外强迫对热带季节内振荡影响的模拟研究

应用经过修改的NCAR CCM3模式和CAM2模式进行的数值实验结果以及NCEP的GFS模式的输出结果讨论了海温等外强迫作用对热带季节内振荡的影响.结果表明,热带季节内振荡是热带大气固有的内部变率.它是由大气内部过程的相互作用决定的.但外强迫对热带季节内振荡的强度、传播方向等有明显的影响.当外强迫没有变化时,模式可以模拟出与观测近似的低频振荡.当作为外强迫的海温和太阳辐射有年内季节变化时,模式模拟的季节内振荡则明显减弱.当海温与辐射不仅有季节变化而且有年际变化时,模式模拟的季节内振荡会进一步减弱.具有长周期的外强迫还会削弱季节内振荡中东移波动的能量而增加静止波的强度.在与海洋模式耦合的状态下,模式不受来自海洋的外强迫影响,而是与海洋构成一个耦合系统,可以产生最强的季节内振荡.

A Simulation Study of the Influence of External Forcing on the Tropical Intraseasonal Oscillation

Using the model experiment results from modified NCAR CCM3,modified NCAR CAM2 and NCEP GFS models,the influences of external forcing of SST and solar radiation(SR) on the tropical intraseasonal oscillation(TIO) are discussed.These models are run under various SST conditions and solar radiation conditions.The output variables such as U wind at 850 hPa and precipitation are undergone spatial-temporal spectral analysis by two-dimensional Fast Fourier Transform(FFT).The spatial-temporal spectral power is compared between the model results and observation and between different model cases.When running under the perpetual external SST and radiation the NCAR CCM3.6 model with McRAS convection scheme can simulate pretty well the tropical intraseasonal oscillation which is similar to the observation.This indicates that the TIO is an internal variability of the atmosphere.In other words,the intraseasonal oscillation in the tropical atmosphere is determined by the internal physical processes of the atmosphere.When the same model is driven by the 12 month climate SST and solar radiation with annual cycle the tropical intraseasonal oscillation produced by the model is substantially weakened.In this case the external forcing have only the annual oscillation,it will force the model to have the annual oscillations which have longer periods than the intraseasnal oscillation.When the SST data of the low boundary condition are changed from 12 month climate value to the observed monthly mean SST,the external forcing has both annual cycle and interannual variability.Driven by this low boundary condition the model will be forced to generate longer period oscillation and the TIO is further weakened.The experiments also show that the external forcing with long period variation will also weaken the eastward propagating components of the TIO and increase the standing part of the TIO.Run the NCAR CAM2 model with RAS convection scheme under 12 month climate SST and run it coupled with a slab ocean model and compare the results from the two cases.The results show the simple air-sea coupling run can produce much stronger tropical intraseasonal oscillation than that produced by the run under the climate SST external forcing.Another comparison with NCEP GFS model run under the observed monthly SST forcing(AMIP II type run) and run coupled with a GCM ocean model also show that the coupling model can produce the most strongest TIO and the external forcing with interannual and annual variability will weaken the TIO substantially.