Secular changes of the M tide in the Gulf of Maine

Analyses of long time series of hourly tide-gauge data at four stations in the Gulf of Maine reveal that the amplitude of the M 2 tide underwent a nearly linear secular increase throughout most of the 20th century. In the early 1980s, however, the amplitude of M 2 abruptly dropped. Sea level changes alone appear inadequate to explain either the long-term trend or the recent trend discontinuity. Tidal models that account for Holocene sea level rise do predict an amplification of M 2 , but much smaller than the currently observed trends. Nor do recent annual mean sea levels correlate with the recent trend discontinuity. Some unknown fraction of the open Atlantic may be similarly affected, since the M 2 discontinuity, but not the long-term secular increase in the tide, is evident also at Halifax.