Artificial skill in monsoon onset prediction: two recent examples

Abstract

For two cases of empirical monsoon onset prediction it is argued that current verification practice leads to optimistically biased skill, caused by the intricacy of the model setup. For the case of the operational forecasts by the Indian Meteorological Department (IMD) it leads to an overlap of model definition and verification data. A more seriously flawed verification was used in a recent method based on trend extrapolations of 'tipping elements' (TE). Claims of TE of predicting onset 2 weeks earlier than other methods are unjustified. On the contrary, the correlation between TE forecasts and observations is as low as 0.24 and compares poorly to the reported IMD correlation of 0.78. That latter value likely being artificially inflated, currently the best and most reliable monsoon onset predictions come from a dynamical model with more reliable skill values of about 0.7.