Variations of zonal circulation intensity over the North Atlantic and displacement of centers of atmospheric action lead to significant climate changes over Europe. The pressure gradient between the Azores High and the Icelandic Low determines the strength of the mid-latitude westerly and, hence, the transport of relatively warm and humid air from the North Atlantic to Europe. During the mid-1990s James Hur-rell and others showed that, on the interannual-to-decadal scale, the NAO intensification (or positive phase of NAO) is accompanied by displacement of the Azores High and Iceland Low to the north-northeast. At the same time, the zonal atmospheric circulation over the North Atlantic and Europe, and zonal winds in the low troposphere within the band restricted by 50 and 60 degrees N are strengthened. This accompanies displacement of pathways of most North Atlantic cyclones to northern Europe and associated increases in air temperature and precipitation there, while the anticyclonic hydrometeorological conditions prevail over central, southern Europe, and the Mediterranean region. The reverse tendency is observed during negative NAO phases, when the Azores High and Iceland Low shift to the south-southwest, and the westerly weakens. It is accompanied by more frequent cyclonic conditions over European regions close to the Mediterranean Sea and Middle East, while more frequent anticyclones occur over northern Europe.
Climate change is accompanied by a tendency of NAO strength and reduction of distance along latitude between the Azores High and Iceland Low in the 20th century. This and superimposed mul-tidecadal North Atlantic variability lead to much more pronounced impacts of NAO on the European climate after approximately 1960. Some ideas about the NAO focus on the North Atlantic basin itself, invoking, for example, stochastic atmospheric forcing of the ocean, mixed, wind-driven, and thermohaline advection, and fully -oupled ocean-atmosphere modes in the North Atlantic region. It has also been suggested that the NAO may be teleconnected to the tropical Atlantic, as well as a local manifestation of a hemisphere-wide oscillation centered over the Arctic. At the same time, as shown by Gidon Eshel, NAO may be generated by processes in the North Pacific coupled system, which impacts the North Atlantic through the atmospheric bridge.
SEE ALSo: Agulhas Current; Arctic Ocean; Atlantic Ocean; Oceanic Changes; Oceanography.
BIBLioGRApHY. Grant Bigg, et al., "The Role of The Oceans in Climate," International Journal of Climatology (v.23/10, 2003); Gidon Eshel, "Forecasting the North Atlantic Oscillation using North Pacific Surface Pressure," Monthly Weather Review (v.131/5, 2003); James Hurrel, "Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation," Science (v.269/5224, 1995); Hunter Machel, Alica Kapala, and Hans Flohn, "Behaviour of the Centres of Action above the Atlantic since 1881. Part 1: Characteristics of Seasonal and Interannual Variability," International Journal of Climatology (v.18/7, 1998); John Marshall, et al., "North Atlantic Climate Variability: Phenomena, Impacts and Mechanisms (Review)" International Journal of Climatology (v.21, 2001); Alexander Polonsky, Elena Voskresens-kaya, and Dmitry Basharin, "Coupled Ocean-Atmosphere System and its Impacts on European Climate," Climates in Transition (Minuteman Press, 2003); Alexander Polonsky, et al., "North Atlantic Oscillation: Description, Mechanisms, and Influence on the Eurasian Climate," Physical Oceanog raphy (v.4/2, 2004); Sir Walker and E. Bliss, "World Weather V," Royal Meteorology Society (v.36, 1932).
Elena Voskresenskaya Alexander Boris Polonsky Marine Hydrophysical Institute, Sebastopol
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