Geoscience Reference
In-Depth Information
Figure 9.14
The North Atlantic Oscillation is the anomalous difference between the polar low and the subtropical high
during the winter season (December through March). (a) A positive NAO index phase corresponds to a stronger than usual
subtropical high pressure center and a deeper than normal Icelandic low, resulting in more and stronger winter storms
crossing the Atlantic Ocean on a more northerly track. (b) A negative NAO index phase corresponds to a weak subtropical
high and a weak Icelandic low, resulting in fewer and weaker winter storms crossing on a more west-east pathway. (From
http://www.ldeo.columbia.edu/res/pi/NAO/.)
North Atlantic Oscillation
Westerly winds blowing across the Atlantic bring moist air into Europe. In years
when westerlies are strong, summers are cool, winters are mild and rain is fre-
quent. If the westerlies are suppressed, the temperature is more extreme in sum-
mer and winter leading to heat waves, exceptional frosts and reduced rainfall.
A permanent low-pressure system over Iceland, called the
Icelandic Low
, and a
permanent high-pressure system over the Azores, called the
Azores High
, control
the direction and strength of westerly winds into Europe. The relative strengths
and positions of these pressure systems vary from year to year and this variation is
known as the North Atlantic Oscillation (NAO), see Fig. 9.14. A large difference in
the pressure at the two stations, i.e., a year when the NAO index is high, leads to
increased westerly winds and, consequently, cool summers and mild, wet winters
in central Europe. When the NAO index is low, the westerly winds are suppressed
and central Europe has cold winters with the storm tracks farther south, toward
the Mediterranean Sea, resulting in more storm activity and rainfall in southern
Europe and North Africa.
Water vapor in the atmosphere
The global distribution of water vapor in the atmosphere and how that
concentration is changing with time is the most important hydroclimatological
consequence of atmospheric general circulation because it reflects the regional
surface water balance. Over the ocean, and over many land areas where water is
readily available, the specific humidity of air near the surface is strongly linked to
near-surface air temperature and changes from about 18 g kg
−1
at the equator to
