Storm surges are caused primarily by the interaction of strong storm winds with the ocean surface, together with the effects of unusually low atmospheric pressure. When a powerful storm passes over or near the ocean, its wind field exerts frictional stress on the sea surface and pushes water horizontally. If the wind has a strong component directed towards a coastline, seawater accumulates there and the local sea surface rises.
The rise is amplified or reduced by the depth and shape of the ocean, the orientation of the coast, the size and movement of the storm and the timing of the event relative to the tide. Low atmospheric pressure also raises the sea surface because the atmosphere exerts less downward force on the ocean. Wind is generally the dominant contributor in major storm surge events, but the pressure effect is physically important and can add to the total. Storm surge is therefore produced by several coupled processes rather than by one isolated cause.
Wind stress is the most important direct mechanism in many storm surges. Air moving rapidly across the sea surface transfers momentum to the water through friction and turbulent interactions. When strong winds persist over a sufficiently large area, they drive water in the direction of the wind. Near a coast, this can produce convergence of water and an elevated sea surface.
The magnitude of this effect depends on wind speed, the size of the wind field, the duration of the wind, the direction of the wind relative to the coast and the depth of the water. Because wind energy increases rapidly with wind speed, exceptionally strong storms can generate disproportionately large ocean responses. The most important wind for surge is not necessarily the maximum wind recorded anywhere in the storm but the sustained wind acting over the appropriate area and in a direction that efficiently pushes water towards the particular coastline.
Atmospheric pressure contributes to storm surge through the inverse barometer effect. Atmospheric pressure is lower near the centre of an intense low pressure system, so the downward force exerted by the atmosphere on the ocean is reduced. The sea surface responds by rising slightly, with a rough approximation that a decrease of one hectopascal in atmospheric pressure can correspond to about one centimetre of sea level rise under simplified conditions.
This relationship is only an approximation because real oceans respond dynamically to wind, waves, coastal geometry, ocean stratification and other processes. Nevertheless, an intense low pressure centre can produce a measurable contribution to coastal sea level. The pressure effect acts over the region covered by the low pressure anomaly and can combine constructively with wind driven water accumulation.
Ocean depth and continental shelf geometry strongly influence the magnitude of storm surge. Shallow water limits the ability of wind driven water to move vertically and can encourage horizontal accumulation towards the coast. A wide continental shelf can therefore support substantial surge development, particularly when strong winds blow across it towards land. Narrow or steep shelves may produce a different response because water can move into deeper ocean more readily. The coastline itself can further amplify water levels.
Funnel shaped bays and estuaries can concentrate incoming water as the available width decreases inland. Local bathymetric features, islands, channels and underwater ridges can redirect currents and alter surge propagation. Consequently, storm surge is highly location dependent. Two coastal sites separated by a relatively short distance can experience markedly different water levels during the same storm because their coastal geometry and exposure differ.
The path, size, intensity and speed of a storm determine how effectively it generates surge at a particular coastline. A storm moving towards a coast can produce strong onshore winds ahead of or alongside its track, while a small change in track may place the strongest winds offshore or parallel to the coast instead. A larger storm can influence a much wider ocean area, while a smaller but extremely intense storm can produce a concentrated response.
A slowly moving storm can maintain wind stress over the same coastal waters for longer, allowing greater water accumulation in some circumstances. The timing of the storm relative to high and low tide is also crucial for flooding. A large surge coinciding with low tide may produce less flooding than a smaller surge coinciding with an unusually high tide. These relationships make storm surge prediction highly dependent on accurate storm track and intensity forecasts.
Waves and nearshore processes do not usually create the underlying storm surge, but they can substantially increase the total water level experienced at the coast. Breaking waves transfer momentum to the water and can produce wave setup, raising the mean water level near the shore. Individual waves can then run up beaches, dunes and structures, temporarily reaching elevations well above the still water level.
During a storm, this wave contribution can determine whether a coastal defence is overtopped even when the surge and tide alone remain below its crest. Wave conditions depend on wind speed, fetch, storm duration, water depth and coastline orientation. Storm surge and wave processes therefore need to be modelled together when estimating actual inundation. Treating surge as the entire coastal water hazard can underestimate the maximum instantaneous water reach during severe storms.
River discharge, rainfall, tides and drainage can further influence the consequences of a storm surge. In an estuary, a high surge can impede the outward movement of river water, causing water to accumulate upstream. Heavy rainfall during the same storm can increase river discharge at exactly the time when the coast is least able to drain. This produces compound flooding in which marine and freshwater sources reinforce one another.
Tidal phase can either amplify or reduce the coastal water level, while local drainage capacity determines how quickly water can leave flooded areas. Urban surfaces such as roads and paved areas can accelerate runoff, while pumps and drainage gates may become ineffective if their outlets are submerged. These processes demonstrate that storm surge is not only an oceanographic problem but can become a coupled coastal and riverine hazard.
Climate, sea level and long term coastal change affect the consequences of storm surges even when they are not direct causes of individual events. A higher mean sea level provides a higher starting point for storm driven water, so the same meteorological surge can produce greater inundation in the future than it did previously.
Loss of dunes, wetlands or sediment barriers can also reduce natural protection, while seawalls and other structures can change the way waves and currents interact with the coast. Storm surge therefore results from atmospheric forcing acting on an ocean whose response depends on geography and whose impacts depend on coastal elevation and protection. Understanding its causes requires considering wind stress, atmospheric pressure, storm characteristics, bathymetry, coastal geometry, tides, waves, river conditions and background sea level together.
