Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 121

1972 and 1992. In summer (January), the frequency of cyclonicity over the ocean
averages about 12. The formation area in the New Zealand region tends to
disappear, and the activity is significantly reduced over most of the Indian Ocean.
The jet stream is an important steering mechanism to determine both direction
of movement and speed. Cyclones tend to migrate eastward and poleward, but
are influenced by standing ridges associated with the CPV (especially wave
number 3 in winter). Overall speed of movement is about 15m s 1 in winter,
especially in the Indian Ocean. Summer translation speeds are slower, and the
cyclones tend to be weaker overall.
4.6.6 Case study: circulation and teleconnections
in the SW Pacific
The discussion above indicates that the SW Pacific (Australia-New Zealand
area) is an important region for mid-latitude circulation dynamics in the SH. In
this location, correlations between the TPI and local circulation, temperature and
moisture are weak (some tendency toward negative) (Jones et al. 1999 ). There
are significant interactions between upper tropospheric ridging (90-1508 E), and
air stream advection toward Antarctica (Goodwin et al. 2003 ). Temperatures and
moisture regimes in New Zealand are strongly influenced by regional meridional
circulations. For example, increased meridional airflow brings higher precipita-
tion to the north and east of the North Island and to the north of the South Island,
but increased zonal airflow increases precipitation in the west and south of the
South Island, with cooler summers and warmer winters. Influences such as the
f¨hn effect on the South Island can enhance the connection between zonal
circulation and temperature.
Tyson et al.( 1997 ) contrasted the advances and retreats of the Franz Josef
Glacier on the west coast of the South Island of New Zealand, with changes in
area-averaged summer rainfall in Southern Africa. The overall results showed
an out-of-phase relationship (glacial advances related to lower rainfall).
Goodwin et al.( 2003 , 2004 ) used NCEP/NCAR reanalysis (see Section 4.3 )to
assess the relationship between mid-latitude atmospheric circulation and snow
accumulation (determined through Na þ measurements) in Wilkes Land, east
Antarctica, in winter. Stronger precipitation was associated with stronger mer-
idional circulation. These studies illustrate the complexity of mid-latitude cir-
culation relationships in the SW Pacific, and are described in more detail below.
Both studies emphasize the importance of synchronous adjustments and oscilla-
tions associated with wave number 3 in the CPV. A summary of the contrasts
under zonal and meridional conditions is presented in Table 4.2.
Under stronger zonal westerly conditions, and fewer waves in the CPV, there
are glacial advances in New Zealand, drought in Africa, and lower precipitation
amounts in Antarctica. Circumpolar wave number 3, which on average estab-
lishes ridges at 408 E (just to the east of South Africa) and at 1658 E (just to the