Time series analysis of OLR anomalies and rainfall variability across Indian monsoon onset phases

The onset of the Indian summer monsoon is a critical climatic transition characterized by rapid changes in atmospheric convection and rainfall distribution. This study investigates the variability of Outgoing Longwave Radiation (OLR) anomalies and precipitation across three monsoon onset phases: pre-onset (15–29 May), normal onset (30 May–2 June), and post-onset (3–13 June). Daily OLR data from NOAA and gauge-based precipitation datasets were analyzed using time series extraction and seasonal decomposition techniques. Stationarity of the extracted series was assessed through rolling statistics and the Augmented Dickey–Fuller test. Results reveal pronounced temporal fluctuations in both OLR and rainfall during monsoon establishment, with negative OLR anomalies consistently associated with enhanced precipitation across all onset categories. Pre-onset years exhibit rapid convective intensification, while normal onset years show relatively organized seasonal evolution. Post-onset years are characterized by increased irregular variability, reflecting unstable atmospheric conditions during delayed monsoon arrival. The decomposition highlights distinct differences in trend and residual behavior among the three onset phases, indicating varying contributions of large-scale circulation and short-term convective disturbances. These findings demonstrate that monsoon onset timing significantly controls convective intensity and rainfall distribution. The study highlights the usefulness of OLR-based diagnostics combined with time series analysis in understanding monsoon onset dynamics and provides valuable insight into early-season monsoon variability.