Geoscience Reference
In-Depth Information
coupled atmosphere-ocean GCMs, one is
statistical and one uses analogue matching. Each
of the methods shows a comparable level of
moderate skill over three seasons ahead, with a
noticeable decrease in skill in the northern spring.
The ENSO phase strongly affects seasonal rainfall
in northeast Brazil, for example, and other tropical
continental areas, as well as modifying the winter
climate of parts of North America through the
interaction of tropical sea surface temperature
anomalies and convection on mid-latitude
planetary waves.
Summer monsoon rainfall in India is related to
the ENSO, but the linkages are mostly simulta-
neous, or the monsoon events even lead the ENSO
changes. El Niño (La Niña) years are associated
with droughts (floods) over India. Numerous
predictors of monsoon rainfall over all of India
have been proposed, including spring tempera-
tures and pressure indicative of the heat low,
cross-equatorial airflow in the Indian Ocean, 500
and 200mb circulation features, ENSO phase, and
Eurasian winter snow cover. A key predictor
of Indian rainfall is the latitude of the 500mb
ridge along 75
E in April, but the most useful
operational approach seems to be a statistical
combination of such parameters, with a forecast
issued in May for the June to September period.
The important question of the spatial pattern of
monsoon onset, duration and retreat and this
variability has not yet been addressed.
Rainfall over sub-Saharan West Africa is
predicted by the UK Meteorological Office using
statistical methods. For the Sahel, drier conditions
are associated with a decreased inter-hemispheric
gradient of sea surface temperatures in the
tropical Atlantic and with an anomalously warm
equatorial Pacific. Rainfall over the Guinea coast
increases when the South Atlantic is warmer than
normal.
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The tropical atmosphere differs significantly from that in mid-latitudes. Temperature gradients are
generally weak and weather systems are mainly produced by airstream convergence triggering
convection in the moist surface layer. Strong longitudinal differences in climate exist as a result of
the zones of subsidence (ascent) on the eastern (western) margins of the subtropical high pressure
cells. In the eastern oceans, there is typically a strong Trade Wind inversion at about 1km with dry
subsiding air above, giving fine weather. Downstream, this stable lid is raised gradually by the
penetration of convective clouds as the Trades flow westward. Cloud masses are frequently
organized into amorphous 'clusters' on a subsynoptic scale; some of these have linear squall-lines,
which are an important source of precipitation in West Africa. The Trade Wind systems of the two
hemispheres converge, but not in a spatially or temporally continuous manner. This Intertropical
Convergence Zone also shifts poleward over the land sectors in summer, associated with the
monsoon regimes of South Asia, West Africa and northern Australia. There is a further South Pacific
Convergence Zone in the southern summer.
Wave disturbances in the tropical easterlies vary regionally in character. The 'classical' easterly
wave has maximum cloud buildup and precipitation behind (east of) the trough line. This
distribution follows from the conservation of potential vorticity by the air. About 10 percent of wave
disturbances later intensify to become tropical storms or cyclones. This development requires a
warm sea surface and low-level convergence to maintain the sensible and latent heat supply and
upper-level divergence to maintain ascent. Cumulonimbus 'hot towers' nevertheless account for a
small fraction of the spiral cloud bands. Tropical cyclones are most numerous in the western oceans
of the Northern Hemisphere in the summer to autumn seasons.
