Wave climate classification according to wind climate
The different wind climates, which dominate different oceans and regions, cause correspondingly characteristic wave climates. These characteristic wave climates can be classified as follows:
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Storm wave climate
This is related to subtropical, temperate and arctic climates dominated by the passage of many depressions. At an exposed, open coast this climate is characterised by very variable wave conditions, both with respect to height, period and direction distributions. This type of climate often results in a wide littoral zone dominated by a sandy coastal profile with bars and a wide sandy beach backed by dunes.
Swell climate
This typically occurs along coastlines near the equator, where the swell is generated by the so-called trade winds. Near the equator the heating of the air masses is particularly high. This causes the air masses to rise, which in turn generates a thermal depression near the surface. This depression induces an atmospheric circulation directed towards the equator. The area where the southern and northern hemisphere circulations meet is called the Inter Tropical Convergence Zone (ITCZ). The related wind fields are called trade winds. Due to the rotation of the earth, their directions are NE north of the ITCZ and SE south of the ITCZ. Near ITCZ the wind climate is predominantly calm; this area is called the doldrums. The trade winds mainly occur over the oceans as they are overruled by the monsoons near the continents. Trade winds are moderate and persistent. The wave climate generated by the trade winds is also moderate and persistent throughout the year. As it mainly occurs over oceans away from the coastlines, the associated wave climate along adjacent coastlines is mainly in the form of swell characterised by relatively small and long persistent waves travelling in a constant direction. A swell climate normally gives rise to a relatively narrow sandy littoral zone with an abrupt shift to a gently sloping outer part of the littoral zone dominated by finer sediments. Swell climates occur also in other areas. They originate from waves fields produced by extreme wind conditions in areas far from the coast. The longest waves move faster and dissipate less energy in the ocean than the shorter (steeper) waves. Extreme winds therefore produce mainly long-period swell waves at distant coasts. This is, for example, the case on the north-west coast of Mexico and the coast of California, where the swell wave climate dominates during the summer months with swells developing from tropical storms in the south and from waves originating in the southern hemisphere. This is an important wave climate from a coastal point of view since these waves tend to move sand from the shoreface onto the beach.
Monsoon wave climate
The monsoon climate is characterised by seasonally changing wind directions. During the summer, local depressions over tropical landmasses cause the wind to blow from the sea towards land. The Inter Tropical Convergency Zone intensifies these tropical summer depressions. In Southeast Asia the summer monsoon is referred to as the SW-monsoon. The summer monsoon is warm and humid. The winter monsoon, which is caused by local high pressure over land, blows from the land towards the sea. In Southeast Asia the winter monsoon is referred to as the NE-monsoon. The winter monsoon wind is relatively cold and dry. The monsoon wind climate is thus characterised by winds from the sea during the summer and winds from land during the winter. The above phenomenon holds for major continental landmasses only, whereas minor landmasses within the monsoon area can experience onshore winds during winter. An example of this is the East Coast of the Malaysian Peninsula, which is predominantly exposed during the NE-monsoon. Monsoon winds are relatively moderate and persistent for each monsoon season. This means that the corresponding wave climates are also seasonal and normally characterised by a relatively rough summer climate and a relatively calm winter climate. The summer climate can, in absolute terms, be characterised as moderate and relatively constant in direction and height. The monsoon climate typically results in a fairly narrow sandy inner littoral zone, shifting to a gently sloping outer part of the littoral zone dominated by finer sediments.
Tropical cyclone climate
Tropical storms are called hurricanes near the American continents, typhoons near SE-Asia and Australia, and cyclones when occurring near India and Africa. Tropical storms are generated over tropical sea areas where the water temperature is higher than 27oC. They are normally generated between 5oN and 15oN and between 5oS and 15oS. From there they progress towards the W–NW in the Northern Hemisphere and towards the W–SW in the Southern Hemisphere. Cyclones do not penetrate the area between 5oN and 5oS, as wind circulation cannot occur so close to the equator. An average of 60 tropical cyclones is generated every year. Tropical cyclones are characterised by wind speeds exceeding 32 m/s and they give rise to very high waves, storm surge and cloudburst. Tropical cyclones occur as single events, peaking during September in the Northern Hemisphere and similarly peaking during January in the Southern Hemisphere. Tropical cyclones are rare and therefore recording programmes seldom document the resulting waves. A tropical storm will normally have great impact on the coastal morphology when it hits, but the coastal morphology will first and foremost be determined by the normal wave climate, which can be either monsoon or swell climates.
Further reading
Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines.
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