Geoscience Reference
In-Depth Information
vertical motion. On one hand, air rises over heated
continents and the warmer parts of the oceans; on
the other, air subsides over cooler parts of the
oceans, over continental areas where deep high
pressure systems have become established, and in
association with subtropical high pressure cells.
Sir Gilbert Walker first identified these circula-
tions in 1922-1923 through his discovery of an
inverse correlation between pressure over the
eastern Pacific Ocean and Indonesia. The strength
and phase of this so-called
Southern Oscillation
is
commonly measured by the pressure difference
between Tahiti (18
•
positive
when there is a strong high pressure in
the southeast Pacific and a low centered over
Indonesia with ascending air and convective
precipitation;
•
negative
(or low) when the area of low pressure
and convection is displaced eastward towards
the Date Line.
Positive (negative) SOI implies strong easterly
trade winds (low-level equatorial westerlies) over
the central-western Pacific. These Walker circula-
tions are subject to fluctuations in which an
oscillation (known as the El Niño-Southern
Oscillation: or ENSO) between high phases
(i.e., non-ENSO events) and low phases (i.e.,
W) and Darwin,
Australia (12°S, 130°E). The Southern Oscillation
Index (SOI) has two extreme phases (
Figure 7.22
):
°
S, 150
°
(A)
High phase SOI
H
H
L
H
COLD
30°S
Atlantic
Ocean
Indian
Ocean
Pacific
Ocean
90°W
0°
90°E
180°
90°W
(B)
Low phase SOI
H
L
WARM
30°S
Atlantic
Ocean
Indian
Ocean
Pacific
Ocean
90°W
0°
90°E
180°
90°W
Figure 7.22
Schematic cross-sections of the Walker circulation along the equator (based on computations of
Y. M. Tourre) during the high (A) and low (B) phases of the Southern Oscillation (SO). The high (low) phases
correspond to non-ENSO (ENSO) patterns (see text). In the high phase there is rising air and heavy rains over the
Amazon basin, central Africa and Indonesia-western Pacific. In the low phase (ENSO 1982-1983) pattern the
ascending Pacific branch is shifted east of the Date Line and elesewhere convection is suppressed due to
subsidence. The shading indicates the topography in exaggerated vertical scale.
Source:Based on K.Wyrtki (1985). By permission of the World Meteorological Organization.
