Observations of the Dispersion Characteristics and Meridional Sea Level Structure of Equatorial Waves in the Pacific Ocean

Abstract

Spectral techniques applied to altimetry data are used to examine the dispersion relation and meridional sea level structure of wavelike variability with periods of about 20 to 200 days in the equatorial Pacific Ocean. Zonal wavenumber-frequency power spectra of sea surface height, when averaged over about 7°S-7°N, exhibit spectral peaks near the theoretical dispersion curves of first baroclinic-mode equatorial Kelvin and Rossby waves. There are distinct, statistically significant ridges of power near the first and second meridional-mode Rossby wave dispersion curves. Sea level variability near the theoretical Kelvin wave and first meridional-mode Rossby wave dispersion curves is dominantly (but not perfectly) symmetric about the equator, while variability near the theoretical second meridional-mode Rossby wave dispersion curve is dominantly antisymmetric. These results are qualitatively consistent with expectations from classical or shear-modified theories of equatorial waves.The meridional structures of these modes resemble the meridional modes of equatorial wave theory, but there are some robust features of the meridional profiles that were not anticipated. The meridional sea level structure in the intraseasonal Kelvin wave band closely resembles the theoretically expected Gaussian profile, but sea level variability coherent with that at the equator is detected as far away as 11.75°S, possibly as a result of the forced nature of these Kelvin waves. Both first and second meridional-mode Rossby waves have larger amplitude in the Northern Hemisphere. The meridional sea level structure of tropical instability waves closely resembles that predicted by Lyman et al. using a model linearized about a realistic equatorial zonal current system.

Barotropic Rossby Waves Radiating from Tropical Instability Waves in the Pacific Ocean

“…2a). The 33-day TIWs also have a signal south of the equator, near 58S, but its amplitude is several times smaller, as expected from previous work (e.g., Chelton et al 2000;Lyman et al 2005;Farrar 2008). …”

“…1; see also Farrar 2008;Shinoda et al 2009). This spectral peak is associated with TIWs, but a complete description cannot come from sea level fluctuations alone.…”

“…The waves extract energy from the large-scale, wind-driven currents and have been extensively studied. TIW variability is one of the most prominent features of the surface dynamic topography of the tropics and the global ocean, with peak amplitudes in sea surface height occurring near 58N in the Pacific (e.g., Miller et al 1985;Malardé et al 1987;Musman 1989;Périgaud 1990;Lyman et al 2005;Farrar 2008;Shinoda et al 2009). Almost all previous studies of tropical instability waves have focused on the immediate vicinity of the equator (about 88N-88S).…”

“…The theoretical and observed meridional SSH structure of these 33-day TIWs in the Pacific Ocean has been addressed recently by Lyman et al (2005) and Farrar (2008), but the latitude where this variability has its maximum amplitude can be assessed directly by bandpass filtering the SSH anomaly to pass westward-propagating variability with periods of 29-36 days and zonal wavelengths of 108-208 of longitude (half-power points shown in Fig. 1).…”

Observed Dispersion Relation of Yanai Waves and 17-Day Tropical Instability Waves in the Pacific Ocean

“…It is demonstrated that wavenumber-frequency spectral analysis is useful for identifying signals of atmospheric equatorial waves including Yanai waves, and for isolating their structure. While the observed dispersion relationships of equatorial Kelvin and Rossby waves in the ocean were identified recently by wavenumber-frequency spectra of satellite-derived sea surface height (SSH) (Wakata 2007;Farrar 2008;Shinoda et al 2009), the resolution of the satellite altimeter data was insufficient to describe oceanic Yanai waves in frequency-wavenumber space until recently. This situation has begun to change because of multiple satellite launches in recent years.…”