The Impact of the Madden-Julian Oscillation on Spring and Autumn Afternoon Diurnal Convection in Sri Lanka

This study examines how the Madden-Julian Oscillation (MJO) phases affect afternoon diurnal convection (ADC) patterns in Sri Lanka during 2001-2020. Sri Lanka experiences the highest frequency of ADC events in the Indian subcontinent region while located in a pivotal position within the propagation pathway of the MJO. To address the research gap regarding the MJO’s impact on seasonal diurnal rainfall in Sri Lanka, we analyze both the spring and autumn seasons, which are the two seasons with greater diurnal rainfall variability, focusing on strong MJO phases (P1-P8). Our findings show that daily rainfall increases during the P2-to-P3 phases and decreases during the P6-to-P7 phases in both seasons. The diurnal rainfall patterns, however, show seasonal differences. In spring, the diurnal rainfall amplitude peaks during P2-to-P3 phases, while in autumn, it peaks during P8-to-P1 phases. ADC events are more frequent and intense during these respective phases. The MJO's effect on both diurnal rainfall amplitude and ADC events is stronger in autumn compared to spring. During active MJO phases, we observe enhanced westward propagation of diurnal rainfall linked to ADC events, driven by moisture convergence and increased upward motion. The combination of mid-to-upper level easterly winds and deep convection over Sri Lanka leads to more distinct westward propagation during P2-to-P3 phases in spring and P8-to-P1 phases in autumn. These findings enhance our understanding of how the MJO influences local rainfall patterns and can aid in improving regional weather forecasting.