Comparisons of sea level and bottom pressure measurements at Drake Passage

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Abstract
Tide gauge and atmospheric sea level pressure measurements from each side of Drake Passage are compared with multiyear time series of bottom pressure at 500 m depth. Synthetic subsurface pressure (SSP), smoothed with a 40‐hour low‐pass filter, derived from tidal and sea level pressure records from the British Antarctic Survey base Faraday is coherent with bottom pressure from southern Drake Passage for periods of 6 to 600 days, but it is about 60° out‐of‐phase at the annual period. The difference between the two series for periods longer than 60 days is mainly annual and is as large as, or larger than, the seasonal variability of bottom pressure itself. Annual variations in open ocean mixed‐layer density near Antarctica are too small to account for the difference. The Faraday SSP series spans 21 years, slightly longer than the 18.6‐year lunar nodal tide, which appears to exist in the record. At the northern side of Drake Passage, SSP from Puerto Williams, Chile, does not compare well with nearby bottom pressure for any band of periods, apparently due to local winds for periods under 100 days and to annual cycles in upper layer density. Barotropic changes in transport of the Antarctic Circumpolar Current cannot be directly estimated using these surface observations, even though over seasonal periods they are an order of magnitude larger than the baroclinic changes due to variations in upper layer density.