226
7 Ocean Currents
given in Fig. 7.9, in which the time series of monthly averages of SOl are given
for the period 1970-1990. The years with El Nino events (negative values of
SOl) are marked in the figure. The close relationship between SOl and these
events is quite clearly visible. In particular, the strong 1982-85 El Nino is very
well correlated with the high negative value of SOL
7.6.2 Physical Mechanisms Linking EI Nino and Southern Oscillation Phenomena
An excellent review of the El Nino events and physical processes responsible for
the El Nino onset may be found in Philander (1990). A key element of these
processes is an interaction between the atmosphere and ocean in the Pacific,
synthesized as the Walker Circulation (Fig. 7.10, see colour plate p.565).
According to Bjerkness (1969), a typical Walker Circulation is forced by the
easterly Trade Winds off South America which drive surface water away from
the coast. The compensating upwelling of cool subsurface water, which is
typically 5°C or more below the zonal average, appears along the coasts of
Ecuador, Peru, and northern Chile. This water is maintained along the Equator
by the Coriolis force which is to the left in the Southern Hemisphere (coastal
upwelling is described in some detail in Sect. 7.7). Dry air from the Eastern
Pacific flows along the Equator as a part of the Trade Winds. On this journey,
air is warmed and moistened as it moves over the progressively warmer waters.
As the westward Trade Winds drive the warm surface water westward and
expose cold water to the surface in the east, the thermocline becomes deep in
the western and shallow in the eastern regions of the Pacific (see Fig. 7.11a).
The pressure gradient associated with the zonal slope of the thermocline balances the wind stress. At the western end of the equatorial Pacific Ocean
lies the pool of the warmest ocean water on Earth, which provides for sustained upward motion of air and rain clouds. The return flow in the upper
troposphere closes the typical Walker Circulation (Fig. 7.lOa, see colour plate
p.565).
However, occasionally the Eastern Pacific warms up, the Walker
Circulation weakens and causes the convective zone of heavy rainfall to move
eastward, into the central and eastern tropical Pacific (Fig. 7.10b, see colour
plate p.565), and an El Nino cycle commences.
All El Nino events are different, but historical data provides a composite
picture of the canonical event. This composite is based on the fact that many
aspects of El Nino are closely linked to the annual cycle. The main phases of
the canonical El Nino can be summarized as follows (Cane, 1984):
• Prelude: The easterly winds, stronger than average, appear in the western
equatorial Pacific for at least 18 months before an El Nino event and move
water toward the west. Sea lever is unusually high in the west and low
in the east. This results in deepening of the thermocline in the west and
warming sea surface temperature (SST) above average in the far west.
• Onset: In the fall of the year proceeding an El Nino, a warm SST anomaly
extends across the South Pacific between 15°S and 30
0
S. In September
7 Ocean Currents
given in Fig. 7.9, in which the time series of monthly averages of SOl are given
for the period 1970-1990. The years with El Nino events (negative values of
SOl) are marked in the figure. The close relationship between SOl and these
events is quite clearly visible. In particular, the strong 1982-85 El Nino is very
well correlated with the high negative value of SOL
7.6.2 Physical Mechanisms Linking EI Nino and Southern Oscillation Phenomena
An excellent review of the El Nino events and physical processes responsible for
the El Nino onset may be found in Philander (1990). A key element of these
processes is an interaction between the atmosphere and ocean in the Pacific,
synthesized as the Walker Circulation (Fig. 7.10, see colour plate p.565).
According to Bjerkness (1969), a typical Walker Circulation is forced by the
easterly Trade Winds off South America which drive surface water away from
the coast. The compensating upwelling of cool subsurface water, which is
typically 5°C or more below the zonal average, appears along the coasts of
Ecuador, Peru, and northern Chile. This water is maintained along the Equator
by the Coriolis force which is to the left in the Southern Hemisphere (coastal
upwelling is described in some detail in Sect. 7.7). Dry air from the Eastern
Pacific flows along the Equator as a part of the Trade Winds. On this journey,
air is warmed and moistened as it moves over the progressively warmer waters.
As the westward Trade Winds drive the warm surface water westward and
expose cold water to the surface in the east, the thermocline becomes deep in
the western and shallow in the eastern regions of the Pacific (see Fig. 7.11a).
The pressure gradient associated with the zonal slope of the thermocline balances the wind stress. At the western end of the equatorial Pacific Ocean
lies the pool of the warmest ocean water on Earth, which provides for sustained upward motion of air and rain clouds. The return flow in the upper
troposphere closes the typical Walker Circulation (Fig. 7.lOa, see colour plate
p.565).
However, occasionally the Eastern Pacific warms up, the Walker
Circulation weakens and causes the convective zone of heavy rainfall to move
eastward, into the central and eastern tropical Pacific (Fig. 7.10b, see colour
plate p.565), and an El Nino cycle commences.
All El Nino events are different, but historical data provides a composite
picture of the canonical event. This composite is based on the fact that many
aspects of El Nino are closely linked to the annual cycle. The main phases of
the canonical El Nino can be summarized as follows (Cane, 1984):
• Prelude: The easterly winds, stronger than average, appear in the western
equatorial Pacific for at least 18 months before an El Nino event and move
water toward the west. Sea lever is unusually high in the west and low
in the east. This results in deepening of the thermocline in the west and
warming sea surface temperature (SST) above average in the far west.
• Onset: In the fall of the year proceeding an El Nino, a warm SST anomaly
extends across the South Pacific between 15°S and 30
0
S. In September