Discovery of an "Atmospheric Bridge" Extending from the North Pacific
to the North Atlantic: Seesaw between the Aleutian and Icelandic Lows

4 December, 2001
NASDA
JAMSTEC

 Dr. Hisashi NAKAMURA, Group Leader, and Dr. Meiji HONDA, researcher, et al. of the Climate Diagnosis Group, Climate Variations Research Program, the Frontier Research System for Global Change (a joint project of NASDA and JAMSTEC) have been analyzing the atmospheric circulation data of the past thirty years. Through this research they have clarified for the first time that the intensities of the Aleutian low (AL) over the North Pacific and the Icelandic low (IL) over the North Atlantic exhibit a distinct inverse (seesaw) relationship in late winter and that this seesaw is formed due to the influence exerted on the North Atlantic by the intensity of the AL in association with the propagation of large-scale atmospheric wave in the upper atmosphere (fig. 1 and explanation). Furthermore, the researchers have discovered that this seesaw that is linked by an "atmospheric bridge" crossing over the North America, will surpass the Arctic Oscillation, which received much attention last winter, to become the most prominent pattern of circulation variation in the atmosphere (fig. 2). The results of this research have become a focus of attention as they suggest for the first time that the changes in weather and climate over the North Pacific in winter not only affect Japan and other regions of the Far East, but also potentially have an impact as far as Europe due to an "atmospheric bridge."
 The discovery of the "atmospheric bridge" will be published in the December 15 issue of the Journal of Climate, a journal of the American Meteorological Society.

Background
 The AL and IL are large low-pressure systems that remain stationary over the above-mentioned oceans for the duration of winter (fig. 1). Climatologically, these lows are extremely important, as cold air breaks off the continent towards Far East and the east coast of North America to cause severe winters, while on the west coast of North America and in Europe the low-pressure cells carry warm air from the ocean resulting in mild winters. Approximately twenty years ago, a group of American researchers suggested the existence of a seesaw relationship between the two low-pressure intensities, but the discussion did not extend to the importance attached to the mechanisms forming this relationship and the circulation variation as a whole. Therefore, until now, the theory was not given much consideration and no relationship was believed to exist between the variations in atmospheric circulation over the North Pacific and North Atlantic.

Discovery and Considerations
 The investigation of the interannual intensities of both low-pressure centers revealed that the inverse relationship was most pronounced from February through March (fig. 1a, 1b, ref. 1). An investigation of how this seesaw was formed every winter showed that in midwinter (January) the circulation anomaly which develops over the North Pacific (corresponding to the AL anomaly) induces large-scale wave in the atmosphere that extends over the North America as far as the North Atlantic, thereby causing a change in the intensity of the IL for approximately one month after the event.
 Our research clearly confirms that the variation in the AL affects the North Atlantic Oscillation (NAO) through this "atmospheric bridge" and forms a large-scale, dominant variation pattern extending from Far East over North America and Europe (fig. 2, ref. 2, ref.3). The discovery of the "atmospheric bridge" suggests that an explanation of the processes of change in the Pacific atmosphere-ocean systems-research into which is currently being actively undertaken by the FRSGC and JAMSTEC-could serve to elucidate climate variations in the North Atlantic and Europe, and lends renewed relevance to the promotion of such research.

Contacts
Frontier Research System for Global Change, Joint Promotion Office Contact: Ms.Akiba Tel: +81 (0)45-778-5684 (direct)
Homepage: http://www.jamstec.go.jp/frsgc/jp/index.html
NASDA, Public Relations Office Tel: +81 (0)3-3438-6107, -6108, or -6109
Homepage: http://www.nasda.go.jp/
JAMSTEC, Public Relations Office, General Affairs Department Tel: +81 (0)468-67-9066
Homepage: http://www.jamstec.go.jp