Discussion: The Atlantic multi-decadal oscillation (AMO) is a natural mode of variability occurring in the North Atlantic basin with an estimated period of 60-80 years. During 1925 to 1965, the AMO was in the warm phase followed by a cool phase lasting from 1965 to 1994 to complete the warm/cool cycle and in this case lasting over 70 years.
Since the middle 1990’s, AMO has returned to the warm phase (Fig. 1). The North Atlantic has shown a tendency for a warmer than usual signature during this warm cycle likely related to the constricting polar ice cap. The smaller and less dense polar ice cap contributes less cooling to the upper atmosphere and therefore the thermal contrast between the polar and subtropical/tropical atmosphere is lower and forces a weaker jet stream that becomes more meridional in character.
Upper-level high-pressure ridge areas are stronger and allow more sunlight to cause the mid-latitude oceans to warm. The warming SSTA trend described is certainly evident on Sep. 27th for 2022 and dating backwards in 10-year intervals to 1982 (Fig. 2-6).
High-pressure areas elongate during summertime in this pattern and can often lead to lengthy dry and hot periods triggering drought as observed during summer 2022 in China, the U.S. and Europe.
However, when a low-pressure trough or a tropical cyclone moves across the warmer-than-normal waters, above normal low-atmosphere moisture (due to the warm SSTA) is evaporated and rinsed out by convective clouds as heavy to extreme rainfall.
Fig. 1: Monthly observations of the Atlantic multi-decadal oscillation since 1982.
Fig. 2-3: The North Atlantic SSTA for 9/27/22 and 9/27/12.
Fig. 4-5: The North Atlantic SSTA for 9/27/02 and 9/27/92.
Fig. 6: The North Atlantic SSTA for 9/27/82.