When rain, not wind, kills: The new cyclone threat

Warm oceans, active equatorial systems, prolonged storms, deforestation and limited preparedness increase deaths due to rainfall, writes B K Singh

Warming seas and a warmer atmosphere are increasingly contributing to stronger storm surges, more intense rainfall, and higher wind speeds, disrupting the lives and livelihoods of coastal populations. The Indian coastline, in particular, has experienced frequent cyclonic disturbances in recent decades. This repeated exposure has enabled governments at the Centre and in coastal states to develop relatively efficient systems for early warning and evacuation. As a result, large-scale loss of human life has been reduced even during severe cyclonic events.

However, while improved evacuation has saved lives, damage to agriculture, housing, and public infrastructure — such as roads, railways, bridges, transmission lines, and communication networks — has remained substantial. Coastal agricultural lands are often the worst affected. Storm surges and flooding render fields saline, making them unfit for cultivation for long periods unless expensive reclamation measures are undertaken. In many areas where cyclones are frequent, farmers increasingly abandon crop cultivation and turn to shrimp farming instead. Such livelihoods are themselves vulnerable to external shocks, including fluctuations in international trade and tariffs imposed by major importing countries.

Recent cyclonic systems such as Senyar and Ditwah, which affected parts of the Bay of Bengal and the eastern Indian Ocean region, illustrate that storms not classified as “super cyclones” can nevertheless cause disproportionate destruction. Despite relatively modest wind speeds compared to the most intense cyclones, these systems were associated with exceptionally heavy rainfall over short periods. This resulted in widespread flooding, landslides, and infrastructure damage across parts of Southeast and South Asia, with severe humanitarian consequences.

Cyclone Senyar developed from a low-pressure area over the Strait of Malacca and the southern Andaman Sea near the end of November, close to 5 degrees north latitude. Cyclone formation is generally less common near the equator because the Coriolis effect — arising from the Earth’s rotation — is weak at low latitudes. However, under certain conditions, including active low-pressure systems, favourable atmospheric instability, and the presence of equatorial waves, cyclonic circulation can still develop. Senyar intensified under such conditions and brought heavy rainfall over parts of Sumatra.

Cyclone Ditwah formed a few days later over the southwest Bay of Bengal, around 6 degrees north latitude. It tracked along the eastern coast of Sri Lanka before moving northwestward and eventually making landfall along the North Tamil Nadu coast in India. Unlike many systems that weaken rapidly after interacting with land, Ditwah lingered close to the coast for an extended period, repeatedly drawing moisture from warm ocean waters and releasing intense rainfall over the same regions. This persistence significantly worsened flooding and associated damage.

Sri Lanka, due to its proximity to the equator, experiences fewer cyclones than regions farther north in the Bay of Bengal, as the Coriolis effect required for strong cyclonic rotation is relatively weak. When intense rainfall occurs over short durations, response times are limited, and the impact on vulnerable populations can be severe, particularly in floodplains and hilly terrain.

The late-November period during which these cyclones developed coincided with an active Inter-Tropical Convergence Zone (ITCZ), where northeast and southeast trade winds converge near the equator. This zone is naturally associated with deep cloud formation and heavy rainfall. In addition, equatorial atmospheric disturbances, which periodically generate large-scale wave patterns, were particularly strong at the time. Together, these systems provided abundant moisture and atmospheric energy, fuelling prolonged and intense rainfall rather than extreme winds.

In Indonesia, Senyar’s heavy rains triggered landslides that buried homes, damaged infrastructure, and cut off entire districts. In hilly regions, intense rainfall led to slope failures and downstream flash floods. Large-scale deforestation for palm oil cultivation in parts of Indonesia and Malaysia has significantly reduced the capacity of landscapes to absorb rainfall. The loss of natural forest cover accelerates soil erosion, increases surface runoff, and magnifies flooding. Visuals showed extensive mudflows and unstable slopes, highlighting the consequences of degraded catchments.

Sri Lanka experienced exceptionally heavy rainfall over short periods, with several regions recording rainfall totals far above seasonal averages. The rapid intensification of rainfall left little time for affected communities to seek safety after receiving warnings. In a warming climate, the interval between alerts and peak flooding is shrinking. Prolonged rainfall saturates soils, destabilises slopes in hilly areas, and weakens makeshift housing structures, making them highly vulnerable to collapse or being washed away.

Warmer oceans play a central role in this evolving risk. Sea surface temperatures across the Bay of Bengal and adjoining regions frequently exceed 28°C, allowing the atmosphere to hold more moisture. This moisture is released in the form of intense downpours during storms. The ongoing La Niña phase further enhances moisture transport toward South and Southeast Asia, increasing the likelihood that even moderate cyclonic systems can produce extreme rainfall. Large volumes of fast-moving water overwhelm drainage systems, breach riverbanks and reservoirs, and trigger landslides, disproportionately affecting vulnerable populations.

We must strengthen disaster preparedness beyond evacuation and invest in stabilising vulnerable slopes, improving drainage networks, reinforcing river embankments, and protecting natural ecosystems that buffer floods. Disaster management agencies must enhance their capacity for rapid, localised evacuations, particularly in landslide-prone and flood-prone regions. However, the long-term solution to reducing the severity of such disasters lies in addressing the root cause: global warming.

(The author is a former head of the forest force, Karnataka)