Characterization of wet and dry periods over southern South America

Abstract

This study characterizes wet and dry conditions in southern South America, defined through the standardized precipitation evapotranspiration index (SPEI), considering short and long timescales. Prior to analysis, a regionalization was conducted considering the area’s diverse climatological regimes, employing a combination of the k-means clustering algorithm and principal component analysis. Dry and wet events were analyzed over six homogeneous regions, defined as wet or dry conditions with time and spatial coherence. Generally, smaller events (< 100,000 Km²) were prevalent monthly, while larger events (> 50% of a region) occurred less frequently and varied in size over time. Event intensity varied, with no prolonged preference for a specific intensity. Certain regions, notably southern Bolivia and northwestern Argentina showed a discernible tendency towards drier conditions over time, with wet events becoming more intense despite reduced coverage. The events defined with shorter SPEI timescales showed similar behavior to those defined with longer SPEI timescales but with higher month-to-month variability and less spatial coherence. A wavelet analysis revealed punctual, significant periods on the interannual timescale and more persistent ones on the interdecadal and decadal timescales. Mechanisms associated with event behavior were analyzed. Characterizing events in terms of their intensity, duration, and spatial extent revealed that moderate events were predominantly short (< 3 months) while longer and more extended events were often severe. Events calculated with shorter SPEI timescales showed smaller durations than those calculated with longer SPEI timescales. In addition, wet events showed a similar proportion of moderate and severe cases, while dry events were predominantly moderate.