Interannual Seesaw between the Aleutian and Icelandic Lows. Part I: Seasonal Dependence and Life Cycle
The seasonal dependence and life cycle of the well-known interannual seesawlike oscillation between the intensities of the surface Aleutian and Icelandic lows (AL and IL, respectively) are investigated, based on the National Meteorological Center operational analyses for the period from 1973 to 1994. It is found that the correlation between the AL and IL intensities is significantly negative only from February to mid-March. It is also found that the seesaw exhibits an equivalent barotropic structure within the troposphere. For this late-winter period an index is defined that measures the intensity difference between the two lows. A linear lag regression analysis between this index and circulation anomalies averaged in each of the nine 45-day periods from early winter to midspring reveals that the stationary AL and IL anomalies constituting the seesaw do not start developing simultaneously over the respective ocean basins in the course of a particular winter season. Rather, the seesaw formation is initiated by the amplification of the AL anomalies with wave-activity accumulation in early through midwinter. In midwinter, part of the wave activity accumulated over the North Pacific propagates across North America in the form of a stationary Rossby wave train, which appears to trigger the formation of stationary anomalies over the North Atlantic. The IL anomalies thus initiated amplify and then become matured by late winter through the persistent feedback forcing from migratory eddies around the Atlantic storm track, while the AL anomalies remain strong until late winter through the continual feedback forcing from the Pacific storm track. It is suggested that interannual variability in the IL intensity for late winter tends to be strongly influenced by the AL anomalies that develop over the North Pacific in early through midwinter. The AL-IL seesaw is robust in a sense that it is apparent even after the influence of El Nino-Southern Oscillation is statistically removed from the data, suggestive of the importance of midlatitude processes in the seesaw formation.