But no one can say for certain. Now we ignore that episode when looking at the larger record. But at the time, it might have seemed as if we were entering a warm period. No one knows why the PDO occurs, although presumably it has something to do with rhythmic alterations in Pacific currents. Survival rates for young salmon are directly tied to environmental conditions, particularly water temperature.
Lower temperatures favor the fish.
So when the PDO is in its warm phase, salmon populations decline in the Pacific Northwest but rise in Alaska. During the cool phase, the pattern reverses. The PDO has been identified and studied for such a short time that its worldwide effects are largely unknown. Leetmaa believes it is possible that PDO phase affects such remote events as rainfall in China and the periodic droughts in the Sahel region of Africa. Locally, a repeat of the 1950s and '60s would mean lower minimum and maximum temperatures, more snow and more rain, according to National Weather Service archives.