Free Mode And Low-frequency Oscillation In Extratropical Latitude

Using variational memod,the effect of the observed free mode is obtained and a type of analytical free modes(ideal free mode) are designed in the text.Normal-mode analysis is performed to examine the stability of the observed free mode and the ideal free mode.The results indicate that the two kinds of free modes are unstable and are quasi-stationary or long-periodic and low-frequency.Especially,the quickest increasing free mode approaches to be quasi-stationary and is of great local.Using two kinds of free modes as forcing fields respectively,and 500hPa streamfunction for July 1,1993 as a initial field,then the numerical experiments are run for 300 days by a forced and dissipative global spectral rnodel,the results show that:(l)When using the observed free mode of December 27-28,1992 as a forcing field:When a = y =k =50.02,that is to say,when the forcing and dissipative coefficient is equivalent and great enough,model atmosphere always approaches to the forcing free mode as time evolves;When the forcing coefficient a =0.1 and thedissipative coefficient y =0.02, the local quasi-biweekly oscillation of the conversionbetween meridional circulation and zonal circulation in extratropical latitude will occur;(2)When using the free mode of idealized atmosphere as a forcing field, and making the range of forcing coefficient 0.0095≤ a≤0.011, the dissipative coefficient 0.01,the quasi-two weeks index cycle caused by the conversion between meridional circulation and zonal circulation in extratropical latitude will appear and the period of oscillation has a positive correlation with the value of the forcing coefficient;(3)If the two kinds of free modes are used as a forcing field respectively and the forcing iterm cann't he counteracted by the dissipative iterm,the model atmosphere will explode ultimately.When using the ideal free mode as a initial field without the dissipative iterm existing,the quasi-two weeks oscillation will take place in extratropical latitude.If the two kinds of free modes are used as a initial field separately and there is no the forcing iterm but the dissipative iterm,the system of model atmosphere will move westwards and the value of streamfunction will minish as time evolves.To sum up,whichever free mode is used as a forcing field,when the forcing and dissipative coefficient is chose in proper range,the quasi-two weeks index cycle caused by the conversion between high-index meridional circulation and low-index zonal circulation in extratropical latitude will appear.The new physical mechanism is put forward accordingly that the low-frequency fluctuation may be caused by the quasi-two weeks circulation between the high index and low index.