Spatio-temporal variation of wet and dry spell characteristics of tropical precipitation in Singapore and its association with ENSO

Climatology and statistical variability of wet and dry spell characteristics of precipitation provide an effective tool to understand the impacts of natural climate variability such as El Nino/Southern Oscillation (ENSO) and climate change on the water availability over a region. However, to date, detailed studies about precipitation spell characteristics in the tropical urban context are still limited. Therefore, this study presents a comprehensive framework to characterize the wet and dry spells of precipitation in a tropical urban city-state (i.e. Singapore), based on daily data from 26 precipitation stations. In addition, 14 wet and dry spell indices are defined, and spatial and temporal variations of the indices are analysed based on ordinary kriging method and an improved Mann-Kendall (MK) test method, respectively. Lastly, the influence of ENSO condition on the wet and dry spell indices is investigated utilizing a regional regression method. Results indicate that: (1) the region over Singapore is predominantly characterized by 1-day wet and dry spells, while the main contributions to total duration of wet and dry spells come from 3-4-day wet spells and 3-6-day dry spells, respectively. The major contribution to total accumulated depth of wet spells comes from 3-4-day and 20-80 mm wet spells; (2) significant increasing trends are detected in general and extreme characteristics of wet spells at annual time scale, however, these trends are not significant during the two monsoon seasons. Spatial variation of wet and dry spell indices is also evident; (3) Singapore's precipitation spells are influenced by ENSO, but mainly during the southwest monsoon season. During the El Nino phase, precipitation spells generally become drier. The predicted changes of wet and dry spell characteristics in the contexts of ENSO and climate change have implications for water resources management in tropical urban cities.