Bermuda sea level in relation to tides, weather, and baroclinic fluctuations

Abstract

The sea‐level record over an 8‐year period at the Biological Station, Bermuda, has been analyzed in an attempt to deduce the complete physics of sea‐level variations in the frequency band of 1 cycle per 8 years to 0.5 cycles per hour. The admittance of the linear tidal lines was calculated, and, on the basis of certain assumptions about a linear system, the Hilbert transform relation between real and imaginary parts of the admittance function was used to deduce the function between diurnal and semidiurnal tides. The North Atlantic appears to be resonant with a period of 14.8 hours having a Q exceeding 3.3. An overtone of the resonance is also possibly present. An examination of the overtides in the record indicates that the hydrodynamical regime plus measuring system is highly linear. The spectra of observed atmospheric pressure, temperature, and winds were calculated, and the transfer functions between atmospheric narrow‐band processes (tides) and the continuum with sea level were computed. The tidal constituent S2 is about 10 per cent radiational in nature. The continuum of sea level is found to be dominated by pressure in an inverted barometer sense for periods shorter than a year. At longer periods, the winds appear to dominate, but low‐frequency sea‐level power rises more rapidly than any other weather variable, so that the sea‐level spectrum essentially decouples from the atmosphere. The effect of atmospheric temperature is found to be negligible. A comparison between temperature measured in the main thermocline and the sea‐level record yielded no coherence in any part of the spectrum. Approximately 0.1 cm of the semidiurnal tide is attributed to motion of the sea surface by the baroclinic tide. Otherwise, the rest of the internal wave spectrum can be ignored. The Panulirus hydrographic station sequence shows coherence between dynamic heights and the residual (nonweather) sea‐level record for periods between 6 months and a year. At very low frequencies, the dynamic height spectrum tapers off, indicating that the lowest‐frequency energy in the sea‐level record is probably in barotropic motions. Examination of the coherence between the sea‐level record and another record made nearby on Bermuda indicates that the tide‐gage record used in this study is representative of sea‐level motion over a scale of a few miles. Approximately 87 per cent of the total sea‐level variance is explicitly accounted for. The remainder of the energy is in errors of the analysis and in barotropic motions.