Further Evidence for Coastal Trapped Waves along the Peru Coast

Abstract

Time series of coastal sea level during 1976–77, from 2°12′S to 17°S along the west coast of South America, show that low-frequency, ω < 0.25 cycles per day (cpd), fluctuations propagate poleward with the phase speed of baroclinic Kelvin waves (2–3 m s⁻¹). The alongshore coherence is highest in the frequency band 0.1–0.2 cpd. Computing the frequency-domain empirical orthogonal functions (EOF) for alongshore current, from an army of current meters extending from 5°S to 15°S during March-May 1977, gives 70% of the variance in the 0.1–0.2 cpd frequency band to an EOF mode with poleward phase propagation at 2.75 m s⁻¹. The vertical structure of the alongshore current fluctuations (0.1–0.2 cpd) over the continental slope at 5°S and 15°S is consistent with a first-mode baroclinic Kelvin wave. The current and sea-level fluctuations are coherent and propagate poleward through latitudes where their frequency equals the local inertial frequency. The fluctuations are not significantly coherent with coastal winds from 4°S to 15°S and am therefore presumed to have an equatorial origin. Intermittent sea-level data at the Galapagos Islands during the period provide tenuous evidence that these fluctuations, propagating poleward as coastally trapped waves, previously traveled in the equatorial wave guide.