Sub-surface pressure (SSP) data from tide gauges at three bases on the Pacific coast of the Antarctic Peninsula, together with SSP information from a bottom pressure recorder deployed on the south side of the Drake Passage, have been used to study the relationships between SSP, Drake Passage transport, and the strength of Southern Ocean zonal winds as represented by the Southern Annular Mode. High correlations were obtained between all parameters, confirming results obtained previously with independent data sets, and demonstrating the value of information from the permanent Rothera base, the southern-most site considered. These are important findings with regard to the design, installation and maintenance of observation networks in Antarctica. In particular, they provide the necessary justification for Antarctic Peninsula tide gauge infrastructure investment in the lead up to International Polar Year. Data delivery from Rothera and Vernadsky is currently being improved and should soon enable the first near real-time system for monitoring Drake Passage transport variability on intraseasonal timescales, an essential component of a Southern Ocean Observing System.
The secular trend of mean sea level at Southampton (1951-55 and 1957-66) and Portsmouth (1962-76) has also been investigated by us using multiple regression analysis. With local mean barometric pressure and time as independent variables it is estimated that the rise in mean sea level at Southampton and Portsmouth is 1.10 mm/year and 5.96 mm/year, respectively. There is good agreement with regard to the former, but the latter value is significantly lower than that derived by the Authors (8.35 mm/year). and lower values of mean sea level in 1973-74 than used by the Authors." Nevertheless, the discrepancy between the two nearby ports is still quite large. However, the problem of comparing secular sea level trends at different places and for different epochs is well known.1a A more uniform pattern would doubtless emerge if corresponding periods were analysed. The explanation almost certainly lies in the inclusion of more recent data27. The concept of a regional dimensionless factor has obvious appeal and the factor used in Table 4 was introduced by Lennon' to establish the regional distribution of extreme levels on the west coast of England. However, in this case it would appear to be justifiable only if the numerator of the factor (i.e. hn=n year level-MHWS) is proportional to the spring tidal range within a region. For the Solent region, where the spring range in Christchurch Bay is about one half of that at Spithead, there is evidenceI3 to suggest that this is not so.28. It would seem that, for design purposes, h. may be a more appropriate regional criterion. If this procedure were adopted the 50 year level would be MHWS+RS0, and so on. This would take account of those cases where-such as near an amphidromic point-the range of the tide may differ appreciably over a relatively short distance. Mr Blackman and Mr GraffWe are interested in the comments of Mr Davies and Mr Webber concerning the use and interpretation of empirical factors as a measure of assessing more strictly the regional distribution of extreme level occurrences. We accept these comments; indeed, the sensitivity of the Lennon type measure is discussed in references 9 and 14. Further r e s e a r~h '~* '~ has highlighted the sensitivity of the analysis procedure as used in a conventional way in the computation of return period heights as presented in the Paper. These recent findings indicate that there is a considerably less strict value associated with estimated heights relating to defined return periods and that these values can differ in response to the port location and any period of data analysed. As such it would seem inappropriate at present to enhance the use of any regional factor which is a measure that would involve a strictly defined estimate of a return period height.
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