4st PART








P.D.O. (Pacific Decadal Oscillation)

In 1996 the scientist Steven Hare showed that a second oscillation existed in the Pacific with ENSO : it is the Pacific Decadal Oscillation (PDO). This is a variation of the surface temperature anomaly in the Pacific between 40 and 50 years with 2 phases of about 20 to 30 years in contrast to the ENSO that persists for 6 to 18 months. The P.D.O. phenomenon is comparable to E.N.S.O. but with a much larger amplitude and which is characterized by a distribution of the temperature anomalies of the ocean in the form of the "horseshoe", while ENSO has an impact along the Pacific between the tropics.

The causes of the PDO are currently not well known because this oscillation is long. It is also difficult to predict in contrast to El Niño and La Niña. On the one hand the identification of the PDO is important because it shows that climatic conditions can change over long periods in the Pacific.


There are two phases :

- During the warm phase (positive), in the North East Pacific the waters are colder than normal. But the temperature of the ocean along the North western America is warm as well as along the equator from the central America to the Marquesas Islands. This creates similar anomalies at El Niño in Western North America the temperatures are above the average in winter and spring, but they are below the average in the southeast of the USA. Mexico and the Pacific Northwest rainfall is less important.

- In the cold phase (negative), the PDO looks like a warm water arc going from North to South through the Pacific near the Japan, and that surrounds a large area of cold water near the equator. This phase is the opposite of the previous which therefore has the opposite effect, so that is similar to La Niña : The North Western America and the Great Lake have significant rainfall from March to October and often in the spring it is possible that it snows a lot.

The temperature anomalies of the sea surface during the two phases of the PDO

When comparing the phase of El Niño and the positive phase of the PDO
we can see that they are quite similar and similarly during the negative
phase and La Niña. During the El Niño and La Niña, the anomalies
are more important at the equator than during the PDO.

The PDO phases can be associated with El Niño or La Niña. The positive phase of the PDO which has the water surface with warm temperatures in the equatorial Pacific, can enhance an El Niño episode, amplifying the effects of the latter. This same phase of the PDO would weaken the La Niña events that would occur during this period. Similarly, during a negative phase of the PDO, events La Niña would be rather enhanced, and El Niño events would be weakened.

1947 to 1977
1978 to 1998
1999 to now
The number of El Niño and La Niña in
the last three phases of the PDO since 1947


By studying tropical corals it was possible to recompose the index of the P.D.O. as there is a correlation between the latter and the isotopic composition of oxygen in corals collected in Fiji islands. And several trees ring resulted it also.

The evolution of the PDO on a long-term maturity calculated from 17 tree
rings of Northeast Asia. This graphic is created according to the NOAA data

The P.D.O. index is defined as the monthly variability of the surface temperature of the North Pacific. Is calculated every 2 to 3 months.

Cold Phase Warm Phase Cold Phase Warm Phase
Cold Phase
Click to enlarge
The P.D.O. experienced a cold phase from 1900 to 1924 and then a warm
phase from 1925 to 1946 followed by a cold phase from 1947 to 1976
and then another warm phase from 1977 to 1998 and a cold phase
began around since 1999. This graphic is created and updated with :
Joint Institute for the Study of the Atmosphere and Ocean (JISAO)


    • PDO has an impact on the change of the regional atmospheric circulation and therefore is associated with anomalies on precipitations and temperatures... of the United States. During the PDO+ the droughts are more frequent in the North and Northwest of the USA and less frequent in the Southwest and the North American Center. And during the PDO- all the reverse is facing the United States.

    Anomaly of precipitations depending on the PDO

The climate of North America is influenced by long-term PDO but also by AMO. And so these two oscillations long periods may contradict or vice versa and thus amplify the anomalies in rainfall, temperatures and atmospheric pressure.

On the right you have a comparison of the indices of the two climatic phenomena : "A" PDO and "B" AMO.

Click here for more info about the AMO and the consequences of the association of two climatic phenomena


    • Owing to the change in the Pacific SST the number of certain fishes varies.

The abundance of salmon showed a clear correlation with the variations in climate of the 20th century in the Pacific. In the North West Pacific the salmon is more abundant during the cold phase of the PDO and less abundant during the warm phase while it is quite the opposite in Alaska..

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