In my previous post I describe how the Mann Eddy is driven by the North Atlantic Current (NAC). It was one of the things we learned thanks to a major RAFOS float program we started in 1993 to examine the structure and coherence of the entire NAC as a western boundary current - from where it branches off from the Gulf Stream in the south to the Northwest Corner (NWC) in the north where it turns sharply to the east. We knew it was a meandering current, but I’m not sure we appreciated how steep and stationary the meanders were.
The black line in the panels was my estimate of the mean path of the NAC at the time. The red track (float 307) is perhaps the one that best stays in the NAC from south to north. There is no question that topography dictates the path of the NAC. From south to north we have the SE Newfoundland Rise (1), the Newfoundland Seamounts (2), Milne Seamounts (3), Flemish Cap (4) and Orphan Knoll (5). It is rather curious that the trough at 43-44°N extends so far east (beyond the Seamount chain) before sharply turning back west forming a meander crest just south of Flemish Cap. The left panel shows mean dynamic topography (MDT, the figure was made in 2009), and the right panel shows all looper float tracks from the NAC study.
The MDT shows clearly the NAC working its way north as well as the cold Labrador Current flowing south along the continental slope – even through the narrow, roughly 1000 m deep trough that separates Flemish Cap from the Grand Banks. It continues south passing between the Slope and the Newfoundland Seamounts before turning west at the Tail of the Grand Banks. A lot of detail in the MDT shows up in the float tracks. Here we focus on floats that revealed some form of looping motion, which we define as when a float it executes at least two loops. We have already discussed the Mann Eddy. Notice the presence of cyclonic (blue) loopers inside of all three troughs and anticyclonic (red) loopers inside the Mann Eddy and the NWC. That is what we would expect but notice also red loopers outside the troughs at 47 and 50°N. Anticyclonic motion appears to dominate at these sites given that they show in the MDT. Why is this? I would guess that a lack of mean flow and the presence of the NAC to its west is the source of anticyclonic vorticity. This would certainly be the case at 49-50°N in the constriction between the north-flowing NAC and the retroflected current after rounding the NWC.
Other loopers in the figure show that coherent eddy motion, both cyclonic and anticyclonic, is commonplace. I like the little cyclonic looper drifting south for nearly 3 months in the Labrador Current! My August 12, 2023 post discusses loopers (aka spinners) throughout the northern North Atlantic. The main point of this brief post is that the NAC meanders remain in place, they do not propagate as in the Gulf Stream. We have found no evidence (so far) that the NAC meanders can pinch off to create warm and cold core rings as in the Gulf Stream.
Andersson-Fontana, S., M. Prater and T. Rossby (1996). RAFOS float data report of the North Atlantic Current study 1993-1995. GSO Technical report 96-4. University of Rhode Island.