level using a t-test
Thailand summer monsoon rainfall was correlated with the standard ENSO indices and IOD index from
pre-monsoon seasons and also with the spring (MAM) Thailand air temperatures (SAT). The latter are believed
to be an indicator of the land–ocean thermal gradient, which is important for the strength of the monsoon
(Singhrattna et al., in press). The correlations are computed for the post-1980 period and shown in Table I.
Correlation values that are statistically significant at the 95% confidence level using a t-test (Helsel and Hirsch,
1995) are shown in bold in the table. It can be seen that SOI, the SLP-based ENSO index, shows a strong
correlation with monsoon rainfall during the concurrent season and also one or two seasons prior. The spring
land temperatures also exhibit a significant correlation, as expected. The IOD shows a strong correlation with
the monsoon rainfall at a two-season lead time. All these brighten the prospects for a long-lead forecast.
To confirm that the correlations are strong only during the post-1980 period (as in Table I), selected
predictors from pre-monsoon seasons (January–March (JFM) NINO3, May–July (MJJ) SOI, MAM IOD and
MAM SAT) were correlated with monsoon rainfall on a 21 year moving window (Figure 1). It can be seen
that the predictors show correlations with summer rainfall only in recent decades, much as the correlations
between the rainfall and ENSO (shown as a solid line between summer rainfall and ASO SOI) and seen by
Singhrattna et al. in press. Similar shifts have been seen (Miyakoda et al., 2003) in pre-monsoon signals of