Cyclone Arnab Explained: El Niño, Rapid Intensification and Coastal Risk

Verified event: Arnab crossed the Andhra Pradesh–Yanam coast on 28 October 2026 as a very severe cyclonic storm and subsequently weakened. IMD information carried by public broadcaster Akashvani placed the crossing near Narsapur, southwest of Yanam, with sustained winds of 125–135 km/h and gusts reaching 150 km/h.

Geographical setting: The Bay of Bengal lies east of India. Yanam is part of the Union Territory of Puducherry, although it is geographically situated on the Andhra coast. Bapatla, Narsapur, Kakinada and Visakhapatnam are useful locations to identify on a map.

Study relevance: The event connects physical geography with risk reduction: warm seas and atmospheric conditions affect the storm, while coastal elevation, settlement patterns and preparedness affect the damage. Reports of interrupted transport and electricity supply also show how impacts spread beyond the immediate landfall area.

Organised low pressure: A tropical cyclone is a rotating storm system that develops over tropical or subtropical waters, with thunderstorms organised around a low-pressure centre. Cyclones, hurricanes and typhoons are regional names for the same broad type of system.

Heat engine: Evaporation transfers moisture and energy from the ocean to the atmosphere. As moist air rises and water vapour condenses into clouds, latent heat is released. This supports rising air and the circulation that draws in more moist air.

Structure: In a mature strong cyclone, the eye can be relatively calm, while the surrounding eyewall contains intense convection, rain and powerful winds. The wider spiral rainbands can affect areas far from the centre.

Ocean warmth: Sea-surface temperatures around 26.5°C or higher over a sufficiently deep warm layer are a useful conventional guideline. They are not a guarantee that a cyclone will form.

Moisture and instability: Moist air and an atmosphere that allows vigorous rising motion support thunderstorms. A pre-existing disturbance helps organise the inflow.

Low vertical wind shear: Wind shear is a change in wind speed or direction with height. Strong shear can displace thunderstorms from the circulation centre; relatively low shear helps the storm remain organised.

Coriolis effect: Earth’s rotation deflects moving air. Tropical cyclones rarely form very close to the equator because this effect is weak there. Their circulation is generally anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Meaning: Rapid intensification concerns a sharp increase in a cyclone’s maximum sustained wind, not its speed of travel. A widely used NOAA benchmark is an increase of at least 30 knots, about 55.6 km/h, in 24 hours.

Warm layer: A cyclone mixes the upper ocean and can bring cooler water to the surface. If warm water extends deeper, this cooling can be weaker, allowing continued heat and moisture supply. Ocean heat content therefore adds information beyond a surface-temperature reading.

Forecast challenge: Local moisture, wind shear, ocean structure and changes within the eyewall interact. Small errors can produce large differences in intensity forecasts, particularly near landfall.

Evidence boundary: Reporting describes Arnab as strengthening rapidly. The precise numerical increase over a particular 24-hour period should be established from an official time series; reaching a very severe category does not, by itself, prove that benchmark was crossed.

Pacific origin: El Niño is the warm phase of the El Niño–Southern Oscillation, or ENSO, a coupled ocean–atmosphere pattern centred on the equatorial Pacific. It is not simply another name for warm water in the Bay of Bengal.

Atmospheric connection: Pacific warming changes the distribution of tropical thunderstorms and the Walker circulation, an east–west overturning circulation. These changes can affect winds, pressure, moisture and wind shear in distant regions. Such distant links are called teleconnections.

Regional influence: Indian Express cites the IMD chief’s explanation that El Niño conditions can favour lower-latitude cyclone formation and tracks towards Andhra Pradesh and Tamil Nadu. Research published in IMD’s MAUSAM journal also discusses a statistical association between ENSO and the latitude of post-monsoon cyclone tracks.

Necessary qualification: A statistical tendency is not a fixed route for every storm. El Niño does not guarantee a cyclone, a particular landfall or rapid intensification. Local ocean conditions, the Indian Ocean Dipole, shorter weather fluctuations and the storm’s internal structure also matter.

Current outlook: WMO’s October seasonal updates describe a strengthening El Niño expected to peak towards the end of 2026, with effects extending into 2027.

