Upper-Ocean Eddy Transports of Heat, Potential Vorticity, and Volume in the Northeastern North Atlantic—“Vivaldi 1991”

Abstract

Mesoscale eddies in the northeast North Atlantic were investigated using the SeaSoar towed CTD and ADCP data from the 1991 Vivaldi cruise. These data cover an area of 1700 km ? 1500 km between 39° and 54°N and between 35° and 10°W. To maximize statistical significance, but retain the possibility of determining north?south gradients, statistics of eddy quantities were calculated separately for the northern and southern halves of the cruise area. The mean flow in the south is essentially zero; in the north the flow is dominated by the North Atlantic Current (NAC) with a mean speed of 6.5 cm s?1. The eddy kinetic energy in the south, 205 cm2 s?2, is, however, only slightly less than in the north, 272 cm2 s?2. The eddy momentum transports, or Reynolds stresses, u???, show a poleward decrease, corresponding to an acceleration of the mean eastward flow associated with the NAC of 0.03 cm s?1 day?1. The eddy heat transports, u?T?, are not significantly different from zero in the south but show a clear poleward transport in the north of 5.5 K cm s?1, or 0.1 PW for the 365-m layer 1500 km wide. The depth-averaged eddy potential vorticity fluxes, u?q?, show a convergence toward the source region of the low-potential-vorticity eastern North Atlantic Central Water west of Biscay. The residual or rectified eddy transport velocity implied by the eddy potential vorticity flux, u* = ?u?q?/q, is 0.7 cm s?1 toward the southwest in the south, while in the north it is 0.9 cm s?1 toward the northwest crossing the property isolines. The directions correspond to a divergence from the formation region of the eastern North Atlantic Central Water. An assessment of the overall volume transport of the region suggests that the westward eddy volume transport (?4 Sv; Sv ≡ 106 m3 s?1) is almost balanced by an eastward geostrophic flow (?3 Sv) with the remainder being supplied by a smaller contribution leaving the northward-flowing eastern boundary current (?1 Sv).