Influence of PDO and ENSO on Indian summer monsoon rainfall and its changing relationship before and after climate shift

Indian summer monsoon provides rainfall over a large area during 01 June to 30 September and it plays vital role for the water needs of the population of India. It is intense because strong differential heating prevails over the region due to geographical features of India. Further, it can be viewed as a synoptic scale ocean - atmosphere interactive system. In this study, we investigated the possible relation between the Indian summer monsoon and the combination of the different phases of Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) before and after the climate shift in 1976. This study is carried out using IMD’s precipitation dataset, HadISST v1.1 dataset and twentieth century reanalysis dataset by comparing anomalies of the respective parameters from 1901 to 2020. It is found that when positive (negative) phases of PDO and El Niño (La Niña) co-occur, deficit (surplus) rainfall are likely to occur over entire India. SST signatures of both phenomena are evident in this context. However, when negative (positive) PDO and El Niño (La Niña) co-occur, the signal is mixed and it is unlikely that either surplus or deficit rainfall conditions will occur over entire India. SST signatures are disrupted and minimized. In other words, when ENSO and PDO are in (out of) phase they enhance (counteract) the conventional monsoon-ENSO relation. Further, the study periods were divided into pre and post climate shift periods based on Niño 3.4 index and PDO index and analysed their impact on the Indian summer monsoon rainfall. In the pre-shift period, in-phase conditions exhibit similar qualities to those described above. Rainfall patterns are more indicative of ENSO than PDO. In the post-shift situation, the positive anomaly of SST in the PDO and Niño region is significantly stronger than that of the pre-shift phase. When compared to the pre-shift, positive rainfall anomalies are amplified during positive PDO and El Niño,  while negative PDO and La Niña show a weakening of positive rainfall anomalies. The out of phase condition has a balancing effect due to the counteracting impact, but with an increased positive anomaly of SST. In that combination, rainfall patterns with PDO characteristics rather than ENSO characteristics emerge. Significant warming of the Indian Ocean basin was also evident in the above combinations after the climate shift in 1976. Low level wind anomalies and other circulation features are consistent with the above result.