Evidence for equatorial trapped waves at the Galápagos Islands

Abstract

From June 28 to November 23, 1979, sea level fluctuations at three locations on the western side of the Galápagos Islands were measured with shallow bottom‐moored pressure and temperature gauges. Gauges were deployed at 1°24′N, 0°, and 1°S. The sea level variations were dominated by a highly coherent low frequency fluctuation with a period of the order of 100 days. In addition, high‐frequency spectral peaks were observed with periods of 5.3 and 2.8 days. Temperature fluctuations also showed the dominant low‐frequency change and the high‐frequency spectral peaks; however, the temperature fluctuations were less coherent because of the thermal front in this region. Empirical orthogonal functions of the pressure gauge data showed that most (95%) of the low‐pass filtered data could be explained by the first mode. This mode had a meridional shape consistent with a first baroclinic mode equatorial Kelvin wave. The second mode accounted for about 3.6% of the variance with periods from 15 to 5 days. Its shape was consistent with a first baroclinic mode mixed Rossby‐gravity wave. The higher frequency peaks occur at frequencies where equatorial trapped inertial‐gravity waves have zero zonal group velocity.