They are the magnitude and position of the Aleutian Low during winter and spring. The Aleutian Low is a statistical feature of the climate field, generated by averaging North Pacific sea level pressure for long periods. Because this is a region of frequent storm progression, the averaged pressure appears as a low-pressure area, much like a weather map showing an individual storm. These winter and spring patterns are somewhat independent and have different oceanographic consequences. There is a shift from high to low values of the index in 1925, a shift to high values in 1946, and a shift back to low in 1977.
Lower pressure implies stronger winds and warmer temperatures over the Bering Sea.
In spring, the index is a displacement in pressure northward or southward. The shift to lower values in the 1970s occurred earlier than in the winter pattern. A shift to positive values is clear in the 1990s. This provides for higher pressure and increased number of days with clear skies and increased Bering Sea surface temperature due to solar heating. This may have been a factor in developing oceanic conditions on the Bering Sea shelf that contributed to the coccolithophorid bloom during summer 1997.