M. P. Meredith*,**, J. M. Vassie*, P. L. Woodworth*, C. W. Hughes*,
R. Spencer* & K. J. Heywood**.
*Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside L43 7RA, U.K.
**School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
Six years of bottom pressure data have been used for the monitoring of the temporal variability of the Antarctic Circumpolar Current (ACC) transport at Drake Passage (DP), the region of greatest constriction of ACC flow. Figure 1 shows the locations of the bottom pressure recorder (BPR) deployments used for this monitoring. The FS1 and FS3 positions (approximately 2800 and 2300 m respectively) were first instrumented in November 1988, with the BPRs being moved to the DN93 and DS93 positions (both approximately 1000 m depth) in November 1992. Monitoring is continuing with the gauges in these locations. Additional bottom pressure data from the DN92 and DS92 positions (both almost 4000 m depth) were obtained for December 1991 to November 1992.
Under the assumption that the ACC is in geostrophic balance, the current speed variability at the depth of BPR deployment is able to be deduced from the north-minus-south horizontal pressure gradient. If it is assumed that the current speed variability is barotropic (i.e. independent of depth), the transport variability can be derived from the current speed variability and knowledge of the mean depth between the BPRs. Predominantly barotropic ACC variability was a result of the International Southern Ocean Studies (ISOS) program, which took place at DP during the mid-1970s and early 1980s (Whitworth & Peterson, 1985). This conclusion was supported by analysis of the more recently recovered data, where deep bottom pressure from the FS3 position was observed to be remarkably similar to shallow bottom pressure recorded nearby at Signy Island. Consequently, the change in current speed associated with the change in pressure would be equal at these different depths, and hence barotropic.
Fig. 1. Locations of POL bottom pressure recorder (BPR) deployments at Drake Passage.
remove the influence of the most dominant (i.e. lunar diurnal and semi-diurnal) tides. The solid line shows the same data smoothed with a 31 day moving-average filter to illustrate the lower-frequency behaviour of the time series. Gaps between each individual year of data are the result of recovering and redeploying the BPRs ready for the next yearsü data collection. In Figure 2, all data are presented about the mean of that yearüs deployment, as per Vassie et al. (1994). Geostrophic calculations on these data yield values for the standard deviation in transport ranging from 5.3 Sv in 1993 to 8.9 Sv in 1990 (1 Sv = 1,000,000 m 3/s). These values are all less than the 10 Sv obtained during the (ISOS) program. Some of this discrepancy is due to procedural differences in dealing with the gaps in the data - in the ISOS program, the ends of each yearüs data were matched to the start of the next yearüs to produce continuous long time series. However, the most significant difference is that the ISOS data contained two very sudden shifts in pressure difference which led to difference which led to the derived ACC transport changing by approximately half its mean in the space of just a couple of weeks. No such shifts were observed in the present data. Examination of the ISOS data revealed that whilst the first large shift in pressure difference was caused by changes of comparable magnitude in both north and south side pressure, the second was caused by a change of unprecedented magnitude at the south side with no corresponding change at the north side.Furthermore, whilst the first shift in ISOS pressure difference coincided well with various proposed mechanisms for ACC wind forcing (such as outlined by Wearn and Baker, 1982; Peterson, 1988), the second did not. It is thus thought that the second shift in ISOS pressure difference is very unlikely to be a genuine reflection of a change in ACC transport. Even if it were, it is concluded that the ACC can be persistently less variable over an extended period than previously observed.
Fig. 2. Drake Passage bottom pressure time series (a) from the north side (b) from the south side and (c) north-minus-south pressure difference. Dashed line is data smoothed with 1 cycle/day cut-off filter, solid line is same data smoothed with 31-day moving average filter.
Since 1992, the DP BPR deployments have also featured Inverted Echo Sounders (IESs) on the instrument rigs. Data from these have been used to empirically obtain parameters such as dynamic height and vertically-averaged density at the north side of the passage, from which sea level can be obtained (Woodworth et al., 1995). No such derivation is possible for the south-side IES data, due to a non-linear relationship between measured acoustic travel time and the desired parameters. The IES data indicate that there is significant steric contamination in the BPR data from the north side of DP due to activity of the SubAntarctic Front (SAF), and consequently the north-minus-south pressure difference will overestimate the true transport variability. However, the fact that bottom pressure and acoustic travel time from north DP were observed to be negatively correlated implies that bottom pressure will provide a better measure of transport variability than sea level, since the change in density across the SAF partially compensates for the change in sea level. Overall, it is concluded that whilst the ACC has recently been observed to be less variable than in the previous ISOS program, the standard deviations in transport presented in both studies are best regarded as upper limits to the true transport variabilities.
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