Why does the sea rise
when a typhoon comes?
Typhoon coverage usually focuses on strong winds and heavy rain. But there's another effect: the sea itself swells up. Coastal roads can be swallowed by waves with barely any warning. This is called a storm surge.
A large typhoon is approaching a coastal town at night. The wind isn't especially strong yet, and the rain hasn't set in properly. But beyond the seawall, the water is clearly higher than usual.
Soon waves start topping the seawall, and seawater flows onto the road. It looks sudden, but a quiet change had been building the whole time.
The truth behind this phenomenon: the typhoon itself is lifting the sea and pushing it inland.
Only two things make the sea rise
The typhoon's centre is a low-pressure zone. As the air pressing down from above weakens, the sea rises to fill the gap (the "suction effect").
The powerful winds circling a typhoon keep pushing seawater toward the coast (the "wind setup effect"). This is especially strong at the head of a bay.
Let's look at each one in turn.
Reason 1: falling pressure lifts the sea
Picture sipping a little water from a plastic bottle through a straw. Thin out the air inside the straw (lower its pressure), and the outside air pressure pushes the water up the straw.
A typhoon's centre does something similar over the ocean. Because the pressure there is lower than its surroundings, the air pressing down on the sea surface weakens, and the sea near the centre rises into a dome. This is called the suction effect.
For every 1 hectopascal (hPa) drop in pressure, the sea is said to rise by roughly 1 cm. A strong typhoon's central pressure can easily drop below 950 hPa, which is a difference of 50–90 cm or more from normal levels (around 1013 hPa). We work through the exact calculation in the expandable section at the end.
Reason 2: wind pushes seawater onto land
You may have seen how, if you stand at the edge of a pool and fan the water steadily in one direction with your hand, the water gradually piles up on that side. A typhoon's strong winds do the same thing on the scale of an entire sea.
When wind blows in the same direction for a long time, seawater is pushed along with it, and where the coast is a dead end or a bay narrows inland, the water has nowhere to go and piles up. This is called the wind setup effect. It's said to be strongest at the head of a bay, and when the wind blows straight toward the back of the bay.
Japan has seen cities at the head of a bay suffer devastating damage from record storm surges involving this wind setup effect. The 1959 Ise Bay Typhoon is recorded to have produced a surge of over 3 metres near Nagoya Port.
A storm surge adds on top of the sea's normal tidal rise and fall (high tide and low tide). If the peak of the typhoon's surge happens to coincide with high tide, the two add together for an even higher water level. It's recommended to check the time of high tide alongside typhoon updates.
So what should you do?
- When a typhoon approaches, check whether your home is in a "storm surge flood zone"You can check this on your municipality's hazard map. Take extra care near the sea or in low-lying areas.
- If danger is expected, evacuate before the water arrives, while it's still light outEvacuating after dark, or once the water has started rising, is dangerous — roads become hard to see.
- If evacuating by car, never drive into a flooded roadEven a water depth of just a few tens of centimetres can cause a car to float or stall. Don't push forward — choose another route or higher ground.
If water is already closing in, or a road has begun to flood, don't try to travel far — move to an upper floor of a nearby sturdy building instead (vertical evacuation). Walking through water is dangerous even if it looks shallow — it can sweep your feet out from under you.
A storm surge doesn't only occur when the typhoon is closest — water levels can stay high for a while after it has passed. Don't treat weaker winds as a signal that water levels have dropped. Check emergency services or your municipality's evacuation information, and stay alert until the warning is lifted.
Summary
A typhoon's storm surge comes from two separate mechanisms acting together: ① falling pressure lifts the sea (the suction effect), and ② strong wind pushes seawater onto land (the wind setup effect). The essence of this phenomenon is that the typhoon itself is moving the sea itself.
It isn't just the wind — the sea itself rises up.
That's storm surge: a second kind of flood damage.
The mechanism behind why pressure drops near a typhoon's centre is also covered in our article on the eye of a typhoon.
- Fill a cup with water and put a thin straw in it
- Cover the top of the straw with your finger, sip a little, then lift it without releasing your finger
Thinning the air inside the straw (lowering its pressure) lets the outside air pressure push the water up inside the straw. What happens at a typhoon's centre is the same kind of change, just on a vastly different scale: the sea rises by exactly as much as the pressure has dropped.
Want to go deeper? Terms, formulas, and where this fits in the curriculumLabels show whether each part is middle-school level or university-level
- MSCovered in middle-school science
- HSCovered in high-school "Earth Science Basics"
- HS+Covered in high-school "Earth Science," or treated as advanced/sidebar content in textbooks
- UnivNot covered in high school — university-level content (meteorology, physical oceanography)
- ResearchNot yet settled even at university level — an active research question
MSTerms: vocabulary around storm surge
- Storm surge: an abnormal rise in sea level caused by a typhoon or a developed low-pressure system.
- Suction effect: the effect by which falling pressure lifts the sea surface.
- Wind setup effect: the effect by which strong wind piles up seawater, raising sea level.
- Storm surge flood zone: an area designated by local government as at risk of storm-surge flooding.
HSChecking with a formula: how much does the sea rise when pressure drops?
