Hint: Climate change has somethin’ to do with it.
Schematic diagram illustrating the component parts of the AMOC and the 26◦ N observing system. Black arrows represent the Ekman transport (predominantly northward). Red arrows illustrate the circulation of warm waters in the upper 1100 m, and blue arrows indicate the main southward ﬂow of colder deep waters. The array of moorings used to measure the interior geostrophic transport is illustrated too.
(Above is from Figure 1 of “Observed decline of the Atlantic meridional overturning circulation, 2004–2012”, D. A. Smeed, G. D. McCarthy, S. A. Cunningham, E. Frajka-Williams, D. Rayner, W. E. Johns, C. S. Meinen, M. O. Baringer, B. I. Moat, A. Duchez, and H. L. Bryden, from Ocean Science, 10, 2014, 29–38.)
“AMOC slowdown: Connecting the dots”, by Stefan Rahmstorf, on RealClimate.