|The Pacific Decadal Oscillation|
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.