STOCHASTIC DYNAMICS OF THE MIDLATITUDE ATMOSPHERIC JET

The innate tendency of the background straining field of the midlatitu de atmospheric jet to preferentially amplify a subset of disturbances produces a characteristic response to stochastic perturbation whether the perturbations are internally generated by nonlinear processes or e xternally imposed. This physical property of enhanced response to a su bset of perturbations is expressed analytically through the nonnormali ty of the linearized dynamical operator, which can be studied to deter mine the transient growth of particular disturbances over time through solution of the initial value problem or, alternatively, to determine the stationary response to continual excitation through solution of t he related stochastic problem. Making use of the fact that the backgro und flow dominates the strain rate field, a theory for the turbulent s tate can be constructed based on the nonnormality of the dynamical ope rator linearized about the background flow. While the initial value pr oblem provides an explanation for individual cyclogenesis events, solu tion of the stochastic problem provides a theory for the statistics of the ensemble of all cyclones including structure, frequency, intensit y, and resulting fluxes of heat and momentum, which together constitut e the synoptic-scale influence on midlatitude climate. Moreover, the o bserved climate can be identified with the background thermal and velo city structure that is in self-consistent equilibrium with both its ow n induced fluxes and the imposed large scale thermal forcing. In order to approach the problem of determining the self-consistent statistica l equilibrium of the midlatitude jet it is first necessary to solve th e stochastic problem for the mixed baroclinic/barotropic jet because f luxes of both heat and momentum are involved in this balance. In this work the response to stochastic forcing of a linearized nonseparable q uasigeostrophic model of the midlatitude jet is solved. The observed d istribution of transient eddy variance with frequency and wavenumber, the observed vertical structures, and the observed heat and momentum f lux distributions are obtained. Associated energetics and implications for maintenance of the climatological jet are discussed.