Geoscience Reference
In-Depth Information
westward from Peru, whereas after a major shift in
Pacific basin climate took place between 1976 and 1977,
the warming spread eastward from the western equa-
torial Pacific. The atmosphere-ocean coupling during
ENSO events clearly varies on multidecadal time scales.
ENSO events result from a radical reorganization of
the Walker circulation in two main respects:
ocean circulation during El Niño are facilitated by
the fact that the time taken for ocean-surface currents
to adjust to major wind changes decreases markedly
with decreasing latitude. This is demonstrated by
the seasonal reversal of the southwest and northeast
monsoon drift off the Somali coast in the Indian Ocean.
Large-scale atmospheric circulation is subject to a
negative-feedback constraint involving a negative
correlation between the strengths of the Walker and
Hadley circulations. Thus the weakening of the Walker
circulation during an ENSO event leads to a relative
strengthening of the associated Hadley circulation.
1
Pressure declines and the trades weaken over the
eastern tropical Pacific (Figure 11.50B), wind-driven
upwelling slackens, allowing the ITCZ to extend
southward to Peru. This increase of sea-surface
temperatures by 1 to 4°C reduces the west-east sea-
surface temperature gradient across the Pacific and
also tends to decrease pressure over the eastern
Pacific. The latter causes a further decrease of trade
wind activity, a decrease in upwelling of cold water,
an advection of warm water and a further increase in
sea-surface temperatures - in other words, the onset
of El Niño activates a positive feedback loop in the
eastern Pacific atmosphere-ocean system.
2 Teleconnections
Teleconnections are defined as linkages over great dis-
tances of atmospheric and oceanic variables; clearly the
linkages between climatic conditions in the eastern and
western tropical Pacific Ocean represent a 'canonical'
teleconnection. Figure 11.52 illustrates the coincidence
of ENSO events with regional climates that are wetter
or drier than normal.
In Chapter 7C.1, we have referred to Walker's
observed teleconnection between ENSO events and the
lower than normal monsoon rainfall over South and
Southeast Asia (Figure 11.53). This is due to the east-
ward movement of the zone of maximum convection
over the western Pacific. However, it is important to
recognize that ENSO mechanisms form only part of the
South Asian monsoon phenomenon. For example, parts
of India may experience droughts in the absence of El
Niño and the onset of the monsoon can also depend on
the control exercised by the amount of Eurasian snow
cover on the persistence of the continental high-pressure
cell.
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 rainfall over eastern
Australia in the winter to spring season. During the
latter, a high-pressure cell over Australia brings wide-
spread 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 mech-
anism over southeast Africa and Madagascar, which
results in a decrease of rainfall during ENSO events.
2
Over the western tropical Pacific, the area of max-
imum sea temperatures and convection responds to
the above weakening of the Walker circulation by
moving eastward into the central Pacific (Figure
11.50B). This is due partly to an increase of pressure
in the west but also to a combined movement of the
ITCZ southward and the SPCZ northeastward.
Under these conditions, bursts of equatorial westerly
winds spread a huge tongue of warm water (i.e.
warmer than 27.5°C) eastward over the central
Pacific as large-scale, internal oceanic (Kelvin)
waves. It has been suggested that this eastward flow
may sometimes be triggered off or strengthened by
the occurrence of cyclone pairs north and south of the
equator. This eastward flow of warm water depresses
the thermocline off South America (Figure 11.50E),
preventing cold water from reaching the surface and
terminating the El Niño effect.
Thus, whether La Niña or El Niño develops, bringing
westward-flowing cold surface water or eastward-
flowing warm surface water, respectively, to the
central Pacific, depends on the competing processes of
upwelling versus advection. The most intense phase
of an El Niño event commonly lasts for about one year,
and the change to El Niño usually occurs in about March
to April, when the trade winds and the cold tongue are
at their weakest. The changes to the Pacific atmosphere-
