In view of the relationship between the variability of rainfall during winter season
(DJF) represented by PC1 to SST anomalies in various months, we perform lead-lag
correlation analysis to further investigate. The SST anomalies in the SA of the North Pacific
Ocean were averaged, and used them to determine the relationships with the variability of
rainfall over the Indochina. Whereas, the Nino3.4 area (5°N-5°S, 170°-120°W) representing
ENSO phenomenon (Trenberth and Stepaniak, 2001) was used to determine the lead-lag
correlations. The Fig. 5a shows the positive relation between the variability of rainfall over
the Indochina (PC1) and the SST anomalies over the SA in the Pacific Ocean. The positive
correlation means that the increasing (decreasing) of SST anomalies in that area related to
rising (reducing) of the rainfall variability magnitude indicating more (less) rainfall over the
Indochina. The significant correlations present six months before and after the winter season
that implies the warming SST anomalous over the SA of the Pacific Ocean triggers more
rainfall over the Indochina and after that the rainfall sends signal back to that area of the
Ocean.
For the ENSO, the lead-lag correlation between the rainfall variability and the Nino3.4
shows less significant correlation than that of the SA in the North Pacific Ocean, and the
significant correlation coefficients are negative (Fig. 5b). The negative correlation between
the PC1 and Nino3.4 indicates the increasing (decreasing) of rainfall over the Indochina
6
related to cool (warm) phase of ENSO, but it is not strong, comparing to the SA in the North
Pacific Ocean, in terms of linear relationship.
From these analyses, they show that the forcing of SST anomalous in the Pacific
Ocean affects rainfall variability over the Indochina. The ENSO does not play a greater role
on rainfall variability comparing to the SA in the North Pacific Ocean. These agree with the
relationship between rainfall in the Southeast Asia and ENSO that shows the large variation
of rainfall over the Borneo (Juneng and Tangang, 2005). Nevertheless, the influence of ENSO
on rainfall variability over the Indochina in this study was shown by the data analysis.
Another perspective given by the modeling is important to give more understanding.
Moreover, the Pacific Decadal Oscillation would be of interest for study further due to it
related to the SST in the North Pacific. There are the plenty of understandings required for
more understanding of climate variability over the Indochina such as the mechanism of SST
forcing affects the rainfall variability.