Finite-amplitude Eddy-mean Flow Interaction in the Extratropical Atmosphere

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The mutual interaction between eddies (such as those associated with midlatitude weather systems) and the mean flow (midlatitude jet streams, in particular) is a longstanding topic in atmospheric general circulation. It is clear that jet streams and eddies interact strongly, but traditional theories to account for their interaction are limited because, for mathematical reasons, they assume that the eddies are small-amplitude perturbations of the mean flow. The Principal Investigator (PI) has developed a theory of eddy-mean flow interaction which works for large-amplitude eddies, and work conducted under the award seeks to explore the implications of this new theory. The new theory will be applied to 1) evaluate nonconservative driving of the climate state (radiative forcing, friction, mixing, etc.) by carefully documenting and analyzing the slowly varying reference state; 2) quantify the stabilizing effects of baroclinic eddies ("baroclinic adjustment") by comparing the linear stability of the observed zonal-mean time-mean state and that of an eddy-free reference state derived from the theory; 3) characterize annular mode variability as co-variation of the zonal-mean zonal-wind and wave activity density, and in turn associate it with the variation in the zonal phase speed and energy of the eddies; 4) generalize the criterion for the onset of Rossby wave breaking and testing this theoretical prediction with numerical simulation and meteorological reanalysis; and 5) further generalize the wave breaking criterion for zonally varying mean flows.The broader impacts of this activity are that the eddy-mean flow theory and the diagonstic tools derived from it can be applied to a wide class of flows, including the stratosphere, troposphere, and the oceans. In addition, the project will support and train two graduate students, thereby developing the scientific workforce in this area. Results of the research will also be incorporated into demonstrations performed by the PI in his fluid dynamics laboratory, for audiences at the graduate, undergraduate, and high school levels.