Geoscience Reference
In-Depth Information
Atlantic subtropical high pressure cell and of the
general Trade Wind flow in the Atlantic. This
results in the development of a stronger subsi-
dence inversion layer, as well as subjecting the
western tropical Atlantic to greater ocean mixing,
giving lower sea surface temperatures, less
evaporation and less convection. This tends to:
cells the ocean surface is relatively cold (see
Figure
7.33
). This is the result of: the importation of
water from higher latitudes by the dominant
currents; the slow upwelling (sometimes at the
rate of about 1m in 24 hours) of water from
intermediate depths due to the Ekman effect (see
Chapter 6A.5); and the coastal divergence (see
Figure 7.31
). This concentration of cold water
gently cools the local air to dew-point. As a result,
dry, warm air degenerates into a relatively cool,
clammy, foggy atmosphere with a comparatively
low temperature and little range along the west
coast of North America off California (see
Plate 11.4
), off South America between latitudes
4 and 3
1
Increase drought over northeast Brazil. How-
ever, ENSO events account for only some 10
percent of precipitation variations in northeast
Brazil.
2
Increase wind shear over the North Atlantic/
Caribbean region such that moderate to
strong ENSO events are correlated with the
occurrence of some 44 percent fewer Atlantic
hurricanes than occur with non-ENSO events.
S).
Thus Callao, on the Peruvian coast, has a mean
annual temperature of 19.4
°
S, and off southwest Africa (8 and 32
°
C, whereas Bahia (at
the same latitude on the Brazilian coast) has a
corresponding figure of 25°C.
The cooling effect of offshore cold currents is
not limited to coastal stations, since it is carried
inland during the day at all times of the year by a
pronounced sea-breeze effect (see Chapter 6C.2).
Along the west coasts of South America and
southwest Africa the sheltering effect from the
dynamically stable easterly Trades aloft provided
by the nearby Andes and Namib Escarpment,
respectively, allows incursions of shallow tongues
of cold air to roll in from the southwest. These
tongues of air are capped by strong inversions
at between 600 and 1500m, reinforcing the
regionally low Trade Wind inversions (see
Figure
11.6
) and thereby precluding the development of
strong convective cells, except where there is
orographically forced ascent. Thus, although the
cool maritime air perpetually bathes the lower
western slopes of the Andes in mist and low stratus
cloud and Swakopmund (southwest Africa) has an
average of 150 foggy days a year, little rain falls on
the coastal lowlands. Lima (Peru) has a total mean
annual precipitation of only 46mm, although
it suffers frequent drizzle during the winter
months (June to September), and Swakopmund
in Namibia has a mean annual rainfall of 16mm.
Heavier rain occurs on the rare instances when
°
A further Pacific influence involves the manner
in which the ENSO strengthening of the southern
subtropical jet stream may partly explain the
heavy rainfall experienced over southern Brazil,
Paraguay and northern Argentina during an
intense El Niño. Another Atlantic teleconnection
may reside in the North Atlantic Oscillation
(NAO), a large-scale alternation of atmospheric
mass between the Azores high pressure and the
Icelandic low pressure cells (see Chapter 7C.2B).
The relative strength of these two pressure systems
appears to affect the rainfall of both northwest
Africa and the sub-Saharan zone.
H OTHER SOURCES OF
CLIMATIC VARIATIONS IN THE
TROPICS
The major systems of tropical weather and climate
have now been discussed, yet various other
elements help to create contrasts in tropical
weather in both space and time.
1 Cool ocean currents
Between the western coasts of the continents and
the eastern rims of the subtropical high pressure
