRs), and inverted echo sounders (IESs).It is clear that all devices, either in situ or altimetric, should provide compatible scientific results and that any differences should be understood as far as possible. A related objective is concerned with asking if the altimetric accuracies are "good enough," compared to the in situ values, for perf...

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