Influences Of Angular Momentum Conservation On The Formation And Variation Of The Mean Meridional Circulation

The progress of atmospheric general circulation has been always companied with the study of atmosphere angular momentum (AAM) budget. Whether or not the AAM is balanced is one of the important conditions for evaluating an atmospheric general circulation model. As one of the atmospheric general circulation components, the mean meridional circulation (MMC) plays very important roles in balancing the generation and dissipation of the AAM at the surface and transporting the AAM in the free atmosphere. Based on the atmospheric general circulation model SAMIL developed at LASG/IAP and by using the NCEP/NCAR reanalysis data, the thesis firstly evaluates the AAM transportation and balance in the model; then simulates the formation of the atmospheric MMC; and finally studies the AAM cycle in ENSO events. The main conclusions are as follows:1. The Evaluation of Atmospheric Angular Momentum Transport and Balance in a Numerical ModelThe transport and balance of the AAM in the SAMIL are compared with those calculated from the NCEP/NCAR reanalysis and the observed data (Oort et al. 1983). It was found that the mountain and friction torques which influence the balance of the AAM are reasonably simulated. Their distributions illustrate the source regions of the angular momentum are in the tropics and the sink regions are in mid- and high-latitudes. But the friction torque in the tropics is stronger compared to the observation. This may due to the too much precipitation simulated along the inter-tropical convergent zone (ITCZ) in the model. Secondly, the SAMIL can correctly simulate the angular momentum cycle process. Thirdly, the configuration and feature of the transport and balance of the AAM produced from the SAMIL agree with those calculated from the NCEP/NCAR reanalysis and from observation data in general. Nevertheless there are some differences. These imply that simulations of the convection and tropical precipitation need to be improved.2. The Simulation Study of the Formation of the Atmospheric Mean Meridional CirculationUnder the ideal condition in which the orography is removed and only the sea-land distribution is kept, the SAMIL is used to study how the mean meridional circulation is formed from a static atmosphere. The static model atmosphere is driven by the heating difference between equator and pole with heat source in the tropics and heat sink in high latitudes. At the first time step there is a thermally direct circulation in the model atmosphere that possesses the upward flow in the low latitudes and downward flow in the polar area. Due to the effect of Coriolis force which exerts on the upper branch of the direct circulation, a stronger westerly center appears in the polar region, while a weaker center is form in the subtropical area. In such a one-cell circulation, the atmosphere obtains large westerly angular momentum at the surface from the earth and consumes a little westerly angular momentum in the polar region. Therefore the atmospheric angular momentum as measured by the intensity of the westerly center grows rapidly while the one-cell direct circulation continue to develop. As a result, a weak indirect cell is formed in high latitudes. By the 36th time step, the model atmosphere accomplishes the transition form one-cell circulation to two-cell circulation. There is a momentum source in the upper subtropical region. During the transition, the"polar westerlies jet"gets weakened and vanishes at the end, while the subtropical westerlies jet gets strengthened. At this two-cell stage, the generation and dissipation of westerly angular momentum do not balance each other, and the atmosphere is still at an unstable state with great surplus of angular momentum generation. To the 17th day, the surface easterly belt and westerly belt are located in low and high latitudes reasonably surface the three-cell MMC is established the AAM balance is achieved, and the atmosphere reaches a quasi-state state.3. Atmospheric Angular Momentum Transfer and the Mean Meridional Circulation in ENSO eventsSix ENSO warm events and four ENSO cool events from 1958 to 2004 are selected for composite diagnoses. In winter (December to February) in the Northern Hemisphere, compared with cold events, much more momentum is transported pole-ward over the whole troposphere in low-latitudes during the El Nino episode mainly by stationary eddies and MMC in the upper troposphere but only by the Hadley circulation in the lower troposphere. In the mid- and high-latitudes there is very strong equator-ward momentum transport anomalously mainly by stationary eddies. Therefore the subtropical westerly jet is greatly strengthened in the El Nino episode. In summer (June to August), the differences between warm and cold ENSO events either in angular momentum transfer or in MMC are weak.Results from this study demonstrate that the transfer and balance of the atmospheric angular momentum are important parameters characterizing the atmospheric circulation and climate. They can be used as powerful tools for diagnosing the mean meridional circulation and extreme climate events such as ENSO presented in models or reanalyses.