Observations of the principal tidal currents at Drake Passage

Abstract

Twenty‐one long current records collected at 12 positions in Drake Passage are used to describe the principal diurnal (K1 and 01) and semidiurnal (M2 and S2) tidal current constituents in that region between the Atlantic and Pacific Oceans. The tide is predominantly semidiurnal in the northern passage and of mixed type at other locations. One‐quarter of the fluctuation kinetic energy of the records is between periods of 2 hours and 2 days. Approximately 50% of that energy is contained in two narrow bands centered at the diurnal and semidiurnal frequencies (the semidiurnal band also contains significant inertial current energy); the fraction of energy in these bands increases southward across Drake Passage from 39 to 66%. Admittance calculations show that an average of 56% of this band‐passed kinetic energy is coherent with the tidal forcing potential. Least square fitting (by the method of cyclic descent) of 250‐day records at the 10 most energetic short‐term tidal frequencies yields currents essentially coherent with the tidal forcing functions. From these fits the average partitioning of coherent tidal energy among the four principal tidal constituents is estimated: 75% at M2, 16% at S2, 9% at K1, and 3% at 01. Mean tidal ellipses for these four tides are constructed from the fits and used to describe depth and latitudinal variation across the passage and to make comparisons between the years 1975, 1976, and 1977. The variances of coherent diurnal tides in the deep water increase southward, whereas those for coherent semidiurnal tides are smallest in midpassage and increase more to the north than to the south. The noncoherent (residual) portions of the band‐passed currents are isotropic, in contrast to the coherent constituents, which are most energetic in the through‐passage (along‐current) direction. In general, these non‐coherent oscillations are most energetic for depth ranges and locations of greatest hydrostatic stability. The squared coherence and phase relationships between principal axis components obtained from the least square fits showed the K1, M2, and S2 tides to propagate with southward components while the 01 has a northward component of propagation across Drake Passage. Coherence between current meters separated vertically on the same mooring was greatest for the diurnal tide K1. Temporal variability of the tides is described by time series of hodographs and of kinetic energy density of currents in narrow pass bands centered on the four major tidal constituents.