The influences of ENSO and IOD on rainfall variability over the

36 find the correlations of both ENSO and IOD indices with rainfall are strong. Table 3.1 also demonstrates significant correlation between Semarang rainfall and DMI during the peak of wet season DJF. The delay of the IOD impact on rainfall in Semarang could be related to the influence of monsoon activities in controlling the difference of wet season onsets. Based on the significant correlation of DMI with rainfall found during DJF, another possibility is the chance of having more rainfall when the IOD condition is negative, leading to more flood occurrence in the region. The sequences of seasonal rainfall time series correspond to floods and droughts occurrences due to both phenomena, i.e. ENSO and IOD, can be explored in Figure 3.1. The figure shows some fluctuations of rainfall as a result of extreme climate events during the years when ENSO andor IOD occurred. In El Nino years during late 20 th century 1972, 1983, 1987, 1994, 1995 and 1997, the records showed significant decreases of rainfall over Semarang city. Since the sensitivity of rainfall variability in this city is quite large to the extreme climatic conditions such as ENSO and the IOD, this is showing the need for more systematic adaptation planning and strategy for the regions to anticipate the impact of extreme events causing floods and droughts Figure 3.1: Series of seasonal rainfall over Semarang

3.1.2 Extreme wind

Despite the influence of extreme climate events caused by large-scale climate variability on interannual time-scales, the Semarang City is also influenced by short- time extreme weather condition, such as extreme wind speed. Based on the record of historical daily data collected from weather stations in Semarang and Ahmad Yani, we found that the extreme wind speeds tend to occur locally Figure 3.2. In the Semarang station, we found extreme wind speed above 60 kmhour 17.2 ms or 62 kmhour occurred on 15 June 1994, while the highest wind speed in Ahmad Yani station occurred in 5 March 1995 12.5 ms or 45 kmhour. Due to data constrain, it is difficult to analyze this extreme event, especially in defining its recurrence period. 400 800 1200 1600 2000 M a r- 1 M a r- 7 M a r- 1 3 M a r- 1 9 M a r- 2 5 M a r- 3 1 M a r- 3 7 M a r- 4 3 M a r- 4 9 M a r- 5 5 M a r- 6 1 M a r- 6 7 M a r- 7 3 M a r- 7 9 M a r- 8 5 M a r- 9 1 M a r- 9 7 Seasonal Time S e a s o n a l R a in fa ll m m - S e m a ra n g 37 Figure 3.2: Daily wind speed in two stations over Semarang city, a Semarang station, b Ahmad Yani station, and c average of both stations 1 January 1994 – 31 December 1999.

3.2 Trends of Historical Climate in Semarang

3.2.1 Rainfall

In general, it was shown in previous study that the observed rainfall in Indonesia experienced decreasing trends after 1970s IPCC 2007. If we investigate the rainfall over different regions in Indonesia, it is possible that the trend will vary. Here we investigate the trends of rainfall in Semarang city as respectively shown in Figure 3.3. Based on the average of observed rainfall from five rain gauge stations over Semarang city during 1970-2000 periods, it is found that there is no considerable trend of rainfall in all seasons Figure 3.3. This is supported by the result of Mann-