The PRIME eddy

An intrathermocline lense with strong anticyclonic circulation and cold surface temperature anomaly (-1°C) is reported in the Iceland Basin at 59.1N 20W. The lens is 1000 m thick, 120 km wide and centered on the 27.45 σ_o isopycnal at 700 m. Its influence, through distortion of density surfaces, is felt throughout the 2800 m water column. At the surface this leads to a shallower mixed layer within the eddy which will have an impact on the local planktonic ecology. At a depth of 700 m and a radius of 40 km the eddy circulation has an azimuthal velocity of ~40 cm s^-1. This flow has a strong barotropic component. Potential temperature (7.8°C) and salinity (35.18) are homogeneous throughout the core-though there is evidence for a three-layer composition-which, together with a high oxygen content (6.4 ml l^-1) and low Brunt Vaisala frequency (~2 × 10^-4 s^-1), suggest that the eddy is formed from mode water created by deep winter mixing. Sea-Surface Temperature (SST) satellite images, displaying the cold surface signature of the eddy, reveal two similar features also in the region. None of the eddies moves appreciably in the five months of satellite coverage, the three forming a line northwest of, and parallel to, the Hatton Bank. This suggests that they may be produced by the North Atlantic Current (NAC) rounding the southern end of the bank. If common, estimates suggest that such features may be transporting significant amounts of salt and heat, from eastern mode waters, westward across the easternmost front of the NAC.