Ocean, current, sea ice and storm surge models
What ocean models forecast besides waves: currents, sea temperature, sea level, surge and ice, and how far to trust them.
What these models forecast
| Type | What it forecasts | Where a sailor meets it |
|---|---|---|
| Ocean circulation model | Currents, sea temperature, salinity and eddies, from the surface down | Gulf Stream, Agulhas, Kuroshio and other big currents; eddies on an ocean passage; sea temperature for fog and squall risk |
| Tidal model | Tidal heights and streams, from the pull of the Moon and the Sun and the shape of the coast and seabed | Tidal streams in the Channel, the Solent, the Danish straits, Pentland Firth, Puget Sound |
| Storm surge model | Sea level raised or lowered by wind and low pressure, added to the tide | Harbour entrances, drying moorings, bridges, shallow bars |
| Sea ice model | Ice cover, concentration, thickness and drift | The northern Baltic in winter and spring, high-latitude passages, the Great Lakes |
| Coupled model | Atmosphere, ocean, waves and ice run together and exchange heat, moisture and momentum | Some global systems and tropical cyclone models, where warm ocean water feeds the storm |
Not every model in the catalog covers these. Check what the model page lists.
How they differ from wave models
A wave model tracks the energy of the sea surface: height, period and direction. An ocean model tracks the water itself, which moves as a current. They are linked but not the same. Waves are pushed by wind and changed by the current they travel through. The current in turn is driven by wind and tide. Many wave forecasts do not include the current, so a strong stream against the wind can make the real sea much steeper than the forecast. See Waves: height, period and swell.
Where their wind comes from
An ocean or surge model does not make its own weather. It is run with wind, pressure and heat fluxes from an atmospheric model. If the wind is late or too weak, the surge and the wind-driven current will be too. Surge in particular reacts to the track of a low. Runs on slightly different tracks can give clearly different water levels, so an ensemble of surge forecasts is the sensible tool for the days ahead. The atmosphere comes first: see Types of weather models.
Tide and surge are not the same thing
- Tide is regular and can be calculated years ahead. Tables are made from long records at each port.
- Surge is the difference between the real sea level and the predicted tide. Low pressure lifts the sea slightly (about 1 cm for each hPa below normal), and wind pushes water towards a coast, especially in shallow or funnel-shaped seas. A surge of half a metre is not unusual in a gale, and much larger values occur in the strongest storms.
- Timing matters. A surge on top of high water is a problem. The same surge at low water may hardly be noticed.
Update cycles and limits
- Cycles. Ocean and surge models usually run once to a few times a day. Surge models are often run again whenever the atmospheric model that drives them has a new run. Global ocean systems are commonly daily, and sea ice is typically updated once a day.
- Resolution. Global ocean models resolve eddies of tens of kilometres at best. Coastal models are finer, but narrow channels, harbours and river mouths are still smaller than the grid.
- Observations are sparse. Satellites see the sea surface, and floats and buoys sample the depths, but far fewer measurements exist than for the atmosphere. Currents are more uncertain than winds.
- Seabed and coast data. Tidal streams depend on the shape of the seabed. Errors in depth or coastline become errors in the stream, especially in shallow water and narrow passes.
- Ice is hard. Ice edges move with wind and current, and thin or new ice is hard to see. For any ice passage use the charts from the national ice services, made from satellite pictures and observers.
Uncertainty is larger than a smooth map suggests. For how to compare several sources, see Using several models together.