Novel approaches in assembling and analysing multi-archive datasets to assess changes in frequency, seasonality and areal extent of large rainstorms in Sabah, Malaysian Borneo 1906-2024

There is growing evidence of worldwide increases in magnitude and frequency of large rainstorms driven by global warming-related rises in atmospheric temperature and moisture content and sea surface temperature – and with major consequences for flooding, landslides, soil erosion and sediment transport.  Assessments of the magnitude of recent change, however, are often constrained via comparisons with short periods of pre-1980 daily rainfall data that may not be representative of the pre-global warming era.   Assessment of change in tropical areas is particularly hampered not only by the short and patchy nature of daily records (in terms of continuity, quality and availability), but also by the localized nature of many tropical rainstorm events and the strong influence of locally variant factors (such as coastal and topographic aspect and associated, often diurnally variant, local wind circulations) on large event occurrence.  This paper focuses on methods used in the assembly and analysis of changes in magnitude-frequency, seasonality, areal extent and other characteristics of large ( >50 mm) daily rainstorms in Sabah, Malaysian Borneo, over the past 120 years.  Archival search, detection and transcription of daily data for 1906-1959, together with Malaysian Meteorological Department data since 1960, enabled continuous daily data series to be constructed for three stations of contrasting coastal aspect (Sandakan – N-facing; Kota Kinabalu - W-facing; and Tawau - S-facing). The only gap was during and immediately following the Japanese Occupation during WWII.  Shorter daily rainfall series at other Sabah stations – notably Labuan, Kudat and the inland stations of Danum Valley and Keningau - were also derived and used.  A previous paper (Walsh et al. 2013) analysed changes up to 2012 at individual stations using > 50mm and  >100 mm threshold daily rainfall series and extreme value analysis. The current paper uses novel approaches to refine and extend analysis in four ways.  First, combined data series of the three key stations are used to derive separate datasets of “local” and “multi-site/regional” events for assessment of changes in their frequencies. Second, it uses the N-facing Sandakan and Kudat records to identify (and assess changes in magnitude-frequency) of “Cold Wave” events - characterized by 4-10 successive days of very high rainfall in December or January  and affecting N/NE-facing coastal areas of Sabah and Peninsular Malaysia. The very recent December 2024 event had particularly severe flooding consequences. Third, changes in seasonality of large events are explored via (1) monthly frequency regimes for 1906-40, 1947-79, 1980-99 and 2000-24 and (2) running means of monthly frequencies.  Fourth, when exploring seasonal and annual changes in frequency, the focus is on change in frequencies of different size categories of rainstorm (50-74, 75-99, 100-124, 125-149, 150-174, 175-199 and >200 mm) rather than the hitherto cruder > 50 mm and 100 mm threshold data. Results demonstrate that rises in magnitude-frequency since 1980 are continuing, but where the magnitudes of change vary with season (NE Monsoon, SW Monsoon and Transition Phases}, between stations of different coastal and topographic aspect, between different storm-size categories, and between localized and regional storm types.