Observations of Vortex Rossby Waves Associated with a Mesoscale Cyclone

Abstract

Short-wavelength (L ? 100 km) Rossby waves with an eastward zonal phase velocity were observed by high-frequency radio Doppler current meters and moored ADCPs west of Oahu, Hawaii, during spring 2003. They had Rossby numbers Ro = |?/f| = O(1), periods of 12?15 days, and phase speeds of 8?9 cm s?1, and they were surface trapped with vertical e-folding scales of 30?170 m. They transferred horizontal kinetic energy to the background flow of a mesoscale cyclone lying 160?190 km west of Oahu, revealed by altimetry. The waves approximately satisfied the dispersion relation of vortex Rossby waves propagating through the radial gradient of potential vorticity associated with the cyclone. Vertical shear of the background currents may also affect wave propagation. Theoretical studies have shown that vortex Rossby waves provide a mechanism by which perturbed vortices axisymmetrize and strengthen and may be important to the dynamics of oceanic vortices.