Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 31

Figure 2.2 Monthly
values for the PDO index,
January 1900 to August
2004.
C. A. Liles of the NWS in Albuquerque, N. Mex., relates New Mexico rainfall to
PDO. He concludes that the PDO cycle in its negative (cool) phase is related to
long-term droughts in the region. Given that we may be on the verge of a new
negative phase, Liles foresees major drought problems, especially with the
burgeoning population of this sunbelt state. A similar result has been derived
for Arizona, where it has been found that winter season precipitation is signifi-
cantly affected by PDO phase. Rainfall was well below normal in the cool phase
and above normal in the warm phase.
Causes of the PDO are not known and hence prediction through modeling
is not well established. However, because of its multi-year persistence, know-
ledge of the PDO improves year-to-year and season-to-season forecasting.
Recognition of the PDO is of value as a societal function for it illustrates how
''normal'' climate conditions can vary over a human lifespan.
2.4 The Pacific North American Oscillation (PNA)
The PNA is an alternating pattern between pressures in the central Pacific Ocean
and centers of action over western Canada and the southeastern United States
(Latif and Barnett 1994 ). Expressed as an index that is both ocean- and land-
based, the PNA is characterized by atmospheric flow in which the west coast of
North America is out of phase with the eastern Pacific and southeast United
States. It tends to be most pronounced in the winter months. The PNA is
associated with a Rossby wave pattern with centers of action over the Pacific
and over North America. It refers to the relative amplitudes of the ridge over
western North America and the troughs over the central North Pacific and
southeastern United States (Leathers et al. 1991a ).
According to Wallace and Gutzler ( 1981 ) the PNA is a ''quadripolar'' pattern
of pressure height anomalies. Anomalies with similar signs are located south of
the Aleutian Islands and the southeastern United States and those with opposite
signs (to the Aleutian center) are in the vicinity of Hawaii and the intermountain
region of Canada. The locations used to derive a PNA Index are 208 N, 1608 W;
458 N, 1658 W; 558 N, 1158 W; 308 N, 858 W, using the normalized 700 mb
height anomalies for a particular season. Other definitions that are used are the
result of a statistical pattern analysis.