The Atlantic storm track and Icelandic low-pressure center are dominant features of northern wintertime climate. Sea-level pressure shows closed contours extending from the subtropics near Cape Hatteras far to the north, to high in the Arctic. This is after time-averaging. Many storms pass along this track northeastward, born 'on the slopes of the Rockies' (from Dr. Fred Sanders) or farther west in the Pacific. Yet the Icelandic Low is not just the average of all these small storm centers, but has a gyre-like nature: storms can circulate around it, and even come back south through the Canadian Archipelago after a visit to the Arctic.

The energy source for this remarkable circulation is the cold dome of air that forms in winter, centered on the North Pole, and also the warm sea surface over which the storms pass. 'Explosive cyclogenesis' is a term indicating the strong influence of oceanic heating. Yet it is complex, because the cold sector of a storm (usually on its northwestern side, where the cold Arctic air is being drawn in) will be 'damped' by ocean heating whereas the weaker warm sector (on the southeastern side) will be energized by heating from below.

The figure shows sea-level pressure averaged over Jan-Feb-March of 1993, a strongly cyclonic winter in the Arctic. Blue=low pressure, red=high pressure. Both the storms (transient energy) and mean winds heat and wet the northern regions, having picked up water from the subtropical ocean. The precipitation produces snow and ice on land and on sea-ice, and a buoyant low-salinity layer that floats on top of the ocean. The storms warm and dampen Europe, then spill over on the northern fringe of Asia, depositing heat and moisture there.

In an animation of this winter, the 'billiard-ball' progress of storms, one seeding another downwind (as described by Chang and Orlanski), is very clear. North Pacific storms follow a bifurcating path as they approach N America, most sliding over the Rocky Mountain ridge/trough, while some go north through Bering Strait into the Arctic. The Beaufort high is absent this winter, and Atlantic cyclones cluster in the Arctic and sometimes reach through Bering Strait and mate with Pacific cyclones. Heat uptake from the Atlantic actively energizes these storms, which continue experience heat gain until hitting the ice north of the Barents Sea.


500mb height field superimposed on SLP; note the westward shift of the mean low pressure, and the backing of winds with height over the Labrador Sea (suggesting cold air advection/subsidence, and heat uptake from the ocean through the veering/heating version of the thermal wind equation). At 500mb and above (perhaps centered at the 250mb level) the jet stream has core windspeeds so great that it is a dominant component of the kinetic energy of the atmosphere. Meanders of the jetstream play a role in the generation of storms, and one often sees them 'shooting off' from the more stationary troughs of the jet.

To experience these events, visit a good weather site like www.atmos.washington.edu and look at animated loops; some of my favorites are the combined 500mb and SLP animations centered over North America, and similar animations centered over the North Pole. You can learn to retrieve older data (for example from an intense part of winter) as well as looking at current weather. Slow the loops down and step through them, and try to imagine where storms originate.

In the fall of 2003 at least 4 tropical cyclones (3 Atlantic hurricanes and 1 Pacific typhoon) came north, and after weakening came underneath the jetstream. [Late in the season, there were 4 major typhoons in November.] There they were 'reborn' as cyclonic storms. In each case one could see the shaking of the jet propagate downwind, eastward, setting off new storms. Typhoon Lupit (force 5 winds reaching about 145 knots) was particularly good at this, since downwind lay the fertile storm territory of the Atlantic. It was a remarkable 'experiment'.

Peter Rhines, December 2003