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南半球热带外准半年振荡及IAP 9L AGCM模拟检验
Semiannual Oscillation of the Extratropical Circulation in the Southern Hemisphere and Its Numerical Simulation
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摘要: 基于1979—2000年的NCEP/NCAR海平面气压和位势高度场资料分析了南半球大气环流的准半年振荡 (半年波) 现象。结果表明:这一现象主要出现在南半球对流层低层的中高纬度和中高层的热带地区。对南半球热带外大气而言, 40°S和65°S是低层大气环流准半年振荡最为显著的两个纬度带, 半年波的贡献都超过了70%, 低层南半球中高纬度海平面气压场季节变化的反位相也主要体现为各自半年波分量变化的反位相。在此基础上, 检验了IAP 9L AGCM (大气物理研究所9层大气环流模式) 对这一现象模拟的能力, 模拟结果显示, 模式成功模拟了65°S处海平面气压场的准半年振荡现象, 其振幅略低于观测结果, 但模式对40°S处气压场准半年振荡的模拟效果较差。Abstract:
The semiannual oscillation (SAO) of the atmospheric circulations in the extratropic regions is one of the unique characteristics in the Southern Hemisphere. Based on the NCEP/NCAR reanalysis datasets of the sea level pressure (SLP) and the geopotential height from January 1979 to December 2000 the oscillation is studied. Results show that the SAO is most active in the extratropic regions in the lower troposphere but the tropics in the middle-higher level. The extratropical SAO of SLP is found to be most significant in 40°S and 65°S, i.e., the two branches of the Antarctic oscillation (AAO) or named as the Southern Hemisphere annular mode (SAM), where each variance percentage of the half-year period exceeds 70%. That is to say that the anti-phase relationship for the annual cycle of SLP between the two latitudes is primarily caused by their SAO components. But the SAO is not remarkable between these two latitudes, especially in 55°S, i.e., the node position of the standing wave of the AAO. In 55°S, the variance percentage of SAO is less than 20% while that of the annual cycle is greater than 70%. Results also indicate that the phases at the latitudes of 40°S and 65°S are inconsistent, with maximam variance percentages occurring in March and September while minimums in June and December respectively in 40°S, but maximums in January and July and minimums in April and October respectively in 65°S. In other words, the phase of the SAO in the near-polar latitude has a one-month lag than in the middle counterpart.At present, most of the general circulation models (GCMs) are poor in simulating the characteristics of the extratropical SAO in the Southern Hemisphere. So the SAO is used widely as a criterion to verify a GCM's simulation skill. Based on the above results, a nine-level atmospheric general circulation model developed in the Institute of Atmospheric Physics (IAP 9L AGCM) is employed to simulate the SAO. In general, the model is successful in simulating the SAO of SLP along 65°S, with its intensity some what weaker than the observation. On the other hand, the IAP 9L AGCM fails to simulate the SAO along 40°S.
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Key words:
- semiannual oscillation;
- Southern Hemisphere;
- sea level pressure
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图 1 1979—2000年南半球3月 (a), 6月 (b), 9月 (c) 和12月 (d) 平均海平面气压场
(单位: hPa, 1020 hPa以下等值线间隔为5 hPa, 1020 hPa以上间隔为10 hPa, 阴影区气压值小于985 hPa)
图 4 1979—2000年40°S (a) 和65°S (b) 平均pSL (粗实线) 及各自年周期分量(细虚线, 方差贡献率为λ1) 和半年周期分量 (细实线, 方差贡献率为λ2)
图 5 1979—2000年南半球500 hPa (a) 和200 hPa (b) 平均位势高度场半年周期分量方差贡献
(单位: %, 阴影区表示半年分量方差贡献大于年分量贡献)
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