Geoscience Reference
In-Depth Information
Figure 11.54
Schematic Pacific-North America (PNA) circulation pattern in the upper troposphere during an ENSO event in
December to February. The shading indicates a region of enhanced rainfall associated with anomalous westerly surface wind
convergence in the equatorial western Pacific.
Source
: After Shukla and Wallace (1983), by permission of the American Meteorological Society.
It is apparent that ENSO teleconnections affect extra-
tropical regions as well as tropical ones. During the
most intense phase of El Niño, two high-pressure cells,
centred at 20°N and 20°S, develop over the Pacific in
the upper troposphere, where anomalous heating of the
atmosphere is at a maximum. These cells strengthen
the Hadley circulation, cause upper-level tropical easter-
lies to develop near the equator, as well as subtropical
jet streams to be intensified and displaced equatorwards,
especially in the winter hemisphere. During the intense
ENSO event of the northern winter of 1982 to 1983,
such changes caused floods and high winds in parts of
California and the US Gulf states, together with heavy
snowfalls in the mountains of the western USA. In the
northern hemisphere winter, ENSO events with equa-
torial heating anomalies are associated with a strong
trough and ridge teleconnection pattern, known as the
Pacific-North American (PNA) pattern (Figure 11.54),
which may bring cloud and rain to the southwest United
States and northwest Mexico.
The Atlantic Ocean shows some tendency towards
a modest effect resembling El Niño, but the western
pool of warm water is much smaller, and the east-west
tropical differences much less, than in the Pacific.
Nevertheless, ENSO events in the Pacific have some
bearing on the behaviour of the Atlantic atmosphere-
ocean system (e.g. the establishment of the convective
low-pressure centre over the central and eastern Atlantic
subtropical high-pressure cell and of the general trade
wind flow in the Atlantic). This results in the develop-
ment of a stronger subsidence 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:
1
Increase drought over northeast Brazil. However,
ENSO events account for only some 10 per cent 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
per cent fewer Atlantic hurricanes than occur with
non-ENSO events.
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 atmos-
pheric mass between the Azores high-pressure and the
