As indicated by rainfall histograms at Tainan (Figure 1(c)) and Keelung (Figure 1(d)), the alternation of
the two rainfall regimes is a result of the alternation in the northeast winter and the southwest summer
monsoons with an apparent enhancing effect by the orography (outlined in Section 1). Of course it may
not be completely legitimate to define two rainfall regions over all of Taiwan based on only two stations.
The following questions are raised: (i) What is the geographic coverage of these two rainfall regimes, and
(ii) how does the transition of two rainfall regimes behave following the alternation of the two monsoons?
These questions will be answered by the following analyses: (i) the EOF analysis of rainfall, and (ii) the
seasonal march of rainfall around the coastal zones of Taiwan. The EOF analysis provides a concise view
of the major seasonal variation modes in Taiwan rainfall, while the second approach illustrates the
transition between these seasonal variation modes. Results are reported in the following sections.
After removing their annual mean values, the rainfall departures of all 15 CWB stations along the
coastal zone are subjected to EOF analysis. Details concerning the EOF analysis are referred to Kutzback
(1967). Displayed in Figure 2 are the eigencoefficient (C1 and C2) time series and eigenvectors (E1 and
E2) of the first two principal eigenmodes. Based upon variance explained by these two eigenmodes (58%
and 34.3%, respectively), the seasonal variation in Taiwan rainfall can almost be portrayed. The temporal
variation of the Cs and the spatial structure of the Es reveal two distinct rainfall modes: southern Taiwan
and northern Taiwan modes. Comparison between the C1 time series with the rainfall histogram of Tainan
(Figure 1(c)) reveals that the summer monsoon rainfall in Taiwan is represented by the first eigenmode.
In contrast, the comparison between the C2 time series and the Keelung rainfall histogram (Figure 1(d))
indicates that the autumn rainfall is represented by the second eigenmode.
Though the majority of the variance (92.3%) of seasonal rainfall variation over Taiwan is explained by
the first two eigenmodes, two interesting features concerning Taiwan rainfall are missin