The TRR 181 seminar is held by Pragallva Barpanda (University of Bergen) on November 5, 2:15 pm in Grindelberg 7, Hamburg, room 4007.

Understanding Rossby-Kelvin wave interactions in the presence of meridional wind shear.

Abstract

Equatorial Rossby and Kelvin waves are prevalent in nature and dominate the tropical weather variability on synoptic to sub-seasonal timescales. These waves were first isolated theoretically by Matsuno (1966) as a subset of orthogonal solutions of the rotating shallow water equations on an equatorial beta plane with a resting state. However, real atmospheric waves seldom occur in the absence of a background flow. Despite extensive research, the influence of non-resting background conditions on equatorial waves and their potential for wave-wave interaction remains unclear. In this talk, I will illustrate how equatorial waves interact in the presence of a subtropical jet using two sets of simplified model experiments. (1) In the first, a spherical nonlinear shallow water model is run to a steady-state while being forced by large-scale thermal/vorticity forcings reminiscent of the Madden-Julian Oscillation (MJO) in the presence of a subtropical jet of varying strength. (2) In the second, the problem is simplified to that of a transient linear shallow water, to investigate how Matsuno’s original free waves are altered under widely varying meridional shear values. In both cases, we observe the coupling of Rossby and Kelvin waves, with the degree of coupling being strongly dependent on the strength of the background shear. With regards to the steady-state MJO-like solutions, this coupling plays a key role not just in modifying horizontal flow structures but also their amplitudes, which generally increase in response to strengthening of the subtropical jet. In the transient case, we recover the emergence of Rossby-Kelvin instability under extreme shear conditions due to wave-wave resonance. The latter is linked to ageostrophic planetary instability relevant to other planetary atmospheres.  Overall, we find that that even in a linear regime, Rossby and Kelvin waves tend to exchange energy with one another which is mediated by the strength of background mean-meridional shear remotely induced by the subtropical jet in the midlatitudes.