The North Atlantic Oscillation and the ocean’s response in the 1990s

The North Atlantic Oscillation (NAO) is the dominant recurrent mode of atmospheric behaviour in the North Atlantic sector, dictating much of the climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during boreal winter. During the 1990s, the behaviour of the NAO became extreme in two main ways, both of which had deepreaching effects on Atlantic hydrography and on the marine ecosystem. First, in the early 1990s (1989-1995 approximately), the NAO Index evolved to its most extreme positive state in a 175-year instrumental record, following a long if irregular amplification over the previous three decades. Then, after a brief return to extreme NAOnegative values in 1996, the NAO dipole pattern in sea-level pressure (sip) showed some tendency to shift eastward as the Index recovered to more positive values. Through the associated variability in the intensity of open-ocean deep convection, in the production-rates and characteristics of the main convectively formed mode waters, in the freshwater accession to the Nordic Seas and in the hydrography of the dense northern overflows, these extreme trends in NAO behaviour have been associated with radical effects throughout the water column of the North Atlantic. The evidence for this is described.