The Pacific Ocean influences long term drought in the Southwestern US through a phenomenon called the Pacific Decadal Oscillation (PDO).

The PDO is characterized by fluctuations in the temperature of the surface waters of the North Pacific Ocean.

PDO has two phases: warm and cool. During the warm phase, the surface waters of the North Pacific Ocean are warmer than average. This leads to changes in the atmospheric circulation that result in increased precipitation along the West Coast of the United States and decreased precipitation in the interior West, including the Southwest.

This prolonged decrease in precipitation contributes to drought conditions in the Southwestern region.

Conversely, during a cold PDO phase, the North Pacific waters cool and shift the precipitation patterns bringing wet weather to the Southwest and drier weather to the Pacific Northwest.