Linkage between the summer hot extremes in Taiwan and Pacific Meridional Mode

The temperature observed in Taiwan has been on the rise for the past 100 years. At the same time, the number of summer days in the 2020s has increased significantly compared with that in the 1960s, while the frequency and intensity of heatwave events are also increasing. Extremely high-temperature (EHT) and heat wave events will cause huge effects on human activities, therefore, pre-warning of EHT and heat wave events is essential.

This research investigates the association between the Taiwan summer temperature and the Pacific Meridional Mode (the PMM), an anomalous north-south sea surface temperature gradient over the northeastern subtropical Pacific. Because of the PMM's 5-6 years cycle, it is a good predictability source on a decadal timescale. It was found that when the PMM was in a positive (negative) state, the summer temperature in Taiwan significantly increased (decreased) on a decadal timescale. The zonal circulation in the sub-tropical north Pacific and the subsidence in Taiwan were considered to be the physical mechanism in the linkage.

Besides, the calendar day 90th percentile of max temperature (CTX) and heatwave magnitude scale (HWMS) were used in this research to trace the extremely high-temperature and heat wave events. The research results indicate that during the PMM-positive state, the frequency and duration of EHT and heat wave events become higher and longer than they are during the negative state.

This linkage found in research could help to improve the decadal prediction of summer temperature as well as EHT and heat wave events in Taiwan and provide climate information for the decision-makers to formulate adaptation strategies.