Tide gauge data analysis of the pole tide in the North Sea

It is generally accepted that the pole tide-the oceanic response to the Chandler wobble-is, in the North Sea, characterized by anomalously large amplitudes. In this paper we test that postulate by extracting amplitudes and phases at the Chandler frequency from sea-level signals recorded over the past century and a half around the North Sea. We have also determined the equilibrium pole tide and tidal admittances over the same time span using long-term polar motion. Our analysis has focused on three sub-intervals of time: 1872-1900; 1900-1964; and 1955-1984. We find that the 'eastward intensification' is not a persistent feature of the North Sea pole tide over all three intervals. Furthermore, the typical pole tide amplitude during these three time spans varies, but not in the same way as the Chandler potential; this supports Naito's (1977) and Daillet's (1981) conclusion, that the variation of the pole tide amplitude with respect to time is at least partially independent of the Chandler forcing. Finally, we have analysed the North Sea annual tide at the same sites, and found that it also exhibits an eastward intensification; however, in contrast to the pole tide, the annual tide's eastward intensification persists clearly over all three time intervals. These data analyses, combined with our earlier theoretical study of the North Sea pole tide (Xie & Dickman 1995), suggest that the observed 'pole tide' signal in the North Sea may be attributed largely to a combination of Chandler wobble and meteorological forcing, but that other causes must exist as well.