The Northwest Corner (NWC) is another member in the pantheon of unique features of ocean currents. It is where the North Atlantic Current (NAC) flowing northeast along the Newfoundland/Labrador escarpment turns east at about 51°N. No other western boundary current reaches more than about 35° latitude before turning east. The reason is that the NAC is a link in the Atlantic meridional overturning circulation (AMOC); it is not a wind-driven current. But why does it turn east just there?
The answer seems to be that near this latitude the Charlie Gibbs Fracture Zone offers a path of least resistance across the mid-Atlantic ridge to the NE Atlantic and the Nordic Seas. While the location of the NWC appears to be stable, it varies significantly in strength. Years ago, we wrote a paper on the reasons for this (Woityra and Rossby, 2008). The NAC approaches the NWC as a narrow rapid flow but broadens substantially after turning east. We c thaonjectured that this convergence of kinetic energy flux (same mass flow but wider and slower) was used to pump up the NWC creating in effect a growing coherent vortex with the NAC circling around it. The warm water in the center is subject to significant cooling at this high latitude such that its increasing density causes it to get overrun by the NAC and in effect restarting the process. The submerged eddy, now an anticyclonic lens, drifts south offshore of the NAC; this we have seen. Its subsequent fate may depend upon a number of factors, but we have strong circumstantial evidence that such a lens crossed the Gulf Stream at depth continuing west and south into the Sargasso Sea (Prater and Rossby, 1999), a 4000 km journey!
It has been customary to apply the NAC label to the continued drift of warm water across the Atlantic into and around the Iceland Basin. Better would be to call this eastward flow the North Atlantic Drift, an expression that has sometimes been used. While its northern limit is reasonbly well defined, the current is not narrow and swift but rather broad and variable with an observed tendency to cross the mid-Atlantic ridge wherever there are openings in it (Bower et al., 2002).
Here are some interesting speculations. If the NAC and NWC are links in the AMOC, then they must disappear during the cold glacial epochs. Which means there might be some telling paleo-climate signals in the underlying sediments. (See Nov. 27, 2024). We also know that the AMOC turns on abruptly at the start of the next Dansgaard-Oeschger event (See Aug. 24, 2024). At which time the entire northern North Atlantic becomes warm - as it is today. The Greenland ice record shows that over the following ~1000 years a gradual cooling sets in before the AMOC collapses altogether. It is tempting to speculate that the cooling reflects the NAC increasingly shifting its mass flux to fracture zones farther south in the mid-Atlantic Ridge (Faraday and Maxwell FZs) with the effect that warm area of the subpolar North Atlantic decreases. Perhaps the final collapse comes when the Nordic Seas component of the AMOC also shuts down?
Bower, A.S., B. Le Cann, T. Rossby, W. Zenk, J. Gould, K. Speer, P. Richardson, M.D. Prater and H.M. Zhang, 2002. Directly-measured mid-depth circulation in the northeastern North Atlantic Ocean. Nature, 419, 603-607.
Prater, M.D. and T. Rossby, 1999. An alternative hypothesis for the origin of the “Mediterranean” salt lens observed in the Bahamas in the Fall of 1976. Phys. Oceanogr., 29, 2103-2109.
Woityra, W. and T. Rossby, 2008. Current Broadening as a Mechanism for Anticyclogenesis at the Northwest Corner of the North Atlantic Current. Geophys. Res. Letters, 35, L05609, doi:10.1029/2007GL033063