It is well known that the surface Aleutian and Icelandic lows (AL and IL, respectively) exhibit a seesaw-like oscillation in winter (van Loon and Rogers 1978; Wallace and Gutzler 1981). We examined seasonal dependence of this interannual seesaw by recording the minimum within each of the North Atlantic and North Pacific in the 31-day moving-averaged sea-level pressure (SLP) for each day over 1973-1994 based on the NMC analyses. The correlation coefficient between the 22-year timeseries of thus-defined AL and IL intensities was evaluated for each calendar day. The correlation was found to be strongly negative only in the late winter (February through mid-March). In the rest of the year it is weakly negative or even slightly positive. A similar seesaw is also evident in the SLP field from which ENSO-related anomalies as inferred from a linear regression analysis with SOI were removed. We defined AI (AL-IL index) as the IL-center pressure subtracted from the AL-center pressure in the 45-day mean SLP for January 31-March 16. In the recognition of the equivalent barotropic structure of the seesaw, we investigated seasonal evolution of the seesaw in the linear lag regression maps between AI and the mean of the 250-hPa height for each of the fourteen 45-day periods, whose central calendar days are mutually 15 days apart. We focus on a winter of the weaker AL with the stronger IL. After positive height anomalies gradually develop over the eastern North Pacific in December, a PNA-like wavetrain emanates from the positive anomalies in early January (Dec. 17-Jan. 30) with negative and positive anomalies appear over western Canada and the southeastern US, respectively. Then, another wavetrain emanates from the latter anomalies with negative and positive centers over western Atlantic to the east of Newfoundland and over northern Europe, respectively. A diagnosis based on a wave-activity flux suggests that these anomalies are associated with stationary Rossby wavetrains. As these wavetrains gradually weaken by early February (Jan .16-Mar. 1), positive anomalies over the North Pacific fully mature and cover the entire North Pacific, which correspond to the weak AL. At the same time the negative anomalies over the North Atlantic further develop while shifting northward until they mature in the "peak period" (Jan. 31- Mar. 16), when the AL-IL seesaw is the most apparent. The positive anomalies over the North Pacific start decaying right after this period, whereas the negative anomalies over the North Atlantic remain strong until early April (Mar. 17- Apr. 30). The sign should be reversed for each of the anomalies in the situation of the stronger AL with the weaker IL. It became apparent through our analysis that the positive and negative anomalies that constitute the AL-IL seesaw in the late winter do not develop simultaneously over the North Atlantic and Pacific. Rather, the seesaw appears to form in association with the propagation of wave activity accumulated over the North Pacific in early to mid-winter into the North Atlantic in the form of a Rossby wavetrain across North America.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics