1. Storm surge is the extra sea level a storm adds on top of the tide
NOAA, the U.S. ocean and weather agency, defines storm surge as the rise in sea level caused by a storm, measured as the height of the water above the normal predicted tide[1]. The ordinary tide, which the Moon and Sun cause (the subject of another article here), is not counted as part of the surge. The surge is the extra that the storm piles on top of it.
Two things push the sea up. One is the storm's low air pressure. The other is its wind. Japan's weather agency and its land ministry both describe these as the two main mechanisms[3][4], and NOAA says the surge is caused primarily by a storm's winds pushing water onshore[1]. How high the water ends up on a given coast then depends on the shape of the land and sea floor, and on whether the tide is high or low when the storm arrives. The next sections take these one at a time.
2. Low air pressure lifts the sea, but only by about one centimeter for each hectopascal
Air has weight, and it presses on the sea surface. In the middle of a typhoon or hurricane the pressure is lower than around it, so the surface there is pressed down less and bulges upward, as if the middle of the storm were sucking the water up. The usual rule of thumb is that for each 1 hectopascal (hPa, the unit weather maps use) that the air pressure falls below normal, the sea rises by about 1 centimeter[3][4]. Normal pressure is around 1,013 hPa. A typhoon with a center at 950 hPa would therefore lift the sea by roughly 50 to 60 centimeters (about 20 to 24 inches) near its middle (the Japan Meteorological Agency's own example starts from 1,000 hPa and gets about 50)[3], and one at 910 hPa by roughly a meter[4].
That is a real rise, but a modest one. A surge that tops 3 meters (10 feet) cannot come from pressure alone. Something stronger has to be at work.
3. Wind does most of the lifting by shoving water toward the coast
When strong wind blows from the open sea toward the land, it drags the surface water along and stacks it against the shore. In the Japan Meteorological Agency's simple rule, the rise this causes grows with the square of the wind speed, so doubling the wind speed makes the rise four times bigger[3]. Real surges also depend on the shape of the bay and the sea floor, as the next paragraph shows. Still, the rule shows why a very strong storm can be so much worse than a merely strong one.
The shape of the sea matters just as much. In shallow water that slopes gently away from the coast, or in a bay that opens toward the wind, the land and sea floor make the rise larger[3]. Japan's land ministry adds that the rise is bigger when the wind is faster, when the bay is longer, and when the water in it is shallower[4]. The U.S. National Hurricane Center's list of factors that shape a surge includes the storm's wind speed, its size and speed of travel, the angle at which it approaches the coast, the width and slope of the sea floor, and the shape of the coastline[2]. A simple way to picture the bay effect is the tray from the start: wind that blows along a long, shallow tray piles up much more water at the far end than wind that blows across a short, deep one.
High waves ride on top of the raised sea. When the water is already high, waves reach places they normally never touch, which makes damage worse[3].
4. A surge on top of high tide is worse, but a surge at low tide is still dangerous
NOAA calls the total water level during a storm the storm tide: the storm surge plus the normal tide[1]. If the surge arrives at high tide, the water gets higher. NOAA adds that tides are strongest around new moon and full moon, so the highest storm tides are often seen when a storm arrives at one of those times[1]. Japan's weather agency makes the same point, and also warns that a storm arriving at low tide is "by no means" a reason to relax. People should watch the weather information as the storm approaches, not only the tide timetable[3].
5. In 1959, Ise Bay in Japan showed all of this at once, even though it was not high tide
Japan lies in the path of many typhoons, and the deadliest storm surge in its government's list of major cases came on September 26, 1959. That evening a very strong typhoon (named the Isewan Typhoon, after the bay it hit) came ashore near the southern tip of the Kii Peninsula, in the middle of Honshu, Japan's main island. It then moved north, west of Ise Bay. This bay is a long, fairly shallow inlet that opens to the south, and it has the city of Nagoya at its head. As the storm passed to the west, the winds over the bay blew from the south, straight up its length[5].
The measurements from Nagoya's weather office fit the picture from sections 2 and 3. Nagoya's lowest air pressure was 958.2 hPa. Its fastest 10-minute-average wind was 37.0 m/s (about 83 mph), and at Irago, at the mouth of the bay, it reached 45.4 m/s (about 100 mph)[5]. By the pressure rule of thumb, a drop of about 55 hPa below 1,013 would lift the sea only about half a meter. The pressure alone cannot account for a rise of several meters, so wind piling water up the bay probably played a large part. That is my own reading of the numbers, and no source I read splits the rise into parts. The Nagoya weather office says only that the wind effect and the pressure effect, among other things, added up[5].
Near the mouth of the bay the water stood just under 2 meters (6.5 feet) high. At the head of the bay, in Nagoya's port, it reached 3.89 meters (12.8 feet) above Tokyo Peil, Japan's standard sea-level reference, at 9:35 in the evening, the highest ever recorded there. The Nagoya office notes that this happened although the tide was not high[5]. The land ministry says the surge raised the sea level of the whole bay by about 2 meters for nearly an hour[6]. The gap between the bay mouth and the bay head is what a long, shallow, south-facing bay would be expected to produce, though these sources do not break the rise down into separate wind and pressure parts.
Japanese government sources report that roughly 5,000 people were killed or missing across the country (the exact total differs by source, depending on what is counted), and the low-lying land behind the bay head was so flooded that it took until late December to pump out the water[6].
6. In 2018, a typhoon again pushed Osaka's sea level past its old record
Typhoon No. 21 of 2018 came ashore in southern Tokushima Prefecture (on the island of Shikoku) on September 4 at "very strong" intensity, and again near Kobe about two hours later[7]. Japan's weather agency reported that in Osaka the highest sea level reached 329 centimeters (10.8 feet) and in Kobe 233 centimeters (7.6 feet), values that beat the previous records at some stations. The agency also lists flooding of one of Kansai International Airport's runways among the effects[7]. The preliminary report I read does not separate wind from pressure or say what the tide was doing, so this article does not either. What it shows is that a record from 1959 can still fall when the storm's path and the shape of the coast line up in a bad way.
Try a small storm surge yourself
- Make a tiny surge. Put a shallow tray of water on a table and blow gently along its length through a straw. Watch where the water piles up. Try a longer tray, or less water, and see if the pile gets bigger or smaller. Wipe up spills so nobody slips.
- Do the pressure arithmetic. Look up the lowest pressure of a recent hurricane or typhoon on a weather service page. Subtract it from 1,013 hPa and use the 1 cm per hPa rule. How much of a 3-meter surge could that explain?
- Read the original. The National Hurricane Center's storm surge overview has animations that compare a small storm with a large one, and a steep sea floor with a gently sloping one.