Geoscience Reference
In-Depth Information
recent years. Using a data set beginning in 1890, the researchers noted that
strong/frequent hourly precipitation occurred more in the 1940s, and although
the 1990s had a considerable number of extreme hourly events, they did not
match the earlier period. The authors did note that precipitation events caused by
tropical cyclones or Baiu fronts differed in the two periods.
From the foregoing, it is fairly clear that there are differences of opinion in
ongoing research. At the same time, the role of teleconnections in precipitation
variability has been considered in a number of ways. In the studies cited above,
teleconnections were used to help explain variances. But also to be considered is
the role of precipitation, in the form of snow, as a possible forcing mechanism in
atmospheric oscillations. Gong et al.( 2002 ) explored this possibility in relation
to snow cover and the AO and NAO. Using large-ensemble atmospheric GCMs,
they compared two different surface snow conditions. They concluded that
their numerical modeling results agreed with earlier studies that link the AO/
NAO mode with the Siberian high pressure cell and associated variations in
snow cover.
4.6 Mid-latitude circulation and teleconnections
in the Southern Hemisphere
The SH climate pattern in the mid-latitudes occurs above a significantly differ-
ent surface structure than that in the NH. Here, the mid-latitudes are located
north of the circumpolar trough (CPT) which surrounds Antarctica (Jones et al.
1999 ). The region is dominated by ocean, with the exception of the southern tip
of South America, the very southern edge of Southern Africa, Tasmania and the
south coast of Australia, and most of New Zealand. Land extends north from
Antarctica into the mid-latitudes at 60-708W longitude, the Palmer Peninsula.
This is the area encompassing the ''Roaring Forties,'' the famous SH westerly
belt, which extends around the globe virtually unchecked. It is the locational
basis of many seafaring stories and novels, such as Moby Dick by Herman
Melville.
The climate and circulation of the SH mid-latitude westerlies is affected by a
range of influences (Trenberth 1987 ; Tyson et al. 1997 ; Bigg et al. 2003 and
others). The QBO and ENSO, so important for the tropical climates (see
Chapter 2 ), have overlapping influences in the mid-latitudes. The circumpolar
vortex (CPV) and the associated Rossby waves determine the frequency and
strength of zonality (wave number 1) or meridionality (wave number 3). In
contrast to the NH, there is one center of circulation control, the East Antarctic
plateau. Biannual seasonal changes and quasi-decadal changes in circulation
patterns and strength determine both spatial and temporal climate variations.
Circulation strength and variability are also influenced by the temperature
gradients between Antarctica and the tropics, and between ocean, land, and
ice. SST variations are also important influences, although not to the extent
Search WWH ::




Custom Search