Assessment of moisture transport and convergence fields in CMIP3 and CMIP5 Global Climate Models in South America

CARLA GULIZIA; INÉS CAMILLONI

Dubrovnik

Workshop; Workshop on CMIP5 Model Analysis and Scientific Plans for CMIP6; 2015

WCRP Working Group on Coupled Modelling (WGCM); European Commission FP7 project EMBRACE (Earth system Model Bias Reduction and assessing Abrupt Climate)

In a previous study we have seen that during austral summer though WCRP/CMIP5 Global Climate Models (GCMs) still underestimate precipitation mainly over southeastern South America, they did so to a lesser extent than those of CMIP3, and the dispersion among the new generation of models was smaller than in the previous one. Two sets of 5 GCMs from each intercomparison project have been previously selected based on their reasonable representation of precipitation over South America. The aim of this study is to assess if the improvements found in CMIP5 models to simulate summer precipitation are due to a better representation of moisture transport and convergence over South America. On the other hand, as CMIP5 models still present some difficulties in simulating summer precipitation, another objective is to evaluate if the deficiencies in simulating precipitation can be at least partially explain by an inadequate representation of the atmospheric circulation patterns. Consequently, principal component analysis is applied to austral summer monthly fields for the 1960?1999 period in order to identify the principal patterns of observed vertically integrated moisture transport and evaluate the ability of both sets of GCMs, individually and/or the ensembles, to represent them. Observed spatial patterns are calculated based on NCEP/NCAR reanalysis data. Moreover, we analyzed if the GCMs are able to represent the accumulated precipitation anomalies fields and the convergence and moisture transport associated only with the second and third principal modes.The comparison of both ensembles? performance showed no significant changes in the skill of the art of CMIP5 regarding CMIP3 models in representing the main moisture transport patterns. Therefore, the better representation of summer precipitation in South America should be due to an improvement in simulating other processes/mechanisms not related to moisture transport and convergence. Besides, as moisture transport from tropics to extratropics is a key climate feature in South America and strongly determines both the spatial pattern and the sign of rainfall anomalies, its relatively erroneous representation explains at least partially the deficiencies of GCMs to estimate regional precipitation accurately. Thus, this analysis suggests a possible pathway to improve model rainfall representation in South America.Finally, we aim to advance in the understanding of the role of summer moisture convergence as a possible mechanism to explain precipitation projections. For this purpose, the same two subsets of 5 GCMs each were selected. The A1B scenario (for CMIP3) and the medium-low RCP4.5 (for CMIP5) were used to prepare climate change scenarios for the time slices 2020-2029, 2040-2049 and 2070-2079 with respect 1990-1999. Summer precipitation changes and future convergence projections were analyzed for three particular sub-regions within the continent. Results indicate that projected changes in moisture convergence could be attributed as one of the main mechanisms to explain precipitation future scenarios, especially in the last generation of models. In this sense, although this study did not assess the uncertainties associated with climate projections, it suggests a first attempt to understand the role of one of the mechanisms that explain part of the precipitation, such as moisture convergence.