Satellite and In Situ Equatorially-Trapped Waves

Robert A. Weller and John T. Farrar, Physical Oceanography Department, WHOI

For at least 40 years, researchers have fully appreciated that the vanishing of the Coriolis ‘force’ at the equator causes the near-equatorial regions to act as a waveguide for atmospheric and oceanic waves with frequencies low enough to be affected by the Earth’s rotation. Waves in the equatorial waveguide tend to propagate in the east-west direction and are organized into a discrete set of north-south ‘modes’ with coherent variability spanning a distance of more than 1500 km across the equator. These waves have large wavelengths (>~1000 km) and periods of days to a year or more, and the horizontal currents and vertical motion that they produce within the water column can have dramatic effects on the vertical and horizontal distribution of temperature and nutrients. For example, equatorial waves play a central role in bringing about the physical and biological changes associated with the El Niño/La Niña episodes in the tropical Pacific, which have well-known impacts on weather and fisheries. On faster timescales, equatorial waves with periods of 2-3 months are hypothesized to substantially modulate phytoplankton chlorophyll-a concentration near the Galápagos Islands (Palacios et al., 2006). Despite the importance of equatorial waves in modulating the properties of the tropical oceans, many aspects of the dynamics and behavior of equatorial waves remain poorly understood.

The  work proposed here would continue efforts by Farrar to improve understanding of the properties and dynamics of equatorial waves in the world oceans. Previous work using satellite observations of sea surface height in the Pacific has provided new insight into the spatial structure and dispersion relation (i.e., relationship between wavelength and wave period) of these waves (Farrar, submitted). Furthermore, a previously unobserved (but theoretically expected) wave mode has been identified for the first time (Farrar, submitted). The proposed work would use in situ observations from the Tropical Atmosphere-Ocean and Trans-Ocean Buoy Network (TAO/TRITON; McPhaden et al., 1998) in the Pacific, together with satellite observations, to extend that analysis to study higher frequencies and to examine the causes (e.g., wind forcing) and effects (e.g., chlorophyll-a modulation) of these waves. An exploratory analysis would also be undertaken to study these waves in the tropical Atlantic and Indian Oceans using satellite observations and available in situ data.