Formation of cyclonic eddies in the lee of the Hawaiian Islands

Satellite altimetry shows an enhancement of mesoscale eddy variability in the lee of the Hawaiian Islands. The study of Calil et al. (2008) using the regional ocean modeling system (ROMS) suggests that the increased variability is largely caused by the spatial and temporal variations in the wind forcing. As the northeasterly trade wind blows around the high mountains of the islands of Maui and Hawaii in particular, regions of weak wind leeward of the islands and strong wind jet between the islands are created. These local wind anomalies, positively correlated with the strength of the trade wind, are one of the key forcing mechanisms for the generation of cyclonic and anticyclonic eddies in the lee of the Hawaiian Islands. The study of formation and propagation of cyclonic eddies in the lee of the Hawaiian Islands, in particular, is of great relevance, as the eddies bring nutrient-rich water at depth to the euphotic zone and enhance primary production in a region of generally low productivity in the subtropical gyre. Through numerical experiments performed with the Hybrid Coordinate Ocean Model (HYCOM), we find that high resolution of surface wind, both temporal to capture the intensity of the trade wind and spatial to resolve the strong funneling effect between the islands is necessary for the formation of cyclonic eddies. The HYCOM implementation for the Hawaii region at a horizontal resolution of 0.04 degree is downscaled from the global HYCOM and uses its output for initial and lateral boundary conditions. The wind products used include ECMWF, NOGAPS, QuikSCAT and output from regional atmospheric models; their temporal resolutions range from 3 hourly to daily and spatial resolutions range from approximately 1 to 0.16 degree. The formation of a cyclonic eddy requires the occurrence of strong and persistent trade wind that gives rise to a strong jet between the islands but cyclonic eddies do not always form during periods of strong trade wind. We discuss other factors, such as island topography, local current shear and remote forcing that may contribute to the formation of cyclonic eddies in the lee of the Hawaiian Islands based on our model experiments.