Coastal trapped and Kelvin waves Kelvin waves

Coastal trapped and Kelvin waves

Kelvin waves

Kelvin waves Upwelling and daunvelling lead to the appearance Kelvin waves

Observations of Kelvin waves on the African Atlantic shelf. Top: general geography and location of observation stations. Bottom left: Sea surface temperature (°C) at various locations along the coast. The effect of the wave on the surface temperature is seen as periodic upwelling and downwelling, which produces periodic lowering and rising of the water temperature. The period is approximately 14 days. Bottom right: Temperature and alongshore currents at 12, 42 and 82 m depth at a mooring located about halfway along the heavy line that goes out across the shelf from Tema in the top figure.

Evidence of Kelvin waves along the coast of Peru. The shelf of South America is very narrow and the shelf edge very steep, particularly in the south. Currents were measured for periods of several months at the locations shown as red dots on the map. The data were low-pass filtered to eliminate tides and other variations with periods less than one day. The filtered data from different stations were then compared with each other using cross correlation analysis. It was found that the data compared best when the time series from different stations were shifted ("lagged") against each other in time. The right diagram shows the lags obtained for the best comparison (highest correlation), relative to the northernmost station at 10°S. The regular increase of the lag for the highest correlation indicates that a variation of the current speed observed at that station travels southward along the coast with a speed of about 200 km/day.

Сoastal trapped waves Вasic physical mechanism absolute vorticity: relative vorticity :

Sketch of the situation that can give rise to coastal trapped waves. The key ingredients are • the earth's rotation, indicated in the upper left of the diagram, • a sloping continental shelf, and • variable external forcing such as a periodic wind stress τw. The wind stress drives water in the surface mixed layer (the Ekman layer) away from the coast and back, producing periodic upwelling and downwelling near the coast. This produces alternating onshore and offshore directed flow below the mixed layer. If the shelf region is in the southern hemisphere and north is up, the northward acting wind stress produces downwelling and the southward acting wind stress produces upwelling.

The surface mixed layer is now removed from the diagram, and only the water below the mixed layer is shown. Northward wind stress produces downwelling and pushes the water below the mixed layer away from the coast. Southward wind stress produces upwelling and pulls the water below the mixed layer towards the coast. The effect of variable wind stress on the coastal ocean, demonstrated for a shelf in the southern hemisphere. As a result, water that originally was located at the broken line that runs parallel to the coast is moved into the location shown by the red line. A water column on the central section that was originally located under the broken line is moved into deeper water and gains negative vorticity; the water columns originally located under the broken line at both ends of the diagram are moved into shallower water and gain positive vorticity.

The rotation induced by a change in vorticity drives the water particles away from the coast in some places and towards the coast in others. As a result, the location of the water particles changes from the red line to a new position on the yellow line. The net effect of the change in particle location is a northward shift of the wave pattern from the red to the yellow location. The result of alternative gain of negative and positive vorticity in the water column when the forcing from the wind stress is relaxed. For an observer in a fixed location this appears as a northward propagating wave.