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The role of wave - mean flow interaction and eddy lifecycles for midlatitude atmospheric variability

Final Activity Report Summary - EDDYLC (The role of wave - mean flow interaction and eddy lifecycles for midlatitude atmospheric variability)

The common framework for understanding the observed mid-latitude atmospheric circulation is that of separating the atmospheric flow into a basic state - the planetary scale flow which varies on seasonal or slower time scales - and the deviations from it. The atmospheric Jet Stream, and the weather systems that propagate along it, are a common example of such a separation. There is a complex mutual interaction between the basic state (which is also referred to as the mean flow), and the waves or eddies, which are the deviations from it. The objective of this project is to better understand the processes of cyclone growth and decay, and the mutual interaction with the jet stream, in the context of understanding observed mid-latitude atmospheric variability, both forced and unforced.

Work was done in several projects of varying nature. The most exciting results (in our mind) were from two theoretical studies: In a study with Dr. Eyal Heifetz (Tel Aviv University), we unified the two main theories of shear instability - a central mechanism in fluid dynamics and an underlying process in cyclone formation. While one theory (CRW - Counter propagating Rossby waves) would view cyclone growth in terms of a mutual amplification of surface and tropopause troughs, the other (Overreflection) discusses it in terms of a Laser like amplification, involving a wave process called over-reflection. The two existing theories illuminate different fundamental aspects of shear instability, and while they go a long way towards explaining the instability, a full mechanistic understanding, of the kind we have for buoyant convection (heavy air over light fluid is unstable), is still lacking. We have managed to unify the two theories, by using the building blocks of CRW theory to understand the over-reflection process and its role in shear instability. This has shed light on basic wave processes and the processes contributing to the energy growth of perturbations.

In a following study, with Dr Heifetz and Dr Umurhan (Tel Aviv University) and Dr Lott (LMD, France), we developed a similar kind of understanding for a different prevalent kind of atmospheric waves, called gravity waves, which arise from buoyancy perturbations. Gravity waves are much smaller in scale compared to typical weather systems, yet they influence the global momentum balance, and are thus an important component of the atmospheric general circulation. Realising that the horizontal component of vorticity plays a central role in gravity wave dynamics, we formulated the linear dynamics of stratified shear flow anomalies in terms of a continuous interplay between vorticity and buoyancy anomalies. The resulting formalism is directly analogous to CRW theory for Rossby waves. This has yielded a new, intuitive, mechanistic understanding of gravity waves and related phenomena. We also mechanistically explained an instability arising on two density jumps, highlighting the similarities with classical Rossby wave based instabilities, and examined a particular example of a foam layer, arising at the air-sea interface in strong hurricane winds.

Atmospheric waves grow at the expense of the vertical shear of the background flow, and are in turn modified by these changes. Atmospheric models often exhibit multiple dynamical regimes, which necessarily involve a change in the nature of wave-mean flow interaction. The process of wave growth and decay is called an eddy life cycle. Our underlying hypothesis is that this change is manifest in the type of eddy life cycle. In two studies, differing in the model complexity, we show explicitly that small changes in the basic flow which determine whether the waves get absorbed or are reflected back in the north-south direction, can bring about a regime change, and a change in life cycle type. In a simplified model, we are able to show how small changes in the mean flow in specific tiny critical regions determine the regime in which the models settle.