Indian Monsoon Teleconnections and the Impact of Correcting Tropical Diabatic Heating

Abstract

Abstract The Indian summer monsoon is partly modulated by persistent remote forcing from the tropical Indo-Pacific, evident in the dominant observed teleconnection patterns, namely, El Niño–Southern Oscillation (ENSO) and the equatorial Indian Ocean Oscillation (EQUINOO). In the atmosphere, these teleconnections are presumably driven by diabatic heating, primarily associated with the release of latent heat in condensation with rainfall. However, in coupled atmosphere–ocean models, biases result in large systematic errors in tropical heating. This study seeks to understand the extent that teleconnections are forced by tropical heating and whether or not correcting tropical heating biases improves monsoon prediction skill. We examine a series of reforecasts made with the NCEP Climate Forecast System version 2 in which the “added heating” technique is applied to largely remove tropical heating biases. We isolate the ENSO and EQUINOO signals and examine the ability to reproduce and predict these teleconnections in the model run with and without tropical heating correction. Improving ENSO and EQUINOO-related heating does result in increased prediction skill in monsoon circulation teleconnection patterns. Prediction of other relevant tropical and subtropical circulation indices is improved; however, the impact on the Indian monsoon as a whole is limited. EQUINOO exhibits large internal variability in the model, and despite imposing realistic EQUINOO heating, the monsoon circulation is relatively insensitive in the model. This suggests that either the EQUINOO teleconnection in nature does not emerge as a forced response to tropical heating, and/or the model is unable to reproduce the relationship due to separate deficiencies. Significance Statement India receives over 80% of its annual rainfall during the summer in association with the monsoon. A strong socioeconomic dependence on agriculture makes India sensitive to year-to-year variations in monsoon rainfall, thus predicting and understanding such variations is of great value. Coincident changes in tropical atmospheric heating (and cooling) may be more predictable and presumably impact the monsoon; however, causality has yet to be demonstrated and quantified, particularly for the tropical Indian Ocean. This motivates our modeling study to diagnose the role of tropical heating for the Indian monsoon and whether or not correcting heating errors improves monsoon prediction.