The Pacific decadal oscillation is a robust recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. This phenomenon manifests as warm or cool surface waters in the Pacific Ocean north of 20°N latitude.
Steven R. Hare named the Pacific decadal oscillation while studying salmon production pattern results in 1997. Evidence exists for reversals in the prevailing polarity occurring around 1925, 1947, and 1977.
During the positive phase the wintertime Aleutian Low is deepened and shifted southward to advect warm humid air along the North American west coast. Temperatures are higher than usual from the Pacific Northwest to Alaska while winter precipitation is higher than usual in the Alaska Coast Range Mexico and the Southwestern United States.
MacDonald and Case reconstructed the PDO back to 993 using tree rings from California and Alberta. A signal has been reconstructed as far back as 1661 through tree-ring chronologies in the Baja California area.
The 1976/1977 shift changed the PDO to a warm phase and corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean. In 1997/1998 several changes in sea surface temperature and marine ecosystem occurred in the North Pacific when the PDO changed back to a cool anchovy phase.