Geoscience Reference
In-Depth Information
40N
27.5
C(Aug)
27.5
C(Aug)
27.5
C(Aug)
20N
27.5
C(Feb)
EQ
27.5
C(Feb)
20S
27.5
C(Feb)
40S
60E
120E
180
120W
60W
0
Figure 9.11
Primary regions where tropical storms are initiated and their subsequent primary paths together with the
27.5°C sea-surface temperature isotherm for August. (From Rassmusson
et al
.,1993, published with permission.)
cal storms
when they have sustained winds of
18 m s
−1
and
hurricanes
or
typhoons
(the name changes regionally) when sustained winds reach 33 m s
−1
or greater.
For tropical cyclones to generate, three conditions must occur simultaneously.
Together these restrict the oceanic regions over which tropical cyclones can
originate. The first requirement is that the ocean must be sufficiently warm: SSTs
of at least 26-27°C are required. Second, because significant rotations can only be
generated where there is a significant Coriolis force, tropical storms cannot form
within 5-8° of the equator even if the SST is high enough. Finally, a small change
of wind with height is required if the storm is to persist. Together these three
criteria restrict the potential source areas for tropical cyclones to those shown in
Fig. 9.11. Once established, tropical cyclones generally move westward and toward
the poles. When they reach land they can have a major and usually detrimental
impact before eventually decaying because they no longer have access to the latent
heat energy they extract from warm oceanic waters.
∼
El Niño Southern Oscillation
The annual average sea-surface temperature distribution shown in Fig. 9.8 reveals
that equatorial SSTs are significantly modified by wind-driven oceanic currents. In
particular, the Peru current brings cold polar waters from the Antarctic to the
equator and SST in the eastern Pacific is lower than it would otherwise be. The
trade winds blowing across the Pacific support an easterly equatorial surface cur-
rent and the waters warmed by solar radiation during this transit gather to form
the western Pacific warm pool (Fig. 9.12a). Because the sea surface temperature in
the warm pool is higher than elsewhere in the equatorial Pacific, there is more
convection in the atmosphere above, and higher precipitation and latent heat
release. On average, the prevalent atmospheric ascent in this region draws in air,
including air from across the Pacific. The trade winds are thus enhanced, and an
unstable equilibrium is established in equatorial Pacific air-sea interactions char-
acterized by warmer water and more convection in the west Pacific and cooler
water and less convection in the east Pacific.
