Geoscience Reference
In-Depth Information
A
NAO Positive phase
L
H
B
NAO Negative phase
H
L
H
Cold air
or water
Warm air
or water
Center of
low pressure
Center of
high pressure
L
H
Westerlies
Trades
Area of increased rain
Atmospheric circulation patterns and climatic impacts associated with A) positive and B) negative phases of the North Atlantic Oscillation (based
on Stephenson, 1999).
Fig. 2.13
characterizing this oscillation has been built using
average surface air pressure over the region
30°N-65°N, 160°E-140°W (Figure 2.8). Over the
north Pacific Ocean, atmospheric pressure near
the Aleutian Islands varies out-of-phase with that
to the south, seesawing around the mean position of
the Pacific subtropical jet stream. Over North America,
variations in atmospheric pressure over western
Canada and the north-western United States are nega-
tively correlated with those over the south-eastern
United States; but positively correlated with the
subtropical Pacific. Changes in these centers occur
concomitantly with changes in the amplitude of Rossby
waves and the intensity of mobile polar highs. The
NPO is linked to changes in tropical Pacific sea surface
temperatures associated with the ENSO phenomenon.
20
NAO
0
-20
15
Storms
10
5
0
1960
1970
1980
1990
Fig. 2.14
The number of winter storms in the north Atlantic between 1956
and 1994 with pressure below 950 hPa (Villwock, 1998).
