Why Rossby Wave affects climate so much


Why Rossby Wave affects climate so much

Rossby waves are structures that characterize the motions of geophysical fluids on a synoptic and planetary scale. They can be observed both in the atmosphere, for example in the meanders made by the sub-polar jet stream at mid-latitudes, and in the ocean.

Rossby waves originate from the fact that western zonal flows, flows directed from West to East, can oscillate in a southern direction around the equilibrium position, due to the conservation of potential vorticity, given that the planetary vorticity increases with latitude.

In fact, if a westerly flow has a curved trajectory that takes it north, it initially has positive relative vorticity. Moving northward the planetary vorticity increases, and consequently the relative vorticity decreases.

When the relative vorticity reaches a negative value, the flow curves towards the South. In the movement towards the South, a similar mechanism occurs, due to which the relative vorticity increases again and the flow returns to turn towards the North.

In short, the flow on average is always facing east, and oscillates around this direction. Atmospheric Rossby waves, like Kelvin waves, can occur on any rotating planet with an atmosphere. The Y-shaped cloud feature on Venus is attributed to Kelvin and Rossby waves.

Why Rossby Wave affects climate so much

Oceanic Rossby waves are large-scale waves within an ocean basin. They have a low amplitude, in the order of centimeters to metres, compared with atmospheric Rossby waves which are in the order of hundreds of kilometres.

They may take months to cross an ocean basin. They gain momentum from wind stress at the ocean surface layer and are thought to communicate climatic changes due to variability in forcing, due to both the wind and buoyancy.

Both barotropic and baroclinic waves cause variations of the sea surface height, although the length of the waves made them difficult to detect until the advent of satellite altimetry. Satellite observations have confirmed the existence of oceanic Rossby waves.

Baroclinic waves also generate significant displacements of the oceanic thermocline, often of tens of meters. Satellite observations have revealed the stately progression of Rossby waves across all the ocean basins, particularly at low- and mid-latitudes.

These waves can take months or even years to cross a basin like the Pacific. Rossby waves have been suggested as an important mechanism to account for the heating of the ocean on Europa, a moon of Jupiter.