E7 Atmospheric Waves - Planetary and Equatorial

Topic

Large-scale atmospheric waves fall into two main families based on where they originate and the restoring force acting upon them: planetary waves (also known as Rossby waves) and equatorial waves. Planetary waves are large-scale oscillations driven by the meridional variation of the Coriolis parameter—specifically, how this parameter changes with latitude (known as the "beta gradient"). It is this variation that pushes back against any flow perturbation, thereby generating the wave. Planetary waves propagate westward relative to the background wind carrying them; they are responsible for the meandering of jet streams, the blocking systems that can result from these meanders, and the teleconnections that transport climate signals thousands of kilometers from their point of origin. The quantity governing this entire dynamic—and which remains conserved as the wave propagates—is Ertel potential vorticity.

The second family, equatorial waves, does not propagate freely across all latitudes like planetary waves; instead, these waves remain trapped near the equator. This occurs precisely because the Coriolis parameter changes sign there, and its latitudinal variation acts as a sort of waveguide, confining the wave to the equatorial belt.

Within this family of equatorial waves, three types are particularly significant for tropical circulation. Kelvin waves propagate exclusively eastward, without any meridional wind component, and are critical to the propagation of the El Niño–La Niña oscillation in the tropical Pacific. Equatorial Rossby waves, by contrast, propagate westward—much like the planetary waves from which they take their name—and are linked to the convection associated with the Madden-Julian Oscillation. Mixed Rossby-gravity waves—also known as Yanai waves—also propagate westward, combining characteristics of both wave types.

Together, these three types of equatorial waves modulate the organization of tropical convection, determining where and when storms cluster along the equatorial belt; they also modulate the semiannual cycle of the tropical stratosphere, imposing their own rhythm on the circulation within that atmospheric layer.

Both planetary wave activity and equatorial wave activity can be tracked using the same diagnostic tool: Eliassen-Palm vectors. These vectors show how wave activity propagates across the plane spanning different latitudes and altitudes, while also revealing the effect the wave has on the surrounding mean flow. The divergence of these Eliassen-Palm vectors measures precisely the extent to which the combined activity of planetary and equatorial waves accelerates or decelerates the mean zonal flow. Thus, from the jet stream meanders caused by planetary waves to the rhythm equatorial waves impose on tropical convection and the stratosphere, these two families of large-scale atmospheric waves are ultimately described in the same terms: the amount of propagating wave activity and the extent to which that activity accelerates or slows the surrounding mean flow.

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