Physical and bio-optical observations of oceanic cyclones west of the island of Hawai’i

Interdisciplinary observations of mesoscale eddies were made to the west of the island of Hawai’i. A central goal of the studies is to improve our understanding of the coupling of physical, biological, and biogeochemical processes that occur within these eddies. A specific objective was to test the hypothesis that the physical mechanisms of mesoscale eddies result in increases in nutrient availability to the euphotic layer, increases in primary production, changes in biological community compositions and size distributions, and increases in carbon flux to the deep sea. Data were obtained from ships, surface drifters, and satellite sensors during three separate field experiments. Variability was associated with two well-developed cyclonic, cold-core mesoscale eddies, Cyclone Noah and Cyclone Opal, which were observed during the E-Flux I (survey November 6–20, 2004) and E-Flux III (survey March 10–27, 2005) field campaigns, respectively. No mesoscale eddies were found during E-Flux II (survey January 10–19) when winds were erratic in magnitude and direction, supporting the hypothesis that persistent trade winds drive the production of cold-core mesoscale eddies in the lee of the Hawaiian Islands. Cold-core eddies were present in the E-Flux study area for about 2/3 of a year beginning May 1, 2004 and trade winds prevailed for about 3/4 of the same year.Both Cyclone Noah and Cyclone Opal were generated during strong, persistent northeasterly trade wind conditions and appeared downwind of the ’Alenuihaha Channel separating the islands of Maui and Hawai’i. The likely production mechanism for both mesoscale cold-core eddies involves localized wind-stress-curl-induced upwelling produced by trade wind forcing. Cyclone Noah   was likely spun up by strong trade winds just to the southwest of the ’Alenuihaha Channel (∼20.10∘N∼20.10∘N, 156.40∘W156.40∘W) between August 13 and 21, 2004 based on MODIS satellite sea-surface temperature (SST) imagery and QuikScat satellite wind data, and apparently began to dissipate by mid-December 2004. Cyclone Opal   was likely spun up by strong trade winds between February 2 and 18, 2005 southwest of the ’Alenuihaha Channel (∼20.30∘N∼20.30∘N, 156.30∘W156.30∘W), but was no longer evident in April 2005.Both Cyclone Noah and Cyclone Opal   had strong physical, chemical, and biological expressions and displayed similar maximum tangential current speeds of ∼60cms-1. However, Cyclone Opal was more symmetric and larger in scale (roughly 180–200 km in diameter compared to ∼160km in horizontal scale for Cyclone Noah). Both mesoscale eddies displayed significant doming in their centers and in some cases outcroppings of isothermal, isopycnal, nutrient, and chlorophyll a isopleths. After formation and a slow drift southward, Cyclone Noah   remained in nearly the same location (roughly 19.60∘N19.60∘N, 156.50∘W156.50∘W) during the 3-week in situ sampling period, whereas Cyclone Opal   drifted southward by ∼165km over a similar time span of sampling. Interestingly, the physical manifestations of both features were relatively unchanged during the ship-based surveys; however, the biology appears to have evolved within Cyclone Opal. The present report sets the context for several other E-Flux studies.