Causal Analysis Discovers an Enhanced Impact of Tropical Western Pacific on Indian Summer Monsoon Subseasonal Anomalies

Existing studies have shown changes in the impact of atmospheric teleconnections on Indian Summer Monsoon (ISM) at interannual time scales due to the changing background state. However, the exploration of potential changes at subseasonal time scales remains limited. In this study, we use a causal discovery method to find the tropical atmospheric drivers of ISM subseasonal anomalies, and quantify the causal effects of the drivers on ISM, as well as the changes in causal effects over the past 40 years. We find the impact of tropical western Pacific on ISM subseasonal anomalies has strengthened, while the ISM self-feedback has weakened. Wavenumber-frequency analysis shows that the Rossby wave energy over 15 degrees N-30 degrees N at periods of 10-20 days has increased over the past four decades, which offers a plausible explanation for the enhanced tropical atmospheric teleconnection between western Pacific and ISM at subseasonal time scales. The subseasonal variability of Indian Summer Monsoon (ISM) is vital to agriculture and economy. Therefore, it is important to understand the drivers of ISM subseasonal anomalies, how strong an impact of a driver is, as well as potential changes in the strength of such an impact. In this study, we explore the possible drivers for ISM subseasonal anomalies by using a causal discovery method, and taking into account the tropical geopotential height anomalies together with the ISM. Two tropical atmospheric drivers of ISM subseasonal anomalies are detected: one is the geopotential height anomalies over the tropical western Pacific, and the other is the ISM itself. Over the past four decades, the tropical western Pacific has exerted an enhanced influence on ISM subseasonal anomalies, while the self-feedback of ISM has weakened. Under global warming, more anomalous convection signals can emanate over the western Pacific, and propagate westward to influence the ISM region, which might explain the physical mechanism behind the enhanced impact of the western Pacific on ISM subseasonal anomalies. Causal analysis reveals the causal impact of the tropical western Pacific on Indian Summer Monsoon (ISM) and of ISM on itself at subseasonal time scales The causal impact of the western Pacific on ISM has strengthened, while the ISM self-feedback has weakened over the past 40 years The impact of the tropical western Pacific on ISM subseasonal anomalies is strengthening due to amplified Rossby wave energy over 15 degrees N-30 degrees N