Geoscience Reference
In-Depth Information
drought in southern Peru, Bolivia and north-eastern
Brazil. For example, in 1982-1983, in the coastal
desert region of Ecuador and Peru, flash floods ripped
out roads, bridges and oil pipelines. Towns were buried
in mud, and shallow lakes appeared in the deserts.
Irrigation-based agriculture was devastated, first by the
flooding, and then by swarms of insects that prolifer-
ated under the wet conditions. The warm water also
forces fish feeding on plankton to migrate to greater
depths, and causes vast fish kills. Guano-producing
birds feeding on anchovies die in their thousands,
causing the collapse of the anchovy and fertilizer
industries in Peru.
These climatic changes worldwide can be responsible
for the increase of such isolated phenomena as
snakebites in Montana, funnel-web spider bites in
Sydney, and schizophrenia in Brazil (the latter caused by
a lack of UV light due to increased cloudiness). More
significantly, these changes can generate extreme
economic and social repercussions, including damage
to national economies, the fall of governments, and
the deaths of thousands through starvation, storms and
floods.
and a longer period cycle centered on a periodicity of
five years. Fourth, changes in the behavior of southern
hemisphere, mobile polar highs are linked to ENSO
events. Strong Walker circulation is related to strong
westerlies between 35° and 55°S. These westerlies
appear to lock, over the Indonesian-Australian
'maritime' landmass, with heat lows developing in the
austral spring. If the normally strong latitudinal flow in
the mobile polar highs is replaced by
longitudinally
skewed circulation in the Australian region, then low
pressure can be forced into the Pacific during the tran-
sition from northern to southern hemisphere summer.
Increased southerly and south-westerly winds east of
Australia preceded the 1982-1983 ENSO event, and
flowed towards a South Pacific Convergence Zone that
was shifted further eastwards. While highs were as
intense as ever, they had a stronger meridional compo-
nent, were displaced further south than normal, and
were stalled over eastern Australia. Fifth, ENSO events
are more likely to occur one year after the south polar
vortex is skewed towards the Australian-New Zealand
sector. A significant
correlation
has been found amongst
this eccentricity, the Southern Oscillation, and the
contemporaneous occurrence of rainfall over parts of
southern hemisphere continents. Sixth, excess salinity
north and south of the equator is correlated with
warmer waters, presaging an ENSO event about twelve
months later. The sinking of this cold salty water around
the equator draws in warmer, less saline, surface water
triggering the ENSO event. Finally, the causes may be
interlinked. For example, more snow in Asia the winter
before Walker circulation collapses may weaken the
summer jet stream, leading to failure of the Indian
monsoon. Alternatively, more sea-ice in the Antarctic
may distort the shape and paths of mobile polar highs
over the Australian continent, or displace the south
polar vortex.
Whatever the cause, the movement eastward of low
pressure, beyond the Australian continent, leads to
westerly airflow at the western edge of the Pacific
Ocean. Because there is no easterly wind holding supra-
elevated water against the western boundary of the
Pacific, warm water begins to move eastward along
the equator (Figure 2.7b). Normally, the
thermocline
separating warm surface water from cooler water below
is thicker (200 m) in the west Pacific than in the east
(100 m). As warm water shifts eastward, the thermocline
rises in the west Pacific. One of the first indications of an
ENSO event is the appearance of colder water north of
El Niño-Southern Oscillation (ENSO) events
(Philander, 1990; Glantz et al., 1991; Allan et al., 1996;
Glantz, 1996; Bryant, 1997; Couper-Johnston, 2000)
Zonal, ocean-atmosphere feedback systems in the
southern hemisphere are powered by three tropical
heat-sources located over Africa, South America and
the Indonesian-Australian maritime continent. The
latter site, by far the strongest of the three systems,
initiates Walker circulation. While it is relatively stable,
Walker circulation is the least sedentary, because it is
not anchored directly to a landmass. It is highly migra-
tory during transition periods of the Indian monsoon,
in either March-April, or August-September. Six
pre-
cursors
to the failure of Walker circulation stand out.
First, sea-ice in the Antarctic and snow cover in central
Asia tend to be more extensive beforehand. Second,
the strength of the Indian monsoon relies heavily upon
formation of an upper air, thermally driven anticyclone
(between 100 and 300 hPa) over the Tibetan Plateau
that leads to the formation of an easterly upper jet stream
over southern India. Zonal easterly winds at 250 hPa over
the Indian monsoon area also have been found to
weaken up to two months before the onset of an ENSO
event. Third, the Southern Oscillation appears to be a
combination of the 2.2 year Quasi-Biennial Oscillation
