Interaction of waves and currents in the ocean
 | This article needs attention from an expert in Physics. (October 2018) |
In the ocean, the waves do not propagate on a still surface but on the current which varies on time and space. As a consequence, the interaction between them leads to the change of their evolution. It is the cause of many well-known phenomena such as rip current, Langmuir circulations, longshore currents, and undertow current.
Rip currents[edit]
The rip current is caused by complex interaction of the waves with the mean currents, water level, and bathymetry. This is a powerful current that running from the coast to the offshore. This current is usually found at the locations where there is a gap between sandbars. The rip currents are very dangerous for swimmers who lack of swimming skill or the basic knowledge about this type current. In the United States, there are over 100 deaths per year caused by rip currents according to the National Oceanic and Atmospheric Administration (NOAA).[1] Besides, this type of current is also responsible for more than 80% of the rescue carrying out by the beach lifeguards.[2]
Langmuir circulation[edit]
The Langmuir circulation is the result of the interaction between the surface wave and the mean currents[3]. The axes of this circulation is parallel to the wind’s direction at the water surface. This current is the result of the interaction between the waves and the currents. The vertical velocity of this circulation is quite strong up to 25% of the surface current according to Craik and Leibovich (1976). Therefore, it has important role in the contribution of nutrient, oxygen, and other material in the vertical direction of water body. This circulation can be observed where the foam and buoyant material converge into bands at the surface.
Longshore current[edit]
The other phenomenon related to the wave and current interaction is longshore current[4]. It flows parallel to the coastline, and varies depending on the characteristics of the waves. This current causes the alongshore transport of floater, sediment, and nutrient. A strong longshore current might be dangerous for the swimmer because it prevent swimmers keeping their feet on the bed leading to the difficulty in returning the shore. The longshore current is also responsible for the rescues on the coast by sweeping people into the rip currents, piers, or other hazardous areas.
Undertow[edit]
The undertow current is also one of the phenomena cause by the interaction between waves and current. This current is relatively strong and has seaward direction. This current plays a important role in the formation of the sandbars in the surf zone. According to the Coastal Engineering Manual (2003), this current is driven by wave stress, surface slope, and the vertical mixing. It compensates the mass flux transport to the coast caused by the surface wave. In popular usage, many people misunderstand the rip current with undertow current. The undertow occurs anywhere there is the waves approaching to the coast when the rip current only takes place in certain areas.
Radiation stress and vortex force[edit]
The above presented some well-known phenomena related to the interaction of waves and currents in the ocean. It shows that the wave-current interaction in the ocean is very important. A good method to calculate and predict these phenomena is necessary both for the development of ocean science as well as for human activities on and by the sea.
The effect of the wave on the current can be expressed in term of radiation stress or vortex force representations. Both of them are equivalent mathematically. The wave radiation stress concept is firstly introduced by Michael S. Longuet-Higgins and R.W. Stewart in their paper published in 1964. The wave radiation stress is the excess momentum caused by the presence of the wave. That concept is successful in explaining the formation of many phenomena in the ocean such as wave set-up, wave set-down, surf beat, or the longshore current.
The effect of the wave on the current can be expressed in term of vortex force. It was first introduced by Craik and Leibovich in their paper in 1976. Using this concept, the formation of Langmuir circulation is explained much easier than using radiation stress concept. This is presented detail in the paper of Leibovich in 1980. The vortex force representation is also employed by many other researcher such as Huang (1979) in deriving the ‘Coriolis vortex force’, McWilliams et al. (2004) in developing an asymptotic theory for the waves and currents interaction, or Ardhuin et al. (2008) in developing an explicit wave-averaged equations of motion.
Calculation and prediction[edit]
To predict the movement of fluid particle in the ocean the equations of mean motion are applied. In which, the effect of the waves on the currents can be included either in radiation stress or in vortex force representations. Two methods are usually applied to obtained the equations of mean motion called Eulerian mean and Generalized Lagrangian Mean (GLM). In the Eulerian mean method, the equations of motion are averaged at fixed positions. The resulting equations are called Reynolds averaged Navier-Stokes (RANS) equations. This is a well-known set of equations and is applied widely in numerical ocean models such as Regional Ocean Modeling System (ROMS model), Princeton Ocean Model (POM model), Mike model, etc. The GLM method was proposed by Andrews & McIntyre (1978). In this, the GLM quantities is defined by averaging quantities over disturbance position of fluid particles. This method was employed in well-known numerical models such as Delft3D model, MARS3D model, TELEMAC-3D model, and SYMPHONIE model.
However, the RANS equations is only valid from the bottom to the wave trough because the presence of the air in the area from the wave trough to the mean water level in a part of the wave period. Therefore, this method is only suitable if the surface wave amplitude is infinitesimal. In contrast, the GLM method is valid until the mean water level. Then, it is applicable for a wide range of applications from the deep sea to the coastal areas where the surface wave amplitude is finite.
See also[edit]
References[edit]
Citations
- ↑ Administration, US Department of Commerce, National Oceanic and Atmospheric. "What is a rip current?". oceanservice.noaa.gov.
- ↑ "USLA". United States Lifeguard Association.
- ↑ "ScienceDirect". www.sciencedirect.com.
- ↑ Administration, US Department of Commerce, National Oceanic and Atmospheric. "Longshore Currents - Currents: NOAA's National Ocean Service Education". oceanservice.noaa.gov.
Journal articles
- Andrews, D. G., & McIntyre, M. E. (1978). An exact theory of nonlinear waves on a Lagrangian-mean flow. Journal of Fluid Mechanics, 89(4), 609-646.
- Ardhuin, F., Rascle, N., & Belibassakis, K. A. (2008). Explicit wave-averaged primitive equations using a generalized Lagrangian mean. Ocean Modelling, 20(1), 35-60.
- Army, U. (2003). Coastal Engineering Manual. Chapter II-2, Meteorology and Wave Climate. Engineer Manual 1110-2-1100. US Army Corps of Engineers, Washington, DC.
- Craik, A. D., & Leibovich, S. (1976). A rational model for Langmuir circulations. Journal of Fluid Mechanics, 73(3), 401-426.
- Huang, N. E. (1979). On surface drift currents in the ocean. Journal of Fluid Mechanics, 91(1), 191-208.
- McWILLIAMS, J. C., Restrepo, J. M., & Lane, E. M. (2004). An asymptotic theory for the interaction of waves and currents in coastal waters. Journal of Fluid Mechanics, 511, 135-178.
- Leibovich, S. (1980). On wave-current interaction theories of Langmuir circulations. Journal of Fluid Mechanics, 99(4), 715-724.
- Longuet-Higgins, M. S., & Stewart, R. W. (1964, August). Radiation stresses in water waves; a physical discussion, with applications. In Deep Sea Research and Oceanographic Abstracts (Vol. 11, No. 4, pp. 529-562). Elsevier.
This article "Interaction of waves and currents in the ocean" is from Wikipedia. The list of its authors can be seen in its historical and/or the page Edithistory:Interaction of waves and currents in the ocean. Articles copied from Draft Namespace on Wikipedia could be seen on the Draft Namespace of Wikipedia and not main one.
