ABSTRACT A key step for any modeling study is to compare model-produced estimates with observed/reliable data. The original index of agreement (also known as original Willmott index) has been widely used to measure how well model-produced estimates simulate observed data. However, in its original version such index may lead the user to erroneously select a predicting model. Therefore, this study compared the sensibility of the original index of agreement with its two newer versions (modified and refined) and provided an easy-to-use R-code capable of calculating these three indices. First, the sensibility of the indices was evaluated through Monte Carlo Experiments. These controlled simulations considered different sorts of errors (systematic, random and systematic + random) and errors magnitude. By using the R-code, we also carried out a case of study in which the indices are expected to indicate that th empirical Thornthwaite’s model produces poor estimates of daily reference evapotranspiration in respect to the standard method Penman-Monteith (FAO56). Our findings indicate that the original index of agreement may indeed erroneously select a predicting model performing poorly. Our results also indicate that the newer versions of this index overcome such problem, producing more rigorous evaluations. Although the refined Willmott index presents the broadest range of possible values, it does not inform the user if a predicting model overestimate or underestimate the simulated data, resulting in no extra information regarding those already provided by the modified version. None of the indices represents the error as linear functions of its magnitude in respect to the observed process.

Resumo O El Niño (EN) é caracterizado pelo aquecimento anômalo das águas superficiais do oceano Pacífico Tropical central e leste que afeta o clima global e regional através das anomalias da circulação da atmosfera. O EN 2015/2016 foi classificado como um evento muito forte. Portanto, esse estudo tem como objetivo mostrar as características da atmosfera durante a ocorrência desse evento nos meses de setembro, outubro e novembro de 2015. Entre os resultados, têm-se o deslocamento para leste da célula de circulação zonal de Walker, intensificação dos jatos subtropical e polar, presença de anomalias positivas de altura geopotencial no oceano Pacífico próximas ao sul da América do Sul e anomalias negativas no sudeste desse continente e no oceano Atlântico. Além disso, há maior frequência de ciclones nas latitudes subtropicais do oceano Atlântico e convergência do fluxo de umidade integrado na vertical sobre o sudeste da América do Sul. Essas características contribuem para anomalias de precipitação e temperatura do ar na América do Sul.

Abstract The El Niño (EN) is characterized by an anomalous warming of the central and eastern surface of the Tropical Pacific Ocean waters, which affects global and regional climate through the circulation anomalies in the atmosphere. The EN 2015/2016 was classified as very strong. Therefore, this study aims to show the atmospheric characteristics during the occurrence of this event in September, October and November of 2015. Among the results, there are the shift to eastern of the zonal Walker Cell, the intensification of the subtropical and polar jets, the presence of positive anomalies of geopotential height in the Pacific ocean near the southern of South America and negative anomalies in the south and southeast of the continent and the Atlantic ocean. Moreover, there is an increase of the cyclone frequency in the subtropical latitudes of the Atlantic Ocean, and convergence of vertically integrated flux moisture on the southeastern South America. These characteristics contribute to precipitation anomalies and air temperature in South America.