The rise in sea level that a tropical cyclone drives onto a coast, pushed by onshore winds and drawn up by low pressure. Historically the deadliest part of a cyclone, and nowhere on Earth more so than the northern Bay of Bengal.
PhenomenaWhat is storm surge?
Storm surge is the rise in sea level that a tropical cyclone pushes onto a coast — water heaped up by the wind and drawn upward by the low pressure, arriving over a few hours as a moving dome of sea that can flood kilometres inland. It is measured as the height of the water above the normal predicted tide. When the surge coincides with high tide, the combined water level is called the storm tide, and that is what floods the land.
Across the history of tropical cyclones, storm surge — not wind — has been the biggest killer. And no stretch of coast on Earth has lost more people to it than the northern Bay of Bengal.
What makes the sea rise
Three processes combine:
- Wind setup. Persistent onshore winds drag surface water toward the coast. In deep water the excess can sink and flow back underneath; over a shallow continental shelf it has nowhere to go and piles up against the shore. This is usually the largest part of the surge, and it is why shallow coasts suffer far more than steep ones.
- The pressure effect. The sea surface rises by roughly 1 cm for every hectopascal the air pressure falls — the ocean behaving like a giant inverted barometer. A strong cyclone with a central pressure 60–70 hPa below normal lifts the sea beneath it by 60–70 cm before the wind has done anything at all.
- Wave setup. Large breaking waves push extra water shoreward, raising the mean level further, and the waves themselves ride on top of the surge.
Then comes the tide. The same surge that is manageable at low tide can be catastrophic at high tide, so the timing of landfall relative to the tide is one of the first things forecasters check.
Where along the coast matters too. In the northern hemisphere the highest surge usually occurs to the right of the cyclone’s track at landfall, where the winds blow onshore; to the left, winds blow offshore and the sea can briefly retreat before rushing back. The peak often arrives near where the eyewall crosses the coast.
Why the Bay of Bengal is the world’s surge hotspot
- A shallow shelf. The continental shelf off Bangladesh and West Bengal is broad and shallow, so wind setup builds unusually high.
- A funnel. The Bay narrows toward the north, concentrating water into its head like a funnel.
- Low, flat deltas. Much of the Ganges–Brahmaputra–Meghna delta and the Sundarbans lies only a few metres above sea level, cut by river mouths and channels that carry the surge far inland, with low islands (chars) directly in its path.
- Large tides. The tidal range at the head of the Bay is high, so a surge on a spring tide adds to an already high sea.
- People on the coast. Millions live on the delta and its islands, within a few kilometres of the sea.
The record that follows from those facts is grim:
| Year | Cyclone | Where | Deaths |
|---|---|---|---|
| 1970 | Bhola cyclone | East Pakistan, now Bangladesh | an estimated 300,000–500,000 — the deadliest tropical cyclone ever recorded |
| 1991 | Bangladesh cyclone | Chittagong coast | about 138,000 |
| 1999 | Odisha super cyclone | Odisha, India | around 10,000 |
| 2007 | Sidr | south-west Bangladesh | more than 3,000 |
| 2008 | Nargis | Irrawaddy delta, Myanmar | about 138,000 dead or missing |
| 2020 | Amphan | Sundarbans, India and Bangladesh | around 130 |
Most of those deaths were drowning in surge water. Accounts of 1970 describe a surge of up to about 10 metres sweeping over offshore islands at night; the 1991 cyclone drove a surge of around 6 metres onto the Chittagong coast.
How the death toll was cut
The falling numbers for Bangladesh in that table are not luck. After 1970, Bangladesh built the Cyclone Preparedness Programme (1972) — a warning system relayed village by village by tens of thousands of trained volunteers with megaphones and flags — along with thousands of raised concrete cyclone shelters, coastal embankments and mangrove belts. When Sidr struck in 2007 with a surge of several metres, the death toll was a small fraction of 1970’s.
Odisha followed the same path after 1999, creating India’s first state disaster management authority and a network of shelters and evacuation plans. When Phailin (2013) and Fani (2019) struck with comparable strength, around a million people were moved out of harm’s way each time and deaths were counted in dozens rather than thousands.
The lesson is consistent across every one of these storms: surge kills people who are still in low-lying homes when the water arrives. Evacuation works.
The Arabian Sea coast and Pakistan
Arabian Sea cyclones generally produce smaller surges along most of India’s west coast, where the continental shelf drops away more steeply. But the flat, low-lying coasts of Kutch in Gujarat and Sindh in Pakistan, including the Indus delta and the shoreline around Karachi, are exposed. Ahead of Cyclone Biparjoy in June 2023, more than 100,000 people were evacuated in Gujarat and tens of thousands more in Sindh.
Surge and a rising sea
Sea-level rise lifts the baseline that every surge starts from, so the same storm pushes water further inland than it would have a century ago. On the Bangladesh coast and in the Sundarbans, where the land is also subsiding in places, this is one of the most direct ways a warming climate raises cyclone risk — and one reason saline inundation of farmland after a surge now lasts longer.
Frequently asked questions
What is the difference between storm surge and a tsunami? A storm surge is driven by the wind and low pressure of a storm and arrives over hours as the cyclone approaches. A tsunami is generated when an earthquake or landslide displaces the seabed; it crosses oceans at the speed of a jet aircraft and can strike on a calm, sunny day with no storm in sight.
How high can storm surge get in the Bay of Bengal? In severe cyclones, a surge of a few metres is common at the worst-hit point, and historical accounts of the most extreme events, such as 1970, describe up to about 10 metres. Add a high tide and waves on top, and water can reach far higher than the surge figure alone suggests.
Why does the water rise before the worst wind arrives? Onshore winds start piling water against the coast hours before the eye makes landfall, so low roads and bridges can flood — cutting off escape routes — while the wind is still building. This is why evacuation orders come early, and why waiting to see how bad it gets is so dangerous.
Is it safe to stay upstairs in my house? In surge zones, authorities advise moving to a designated cyclone shelter or inland. Surge water carries debris and waves that can destroy the ground floor of an ordinary house. Follow local evacuation orders.
Where can I check conditions for coastal cities? Mausam Online shows live wind, rain and the hourly forecast for coastal cities including Kolkata, Chittagong, Khulna, Barisal, Puri, Paradip and Karachi. Surge warnings themselves come from IMD, the Bangladesh Meteorological Department and the Pakistan Meteorological Department.