Relation between intraseasonal and interannual variability of the South Asian monsoon in the National Centers for Environmental Predictions forecast systems

This study has investigated the extent to which the state-of-the-art general circulation models are able to reproduce the observed relation between the intraseasonal and interannual variability of the South Asian monsoon. Long simulations from two coupled models with different horizontal resolutions and one uncoupled model of the National Centers for Environmental Prediction Climate Forecast System are analyzed. Specifically, the observed feature that the interannual variability of the seasonal monsoon is largely determined by large-scale seasonally persistent modes rather than the nature and frequency of the intraseasonal oscillations has been examined in the model simulations. The importance of ocean-atmosphere interaction has also been examined by comparing the coupled and uncoupled model simulations. The models simulate the spatial structure of the mean and variability of the monsoon fairly well, although the details over the Indian land region are inadequate in all simulations. The variability of the rainfall is overestimated in the models, especially in the uncoupled version. The coupled simulations produce more realistic precipitation and circulation patterns, while the improvement in the mean and variability due to higher horizontal resolution is marginal. Composite and pattern correlation analyses indicate the presence of a large-scale seasonally persistent structure that largely determines the interannual variability of the seasonal mean monsoon. Consistent with the observations, the probability distributions of the pattern correlations indicate that the nature and frequency of the intraseasonal oscillations remain unchanged between strong and weak monsoon years. Similar to the observations, the model results provide hope for seasonal prediction of monsoon.