Nature 378, 706 - 708 (14 December 1995); doi:10.1038/378706a0

Influence of mountain ranges on the mid-latitude atmospheric response to El Niño events

Eric DeWeaver & Sumant Nigam

Department of Meteorology, University of Maryland College Park, Maryland 20742-2425, USA

TROPICAL heating associated with El Niño events influences weather patterns around the globe^1–4, in part by generating wave-like disturbances of vorticity (a measure of local fluid circulation about the vertical) in the upper troposphere which extend into the mid-latitude regions. But these waves do not account well for the observed mid-latitude consequences of E1 Niño^5–8 events. Here we show that a secondary interaction of these waves with mid-latitude mountains contributes significantly to the observed flow patterns. On encountering a mountain, a column of rotating air is compressed vertically, spreads horizontally, and its net vorticity is thereby reduced. For a realistic distribution of E1 Niño-related tropical heating, we find that vortex compression, primarily in the Himalayan–Tibetan region, generates a vorticity contribution at mid-latitudes with an amplitude up to one-half that of the directly propagating wave-train. Mountains therefore play a significant role in determining the structure of the extratropical response to E1 Niño.

References

1.	Bjerknes, J. Mon. Weath. Rev. 97, 163−172 (1969). | ISI |
2.	Ropelewski, C. F. & Halpert, M. S. Mon. Weath. Rev. 115, 1606−1629 (1987). | Article |
3.	Hoskins, B. J. & Karoly, D. J. J. atmos. Sci. 38, 1179−1196 (1981). | Article | ISI |
4.	Lau, N.-C. Mon. Weath. Rev. 113, 1970−1996 (1985). | Article | ISI |
5.	Geisler, J. E., Blackmon, M. L., Bates, G. T. & Munoz, S. J. atmos. Sci. 42, 865−883 (1985).
6.	Held, I. M. & Kang, I.-S. J. atmos. Sci. 44, 3576−3586 (1987). | Article |
7.	Kok, C. J. & Opsteegh, J. D. J. atmos. Sci. 42, 677−694 (1985). | Article |
8.	Held, I. M., Lyons, S. W. & Nigam, S. J. atmos. Sci. 46, 163−174 (1989). | Article |
9.	Holton, J. R. An Introduction to Dynamic Meteorology (Academic, New York, 1992).
10.	Sardeshmukh, P. D. & Hoskins, B. J. Q. J. R. met. Soc. 113, 339−360 (1987).
11.	Schneider, E. K. Pure appl. Geophys. 126, 137−140 (1988). | Article |
12.	Held, I. M. in Large-Scale Dynamical Processes in the Atmosphere (eds Hoskins, B. J. & Pearce, R. P.) 127−168 (Academic, New York, 1983).
13.	Hoskins, B. J. & Ambrizzi, T. J. atmos. Sci. 50, 1661−1671 (1993). | Article |
14.	Simmons, A. J., Wallace, J. M. & Branstator, G. J. atmos. Sci. 40, 1363−1392 (1983). | Article | ISI |