Intraseasonal convection in the Monsoon onset region and its link to northward-propagating MISO Phases: Implications for subseasonal monsoon prediction

Kerala, often termed the gateway of the Indian Summer Monsoon (ISM), receives the first surge of organized convection each year. Understanding how intraseasonal processes in this onset region influence larger monsoon systems is essential for improving early warnings and subseasonal predictions. Although the 30–60-day Monsoon Intraseasonal Oscillation (MISO) is well established, the contribution of higher-frequency intraseasonal convection within the onset region has been relatively understudied.

In this work, 45 years of IMD rainfall, ERA5 winds, CERES OLR, and continuous ST radar observations from southwestern India were used to explore this relationship. The onset region consistently exhibits a 10–30-day oscillation, which we identify as the Kerala Intraseasonal Oscillation (KISO). Wavelet and coherence analyses indicate that this signal often aligns with the broader MISO cycle rather than existing independently.

A clear lead-lag relationship emerges, peaks in KISO convection lead rainfall enhancement over the Core Monsoon Zone (CMZ) by approximately 10–20 days. This suggests that intraseasonal signals originating in the gateway region may provide an early indication of how MISO phases will evolve farther north.

Composite analyses of high-frequency active phases reveal deeper convection (lower OLR), stronger low-level westerlies, and a more coherent northward propagation pathway-features consistent with KISO acting as an initiating or reinforcing node within the monsoon intraseasonal circulation. Interestingly, years with major Kerala flood events show a stronger expression of this KISO–MISO coupling, suggesting that high-frequency intraseasonal convection in the onset region can intensify the large-scale moisture environment that supports extreme rainfall.

Overall, the study provides observational evidence that subseasonal monsoon variability over central India is partly predictable from high-frequency signals in the monsoon onset region. This results highlight a simple but useful precursor that could support subseasonal-to-seasonal forecasting  and improve early-warning capabilities in regions sensitive to active–break monsoon cycles.