An overlooked role of the subtropical gyre circulation in regulating the AMOC

It is a common practice in Physical Oceanography to separate the Atlantic Ocean circulations into the meridional overturning and wind-driven gyre components with an assumption that the two components are largely independent of each other. An article published in Nature Communications suggests that the two components are not at all independent. The study shows that the upper-ocean water that flows northward into the North Atlantic circulates the subtropical gyre at least once before leaving the gyre. This process is necessary to achieve an increase in water density and depth through air-sea interaction (i.e., loss of heat and freshwater to the atmosphere) and interior mixing processes, before the water can escape the latitudes of the gyre and join the northern upper branch of the Atlantic Meridional Overturning Circulation (MOC). This suggests that the wind-driven subtropical gyre actively contributes to dense (salty and cold) water production, and thus may serve as a precursor to AMOC variability. For instance, a stronger-than-normal subtropical gyre may increase the retention period of the upper-ocean water increasing the cooling and saltening. A subsequent release of the preconditioned (cold and salty) water may in turn increase deep water formation. On the other hand, a weaker-than-normal gyre may decrease the retention period releasing less dense water (warm and fresh) northward, which may, in turn, contribute to a decrease in deep water formation. 

Figure 2a from Berglund et al. (2022). Left: The Eulerian Atlantic Meridional Overturning stream function computed for the years 1850–2014. The rectangle indicates the area shown in the figure to the right. The contour interval is 2 Sv. Right: The Lagrangian Meridional Overturning stream function for waters that move directly northwards without spiralling in the Gyre (black contours) and for those waters that spiral in the Gyre at least once (red contours). The contour interval is 1 Sv.

Berglund, S., Döös, K., Groeskamp, S., & McDougall, T. J. (2022). The downward spiralling nature of the North Atlantic Subtropical Gyre. Nature Communications, 13, 2000. https://doi.org/10.1038/s41467-022-29607-8