in the Marine Environment. Amsterdam: Elsevier,
pp. 1535–1597.
Sigman, D. M., Karsh, K. L., and Casciotti, K. L., 2009. Nitrogen
isotopes in the ocean. In Steele, J. H., Turekian, K. K., and
Thorpe, S. A. (eds.), Encyclopedia of Ocean Sciences. London:
Academic, pp. 40–54.
NORTH ATLANTIC OSCILLATION (NAO)
Erik Kjellström
Swedish Meteorological and Hydrological Institute,
Norrköping, Sweden
The large-scale atmospheric circulation in the North
Atlantic sector is characterized by westerly winds that
result from the meridional pressure gradient between the
Azores High in the south and the Icelandic Low in the
north. As the strength of this pressure gradient varies over
time, also the strength of the westerlies varies. In periods
of strong north-south gradient, the westerlies are more
pronounced and vice versa when the gradient is weak.
Fluctuations between these different states are known as
the North Atlantic Oscillation (NAO).
The strength and phase of the NAO can be summarized
by an index (NAOI). There exist a few different NAOI
definitions. The most commonly used is likely the one
comparing normalized mean sea-level pressure (MSLP)
in Iceland and in Portugal (Hurrell, 1995). One of its main
strengths is that this type of NAO index can be derived
also for historical times as surface pressure readings exist
at some locations dating back to the nineteenth century.
Other definitions, such as those based on principal component analysis (PCA), are also commonly used (e.g.,
Barnston and Livezey, 1987). A benefit of the latter type
of methods is that they can better depict variability/
changes in the center of action of the NAO.
The NAO has a profound impact on the climate in the
North Atlantic region. During winter the positive phase
is associated with temperatures below average in western
Greenland and southern Europe and above average in
large parts of the eastern USA and northern Europe. At
the same time, the precipitation is above average in northern Europe and in the eastern USA and below average in
southern Europe and western Greenland. In the negative
phase of the NAO, opposite anomalies are found in temperature and precipitation in these areas. Also in summer,
there are impacts on the climate with distinct differences
between northern and southern Europe in the respective
phases of the NAO (Folland et al., 2009).
As the large-scale pressure pattern varies on different
time scales, fluctuations in the NAOI are seen starting
already at daily time scales ranging over weeks and
months to seasons or years or even decadal time scales.
Studies of the temporal behavior of the NAOI show some
weakly significant and intermittent oscillations on quasibiennial and quasi-decadal time scales (see review of
Wanner et al., 2001). This shows that the NAO is not a
pure random process indicating that there may be predictive potential associated with it.
Bibliography
Barnston, A. G., and Livezey, R. E., 1987. Classification, seasonality and persistence of low frequency atmospheric circulation patterns. Monthly Weather Review, 115, 1083–1126.
Folland, C. K., Knight, J., Linderholm, H. W., Fereday, D., Ineson,
S., and Hurrell, J. W., 2009. The summer North Atlantic oscillation: past, present and future. Journal of Climate, 22,
1082–1103.
Hurrell, J. W., 1995. Decadal trends in the North Atlantic oscillation: regional temperatures and precipitation. Science, 269,
676–679.
Wanner, H., Bronnimann, S., Casty, C., Gyalistras, D., Luterbacher,
J., Schmutz, C., Stephenson, D. B., and Xoplaki, E., 2001. North
Atlantic oscillation – concepts and studies. Survey in Geophysics, 22, 321–382.
540
NORTH ATLANTIC OSCILLATION (NAO)
pp. 1535–1597.
Sigman, D. M., Karsh, K. L., and Casciotti, K. L., 2009. Nitrogen
isotopes in the ocean. In Steele, J. H., Turekian, K. K., and
Thorpe, S. A. (eds.), Encyclopedia of Ocean Sciences. London:
Academic, pp. 40–54.
NORTH ATLANTIC OSCILLATION (NAO)
Erik Kjellström
Swedish Meteorological and Hydrological Institute,
Norrköping, Sweden
The large-scale atmospheric circulation in the North
Atlantic sector is characterized by westerly winds that
result from the meridional pressure gradient between the
Azores High in the south and the Icelandic Low in the
north. As the strength of this pressure gradient varies over
time, also the strength of the westerlies varies. In periods
of strong north-south gradient, the westerlies are more
pronounced and vice versa when the gradient is weak.
Fluctuations between these different states are known as
the North Atlantic Oscillation (NAO).
The strength and phase of the NAO can be summarized
by an index (NAOI). There exist a few different NAOI
definitions. The most commonly used is likely the one
comparing normalized mean sea-level pressure (MSLP)
in Iceland and in Portugal (Hurrell, 1995). One of its main
strengths is that this type of NAO index can be derived
also for historical times as surface pressure readings exist
at some locations dating back to the nineteenth century.
Other definitions, such as those based on principal component analysis (PCA), are also commonly used (e.g.,
Barnston and Livezey, 1987). A benefit of the latter type
of methods is that they can better depict variability/
changes in the center of action of the NAO.
The NAO has a profound impact on the climate in the
North Atlantic region. During winter the positive phase
is associated with temperatures below average in western
Greenland and southern Europe and above average in
large parts of the eastern USA and northern Europe. At
the same time, the precipitation is above average in northern Europe and in the eastern USA and below average in
southern Europe and western Greenland. In the negative
phase of the NAO, opposite anomalies are found in temperature and precipitation in these areas. Also in summer,
there are impacts on the climate with distinct differences
between northern and southern Europe in the respective
phases of the NAO (Folland et al., 2009).
As the large-scale pressure pattern varies on different
time scales, fluctuations in the NAOI are seen starting
already at daily time scales ranging over weeks and
months to seasons or years or even decadal time scales.
Studies of the temporal behavior of the NAOI show some
weakly significant and intermittent oscillations on quasibiennial and quasi-decadal time scales (see review of
Wanner et al., 2001). This shows that the NAO is not a
pure random process indicating that there may be predictive potential associated with it.
Bibliography
Barnston, A. G., and Livezey, R. E., 1987. Classification, seasonality and persistence of low frequency atmospheric circulation patterns. Monthly Weather Review, 115, 1083–1126.
Folland, C. K., Knight, J., Linderholm, H. W., Fereday, D., Ineson,
S., and Hurrell, J. W., 2009. The summer North Atlantic oscillation: past, present and future. Journal of Climate, 22,
1082–1103.
Hurrell, J. W., 1995. Decadal trends in the North Atlantic oscillation: regional temperatures and precipitation. Science, 269,
676–679.
Wanner, H., Bronnimann, S., Casty, C., Gyalistras, D., Luterbacher,
J., Schmutz, C., Stephenson, D. B., and Xoplaki, E., 2001. North
Atlantic oscillation – concepts and studies. Survey in Geophysics, 22, 321–382.
540
NORTH ATLANTIC OSCILLATION (NAO)