[A-CG29] Extratropical oceans and atmosphere

[ACG29-P04] Year-to-year variations of the extratropical direct circulation associated with the Aleutian-Icelandic lows

*Masaya Kuramochi¹, Hiroaki Ueda² (1.Graduate School of Science and Technology, University of Tsukuba, 2.Faculty of Life and Environmental Sciences, University of Tsukuba)

Keywords:Extratropical direct circulation, Aleutian-Icelandic lows, ENSO

The mass-weighted isentropic zonal mean (MIM) describes the Lagrangian mean meridional circulation, which has an advantage in expressing the lower boundary condition in comparison with those in the Transformed Eulerian-Mean (TEM) method. The extratropical direct (ETD) circulation, in the diagnosis of MIM scheme, is mostly ascribed to the divergence of the Eliassen–Palm (E-P) flux based on the MIM. In this study, the spatiotemporal variations of atmospheric circulation corresponding to the interannual variations of the ETD circulation are examined in the context of wave–mean flow interactions associated with the E-P flux divergence. The results clearly show that the enhanced ETD circulation is closely connected with deepening of the Aleutian low (AL) and weakening of the Icelandic low (IL), and vice versa. Although the AL and IL exhibit the opposite fluctuations, the upward wave-activity flux intensifies both over the eastern North Pacific and the North Atlantic during the strong phase of ETD circulation. The intensified upward flux is concurrent with the enhancements of the meridional heat transport by planetary wave, which is mainly caused by the modulation of circulation in the Aleutian and Icelandic sectors. These results indicate that the variability of ETD circulation can be explained by the coherent fluctuation of the AL and IL accompanied by modulations of planetary wave activity. The physical process involved in the variations of E-P flux will be discussed in terms of the tropical-extratropical linkage associated with the tropical external forcing such as the El Niño-Southern Oscillation (ENSO).