The seasonal dependence and life cycle of the interannual seesaw between the surface Aleutian and Icelandic lows (AL and IL, respectively) are investigated. 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. The AL and IL anomalies constituting the seesaw do not develop simultaneously over the North Pacific and the North Atlantic, respectively. Rather, the seesaw appears to form in association with the propagation of wave activity accumulated in early winter over the North Pacific into the North Atlantic in midwinter in the form of a Rossby wavetrain across North America. The anomalies over the North Atlantic become matured and are maintained through the feedback forcing from high-frequency transients along the Atlantic stormtrack. The AL-IL seesaw is robust in a sense that it appears even after removing either the Arctic Oscillation or El /Southern Oscillation signal.
  The surface Aleutian and Icelandic lows (hereafter referred to as AL and IL) are wintertime semi-permanent low-pressure cells over the northern parts of the North Pacific (NP) and the North Atlantic (NA), respectively (Fig.1). Several previous studies pointed out that a seesaw-like oscillation exists between the AL and IL from one winter to another (e.g., van Loon and Rogers 1978; Wallace and Gutzler 1981). In the present study (Honda et al. 1999), we shall investigate seasonal dependence and life cycle of the seesaw (AL-IL seesaw).

1.Meiji Honda
IGCR, Frontier Research System for Global Change, Tokyo, Japan;
e-mail:
meiji@jamstec.go.jp
2.Hisashi Nakamura
IGCR, Frontier Research System for Global Change, Tokyo/ Dept. Earth and Planetary Physics, Univ. Tokyo, Japan;
e-mail:
hisashi@geoph.s.u-tokyo.ac.jp
3.Jinro Ukita
IARC, Frontier Research System for Global Change, Tokyo, Japan;
e-mail:
ukita.jinro@nasda.go.jp


Fig.1
Climatological-mean sea-level pressure (hPa) for January over the Northern Hemisphere. AL and IL denote the Aleutian and Icelandic lows, respectively. Two regions are shaded where the lowest pressures were identi゙ed, separately, for de゙ning their intensities.




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