A new set of 12 daily weather types for the New Zealand region has been der
ived from the 40-year NCEP/NCAR reanalysis dataset. Cluster analysis of the
monthly frequencies of these patterns has led to the definition of three '
regimes', characterized by (1) frequent troughs crossing the country, Oil h
ighs to the north with strong zonal flow to the south of the New Zealand, a
nd (iii) blocking patterns with highs more prominent in the south.
Blocking regimes are more frequent in summer and autumn and are associated
with above-normal temperatures, less precipitation in the southwest of the
country and more precipitation in the northeast. The Zonal regime, which br
ings below-normal precipitation to the northeast and milder conditions in t
he south, is less common in summer. The Trough regime is less frequent in a
utumn and is linked to cooler temperatures in the west and above-normal pre
cipitation over the entire country.
The monthly frequencies of individual synoptic types are only weakly relate
d to the Southern Oscillation Index (SOI) and other indices of the hemisphe
ric-scale flow, with variance reductions from regression equations ranging
from between 3 and 33%. Similar predictions of weighted combinations of the
synoptic types corresponding to each regime gave mean variance reductions
70-80% higher. The correct regime could be specified from the hemispheric i
ndices by the use of discriminant analysis for 56% of the 474 months in the
dependent dataset.
The monthly frequency of synoptic types was also tested as an alternative t
o monthly mean 1000 hPa patterns in the selection of analogues over the New
Zealand region. The success of the selection process was measured by the v
ariance of temperature and rainfall patterns over homogeneous regions for s
ets of two, four and eight analogues. The mean 1000 hPa patterns scored bet
ter in nearly all cases, but gave variance reductions of only 13-19% over r
andomly chosen analogues. Copyright (C) 2000 Royal Meteorological Society.