Downstream and In Situ Genesis of Monsoon Low-Pressure Systems in Climate Models

Abstract

The monsoon low-pressure systems (LPSs) are a major contributor to the rainfall over India. The genesis of LPS in climate models is not well understood. Here, we track the LPS activity in 11 coupled climate models using an automated tracking algorithm and classify their genesis mechanism broadly into two categories—in situ and downstream. We find that the in situ genesis mechanism dominates in all models, with an average of 56% systems categorized under this category, while 63% of the observed LPS had in situ genesis. The average downstream genesis in the models is 32%, closer to the observed 30%. About 12% and 7% of the LPS genesis could not be attributed to either of the categories in the models and observations, respectively, due to the presence of both types of genesis mechanisms. Although the bulk statistics of the in situ and downstream LPS genesis across the models in boreal summer is comparable to that of observations, substantial inter-model variability is observed. Also, we find significant differences in the temporal distribution of downstream LPS genesis in models. Although the models realistically capture the percentage of downstream LPS for the whole monsoon season, they tend to simulate a higher number of genesis in the early phase of monsoon as opposed to the observed peak in August and September, which is linked to a stronger Rossby wave activity in the models in June.