Mechanisms of Climate Variability - Natural Hazards - page 21

Cha nges in jet stream paths

A

Strong winds exist in the upper atmosphere adjacent
to the tropopause boundary (Figure 2.2). The more
significant of these is the polar jet stream on the equa-
torial side of the polar front. This jet stream consists of
a zone of strong winds no more than 1 km deep and
100 km wide, flowing downwind over a distance of
1000 km or more. Wind speeds can reach in excess
of 250 km hr -1 . The polar jet stream is most prominent
and continuous in the northern hemisphere. Here, it
forms over the Tibetan Plateau, where its position is
linked with the seasonal onset and demise of the
Indian monsoon . The jet loops northward over Japan
and is deflected north by the Rocky Mountains. It then
swings south across the Great Plains of the United
States, north-eastward (parallel to the Appalachian
Mountains), and exits North America off Newfound-
land, dissipating over Iceland. Both the Tibetan
Plateau and the Rocky Mountains produce a resonance
effect in wave patterns in the northern hemisphere,
locking high pressure cells over Siberia and North
America with an intervening low pressure cell over the
north Pacific Ocean. Barometric pressure, measured
along the 60°N parallel of latitude through these cells,
reveals a quasi-stable planetary or Rossby wave .
In the northern hemisphere, the jet stream tends to
form three to four Rossby waves extending through
5-10° of latitude around the globe (Figure 2.4). Rossby
waves move a few hundred kilometres an hour faster
than the Earth rotates and thus appear to propagate
from west to east. Any disturbance in a Rossby wave
thus promulgates downwind, such that a change in
weather in North America appears over Europe
several days afterwards. This aspect gives coherence
to extreme events in weather across the northern
hemisphere. Because there is a time lag in changes to
Rossby waves downwind from North America, fore-
casters can predict extreme events over Europe days in
advance. In some winters, the waves undergo amplifi-
cation and loop further north and south than normal.
Many researchers believe that changes in this looping
are responsible, not only for short-term drought
(or rainfall), but also for semi-permanent climatic
change in a region extending from China to Europe.
Sometimes the looping is so severe that winds in the jet
stream simply take the path of least resistance, and
proceed zonally (parallel to latitude), cutting off the
loop. In this case, high and low pressure cells can be

B

Path of jet stream in the northern hemisphere showing
likely Rossby wave patterns A) for summer, B) for extreme
winter (Bryson & Murray, ©1977 with the permission of
The University of Wisconsin Press).

Fig. 2.4

left stranded in location for days or weeks, unable to
be shifted east by the prevailing westerly airflow.
High-pressure cells are particularly vulnerable to this
process and form blocking highs . Blocking highs
deflect frontal lows to higher or lower latitudes than
normally expected, producing extremes in weather.
Blocking is more characteristic of winter, especially in
the northern hemisphere. It also tends to occur over
abnormally warm seas, on the western sides of oceans.
Expansion of the westerlies because of enhanced
looping of Rossby waves can be linked to recent failure