he Aleutian and Icelandic lows (AL and IL, respectively) are the major semi-permanent low-pressure systems at the surface that reside over the North Pacific and Atlantic, respectively, in the cold season (Fig. 1). The cold northwesterly winds that persistently blow from the Asian and North American continents along the upstream flanks of the AL and IL, respectively, give rise to severe winter weather over the east coasts of the respective continents.
In contrast, wet and mild weather condition prevails along the west coasts of those continents, including Europe, to which the southwesterly winds bring warm and moist air along the downstream flanks of the two lows. The AL and IL are thus deterministic on the winter climate over the mid-latitude coastal regions of the two ocean basins.
|Fig.1a Map showing the typical sea level pressure distribution in years when the intensity of theAleutian low (AL) isstrong. (Mean for Feb 10 to Mar 12.)|
|Fig. 1a: Indicates a winter exhibiting a strong AL and a weak IL associated with the AL-IL seesaw. The developed AL breaks a strong cold air mass off the continent, and resulting in a colder winter in Japan. In Europe, the weaker winds from the ocean produce a severe winter as well. In contrast, winter of the west and east coast of North America tends to be milder. |
|Fig.1b Map showing the typical sea level pressure distribution in years when the intensity of the Icelandic low (IL) is strong. (Mean for Feb 10 to Mar 12.)
|Fig1b:(b) As in (a), but for a weak AL and strong IL. Tendencies of severe and mild winters become opposite. Both are means for Feb. 10 to Mar. 12. |
We found in the time series of the AL and IL intensities over the three recent decades that their seesaw relationship is most apparent in February and March (Fig. 1).
Our analysis has revealed typical evolution of circulation anomalies in the course of the seesaw formation. Specifically, upper-tropospheric stationary Rossby waves that emanated from the circulation anomalies developing in January over the North Pacific, as an upper-level manifestation of the anomalous AL, propagate eastward across North America and then reach into the North Atlantic, acting to change the intensity of the surface IL in February (Fig. 2).
|Fig.2 Schematic showing the process forming the "atmospheric bridge."|
|Fig. 2. Schematic diagram showing the process forming the "atmospheric bridge" for a weak AL and strong IL case. When the circulation (high-pressure) anomalies corresponding to the weak AL over the North Pacific appears in January, the impact is evident in a large-scale Rossby wavetrain in the upper air reaching the North America. Accompanying these events is cyclonic anomalies that form over the western Canada and anticyclonic anomalies that form to the south of the United States.
In February, another wave appears from the latter, forming low-pressure anomalies in the North Atlantic (south of Greenland) and high-pressure anomalies over Europe. The anomalies over the North Atlantic develop and strengthening the IL intensity over the ocean. This process completes the "bridge" that spans from the North Pacific to the North Atlantic.
When the AL is strong and the IL is weak, the anomalies signals (whether pressure is high or low, or temperature is warm or cold compared with that of normal year, respectively) all reverse. The mechanism, however, whereby the impact of variation in the North Pacific extends over the North Atlantic due to the "bridge," is the same.
In other words, the Rossby waves act as a "bridge" that connects the Pacific and Atlantic variability. Furthermore, we demonstrated that the coherent interannual variability over the North Pacific and Atlantic, thus formed by this "atmospheric bridge", dominates over the variability associated with the so-called Arctic Oscillation or the Northern Hemisphere annular mode.
It is implied for the first time that midwinter anomalies in the coupled atmosphere-ocean system over the North Pacific could significantly influence the weather and climatic condition over the Euro-Atlantic sector in late winter and early spring.
Details have been presented in Honda and Nakamura (2001), within a recent issue of the Journal of Climate published by the American Meteorological Society.
Also, refer to a companion paper by Honda et al. (2001), published in the March 15 issue of the same Journal.
 Honda, M., H. Nakamura, J. Ukita, I. Kousaka, and K. Takeuchi, 2001: Interannual seesaw between the Aleutian and Icelandic lows. Part I: Seasonal dependence and life cycle. J. Climate, 14, 1029-1042.
 Honda, M. and H. Nakamura, 2001: Interannual seesaw between the Aleutian and Icelandic lows. Part II: Its significance in the interannual variability over the wintertime Northern Hemisphere. J. Climate, 14, 4512-4529.
The seesaw fluctuation of atmospheric pressure at polar and middle latitudes that is apparent mainly during winter. This phenomenon accompanies the strength of the low pressure area (polar vortex) that reaches from the troposphere to the stratosphere above the North Pole.