Dynamical Studies On The Eddy-Driven Jet And Its Low-Frequency Variability In The Context Of Climate Change

The eddy-driven jet is one of the most prominent features in the midlatitude atmo-spheric circulations. It has significant impacts on the extratropical weather and climate and on the global poleward transports of heat, momentum and moisture. Observations and climate model simulations have shown that, the eddy-driven jet exhibits significant variations under low-frequency time scale and in response to climate change. Under-standing the mechanisms for the eddy-driven jet variation under low-frequency time scale and climate change are helpful not only in predicting the persistent abnormal weather and intraseasonal variability but also in evaluating the atmospheric response to climate forcing. This dissertation investigates the dynamical mechanisms for jet variations under low-frequency time scale and climate change by carrying out group-s of idealized model experiments and diagnosing observational and modeling results. The main conclusions are as follows:1. A baroclinic mechanism through which synoptic and low-frequency eddies working together sustaining the low-frequency variability of the eddy-driven jet is proposed.We first study the eddy-mean flow interaction in the low-frequency variability of the eddy-driven jet using a β plane multi-layer Quasi-Geostrophic (QG) channel model. Our investigation suggests a baroclinic mechanism responsible for the positive feedback in the low-frequency variability of the eddy-driven jet, in which synoptic and low-frequency eddies play different roles. Sensitive studies on the surface friction show that the strength of surface friction could affect the leading mode of the jet variability and its time scale. The transition of jet variability mode under different friction strength could be related to the distinct critical layer distributions of the synoptic and low-frequency eddies.Using the ERA-40reanalysis data, we further explore the baroclinic mechanism of the low-frequency variability of the eddy-driven jet in the real atmosphere. The different roles of synoptic and low-frequency eddies in sustaining the latitudinal shift of the low-level baroclinicity associated with the low-frequency variability of the eddy-driven jet in Southern Hemisphere are investigated. We find that the latitudinal shift of the jet is followed by a latitudinal displacement of the low-level baroclinicity. In addition to the synoptic eddy-induced Mean Meridional Circulation (MMC), the direct eddy thermal forcing by low-frequency eddies is significant in driving the baroclinic anomalies. These two processes together prevail over the direct baroclinicity deduction by synoptic eddies and sustain the baroclinic anomalies associated with the anomalous jet. The different roles of the MMC induced by synoptic eddy momentum and heat fluxes are also emphasized, with the former leading the baroclinic anomalies and the latter acting to extend the baroclinic anomalies.2. An "upper-level control" mechanism for the latitudinal jet shift under climate change is proposed.Observation and climate model simulations have shown that, the eddy-driven jet experiences an equivalent barotropic latitudinal shift in response to climate change, such as global warming, stratospheric ozone depletion. Previous studies suggest that changes of the vertical structure of the baroclinicity could be the cause of such jet variation. Using the nonlinear β plane QG model, the mechanisms for the jet shift under climate change are investigated through systematically displacing the latitude of the upper/lower level thermal forcing away from the channel center. The dynamical processes responsible for the jet shift are explored through the initial value approach by running transient experiments with "switch-on" perturbed thermal forcing and diag-nosing the finite amplitude pseudomomentum budget. We find that the upper and lower level baroclinicity affects the jet shift differently. Changes of the upper level baroclin-icity significantly alter the spatial distributions of the wave breaking and the associated vertical circulation, which further affects the lower-level baroclinic zone and eddy gen-eration, then shift the eddy-driven jet. In contrast, the lower-level baroclinicity affects the eddy-driven jet simply by modifying the upper-level zonal thermal wind distribu-tion, as well as the associated wave propagation and breaking. The relative roles of the upper and lower level baroclinicity in shifting the jet latitude are further investigated by displacing the upper and lower level baroclinicity simultaneously but in opposite directions. The equilibrated responses suggest that upper level baroclinicity eventually controls the jet shift.3. A mechanism through which barotropic and baroclinic dynamical pro-cesses working together causing the change of the low-frequency variability of the eddy-driven jet under climate change is proposed.Using the same QG model and experiments, we further investigate the low-frequency variability of the eddy-driven jet under climate change. Through the diagnosis of finite amplitude wave activity, we find that the barotropic process as the upper-level wave propagation and breaking, and the baroclinic process as the lower level baroclinic ed-dy generation play different roles in maintaining the positive feedback for the low-frequency variability of the eddy-driven jet, with the former strongly driving the eddy momentum flux and jet shift, and the latter acting to extend the zonal wind anomaly. When the background vertical structure of the baroclinicity becomes asymmetric un-der climate change, the low-frequency variability of the eddy-driven jet becomes less persistent and exhibits a phase asymmetry. The less persistence of jet variability is attributable to the weaker meridional wave propagation and breaking aloft and the less persistent shift of lower troposphere baroclinic eddy generation following the jet shift.Finally, using the coupled GFDL_CM2.1model, we analyze the variation of the eddy-driven jet under global warming. The mechanisms for the jet variation with dif-ferent vertical structure of baroclinicity suggested above are demonstrated to be very efficient in explaining the eddy-driven jet variation under global warming. In response to global warming, the eddy-driven jet shifts poleward and its low-frequency variability becomes less persistent. Diagnosis of finite amplitude wave activity suggests that the poleward shift of the eddy-driven jet is associated with the decrease in wave breaking in the poleward side of the jet. As in the QG model simulations, the less persistence of low-frequency variability of the eddy-driven jet is attributable to the weaker meridional wave propagation and breaking aloft and the less persistent shift of lower troposphere baroclinic eddy generation following the jet shift.