Dust impacts on the Indian summer monsoon: chaotic or physical effect?

Abstract. Aerosols have significant impacts on regional climate, which has been widely investigated with numerical experiments. However, the uncertainties of simulated aerosol impact due to the long-standing chaotic effect remain unclear. Here we propose a diagnostic method based on large ensemble simulations and a random sampling algorithm to unveil the chaos-induced uncertainties in simulated aerosol climatic impacts that have been overlooked in previous studies. Taking the dust impacts on the Indian summer monsoon system as a demonstration, our findings reveal that, while dust generally enhances the large-scale summer monsoon circulation consistently among ensemble members, its impacts on regional systems, such as monsoon depressions, exhibit significant chaotic effect: the simulated aerosol impacts on precipitation from individual ensemble members differ substantially, even inversely. Through quantitative analysis, we demonstrate that the magnitude of these chaotic effects diminishes following a N−½ relationship with ensemble size N. Furthermore, our results indicate that statistical significance testing alone may be insufficient for the robust attribution of dust impacts, as even small ensembles can yield statistically significant yet contradictory results. This study emphasizes the necessity of employing adequate ensemble sizes to capture reliable physical impacts of aerosol on the regional climate.