Geoscience Reference
In-Depth Information
maximum. Hess et al.(
1993
) and Numaguti (
1993
) found that the ITCZ location
following SST maximum depended heavily on the cumulus parameterization used.
Numerical modeling has been used also to examine a variety of relationships.
Included among the many studies are an examination of the relationship between
the ITCZ and atmospheric internal dynamics (Waliser and Somerville
1994
),
energy transport by transient waves (Kirtman and Schneider
2000
), and radiative
convective instability (Raymond
2000
). Needless to say, an appreciable amount
of research continues.
Of particular interest to researchers are the oscillations that occur in the
tropical realm. The Quasi-biennial Oscillation (QBO), the periodic reversal of
winds in the lower tropical stratosphere, is discussed later in this chapter in
the essay by Randall Cerveny (Section
3.3
). Less has been written about
the Madden-Julian Oscillation (MJO), a description of which is given in
3.2.2 The South Pacific Convergence Zone
Extending SE from the ITCZ in Southeast Asia (New Guinea) to around 308 S,
1208 W is a persistent convective cloud band called the South Pacific
Convergence Zone (SPCZ). Details of the SPCZ are described in an excellent
review by Vincent (1994). The SPCZ separates the mainly E-NE airflows on the
western side of the South Pacific Subtropical High Pressure (STHP), from the SE
airflows in the mid-latitudes. It is an area of low-level convergence, leading to
moist air confluence and convection. The SPCZ is most evident, and active, in
summer, as a result of the propagation of monsoon convective activity originat-
ing in India in the previous NH summer. Its persistence is also assisted by
location over the western Pacific pool of warm SST. While the northern and
western sections of the SPCZ are strongly tied to the ITCZ and tropical convec-
tion, evidence suggests the southern and eastern parts are instrumental in steer-
ing storm tracks in the central Pacific toward higher latitudes.
Fluctuations in the strength and location of the SPCZ are linked to several time
scales. Aside from seasonal variations, the 30-60 day variations of the MJO
(Section
2.5
) are important influences. On an interannual basis, the primary
influence is ENSO (Section
2.8
). During an El Ni ˜o event, the SPCZ shifts to the
northeast of its average location, and to the southwest during La Ni ˜a. The SPCZ is
also affected by wave activity in the circumpolar vortex (CPV) (see Chapter
4
).
These variations have strong influences on the climate variations, especially pre-
cipitation and temperature, in New Zealand and the rest of the southwest Pacific.
3.2.3 The subtropical high pressure cells and trade winds
The subtropical high pressure systems are located between latitudes 208 and 408
in both hemispheres. Where not modified by continental climates, they are
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