Observations of vortices and vortex Rossby waves in the lee of an island

Abstract

A 9-month deployment of high-frequency radio (HFR) current meters and moored ADCPs in the lee of Oahu, Hawaii, gives some dynamical insights in the generation and evolution of vortices in the lee of islands. For mountainous islands lying in relatively strong and steady winds, such as the Hawaiian archipelago in the trade winds, vortices can be generated by Ekman pumping associated with orographic wind stress curls. An anticyclone generated in the lee of Oahu in October 2002 reaches a negative absolute vorticity for 4 days, before quickly decaying and broadening, possibly as a result of inertial instability. A large cyclone, generated in December 2002 in the lee of Hawaii, drifted northwestward and stalled southwest of Oahu in March-April 2003. Vortex Rossby waves developing on its periphery were observed by the HFRs with a northeastward phase propagation, 110 km wavelength and 16 days period.