Abstract
Drought is one of the important issues in climate studies. A drought index, Taiwan Meteorological Drought index (TMD index), was previously proposed and is applied here to identify historical severe droughts in Taiwan in order to clarify the corresponding large-scale backgrounds as a potential alert to the society in future. Through the TMD index, several historical severe drought cases in Taiwan are detected and characterized by significant seasonal variability in the annual cycle. Composites for large-scale atmospheric and oceanic environments over different periods within the dry season are conducted. From October to December, the colder sea surface temperature (SST) pattern of Pacific Meridional Mode (PMM) and the PMM-induced local anomalous anticyclones over the South China Sea are both in charge of the extremely dry conditions in Taiwan. From January to February, cold SST in the South China Sea and its adjacent oceans dominates local atmospheric conditions above these regions and creates an unfavorable environment for convection systems. From March to May, a massive anomalous anticyclonic circulation centering beside Alaska and extending its properties to East Asia and Taiwan generates a descending environment and in turn suppresses convection systems to develop. Therefore, the extremely dry conditions under this system are expected.
Highlights
In history, water resources allowed several human civilizations to bloom, and evidenced a decline of those civilizations when they experienced serious shortages of water [1]
Despite the huge demand of water in that region, the results in Figure 3 clearly reveal a dilemma that southern Taiwan is much more likely to face the strikes from severe droughts outside the typhoon season, while northern and eastern Taiwan receive more water over the same period (October to the following May)
This situation could be explained by the anomalous high sea level pressure (SLP) patterns around the island because high pressures create an unfavorable condition for convection systems to grow
Summary
Water resources allowed several human civilizations to bloom, and evidenced a decline of those civilizations when they experienced serious shortages of water [1]. These two indices show their own strengths and were applied in various researches like the trend of droughts under global warming [26] and the spatial and temporal evolution of drought conditions [27], some deficiencies were mentioned, such as utilizing simplified model to compute for the PDSI [28] and the requirement of carefully choosing calibration period for the scPDSI [29,30] Another popular index, named the Standardized Precipitation Index (hereafter SPI), was proposed by Reference [31]. Following the results from HS2017, this study introduces the strengths of the TMD index and applies it to track severe drought cases in Taiwan history in order to investigate possible causes by analyzing large-scale atmospheric and oceanic environments. Through the method described in the present study, the large-scale factors which may trigger another severe drought in the nearly future in Taiwan can be clarified, as a potential alert sent to the government and the society to better prepare countermeasures against the coming dryness
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