A numerical expariment of a topographic effect to Arctic oscillation by using barotropic global circulation model

We have developed a barotropic global circulation model to investigate physical and dynamical features of Arctic Oscillation (AO). We first tried to reproduce the observed AO pattern using this model. External forcing of the model is calcurated from the matrix obtained from the past 50-year observation data by using a liner regression. So the model is referred to_@as "Barotropic S-Model". After running this model for 50 years under the perpetual January condition, we applied the empirical orthogonal function analysis to examine the dominant modes in the model atmosphere. As a result, we obtained an AO-like variaton in terms of both spatial pattern and time spectrum. We examined how the external forcing in our model redistributed the energy of each zonal wavenumber. It is found that a part of the steady compornent of the statistical forcing transports the energy of zonal flow to planatary waves. We considered this compornent as topographical effects. Then, to clearfy the topographical effct upon the AO, we conducted numerical experiments in which the topographical compornents are gradually reduced. According to the result, the AO-like annular structure is modified to a di-pole structure with zonal wavenumber 1 as the topographic effect decreases. When the topographic effect is further decleased, the most dominant mode becomes an annular structure again. It seems that the axisymmetric annular structure with the weak topographic effect is different from the AO in the control run. The result suggests that the AO in the Arctic and Antarctic Osciillation (AAO) in the Antarctic can be dynamically different modes.