Unraveling the pertinence of drought indices in the changing climate

Abstract

The increasing divergence/disagreement between the meteorological drought indicators, majorly due to the changing climate conditions, has disputed their pertinency in the monitoring and modeling of drought events. In the present study, we attempted to quantify the divergence between widely used drought indices such as the standardized precipitation index (SPI), standardized precipitation evapotranspiration index (SPEI), and the newly developed standardized net precipitation index (SNEPI) and traced the evolution of this divergence/disagreement. A persistent presence of enhanced dry extremes in the annual divergence of SPEI with SPI and SNEPI was observed, which raises doubts about the utility of traditional drought indices in capturing the changing climatic characteristics. The seasonal dispersion of this annual divergence revealed a strong, spatiotemporally dominant, signal of dryness in the monsoon season by SNEPI, which SPEI failed to unravel. Furthermore, the attribution analysis revealed that the SPI-SPEI disagreements evolved under the influence of the shortwave radiative fluxes. In addition to that, the divergence between SPEI and SNEPI, is driven by the characteristics of wet spells, with the relationship strengthening in the monsoon season and tropical climate zones. All these findings caution us to evaluate the need to incorporate the changing precipitation characteristics before deploying drought indices in operational uses, especially in tropical climate zones.