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The adequacy of reference drought intensity threshold levels based on deviations of monthly precipitation totals from normal climatological conditions is reconsidered. The motivation for this study is the observation that reference classification schemes are fixed for all climatological regions and threshold levels have been proposed without regard for the statistical distribution of accumulated precipitation in space and time. This misrepresentation of precipitation variability may lead to erroneous estimates of meteorological drought onset in specific areas where natural breaks in the cumulative distribution of monthly rainfall do not fit the generalized classification systems. Disproportionate estimates of drought onset may bias the frequency of events and confuse mitigation strategies. In this study, a new optimized classification system based on the non-parametric ``Fisher-Jenks'' algorithm is proposed for the estimation of drought intensity threshold levels from monthly rainfall totals. The optimized classification system is compared using the tabular accuracy index (TAI) to three fixed classification systems that are proposed in the literature and widely applied in the operational setting. Assessing the drought intensity classifications with optimized and fixed threshold levels, (i) six optimized categories most accurately divide precipitation observations into the most appropriate drought intensities, (ii) optimized thresholds always give considerably improved drought intensity category allocations over fixed thresholds with the same number of categories, and (iii) fixed thresholds underestimate the drought onset. A case study on the monthly and long-term drought frequency estimation for Latin America has been conducted for assessing the spatial link between drought intensity categories computed with the non-parametric ``Fisher-Jenks'' algorithm and different climate classifications. The results show a systematic match between climate variability in the region and spatial patterns of drought intensity.JRC.H.7-Climate Risk Managemen

An Optimized System for the Classification of Meteorological Drought Intensity with Applications in Drought Frequency Analysis