4.3.1: Wave height variation
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Ocean waves are generated by wind. It can thus be expected that the global wind systems (Sect. 4.2.2) determine the global wave environments. Global wave environments are those zones of the seas and the oceans that have similar general wave characteristics, such as similar year-averaged significant wave heights and similar seasonality. Besides, the shape and orientation of the oceans determine the fetch and hence influence the propagation of waves.
Figure 4.8 shows annual mean values for the significant wave height as well as monthly means for January and July. For this analysis mean significant wave heights were used obtained by visual observation of individual waves from ships for the period of 1958 to 1997. The significant wave heights vary mostly between 1 m and 5 m.
From Fig. 4.5, Fig. 4.6 and Fig. 4.8, the following general conclusions can be drawn:
- Wave heights are highest at mid-latitudes (north and south). This is the result of the westerlies which are the strongest winds. These winds (and embedded mid-latitude cyclones) are the source of relatively large waves;
- The mid-latitude wave climate in the North Pacific and North Atlantic is especially seasonal with much larger wave heights in the Northern winter than in the summer. This is the result of the strong seasonality in the NH westerlies due to the presence of especially the large Asian land mass (see Sect. 4.2.2);
- The Southern Ocean is characterised by an almost unlimited fetch and vast regions with high waves. Although the waves are highest in the Southern winter, the seasonality is much smaller than for the Northern Hemisphere (as a result of the smaller seasonality of the westerlies in the absence of vast land masses);
- Wave heights in the subtropics associated with the gentle trade winds are moderate;
- In the tropics and subtropics, sources of larger waves are either swell propagating from higher latitudes (originating from the westerlies), or seasonal winds (e.g. monsoons and tropical storms);
- Monsoons have a regional impact. It appears that the highest wave heights in the Arabian Sea coincide with the SW Monsoon (summer). This is because the SW monsoon blows from the sea to land. The NE monsoon blows from the land, except for the Malaysian Peninsula, which is exposed during the NE monsoon;
- Tropical and east coast cyclones can generate large waves but are limited in extent and too seasonal to greatly impact longer-term wave climates.
The wave climate (in terms of wave height) is generally characterised by the mean significant wave height \(H_s\) on a yearly average basis:
- Low wave energy \(H_s < 0.6m\);
- Medium wave energy \(0.6m < H_s < 1.5m\);
- High wave energy \(H_s > 1.5m\)