Absolute Transports of Mass and Temperature for the North Atlantic Current– Subpolar Front System

Abstract

The flow of subtropical waters carried into the northern North Atlantic Ocean by the North Atlantic Current? subpolar front system (NAC?SPF) is an important component of the meridional overturning circulation. These waters become colder and denser as they flow through the subpolar region, both by mixing with the colder subpolar waters and by atmospheric cooling. The relative roles of these two processes remain to be quantified, and the mechanisms driving lateral mixing need to be better understood. To address those questions, a new methodology is developed to estimate the mean absolute transports of mass and heat for the top 1000 dbar in the region of the NAC?SPF for the time period 1993?2000. The transports are obtained by combining historical hydrography with isopycnal RAFOS float data from the area. The mean absolute transport potential field shows an NAC?SPF ?pipe,? defined by two bounding transport potential contours. This pipe transports 10.0 ± 3.5 Sv (Sv ≡ 106 m3 s?1) (top 1000 dbar) from the subtropics into the eastern subpolar North Atlantic. In contrast to earlier studies, the northward-flowing NAC follows a distinct meandering path, with no evidence of permanent branches peeling off the current before reaching the ?Northwest Corner.? As the current enters the Northwest Corner, it loses its tight structure and maybe splits into two or more branches, which together constitute the eastward flow along the SPF. The eastward flow between the Northwest Corner and the Mid-Atlantic Ridge is not as tightly defined because of the meandering and/or eddy shedding of the branches constituing the SPF. As the flow approaches the Mid-Atlantic Ridge, it converges to cross above the Charlie?Gibbs and Faraday Fracture Zones. The mean absolute temperature transport (top 1000 dbar) by the 10-Sv pipe was estimated across 10 transects crossing the NAC?SPF. Because the mean mass flux is constant in the pipe, variations in the mean temperature transports result from lateral exchange and mixing across the pipe's side walls and from air?sea fluxes across the surface of the pipe. The NAC?SPF current loses 0.18 ± 0.05 PW on its transit through the region, most of the loss occuring upstream of the Northwest Corner. The heat loss is 10 times the corresponding heat lost to the atmosphere. We conclude that cross-frontal exchange induced by the steep meanders of the northward-flowing NAC is the main mechanism by which heat is lost along the current in the region between the ?Tail of the Grand Banks? and the Mid-Atlantic Ridge.