Geoscience Reference
In-Depth Information
The eastward movement of the western Pacific
zone of maximum convection in the ENSO phase
also decreases summer monsoon rainfall over
northern Australia, as well as extra-tropical rain-
fall over eastern Australia in the winter to spring
season. During the latter, a high pressure cell
over Australia brings widespread drought, but this
is compensated for by enhanced rainfall over
Western Australia associated with northerly winds
there.
Over the Indian Ocean, the dominant seasonal
weather control is exercised by the monsoon
seasonal reversals, but there is still a minor El
Niño-like mechanism over southeast Africa and
Madagascar, which results in a decrease in rainfall
during ENSO events.
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, centered at 20
to develop near the equator, as well as subtropical
jet streams to be intensified and displaced
equatorward, especially in the winter hemisphere.
During the intense ENSO event of the northern
winter of 1982-1983, 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 equatorial
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 behavior
of the Atlantic atmosphere-ocean system; for
example, the establishment of the convective
low-pressure center over the central and eastern
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 easterlies
°
N and 20
°
(A)
200mb
(B)
Sea level
H
L
H
50°N
50°N
L
L
30°
30°
H
H
10°
10°
0°
0°
H
10°
10°
L
H
30°
30°
50°S
50°S
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). Courtesy of the American Meteorological Society.
