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One of the main sources of climate variability during winter in North America, Europe and Northern Asia is the North Atlantic Oscillation (NAO). It happens due to a large scale fluctuation between subtropical-high-pressure and subpolar-low-pressure. This pressure fluctuation drives the surface winds to blow from west to east across North Atlantic Ocean from New England to Western Europe sometimes even to central Siberia, eastern Mediterranean and southward to West Africa (Figure 1).
Figure 1, Changes in Jet Streams due to Negative and Positive North Atlantic Oscillation
The changes in NAO results a considerable variability in climate which can be inter-seasonal and interannual for several months in the form of both the positive and negative phases (Figure 2).
The degree of NAO is quantified by NAO Index. NAO Index is measured as the anomalies between Icelandic low pressure and Azores high pressure. In case of a positive difference in NAO index, temperature in the northeastern USA increases, North Sea faces an increased number of storms, central USA gets more precipitation and northern Europe also get more precipitation and warmer temperature. However, in case of negative difference of NAO Index northern Europe get less precipitation and Turkey along with other Mediterranean countries get increased precipitation, and increased number of hurricanes occur in Tropical Atlantic and Gulf Coast.
Figure 2, Weather variability during Positive and Negative North Atlantic Oscillation
Figure 3, Variations in North Atlantic Indexes since 1950 till present
Figure 3 shows that negative NAO indexes were more evident during 1960's and 1970's however positive were experienced during 1980's and 1990's.
The change in pattern of NAO is also related to the Arctic Oscillation and El Niño Southern Oscillation. However, there is still no strong evidence of global warming on NAO pattern. One of the examples of how NAO index may affect the weather/climate variables is the negative relationships of seasonal snowfall in North Carolina and positive NAO index (Figure 4). The graph in figure 4 shows that whenever snowfall is more (the yellow lines goes up), NAO index is negative (shaded blue) or vice versa. This clarifies that negative NAO may have some effect on increased snowfall in NC.
Figure 4, Relationship of NAO Index departures (positive in shaded red; negative in shaded blue) and departures of seasonal snowfall in North Carolina
NAO indexes are calculated based on a statistical procedure (Rotated Principal Component Analysis used by Barnston and Livezey, 1987). The data used for the analysis is the monthly means and standard deviations of 500mb height anomalies obtained in the region of 20 N to 90 N between January 1950 and December 2000 (Figure 5). A time series is developed following the statistics indexes are calculated. More information is available at: http://www.cpc.ncep.noaa.gov/data/teledoc/teleindcalc.shtml
Figure 5, 500-hPa Height Anomalies in Northern hemisphere