Abstract The effects of low- and high-frequency eddies (time scales longer and shorter than 10 days, respectively) on the transitional processes of the Southern Hemisphere “Annular Mode” are investigated, based on NCEP–NCAR daily reanalysis data for the period 1979–2001. Special attention is focused on the zonal symmetry/asymmetry and the temporal evolution of the eddy forcing. For the poleward transitional process, the effects of low-frequency eddies precede those of high-frequency eddies in driving the jet transition. Quasi-stationary Rossby waves propagating along the polar jet with wavelengths of 7000 km play an important role. The waves, originally come from the Indian Ocean through the waveguide associated with the polar jet, dissipate equatorward over the eastern Pacific Ocean. This anomalous equatorward dissipation of wave activity induces an anomalous poleward momentum flux, which is responsible for changes in the polar jet over the Pacific Ocean during the beginning stage. Following the low-frequency eddy forcing, momentum forcing anomalies due to the high-frequency eddies rapidly appear. This forcing continues to drive the polar jet poleward over the whole of longitude, while the low-frequency eddies have completed their role of inducing the anomalous poleward momentum flux during the earlier stage. For the equatorward transitional events, the roles of the low-frequency eddy forcing differ from that in the poleward ones. Anomalous equatorward momentum fluxes due to low-frequency eddies appear simultaneously with that due to high-frequency eddies. Quasi-stationary Rossby waves with wavelengths of 7000 km propagate southeastward through the waveguide over the Pacific Ocean. The convergence of their wave activity results in the deceleration of the westerlies over the higher latitudes of the Pacific Ocean. On the other hand, the high-frequency eddy forcing contributes to the equatorward jet drift longitudinally over the whole of the hemisphere.
Lorenz (1968, 1976) stated that regime transition in almost-intransitivity of nonlinear climatic system may play an important role in climatic change, and he suggested that climatic change associated with the transition may appear in interannual variabilities. Referring to Lorenz's suggestion, we will treat abrupt changes of time mean, designated as climatic jumps. A quantitative definition of jump and simple method of its detection are presented, noting that the time of jump appearance can be specified within a margin of several years. Some jumps are detected in time series of seasonal mean data of surface air temperature, sea level pressure, precipitation, sunshine duration and maximum depth of snow-cover averaged spatially over Japan. The fact that jumps appear commonly in various climatic elements around 1950 suggests an association of these jumps with some abrupt changes of the atmospheric general circulation.Concerning the cause of the jumps around 1950, we survey some change in external forcings. Big explosions of several volcanoes over the world occurred almost simultaneously with the jumps around 1950 after a pause of about 30 years. It is inadequate to assume that this volcanic activity would directly cause the jump in transitive system, because the possible climatic effect of volcanic eruption is mainly cooling and the jump of temperature is warming over Japan. However, further studies are needed for any definite conclusion on problem whether this reopening of volcanic activity would be a triggering action of the regime transition or not.
A three-level linear-balance model for studies of the general circulation using the spectral method is presented with some preliminary results of time integration of the model for a specified terrestrial atmospheric condition.The model is, firstly, integrated for 180 model days under a specified condition to obtain a quasi-steady state without eddies, starting from a calm atmosphere. The zonally symmetric state thus obtained is unstable for the disturbance with zonal wavenumber 5 or 6, according to the linear theory of baroclinic instability. Secondly, after adding disturbances with small amplitudes to the above results of zonally symmetric model, the model integration is continued for another 180 model days to obtain a quasi-steady state with including eddies. The baroclinic instability appears to be released around the day 40, and thereafter the near-equilibrium state is obtained. The results are illustrated mainly for the zonal mean field averaged during either the period of the last 100 days or that of release of baroclinic instability (the day 30-50), and for the time evolutions of energy components and energy conversion terms. The results are not necessarily realistic, because somewhat large values are used tentatively for a part of several parameters, such as the vertical eddy mixing coefficient. Nevertheless, reasonable results are obtained and they are consistent under the model assumption.In order to study extensive problems concerning the general circulation and the climate of the planetary atmospheres, such a numerical model as is simple, economical and easy to handle is useful and worth-while to construct. So far as the preliminary results are concerned, we may say that the present model is one of powerful tools for further studies on the above subjects.
A method for analysis of the ultra-long waves in the atmosphere, which are represented by Fourier harmonics of the geopotential field etc., is presented. Firstly, the ultra-long waves are separated into the quasi-steady and the fluctuating parts by applying several band-pass and one low-pass time filters to the time series of Fourier harmonics. Secondly, the fluctuating parts, which pass through the band-pass filters, are devided into the transient part and the quasi-stationary part with temporal change of the amplitude, with the procedure of shifting the cosine and sine-time series by a quarter of the period. This method is applied to the geopotential field, and some preliminary results are given as an example.
For the purpose of studying the atmospheric response (or sensitivity) to the seasonal forcing, we propose a time-space spectral general circulation model. In the present work we use a low-order equivalent Barotropic vorticity equation with forcing and dissipation terms. The original low-order equation consists of three-component in the space-spectral domain: one purely zonal component and two wave components with the same zonal wavenumber but different mode numbers. Further expanding this equation in the space-spectral domain into that in the time-spectral domain, we finally obtain a time-space spectral model, which is a simultaneous nonlinear algebraic equations system.The model used in the present work consists of one steady and one periodic components in the time-domain. The nonlinear equations system is solved by using the "revised Marquardt method" (Levenberg-Marquardt-Morrison method) and a "Continuation method". Addition of the periodic forcing besides the steady forcing brings about a new stable solution together with the stable solutions corresponding to those under the steady forcing only. This multiplicity of solutions with the moderate intensity of steady and periodic forcings is very interesting for understanding climatic change, and suggests that we can expect to obtain useful results, by extending our model to a further larger timespace spectral general circulation model based on the above-mentioned numerical methods.
To investigate the oscillation of ultra-long waves with periods longer than 12 months, a lowpass time filter and periodogram analysis are applied to zonal harmonics of wave numbers 1 to 5 of monthly mean 500mb geopotential fields at latitudes 30, 40, 50 and 60°N from January 1946 to December 1970. Statistically significant quasi-biennial period is found in the oscillation of amplitude for some of wave numbers at each latitude. Some correspondence is found between the oscillation of amplitude and that of monthly mean zonal wind in the equatorial stratosphere.
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