Energy transport and secondary circulations due to vertically-propagating Yanai waves observed in the equatorial Indian Ocean.

Thursday, 18 December 2014
William Smyth, Ted Durland and James N Moum, Oregon State University, Corvallis, OR, United States
Shipboard current measurements in the equatorial Indian Ocean in October and November of 2011 revealed oscillations in the meridional velocity with amplitude ~0.10m/s. These were clearest in a layer extending from ~300 to 600 m depth and had periods near 3 weeks. Phase propagation was upward.

Measurements from a time series at the equator, four meridional transects and one zonal transect are used to identify the oscillation as a Yanai wave packet and to establish its dominant frequency and vertical wavelength. The Doppler shift is accounted for, so that measured wave properties are translated into the reference frame of the mean zonal flow. We take advantage of the fact that, in the depth range where the wave signal was clearest, the time-averaged current and buoyancy frequency were nearly uniform with depth, allowing application of the classical theoretical representation of vertically propagating waves.

Using the theory, we estimate wave properties that are not directly measured, such as the group velocity and the zonal wavelength and phase speed. The theory predicts a vertical energy flux that is comparable to that carried by midlatitude near-inertial waves. Also predicted is an equatorward heat flux that is balanced, in the limit of a linear plane wave, by a wave-driven Eulerian mean flow. The volume transport carried by this mean flow is in turn balanced by the Stokes drift.