The Atlantic Multidecadal Oscillation: Its manifestations and impacts with special emphasis on the Atlantic region north of 60°N

This paper examines the multidecadal variability in the northern North Atlantic and the Arctic. Observations, modeling and paleo data provide evidence of a strong link between the atmospheric and physical oceanographic variability in these northern regions with Atlantic sea surface temperatures farther south as expressed by the Atlantic Multidecadal Oscillation (AMO). Air and sea temperatures over the past 100-150 years reveal cool periods in the late 1800s to early 1900s and in the 1970s to 1980s with warm periods during the 1920s to 1960s and from the 1990s through to the present, similar to the variability in the AMO index where a positive (negative) AMO index represents warm (cold) periods. Sea-ice extent in the north has also varied at multidecadal scales with the ice retreating during the warm periods and expanding during the cold periods. The presence of multidecadal variability is also suggested from marine sediment paleo data as well as ice-core oxygen isotope data. Observations of biological impacts of the multidecadal variability in the northern regions include a general increase in plankton and fish productivity, as well as expansion of the species distributions northward, in conjunction with the AMO warm periods and the opposite during AMO cold periods. In addition, a review of the mechanisms responsible for the AMO and a brief discussion of the linkages between the multidecadal variability in the northern and southern hemispheres, including between the Arctic and Antarctic, are presented.