The unusually cool and foggy weather that we have had through July and now into August has origins in a cold Pacific Decadal Oscillation (PDO) says Dr. Greg Jones, professor of climatology from Southern Oregon University, as quoted by Glenn McCourty in “Grounded Grapegrowing” in the August 2009 issue of Wines and Vines.

PDO is a Pacific Ocean weather oscillation, specific to north of 20 degrees north latitude, that involves surface water temperatures.  Warm temperatures in the Eastern Pacific are accompanied by cool temperatures in the Western Pacific or vice versa.  These temperature swings normally vary over decades.

The Pacific Coast climate is affected by both PDO and the El Nino-Southern Oscillation (ENSO). ENSO involves a larger swath of the earth and includes ocean temperatures and atmospheric changes. ENSO El Nino and La Nina changes are responsive to temperature changes in waters of the eastern Pacific Ocean.

Finally, an explanation that makes sense! When a cold PDO brings cooler and drier-than-normal weather to California AND ENSO is in the La Nina phase, this results in cold springs, and cooler-than-normal growing seasons. La Nina influences are weakening and the forecast is that we will return to a “neutral” pattern later this year.