PNA (Pacific North American) is a natural variation in the climate
with a positive and negative phase which has an impact on the climate
of North America. As the NAO, has
more significant effects in winter than summer and therefore the anomalies
caused by the NAP are visible during the winter months only. During the
summer, abnormal conditions are more difficult to detect.
This is a"quadrupole"atmospheric model of atmospheric pressure
anomalies ranging from subtropical western Pacific to the east coast of
North America, because this oscillation varies with the position of depressions
and anticyclones. The anomalies with similar signs are located at the
south of the Aleutian Islands and at Southeastern United States and those
with opposite signs to these two latter areas are close to Hawaii and
the Rocky Mountains to the west of the North America.
The model of PNA is associated with strong fluctuations in the strength
and location of
Jet Stream coming from the east of Asia.
Position of the jet stream, anticyclones (H)
and depressions (L) during PNA+
The positive phase of the PNA is characterized by pattern of
higher-than-average atmospheric pressure in the vicinity of Hawaii
and over the mountainous region of western North America, and lower-than-average
pressure in south of Alaska and over the southeastern United States.
This phase is associated with a higher upper-level of meridional
The PNA index appears more often positive in years with El
The positive phase of the PNA is associated with higher than
the average temperatures on the West Coast of the United States
and western Canada, but at temperatures lower than the average across
the South-Central and Southeastern States States.
The precipitation anomalies are higher than the average in
the West coast of Alaska and of United States (California ...),
the Gulf States, Florida. Drier conditions persist on the United
States Midwest and the Canadian Prairies as well as in the Pacific
The positive phase is associated with a jet stream from the
East Asia which is reinforced, going tonorth in the eastern Pacific
and then down to the South East over North Dakota. Then the cold
fronts coming from the Hudson Bay bring over an interval of about
ten days of unusual snowfall in the Mississippi Valley on the east
coast of the United States and sometimes as far away as Florida
It is characterized by pattern of higher-than-average atmospheric
pressure off the coast of the Aleutian Islands and the southeast
of the United States and lower-than-average pressure over the Rocky
Mountains to the West of the United States as well as at the bottom
of the Hudson Bay and in the vicinity of Hawaii. This phase is associated
with a higher upper-level of zonal flow.
The PNA index appears more often positive in years with La
Nina but this is not always the case given the other factors
that come into play.
During the negative phase, the West of United States can have
relatively cold temperatures, while the East of United States has
rather temperatures higher than normal.
The PNA- often causes severe droughts in Mt Shasta on the West
Coast of the United States, while the East and the Canadian Prairies
are associated with wet conditions. The negative phase gives more
snow and colder temperatures in the High Plains and the Midwest
of the United States.
There are two different methods
to calculate PNA index, the details them are here :
1°) - According to the formula of Wallace
and Gutzler (1981) using the method step by step PNA index
is calculated :
PNA = 0,25 x
[Z (20°N, 160°W) -
Z (45°N, 165°W) +
Z (55°N, 115°W) -
Z (30°N, 85°W)] where
Z names the average of the
anomaly of each month following
the basis period of 1950-2000
500 hPa geopotential.
The second calculation method
PNA = Z
* (15°N-25°N, 180-140°W)-Z
* (40°N-50°N, 180-140°W)
+ Z* (45 °N-60°N,
* (25°N-35°N, 90°W-70°W),
* () denotes monthly mean
500 mb height anomaly that
is obtained by subtracting
the calendar mean value
for each month during 1950-2000
is normalized using as basis
the statistical period 1950-2000.
in winter (DJF) for the
period 1949 / 50-1999 /
2000 the coefficient of
correlation between the
two calculation methods
graphic is created according
to the reconstruction of
the index by NOAA
the years 1725-1950 and
the PNA index is updated
according to the NOAA
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