A test of the ability of TOPEX/POSEIDON to monitor flows through the Drake Passage

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.

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