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