Ocean Long Waves

Ocean long waves are disturbances caused by the wind that travel an extremely long way and have a profound effect on ocean circulation.  These can be barotropic waves, which propagate along the surface, and baroclinic waves, which move long a sharp density gradient.  Two of the most important types are Kelvin waves and Rossby waves.

Kelvin waves can travel either way, along the surface or along a density gradient.  They are huge and fast, travelling hundreds of kilometres a day and lasting for days.  They are not however very high, while they can be hundreds of kilometres long they tend to be only a few centimetres high. 

They move generally from west to east in the northern hemisphere and east to west in the southern.  This is due to the Coriolis force, caused by the rotation of the earth.   The Coriolis force deflects parcels of water to the right above the equator and to the left below. Coastal Kelvin waves have the coastlines as a wave guide, which constrains the waves within a certain distance of it.

Equatorial Kelvin waves are a little different, because at the equator the Coriolis force is zero.  These waves always move towards the east and any movement north or southwards is deflected back towards the equator.  The wave guide for these waves is not the coastline but the equator itself.  Kelvin waves are implicated in the El Nino effect and tides.

The second main type of ocean long waves are Rossby waves.  These travel in the opposite direction to equatorial Kelvin waves.  Like Kelvin waves they are not at all high, although they can be hundreds of kilometres wide.  Because of these they were only observed relatively recently, although they were known of theoretically for far longer.  A wave of only a few centimetres high is almost undetectable on the surface of the ocean.

Rossby waves are caused by how the Coriolis force changes with latitude, which moves from zero at the equator to a peak at the poles.  They move far more slowly than Kelvin waves at a rate of only a few kilometres a day, adding to the difficulty of detection.  It can take a Rossby wave months to cross an ocean.

Both kinds of these huge, nearly invisible waves have a key role to play in global weather systems.  As meteorological equipment and analysis improve, observations of these waves can help to predict what will happen with regards to major international weather events.