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(a) Walker circulation
E
W
Sinking air
WET
NITCZ
Equator
NITCZ Northern ITCZ
SITCZ Southern ITCZ
SPCZ
DRY ZONE
DRY
WET
SITCZ
Strong trade winds
South Pacific
convergence zone
Surface winds
ITCZ
WET
Ocean water
level higher
0
50m
Upwelling
200m
WET
DRY
Rainfall
(b) El Niño
E
W
Sinking air
Equator
DRY
WET
WET
Atmospheric
pressure falls
DRY
Strong counter current
Ocean level
rises
Figure 6.21 Schematic picture of the South Pacific during the major phases of ENSO. Note the vertical scale is greatly
exaggerated. Australia is at the bottom left.
Source: Partly after Reid (2000)
+4
+3
Cold
event
+2
'88
+1
La Niña
Index
0
El Niño
-1
'76-'77
'57-'58
'65-'66
Warm
event
-2
'72-'73
'86-'87
'97-'98
'82-'83
-3
-4
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Figure 6.22 Time series of the Southern Oscillation Index, from 1950-2006. Negative values indicate El Niño conditions and
positive values indicate La Niña.
follow the pattern of waves over a period of weeks the
waves often undergo change, as shown in Figure 6.23 .
Initially there may be little meridional element in the
flow, with a strong westerly circulation - termed a high
index flow. Gradually the north-south element becomes
more dominant until eventually the flow breaks down into
a series of cut-off troughs and ridges like the meandering
pattern of a river. This is termed a low index flow, in which
north-south exchanges are strong. Low index flow favours
the formation of blocking highs and cut-off lows ( Chapter
7), which have a marked effect on the weather in some
parts of the westerlies of the northern hemisphere. In the
North Atlantic, westerly flow appears to show some
continuity on an annual to decadal scale. A period of
strong flow associated with deep low pressure over Iceland
and intense high pressure over the Azores can be followed
 
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