Physical dynamics and biological implications of Cyclone Noah in the lee of Hawaii during E-Flux I

Quasi-synoptic observations of the horizontal and vertical structure of a cold-core cyclonic mesoscale eddy feature (Cyclone Noah) were conducted in the lee of Hawai’i from November 4–22, 2004 as part of the E-Flux interdisciplinary collaborative research program. Cyclone Noah appears to have spun up to the southwest of the ‘Alenuihaha Channel (between Maui and Hawai’i) as a result of strong and persistent northeasterly trade winds through the channel. Shipboard hydrographic surveys 2.5 months later suggest that Noah weakened and was in a hypothesized spin-down phase of its life cycle. Although the initial surface expression of Noah was limited in scale to 40km in diameter and, as evidenced by surface temperatures, 2–3 1C cooler than the surrounding waters, depth profiles revealed a fully developed semi-elliptical shallow feature ( 200m), 144km long and 90 km wide (based on sigmat 1⁄4 23 kgm ) with tangential speeds of 40–80 cms , and substantial isopycnal doming. Potential vorticity distribution of Noah suggests that radial horizontal flow of the core water was inhibited from the surface to depths of 75m, with high vorticity confined above the sigma-t 1⁄4 23.5 kgm 3 isopycnal surface. Upward displacements of isopycnal surfaces in the eddy’s center ( 50m) were congruent with enhanced pigment concentrations (0.50mgm ). Comparisons of the results obtained for E-Flux I (Noah) and E-Flux III (Opal) suggest that translation characteristics of cyclonic Hawaiian lee eddies may be important in establishing the biogeochemical and biological responses of the oligotrophic ocean to