AMO and THC
The Atlantic Multidecadal Oscillation (AMO), or the North Atlantic’s periodic shift (65-year period) from predominately warm to predominately cool regimes, controls much of the climatic variability in the Northern Hemisphere. During warm (positive) AMO phases, the Northern Hemisphere is generally warmer, by as much as a few degrees Fahrenheit when compared to the cool phases. This cycle appears to be linked to periodic fluctuations in the strength of the thermohaline circulation (THC), or the conveyor belt”" that moves heat and salt around the world’s oceans. In the North Atlantic, a surface current transports warm water north and then “overturns” near Greenland and becomes a deep cold water current that moves south. The northward movement of warm water is strongest at about 30 degrees North (about the same latitude as New Orleans), where over 40 billion gallons of water pass each second! The strengthening of this current, which has been happening since the 1970?s, has corresponded to the rise in North Atlantic sea surface temperatures and Northern Hemisphere land temperatures.
Seasons: Spring, Summer, Fall, Winter
(Source: Knight, JR et al. “A signature of persistent natural thermohaline circulation cycles in observed climate.” Geophysical Research Letters 32 (2005): L20708 and Bryden, et al. “Slowing of the Atlantic meridional overturning circulation at 25 degrees N.” Nature 438 (2005): 655-657.)