Cyclone Arnab: El Niño, Warm Seas And The Factors Behind Its Rapid Intensification

Cyclone Arnab gathered strength over the Bay of Bengal, advanced towards Andhra Pradesh faster than expected and retained its intensity for hours after reaching the coast. Its development brought together the conditions that can make a cyclone strengthen quickly: warm seas, heat stored beneath the surface and an atmosphere that allows the storm’s circulation to organise.

Arnab crossed the Andhra Pradesh and Yanam coasts early on Wednesday, October 28, as a very severe cyclonic storm, uprooting trees and electricity poles and disrupting power supply. More than 1,80,000 people were evacuated across Andhra Pradesh and Odisha before its arrival.

The storm later weakened into a severe cyclonic storm. But its rapid strengthening and changing movement have raised questions about the role of El Niño and the difficulty of forecasting sudden changes in cyclone intensity.

How Arnab Grew Into A Very Severe Cyclone

Arnab began as a low-pressure system over the southeast Bay of Bengal on October 23. By October 26, it had developed into a cyclonic storm before strengthening through the severe and very severe categories.

It was the first cyclone of 2026 in the North Indian Ocean basin, which comprises the Bay of Bengal and the Arabian Sea.

As the storm approached land faster than initially expected, the India Meteorological Department (IMD) revised its landfall forecast late on Tuesday, bringing the expected crossing forward from the forenoon to the early hours of Wednesday.

Arnab crossed between Bapatla and Visakhapatnam with sustained winds of 125–135 kmph, gusting to 140 kmph. Its outer rainbands had already begun moving over land before the centre reached the coast.

The system did not immediately lose strength. It maintained its intensity for at least six hours before weakening into a severe cyclonic storm. According to IMD, it remained nearly stationary close to Narsapur and Yanam from around 4 am, slowing the crossing process.

How El Niño Can Influence A Bay of Bengal Cyclone

El Niño—the abnormal warming of surface waters in the central and equatorial Pacific—can alter weather far beyond the ocean where it develops. It changes global wind patterns, affects rainfall and can contribute to unusual warming in regions such as the North Indian Ocean.

In 2026, El Niño has been intensifying rapidly. Between July and September, sea-surface temperatures in the Pacific’s Niño 3.4 region, used to assess its strength, were 2–2.8 degrees Celsius above normal.

Its influence can also extend to where cyclones develop.

As the southwest monsoon retreats, the monsoon trough—a shifting belt of low pressure—moves southwards. During El Niño years, changes in ocean temperatures, winds and pressure near the equator can affect the conditions in which cyclones form.

IMD Director General Mrutyunjay Mohapatra has previously explained that North Indian Ocean cyclones tend to develop closer to the equator during El Niño years. This makes movement towards Andhra Pradesh and Tamil Nadu more likely than towards Odisha and West Bengal.

Cyclone Hudhud, which struck Andhra Pradesh in 2014, also developed during an El Niño year.

Warm Water Supplies The Energy

A cyclone draws strength from the sea beneath it. Warm water evaporates, carrying moisture into the atmosphere. As that moisture rises and condenses into clouds, it releases heat that strengthens the storm’s circulation.

Ocean temperatures of around 26 degrees Celsius or above, extending to depths of 50–100 metres, are conducive to cyclone formation.

The depth of that warmth matters alongside the temperature at the surface. Ocean heat content—the heat held below the surface—helps determine how much energy remains available as a storm churns the water.

For Arnab, warm seas provided favourable conditions for strengthening. The cyclone reached its highest intensity as a very severe cyclonic storm over a single day.

Why Wind Shear And A Compact Core Matter

Ocean heat alone does not determine a cyclone’s strength. The atmosphere must also allow its thunderstorms and circulation to remain organised.

Low vertical wind shear helps that happen. When winds at different heights are relatively aligned, thunderstorms can stay concentrated around the centre. Stronger differences in those winds can disrupt the structure.

Moist air, vigorous thunderstorms and efficient heat release can further support intensification. A compact inner circulation can allow wind speeds to rise quickly as pressure falls.

Arnab’s development therefore involved several interacting processes, including wind shear, convection, upper-ocean circulation and exchanges of heat and moisture between the sea and atmosphere.

These conditions can change quickly, making the timing and extent of strengthening difficult to predict.

What Does The “Drillbit” Description Mean?

“Drillbit” is an informal term for a compact, tightly wound cyclone with a sharply defined core. It describes the storm’s appearance rather than an official intensity category.

Such systems can concentrate their strongest winds within a relatively small area.

The “Category 4” label mentioned in descriptions of Arnab belongs to the Saffir-Simpson Hurricane Wind Scale, which places storms with sustained winds of approximately 200–250 kmph in that category.

IMD uses a different classification system. Its description of Arnab as a very severe cyclonic storm, with reported landfall winds of 125–135 kmph, should be distinguished from the Category 4 label.

Why Forecasts Can Change Close To Landfall

Cyclone intensity depends on the interaction between ocean conditions, atmospheric winds and the storm’s internal structure. Changes in any of these can alter how quickly it strengthens or weakens.

Satellite observations help forecasters monitor that evolution, but uncertainty persists when a cyclone intensifies over open water.

Arnab’s faster approach prompted IMD to advance its expected landfall time. Its continued strength near the coast also complicated assessments of how much it would weaken before crossing.

Interaction with land, cooler water brought to the surface through upwelling and changes in atmospheric winds can reduce a cyclone’s intensity. An eyewall replacement cycle, if one occurs, can also change its wind structure.

The Danger Extends Beyond The Coast

Arnab brought substantial rainfall before and during landfall. Visakhapatnam recorded 217 mm, Kalingapatam 147 mm, Tuni 123 mm, Kakinada 118 mm, Machilipatnam 115 mm and Gopalpur 94 mm.

Heavy rain was forecast to continue over northern Andhra Pradesh and southern Odisha until Thursday. Andhra Pradesh and Odisha remained under red alerts on Wednesday, while Chhattisgarh was under an orange alert.

As the system turns northwards, its weakened remnants are expected to carry heavy rain into parts of Telangana, Chhattisgarh, Jharkhand, Bihar, Gangetic West Bengal, eastern Madhya Pradesh and eastern Uttar Pradesh.

Landfall cuts off a cyclone’s access to warm seawater, while friction weakens its circulation. But declining winds do not end the threat: torrential rain, coastal flooding and inland inundation can continue after the storm loses its strongest classification.