Robustness of precipitation Emergent Constraints in CMIP6

<p><span>Climate models are fundamental tools to understand the complexity of the climate system, to study  the processes at work and to provide credible future climate projections. </span><span>Unfortunately, models often disagree significantly in the amplitude of different climate change signals and in their representation of the role of important feedbacks. </span><span>In the past years the “Emergent Constraints” methodology has been developed for reducing uncertainties in climate-change projections. </span><span>An Emergent Constraint (EC) is a statistical relationship between the inter-model spread of a measurable aspect of the present-day climate (predictor) and the inter-model spread of a variable projection (predictand), under a climate change scenario. If a significant correlation is found, observations of the predictor can be used to constrain model projections of the predictand and the uncertainties in climate model outputs can be narrowed. </span></p><p><span>In the last two decades, ECs have been identified in different branches of climate science although just a limited number of these ECs is related to the hydrological cycle. Recently, a relevant number of EC in the literature was discovered to lack a satisfying physical explanation and many, developed and tested with CMIP5 ensemble, seem to be not-significant in the new CMIP6 ensemble. However, the analysis regarding ECs related to the hydrological cycle is still incomplete. </span><span>The aim of this work is to test three ECs related to mean-precipitation and extreme precipitation events, originally identified in CMIP3 or CMIP5 data, and to evaluate if their statistical significance survives also in the CMIP6 ensemble: (a) global hydrological sensitivity used to constrain future changes in local extreme precipitation: we find this relationship not to be robust in CMIP6 models; (b) future changes in the Indian summer monsoon precipitation, constrained by Western Pacific mean precipitation: this relationship is not robust with the new ensemble; (c) changes in future extreme tropical precipitation, constrained by the same variable calculated in the past: we find this EC to be robust both in CMIP5 and CMIP6.</span></p>