Observations of Vortex Rossby Waves Associated with a Mesoscale Cyclone*

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 Ro5 jz/f j5O(1), periods of 12–15 days, and phase speeds of 8–9 cm s21, 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 gra-dient 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. 1