Relating Spring Temperatures to Natural Climate Patterns

Smoke obscures the horizon as the High Park Fire in Colorado continues to burn acres of forest, June 2012. (Credit: NOAA)

David Brown, Southern Regional Climate Services Director, together with Kurt Kipfmueller of the University of Minnesota published the manuscript, "Pacific Climate Forcing of Multidecadal Springtime Minimum Temperature Variability in the Western United States" in the May 2012 edition of the Annals of the Association of American Geographers (available with subscription or through pay-per-access). This article documents their research on the connection between interannual spring temperature minima in the West and indices of the El Niño Southern Oscillation (ENSO) and its decadal-scale "signature," the Pacific Decadal Oscillation (PDO).

ENSO is a climate pattern that occurs across the tropical Pacific Ocean roughly every five years. This pattern involves variations in surface temperature and pressure from a warm phase to a cool phase. During the warm phase, El Niño, the water in the eastern and central equatorial Pacific Ocean is warmer than usual. During the cool phase, La Niña, the water in that region is cooler than usual. The PDO is a long-lived ENSO-like pattern of Pacific climate variability. Its phases typically shift every 20 to 30 years and also involve a warm phase and a cool phase. During the warm PDO phase, the west Pacific becomes cool and part of the eastern ocean warms. During the cool phase, the opposite pattern occurs.

This study examined variations in spring temperatures—specifically minimum monthly temperatures—over a large portion of the Western United States from 1925 to 1994. The scientists hypothesized that there might be patterns in minimum temperature variability consistent with major climate drivers like ENSO and the PDO. The PDO in particular experienced three distinct phases between the mid-1920s and the mid-1990s, which is why they chose that period for the analysis.

The study found that temperature patterns associated with cool phases of the PDO were evident in the Pacific Northwest region during March, whereas warm phases were evident in the Southern Coast region during April, suggesting a possible association with recognized ENSO impacts. These results, indicating a strong and thus far uncharacterized relationship between spring temperature minima in the West and the PDO, have implications for an improved understanding of climate pattern dynamics.

Trends toward an earlier spring season onset in the western United States have been increasingly documented and are of interest to many different users of climate information throughout the region. With the climate impacts of this trend—long-term changes in snowpack and snowmelt, growing season timing and length, drought frequency and intensity—growing in importance for the Western United States, it is increasingly vital to "tease out" the evidence of climate variability from long-term climate change.