The Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) influence sea surface temperatures, sea level pressure, and surface winds in very similar ways. The most obvious difference between the PDO and ENSO is the time scale. Whereas ENSO events tend to persist on the order of one year, the PDO signature can last up to 30 years (Mantua, 2001).

Additionally, while the PDO primarily affects the North Pacific region, its effects can be felt near the equator. Conversely, ENSO primarily affects the climate of lower latitudes, but its secondary effects are felt in the North Pacific (Mantua, 2001). A positive, or warm phase PDO, produces climate and circulation patterns that are very similar to El Niño. Likewise, a negative, or cool phase PDO, produces climate and circulation patterns similar to La Niña (Gershunov and Barnett, 1998). This is documented in the following two figures, which show how sea surface temperatures and surface winds are influenced by (1) the positive and negative phases of the PDO and (2) El Niño and La Niña phases of ENSO. Dark red and dark blue indicate temperature anomalies of +0.8 C and -0.6 C, respectively. Wind stress directions and magnitudes are indicated by vectors.

Source: Mantua, 2000

Numerous studies have attempted to determine the effect of the PDO and ENSO on each other. The results have been largely inconclusive and/or contradictory. However, a study by Gershunov and Barnett (1998) shows that the PDO has a modulating effect on the climate patterns resulting from ENSO. The climate signal of El Niño is likely to be stronger when the PDO is highly positive; conversely the climate signal of La Niña will be stronger when the PDO is highly negative. This does not mean that the PDO physically controls ENSO, but rather that the resulting climate patterns interact with each other.