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Considerations when using output from ECWAM
Interaction of wind-sea and swell
Use of the mean wave height and direction is the simplest method of describing the forecast wave regime in a given area and it is easy to be beguiled into just using this output for forecasts to customers. However, the mean wave direction and height is made up of contributions from wind-sea and swell with different wave periods and they interact in a complex manner. It is important to investigate the forecast wind-sea and swell separately to give an understanding of likely sea conditions in an area (e.g. for a ship requiring a particularly smooth passage) or at a location (e.g. an oil rig).
When wind-sea and swell move in similar directions the wave heights can give information on the likely sea state as one is superimposed on the other, particularly where both have a significant and comparable wave height. On occasion the swell and wind-sea may be moving in opposite directions (an opposing sea) and wave heights give information on the likely rougher sea state to be expected. Often the wind-sea and swell are at right-angles (a cross sea). Where the wind-sea and swell heights are similar the sea can be very disturbed and difficult for shipping. An illustration is given in Figs2A.3.2-1(a to e)
Fig2A.3.2-1(a): ecChart of mean wave direction (wave height is indicated by the length of the arrow). On ecCharts, wave height may be shown by use of the probe tool or more graphically by superimposing mean wave heights). This chart gives an overview of wave conditions. Northwesterly waves (i.e. moving towards the northwest) are indicated near point A. Easterly waves i.e. moving towards the east) are indicated near point C. However, it is important to investigate the contributions to the mean wave directions and heights from inspection of the wind-sea and swell at this time.
Fig2A.3.2-1(b): The forecast wind-sea has developed in response to the forecast winds around a depression in mid-Atlantic. Waves move northwestwards near point A and southeastwards near point B. The length of the arrows near points A & B suggest wave heights are around 3m (wave heights are also available as charts, not shown here). Near point C wind-sea waves are relatively small and move towards the east-northeast.
Fig2A.3.2-1(c): The forecast total swell has been developed in response to earlier weather systems elsewhere and has propagated across the Atlantic. Swell is moving northwards near point A and southeastwards near point C with arrow length suggesting wave heights of around 2m (wave heights are also available as charts, not shown here). Near point B swell waves are relatively small and move towards the southeast.
Fig2A.3.2-1(d): The forecast wind-sea (blue) and swell (black) shown on a single chart. To the north of point B the wind-sea and swell waves have a similar direction of travel; to the east of point B wind-sea dominates with only weak swell contribution but at a large angle. Near points A and C the wind-sea and swell waves differ widely in direction but with similar heights (a cross sea).
Fig2A.3.2-1(e):The forecast mean wave directions derived from the wind-sea and mean swell Fig2A.3.2-1(a) superimposed on Fig2A.3.2-1(d). This illustrates the important additional information that is gained from consideration of the wind-sea and mean swell forecasts together. The mean wave directions in Fig2A.3.2-1(a) give no indication of that a sea passage to the west of Portugal is likely to be through confused rough seas.
Waves and swell with a long period
Large swell waves with a long period breaking on a beach slope tend to have a large swash with water washed well up the beach often followed by strong backwash. This is often unexpected, takes people by surprise and can cause damage and casualties. Users should note when large waves with a long period are forecast to run onto an exposed coast. Extreme forecast index products for waves can alert users to the potential problem.
Example 1: Large swell off South Africa
Fig2A.3.2-2: Wave energy forecast VT 09UTC 16 Sep 2023, DT 12UTC 15 Sep 2023. The chart shows extreme wave energy flux (~1300KW/m) being driven towards the exposed southern coast of South Africa.
Fig2A.3.2-3: Wave energy forecast VT 09UTC 16 Sep 2023, DT 12UTC 15 Sep 2023. The chart shows the extreme forecast index (EFI) for significant wave height at 0.9 to 1.0 alerting to the significant nature of the ocean swell.
Fig2A.3.2-4: Wavegram DT 00UTC 16 Sep 2023. Significant wave heights were forecast to exceed 9 m with wave period above 15 s. These values were also observed at a nearby buoy.
Very large waves and swell were induced by a deep depression in southern Atlantic associated with very strong winds. The waves became larger as they approached the coasts and coincided with spring tides. Significant wave heights exceeded 8 m in many places and suggests maximum waves were significantly higher. There was considerable coastal damage and some loss of life.
