Overview Solar and oceanic cycles are the most important drivers of all weather and climate on our planet and the most important ocean of all is the Pacific. This large body of water covers about a third of the planet’s surface and is bigger than all of the Earth’s land masses combined. Both the Pacific and Atlantic Oceans go through sea surface temperature phases that are characterized as cool (negative) and warm (positive). While there are other important factors that contribute to drought conditions in the western U.S. (e.g., hours of sunshine, humidity levels), the given temperature phase of the Pacific Ocean has historically been found to be a major factor in precipitation trends just as it has contributed greatly to global temperature trends. Indeed, the on-going California drought appears to be a natural consequence of the current cool phase of the Pacific Ocean which typically produces an overall dry weather pattern in the western U.S. and, if history is any guide, it is likely to persist for years to come.
Pacific Decadal Oscillation (PDO) The Pacific Decadal Oscillation (PDO) is a climate index based upon long-term patterns of variation in sea surface temperature of the north Pacific. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of about 20°N. During a warm (positive) phase, the west Pacific becomes cool and part of the eastern ocean warms; during a cool (negative) phase, the opposite occurs. These phases result from the direction of winter winds in the northern Pacific: winter winds blowing chiefly from the southwest result in warmer conditions in the northern California Current (CC); conversely, when winds blow primarily from the north, upwelling occurs both in the open ocean and at the coast, leading to cooler conditions in the northern CC.
The PDO temperature phase in the north Pacific tends to have an impact on the shorter-term sea surface temperature cycles of the tropical Pacific in the equatorial region. Specifically, warm phases of the PDO are generally associated with stronger and more numerous El Nino (warmer-than-normal) events in the tropical Pacific and weaker and fewer La Nina (colder-than-normal) episodes. During cold phases of the PDO, La Nina tends to dominate El Nino in the tropical Pacific. In the past several years, during the current cool phase of the PDO, La Nina conditions have indeed dominated the scene in the tropical Pacific region.
Warm and cool phases of the PDO can persist for decades, usually about 20 to 30 years. A warm phase occurred from 1925 to 1946, a cool phase from 1947 to 1976, and then another warm phase from 1977 to 1998. Currently, the Pacific Ocean is in a cool PDO phase that began around the turn of the century and the recent years of 2008 and 2012 actually exhibited the most negative PDO index values since the 1950’s [PDO Index plot below].
PDO Index plot: Time series of shifts in sign of the Pacific Decadal Oscillation (PDO), 1925 to present. Values are averaged over the months of May through September. Red bars indicate positive (warm) years; blue bars negative (cool) years. Note that 2008 and 2012 were the most negative values recorded since 1956.
The PDO and precipitation anomalies and trends Precipitation anomaly charts are shown below for the U.S. with primarily wetter-than-normal conditions seen on a nationwide basis during the warm PDO phase between 1977 and 1998 (top) and generally drier-than-normal conditions experienced during the cold PDO phase that lasted from 1947 to 1976 (bottom). Note the significant dry weather pattern seen across California during the last cold PDO phase with precipitation nearly two inches below the long term average in much of the state.
Precipitation trends in the southwestern U.S. have been shown to have direct correlation to PDO phases in the Pacific Ocean. The plot below displays the PDO index (red) versus the annual precipitation amounts in the southwestern U.S. (blue) all the way back to the year 1900 [source WSI Energy Weather]. In general, when the PDO was in a warm phase, the annual precipitation amounts in the southwestern U.S. tended to climb and in times of a cold PDO phase they typically dropped. This is supporting evidence that the on-going California drought is likely to continue on average as long as the Pacific Ocean’s cool PDO phase persists.
[credit to WSI Energy Weather]