Decadal-scale variations in extreme precipitation and implications for seasonal scale drought

This study examines the relationship between low decadal mean precipitation and monthly-scale wet and dry extremes over the global land area. We characterise how precipitation distributions change on decadal timescales, and how these changes are linked to seasonal scale drought. The relationship between decadal mean precipitation and extremes is assessed at the grid point level via correlations between decadal mean and extreme percentiles, and through an analysis of indices of seasonal scale drought. A novel metric is also used that determines how the first three statistical moments change monthly precipitation distributions during dry decades for several drought-prone regions. Changes to monthly-scale wet extremes are most significantly associated with low decadal mean precipitation for almost 80% of the globe. Monthly-scale dry extremes show significant, but generally weaker, relationships to low decadal-mean precipitation for 55% of the globe. Consistent with the strong link between decreasing wet extremes and decadal dryness, we find that dry decades are predominatly modulated by changes in positive skewness in monthly precipitation distributions, whilst shifts in the mean of these distributions play an important, but typically secondary, role. There is a negligible role for changes in variance. Lastly, we show that a decadal-scale decline in mean precipitation is rarely accompanied by an increase in the severity of seasonal-scale drought, while the impact on seasonal-scale drought frequency and duration varies depending on global location. Our results have implications for how we think about seasonal-scale drought in the context of decadal variability in precipitation.