Kerala's tropical monsoon climate zone has expanded along the western coast over the past seven decades, yet the state recorded a 28% rainfall deficit during this year's June–September southwest monsoon — a stark contrast that underscores the growing gap between long-term climate trends and seasonal variability, according to a new study by Indian meteorologists.

The joint study by scientists from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune, analysed temperature and rainfall data spanning 1951 to 2020. Its findings, released this year, reveal that the tropical monsoon climate zone — which receives the country's highest annual rainfall — has spread into more areas along the western coast, including Kerala.

How Bad Was This Year's Monsoon Deficit

Kerala recorded just 1,446.5 mm of rainfall against the seasonal normal of 2,018.6 mm, a shortfall that placed this year's monsoon among the seventh-lowest on record since official measurements began in 1901. The deficit was not uniform across the state.

Wayanad bore the steepest shortfall at 53% below normal, while Idukki received 47% less rain than expected. Only six of Kerala's districts recorded rainfall within the normal range, leaving the rest in varying degrees of stress. This comes amid broader concerns about reservoir depletion and groundwater stress that could compound agricultural pressures in the months ahead.

The Long-Term Climate Shift Behind the Numbers

Despite this season's poor performance, the IMD–IITM study found that rainfall has generally increased over the long term in southern India, driven by an expansion of the tropical monsoon climate zone. This apparent paradox — a wetter long-term trend alongside a severe seasonal deficit — reflects the influence of year-to-year atmospheric drivers.

IMD scientist Ananya Karmakar, who led the study, and IMD Director General of Meteorology Mrutyunjay Mohapatra both stressed that phenomena such as El Niño and the Indian Ocean Dipole can significantly suppress or amplify rainfall in any given season, independent of the underlying long-term trajectory. A long-term upward rainfall trend, they cautioned, does not guarantee adequate rain in every individual season.

Implications for Water Resources and Agriculture

The researchers warn that shifting climate zones carry direct consequences for water resource management and farm planning. Changes in the geographic extent of climate zones can alter which crops are viable in a region and shift the optimal sowing window — pressures that farmers and planners are not yet fully equipped to handle.

For Kerala specifically, the study highlights the dual risk: the state must prepare for increasingly erratic seasonal rainfall while remaining alert to both prolonged droughts and episodes of extreme precipitation — a combination that is becoming more common across the Western Ghats region. Notably, this is not the first time Kerala has faced a significant monsoon shortfall in recent years, pointing to a pattern that demands institutional responses beyond emergency relief.

What the Researchers Are Calling For

The study calls for long-term agricultural and water planning grounded in scientific climate assessments, rather than relying solely on historical averages. It urges policymakers to integrate seasonal forecast models with evolving climate zone data to build resilience against future rainfall shocks. The findings also reinforce the case for robust reservoir management and groundwater recharge programmes in districts most vulnerable to deficit years.