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

Abstract

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.