Example 2: Large swell off Ecuador and Peru
Giant swell (above 6 meters in height) struck the coasts of Ecuador and Peru causing significant damage to boats and infrastructure. The swell originated from a storm lying off California several days earlier and was sufficiently powerful to propagate some 5000km while while maintaining a significant amount of energy. Long period swell was mainly the reason for the damage caused.
Fig2A.3.2-5: Snapshot of propagation of long period (21-25sec) swell from Californian coastal area towards Ecuador and Peru coasts. The significant wave heights are about 1m to 2m but amplification can be expected in shelving coastal waters where swell of some 6m were observed causing destruction.
Sea-surface currents
Cy50, introduced in autumn 2026, incorporates ocean surface currents provided by the coupled ocean model NEMO5. The interaction of waves with sea-surface currents can be important. In particular areas, (e.g. Gulf Stream or Agulhas current), the current effect may give rise to localised changes of up to a metre in the wave height. Also ocean currents can refract or alter the direction of wave propagation. This can result in wave focusing so waves become larger in some areas and smaller in others.
Users should also note that whilst ECMWF does provide some ocean current output, from its ocean model (as "sea water velocity fields"), the current resolution of ECWAM (~0.25 deg) is insufficient to allow strong gradients in western boundary currents to be captured. This means that stronger currents that are observed around the world tend to be underestimated in this output, sometimes substantially so.
Shoaling
Shoaling is the deformation of waves as they move from the ocean into shallow waters causing the waves to become steeper, increase in height, and have shorter wavelength. The basic equations in ECWAM do represent the effect when the waves propagate from deep to shallow water, but generally the effect is not dramatic over most coastal waters. Waves inshore and at the beach, where shoaling is very strong, are not represented since ECWAM resolution ~9km cannot represent the actual beach slope. Wave products near coasts, and, to a lesser extent, within small and enclosed basins (e.g. Baltic Sea) may be of lower quality than for the open ocean.
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- incomplete resolution by the land-sea mask. Detail of the coast may be deficient.
- unreliable bathymetry information in some areas. Detail of the water depth may be deficient.
- unidentified small islands. This allows shallow waves to propagate unhindered. However, the wave model has a scheme that tries to represent the impact of unresolved islands on the global propagation of waves.
Fig2A.3.2-6: An example chart of wind-wave and swell. Some shoaling is possible towards the French and British coasts as the sea becomes less deep but forecast values cannot be absolutely relied upon. See Fig2A.3.2-7 for detail around the Azores. No parameters are shown on coasts nor where ice cover >30% (i.e. where some of the grid points used in interpolation of wave data for display are on land or ice. Users should identify whether the ice areas or coastal zones are the cause. See section regarding wave parameters near sea ice)
Fig2A.3.2-7: The same example chart of wind-wave and swell as in Fig2A.3.2-6, magnified near the Azores. There are some areas around the islands where wave parameters are not forecast (i.e. where some of the grid points used in interpolation of wave data are on land) but the detail of coast may not be fully resolved. ECWAM shows re-build of wind waves to the lee of the islands as the wind fetch increases and also the penetration of larger waves through the inter-island straits.
Near sea ice
Interactions between waves and sea ice are important across various time scales.
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In earlier versions of ECWAM (Cy49 and earlier), no wave information was provided for sea-ice concentration >30%.
Fig2A.3.2-8: Illustration of the importance of distinguishing between ice cover and shallow water when an area of wave parameters is missing (Cy49 and earlier). But Cy50 now predicts waves within sea-ice cover >30% but care still needs to be taken in areas of shallow water where ice and depth may or may not be significant.
Fig2A.3.2-9: Significant height of combined wind waves and swell (Hs). Example based on Cy49. The coloured areas show the difference between the heights derived from the 2d spectra (used where >30% sea ice cover is forecast) and from the wave model (as if open sea). Some large differences are evident, illustrating the need to treat the values of wave height with caution where sea ice is present.
Waves near tropical storms
There is an active two-way coupling between the atmosphere and ocean waves - surface wind stress generates the waves and in turn the waves modulate the wind stress. ECWAM generally forecasts realistic wave parameters (wave height, period etc).
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When considering forecast wave parameters in the vicinity of typhoons, hurricanes etc., it should be remembered that IFS still has difficulties in producing some intense tropical cyclones and their subsequent motion.
Fig2A.3.2-10: An example from Cy49 and earlier showing deficiency of wave height in coastal waters due to incorrect water depth.
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A better representation of worldwide bathymetry was introduced in Cy50 in May 2026 which reduces this problem.
Further information
- See more information on the introduction of waves and sea ice at ECMWF.
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