| Summary: | The flow of subtropical waters carried into the northern North Atlantic by the North Atlantic Current-Subpolar Front (NAC-SPF) system is an important component of the meridional overturning circulation. Much of the warm-to-cold water preconditioning that allows for deep convection in the Labrador and Nordic Seas takes place in the subpolar North Atlantic. This study addresses two processes that contribute to the warm-to-cold water conversion: Subpolar Mode Water (SPMW) formation, and cross-frontal mixing. Using Gravest Empirical Mode (GEM) projections of historical hydrography, we find that SPMWs are formed on the flanks of the NAC-SPF through winter-time convection. The mean characteristics and approximate location of formation for the different SPMW classes found in the NAC-SPF region are obtained. To quantify the heat lost by the NAC-SPF waters in their transit towards the Eastern Subpolar North Atlantic (ESNA), we develop a technique called the float-GEM method. It combines GEM fields with isopycnal RAFOS float data to obtain the absolute transports of mass and temperature in the top 1000db for the time period 1993–2000. The NAC-SPF appears as a “pipe”, transporting 15.0 ± 3.1 Sv from the subtropics into the subpolar region. The northward flowing NAC follows a well defined meandering path. No evidence of permanent branches pealing off the current are found upstream of the Northwest Corner. The current seems to split into two branches upon entering the Northwest Corner, which diverge as they turn east, constituting the SPF. The two branches converge as they approach the Mid-Atlantic Ridge, crossing it above the Charlie-Gibbs and Faraday Fracture Zones. The NAC-SPF loses 0.27 ± 0.06 PW on its transit towards the ESNA, which is much larger than the corresponding heat lost to the atmosphere (∼0.07 PW). Nearly all the heat lost by the NAC occurs between the meander crest inshore of the Mann Eddy (∼42.5°N), and the following trough (located at 44°N), due to entrainment of cold waters from the inshore side of the stream, and a loss of warm waters to the subtropical side. 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.
|
|---|