A study of dynamic mechanisms of annular modes

Abstract

In extratropical regions, especially during the cold seasons, the most dominant dynamic modes of atmospheric variability are the annular modes, which include the Arctic Oscillation (AO) or the North Atlantic Oscillation (NAO) and the Antarctic Oscillation (AAO). These atmospheric low-frequency modes are often related to the high-frequency storm track variabilities. In this dissertation, a new dynamic framework is employed to study the interaction between the transient eddy and mean flow. It is shown that the annular modes result from the positive synoptic eddy and low-frequency flow (SELF) feedback through the "tilted-trough" mechanism. Based on the Complex Empirical Orthogonal Function (CEOF) analysis, the storm track variability is characterized in terms of spatial structures, variances, decay time scales and propagation speeds, which represent the "normal" storm activity. With this characterization, a dynamic framework that includes the transient eddy and mean flow interaction can be obtained. A simple model on the middle latitude f3 -plane is used to investigate the generation of the AO and AAO-like modes, which have dipole structures in the north-south direction. The dipole mode is the least damped mode in this simple system due to the positive feedback between transient wave (or stationary wave) and mean flow. The dynamic origin of annular modes were further investigated by using a two-dimensional barotropic model; as well as a three-dimensional primitive equation model. Both the barotropic and baroclinic models show that the annular modes are internal modes of the atmospheric low-frequency dynamics, which arise from the positive SELF feedback. The effect of air-sea interaction on the annular modes in the North Atlantic region is also discussed. It is shown that the North Atlantic Oscillation-like dipole structure and the tri-polar pattern Sea Surface Temperature (SST) anomaly are two related components of the leading coupled mode, the positive SELF feedback plays an essential role in the formation of this coupled mode.