Forecast versus observation: A forecast Pacific temperature anomaly is a projected departure from a reference average in a specified ocean region. It is not the observed temperature beneath Arnab and cannot be directly used as the cyclone’s fuel measurement.

Attribution limit: WMO cautions that El Niño’s strength alone does not determine local impacts. Establishing how much El Niño or human-caused warming changed Arnab’s intensity would require dedicated observations and attribution analysis. The defensible conclusion is that large-scale climate conditions can modify the environment in which the storm develops.

Steering flow: A cyclone’s movement depends substantially on surrounding winds through a deep layer of the atmosphere. High-pressure ridges and troughs help determine whether it travels westward, turns northward or recurves.

Spin versus movement: Anticlockwise rotation in the Northern Hemisphere describes the circulation around the centre. It does not mean that all Northern Hemisphere cyclones travel westward.

Post-monsoon setting: As the summer monsoon circulation retreats, the distribution of low pressure and wind shear changes. The Bay can remain warm, providing a favourable setting for some post-monsoon storms.

Operational lesson: Forecast tracks are updated as observations improve. Coastal planning must consider the forecast uncertainty area and hazards outside the centre line.

Wind criterion: IMD classification uses maximum sustained surface wind, rather than a brief gust or the storm’s forward speed. The operational classes in knots are:

• Depression: 17–27 knots.

• Deep depression: 28–33 knots.

• Cyclonic storm: 34–47 knots.

• Severe cyclonic storm: 48–63 knots.

• Very severe cyclonic storm: 64–89 knots.

• Extremely severe cyclonic storm: 90–119 knots.

• Super cyclonic storm: 120 knots or more.

Important correction: The clipping’s 119–221 km/h range for a very severe cyclonic storm combines categories that are separate in the current system. IMD’s present classification distinguishes very severe from extremely severe storms; the latter covers approximately 167–221 km/h. Knots avoid minor differences in rounded km/h boundaries across official tables.

Regional scales: The North Indian Ocean classification should not be mechanically replaced with the Atlantic Saffir–Simpson categories.

Landfall meaning: Landfall occurs when the storm’s surface centre crosses the coastline. Dangerous winds and rain can arrive before this and continue afterwards.

Weakening mechanism: Moving inland cuts off much of the ocean’s heat and moisture supply. Surface friction and terrain also alter the circulation. Weakening is not instantaneous, and remnants can still cause severe rainfall.

Storm surge: This is an abnormal rise of coastal sea level generated mainly by storm winds pushing water shoreward. Storm tide combines surge with the astronomical tide. Waves add further danger. A storm surge is different from a tsunami.

Compound flooding: Heavy rain, swollen rivers, restricted drainage and elevated coastal water levels can coincide. Consequently, a lower wind category does not automatically mean a low flood risk.

Coastal geometry: Shallow continental shelves allow water to accumulate under strong onshore winds. Funnel-shaped stretches, especially towards the head of the Bay, can amplify surge; the effect varies along the coast.

Exposure: Low-lying deltas, dense settlements, fishing communities, farms and critical infrastructure put people and assets in the hazard zone.

Risk distinction: Hazard is the physical threat; exposure is what lies in its path; vulnerability is the susceptibility to harm. Shelters, evacuation routes and resilient construction can reduce disaster losses even when the cyclone itself cannot be prevented.

Regional naming: RSMC New Delhi assigns names from the list agreed under the WMO/ESCAP Panel on Tropical Cyclones for the Bay of Bengal and Arabian Sea. The current list draws from 13 member countries; Arnab was contributed by Bangladesh.

Naming threshold: Names are assigned when a system reaches cyclonic-storm strength. The contributing country does not predict the landfall location or own the cyclone.

Warning stages: IMD’s framework includes pre-cyclone watch around 72 hours before expected adverse weather, cyclone alert at least 48 hours before it, cyclone warning at least 24 hours before it, and a post-landfall outlook at least 12 hours before expected landfall.

Institutional coordination: IMD provides meteorological forecasts. State and district authorities organise local preparedness and evacuation; NDRF and SDRF support response. The Cabinet Secretary-chaired National Crisis Management Committee reviewed Arnab preparedness with affected states and central agencies.

Preparedness implication: Warnings should identify likely impacts and actionable local instructions, including for inland districts that may face heavy rain after the coastal crossing.