The extratropical Northern Oscillation Index (NOI), a new index of climate variability based on the difference in sea level pressure (SLP) anomalies at the North Pacific High (NPH) in the northeast Pacific (NEP) and at Darwin, Australia near a low pressure center, is introduced. Because of the role of these SLP centers in global atmospheric circulation patterns, the NOI reflects the variability in equatorial and extratropical teleconnections. It represents a wide range of local and remote climate signals, and summarizes the atmospheric forcing of climate change in the NEP and more generally the north Pacific-North American region.
The NOI for 1951-99 is more likely than the SOI to show positive and negative extremes, often indicative of La NiŅa and El NiŅo events, respectively. Positive events occur nearly twice as often as negative events. The NOI is predominantly positive prior to 1977 and from 1984 to 1991, and features a series of strong negative events during 1977-83. In 1991 the NOI shifts to a signal dominated by moderate to strong negative events. The negative phase of the NOI is shown to be associated with a number of strong atmospheric and oceanic anomalies in the NEP, including weaker trade winds, weaker coastal upwelling favorable wind stress, warmer SSTs and subsurface temperatures, lower Oregon hatchery salmon production, and reduced macrozooplankton biomass off southern California. The opposite physical and biological patterns generally occur during the positive phase of the index.Global SLP anomaly patterns associated with these NOI phases reflect atmospheric circulation anomalies (wave trains) that link the NEP to the tropical Pacific and south and east Asia. The NOI is a diagnostic of climate change regardless of season or its source region. The relationship between the NOI and atmosphere and ocean measurements suggests this index is a reliable indicator of climate change in the NEP, and may provide insights on the mechanisms linking variations in the physical environment to marine resources on interannual to decadal climate scales.