Vortex formation downstream of a cape: an analysis of anticyclone formation off Hawaii.

As the North Equatorial Current (NEC) passes South Point HI, it separates into a lateral shear layer which is unstable, forming vigorous anticyclonic mesoscale vortices observed west of the island. To examine the dynamics of vortex growth and merging, a historical analysis of the area downstream of the separation point was carried out using drifting buoy tracks, shipboard ADCP sections, satellite altimetry and SST. Drifter tracks revealed two dominant varieties of anticyclones: vortices induced by Ekman pumping and vortices driven by instability, with the majority of drifters trapped in the latter. Vortices induced by Ekman pumping were varied in size, of short duration, and existed primarily near the island. Vortices induced by instability (with Ro=-1) occurred mainly in Fall and Spring, originated near the island on a latitude close to the southern tip of the island and followed a common track away from origin. They were long-duration and extended westward up to 170W. ADCP sections near the origin showed that the vortices had vertical depth of ~150 m. Empirical orthogonal function analysis of satellite altimetry sea surface height yielded two main modes of variability: a dominant annual frequency, and a 100-day cycle in the lee of Hawaii. Drifters trapped in instability anticyclones appeared in the positive or neutral phase of the 100-day cycle, but did not directly correlate with the annual cycle. Initial results suggests that most of the anticyclones formed in the lee of Hawaii are predominantly caused by shear-layer instability, with wind stress curl in the lee playing a secondary, possibly pre-conditioning, role through downward Ekman pumping.