Some Geophysical Background Notes Related to Climate and Weather - The Changing Arctic Environment: The Arctic Messenger - page 375

The jet stream:
The jet streams are narrow, fast-flowing rivers of air in the upper troposphere. The four
strongest jet streams flowing from west to east are the two subpolar jets and the two less
powerful subtropical jets of the Northern and Southern hemispheres, respectively. Both
types are located close to the tropopause, where temperature gradients are very intense. Be-
ing a high-flow core in a “fluid atmosphere”, the jet stream is dynamically unstable and
small perturbations can make it wiggle strongly north and south. These wiggles, or loops,
are Rossby waves that normally move from west to east at a slower speed than that of the
air within the jet stream itself. Because the Coriolis effect varies with latitude, the waves
start tilting south-west to north-east in the Northern Hemisphere and break in mid-latitude
storms at their polar tips. Normal waves (as in the open ocean) do not transfer momentum
(do not move anything), but breaking waves do transfer momentum (and can move things,
as on a beach). Therefore, as the Rossby waves break, they transfer momentum (and, hence,
air) from the tropics to their breaking tips in the mid-latitudes.
The jets are usually continuous over long distances and follow the seasons, with, for
example, the northern jet moving south during the northern winter. The subpolar (or polar)
jet only really exists during the winter. At that point, there is a strong temperature gradient
between areas in sunlight (getting energy from storms spinning off the poleward edge of
the subtropical jet) and areas without any heat source (such as those areas experiencing the
24-hour darkness of Arctic winter). This leads to a strong pressure gradient and, through the
Coriolis effect, a strong perpendicular wind. Although present in the summer, the subtropic-
al jet is again mostly a winter phenomenon. This is because, in summer, Earth's inclination
is such that at the time of the northern summer solstice, the top of the atmosphere at the pole
is receiving slightly more sunlight than is the equator. At Earth's polar surface, less of this
solar radiation can be absorbed because of the higher albedo of ice and snow. Of course, the
solstice conditions do not last long, so there is not really time for a “more tropical” equi-
librium to develop. But the point is that during the summer, there is very little north-south