The suction effect can reportedly be estimated with a simple proportional formula.
| In symbols | Δh = (P₀ − P) ÷ (ρ × g) |
| In words | height the sea rises = pressure drop ÷ (density of seawater × gravitational acceleration) |
| Where it comes from | It comes from the balance of pressure within water (hydrostatic pressure). The sea rises by just enough that the pressure at the same depth, inside and outside the typhoon, stays equal once the air above has become lighter. |
| Pressure drop (1 hPa converted to Pa) | 1 × 100 = 100 |
| Seawater density × gravity (Pa per metre) | 1025 × 9.8 = 10045 |
| Sea-level rise per 1 hPa (m) | 100 ÷ 10045 ≒ 0.00995 |
In other words, the sea rises by roughly 1 cm for every 1 hPa drop in pressure. The "1 cm" rule of thumb in ① above comes from this calculation.
Sea-level rise (cm) ≒ 1 × (1013 − typhoon centre pressure [hPa])
| Sea-level rise | estimated height the sea rises from the suction effect [cm] |
| 1013 | a typical average sea-level pressure in normal conditions [hPa] |
| Typhoon centre pressure | the pressure observed at the typhoon's centre [hPa] |
The relationship is said to be roughly proportional: the sea rises about 1 cm for every 1 hPa drop in pressure.
| Fairly strong typhoon (centre pressure 975 hPa) | 1013 − 975 = 38 → rise ≒ 38 cm |
| Strong typhoon (centre pressure 955 hPa) | 1013 − 955 = 58 → rise ≒ 58 cm |
| Very strong typhoon (centre pressure 920 hPa) | 1013 − 920 = 93 → rise ≒ 93 cm |
※ The central-pressure figures here are illustrative examples of typhoon strength. Actual values vary by typhoon.
| If the seawall is assumed to be 80 cm high | 93 − 80 = 13 → water overtops the seawall by 13 cm |
For a very strong typhoon, the suction effect alone can raise the sea by about the height of an adult's knee (roughly 1 m), by this calculation. Add the wind setup effect and high tide on top of that, and the level rises further. The surge of over 3 m recorded in the Ise Bay Typhoon can't be explained by the suction effect alone — the amplification from the wind setup effect, shaped by the bay's geometry, is thought to have played a major role.
The key point is that pressure calculations alone can't fully explain an actual storm surge's height. The effects of wind and terrain can grow far larger than a simple proportional formula can capture.
HS+Why is the head of a bay especially dangerous?
The wind setup effect is said to grow larger with the fetch (the distance over which wind pushes the water) and with shallower water depth. In terrain like the head of a bay, which narrows and shallows further inland, pushed-up seawater has even less room to escape, amplifying the rise in water level. Regions with deep, narrow bays — such as Ise Bay, Tokyo Bay, and Ariake Sea — are said to be singled out for storm-surge warnings precisely because of this terrain effect.
UnivWhat it takes to forecast a storm surge
The storm-surge forecasts used in actual disaster-prevention information aren't calculated with a simple proportional formula. They come from numerical simulations that incorporate pressure distribution, wind fields, and seafloor topography into the equations of motion for the sea surface (shallow-water equations). This is university-level content in meteorology and physical oceanography, and it comes with a real difficulty: forecast errors in a typhoon's track and strength carry straight through into storm-surge forecast errors.
📖 Detailed explanation of the mechanism: Japan Meteorological Agency, "Storm Surges Associated with Typhoons"
ResearchWhat's still unclear
- Accurately predicting a typhoon's "rapid intensification" just before it happens is still said to be difficult. Typhoons that suddenly strengthen right before landfall are harder to forecast than their track, and this feeds directly into storm-surge forecast error.
- Research into precisely modelling each bay's amplification effect is still ongoing. Seafloor topography and seawall shapes are complex and vary by location, so predicting exactly how much amplification any given area will see requires an accumulation of region-by-region research.
- How climate change will affect future storm-surge risk is still debated among researchers. Research continues into how trends in typhoon intensity, combined with rising average sea levels, will shape future storm-surge risk.
Where this fits in the curriculum, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science — air pressure, atmospheric pressure | Basic idea behind the suction effect |
| HS | Earth Science Basics — pressure and weather | The full calculation in ①②③ above |
| HS+ | Earth Science — ocean and atmosphere motion | Wind setup effect, amplification from bay terrain |
| Univ | Meteorology / physical oceanography | Numerical storm-surge forecasting based on shallow-water equations |
| Research | Typhoon research / climate science (ongoing) | Forecasting rapid intensification, per-bay amplification models, future risk assessment |
| — | Disaster prevention / safety education | Hazard maps, overlap with high tide, vertical evacuation, timing of evacuation |
- Explanatory materials from the Japan Meteorological Agency (気象庁) on how storm surges form and on past storm-surge disasters.
- Publicly available materials from Japan's Ministry of Land, Infrastructure, Transport and Tourism (国土交通省) and local governments on storm-surge flood zones and hazard maps.
- General descriptions of the suction effect, wind setup effect, and shallow-water equations found in meteorology and physical oceanography textbooks.
- Disaster records from the Japan Meteorological Agency and local governments concerning the 1959 Ise Bay Typhoon (伊勢湾台風).
※ Pressure and water-level figures are approximations and assumptions meant to illustrate the mechanism. The actual height of a storm surge varies greatly with a typhoon's strength, track, local terrain, and tidal timing.
※ This article is a general-audience science explainer. For actual evacuation decisions, follow the information and instructions issued by the Japan Meteorological Agency and local authorities. The figures given here are approximations and assumptions meant to illustrate the mechanism.