A trough in the surface pressure field is a frequent synoptic pattern between Iceland and Greenland. A case of the formation of such a trough is studied by numerical modelling. The trough, with a small quasi-barotropic vortex inside, forms in the wake of an extra-tropical cyclone moving to the northeast between Iceland and Greenland. Both the trough and the vortex appear to be a consequence of the Greenland topography, acting to block the advection of cold air at low levels to the rear of the cyclone. In simulation where Greenland is present there is permanent descent of warm airmass in the lower troposphere, over the trough, off the east coast of Greenland.

Large-amplitude waves were observed over the east coast of Greenland during this case, but sensitivity tests indicate that they have little effect on the deformation of the cyclone.