Geography Reference
In-Depth Information
around the sides and this is how the Rossby waves would look if
you could see them around the Earth. Their location is affected by
large mountain ranges so that the waves, particularly in the north-
ern hemisphere (where there is more land) are locked into pre-
ferred locations. In the winter there tends to be two troughs in the
waves near the eastern edges of North America and Asia with
ridges over the Pacific and Atlantic. This corresponds to troughs
being associated with cold air over the winter continental land
masses and ridges over the warmer oceans.
Within the Rossby waves are fast moving bands of air (at least 30
metres per second) called
jet streams
which are caused by sharp
temperature gradients. Jet streams tend to be thousands of kilometres
long, hundreds of kilometres wide and several kilometres deep. Air-
craft can make use of the jet streams by travelling within them when
moving west but avoiding them when moving east. For instance, the
flight from the west coast of the USA to Europe can be an hour
shorter than the reverse trip. The shape of the Rossby waves in the
high level westerlies and the location of jet streams are used in
weather forecasting as they are associated with the formation of large
circulating air masses across the Earth's surface in the mid-latitudes.
These circulating surface air patterns bring warm and cold air masses
together and can lead to different calm/windy or dry/wet conditions.
LARGE SCALE OCEANIC CIRCULATION
The oceans cover 71 per cent of the Earth. The interactions
between the oceans and the atmosphere play a major role in con-
trolling the climate and weather at the Earth's surface. There are
four primary factors that control ocean circulation:
(ii) water
salinity
;
(ii) water temperature;
(iii) surface winds; and
(iv) the Coriolis effect.
(i) Contributing to water salinity, chemical weathering of rocks on
land produces dissolved materials that enter the oceans. Some of these
dissolved chemicals undergo reactions within the ocean to take them
out of the water and deposit them on the ocean floor. For example,
