chapter - Atmospheric and Oceanic Science

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INDONESIA
Ecuador
Interannual low frequency variability: Background
El Niño conditions La Niña conditions
Neutral conditions
Convective circulation
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ics and extratropics. These changes are basically related to the weakness, intensification
and/or displacements of the large-scale atmospheric circulation in the meridional
and zonal planes, mainly those linked to the Hadley and Walker circulations
(Kidson 1975; Kousky et al. 1984, Souza and Ambrizzi 2002) as well as large-scale
Rossby waves that propagate into the extratropics producing, for instance, changes
in extratropical storm tracks (Trenberth et al. 1998; Ambrizzi and Magaña 1999).
Recent studies have indicated that the extropical response to SST anomalies such
as ENSO-related anomalies is less robust and predictable than the tropical response
due to the chaotic nature of extratropical circulations such as transient baroclinic
waves, fronts, and the SACZ (e.g., Shukla et al. 2000). The Walker circulation is a
result of the “see-saw” in surface pressures between the eastern and western hemispheres
linking these action centres through an atmospheric circulation in the zonal
plan, restricted in the tropical strip, with ascending branch over the western Pacific
and descending branch over the eastern Pacific (Bjerknes 1969). On the other hand,
the differential heating between tropic-extratropics, results in the formation of a
meridional circulation, the Hadley Circulation, with ascending branch over the
equatorial areas and sinking over the subtropical latitudes (around 30° of latitude)
in both Southern and Northern Hemispheres (Hastenrath 1985). Figure 3.2 shows a
schematic view of the Hadley and Walker circulation during ENSO years. For fur-
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Ecuador
120ºE 80º W
120ºE
120ºE
80º W
80º W
Fig. 3.1.
3-D schematic view of the ocean-atmosphere
features during (a) El Niño, (b) La Niña;
and (c) a normal year.
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