南半球対流圏における単一ジェットと二重ジェット構造の間の交互的変動

Abstract

An alternate variation between single and double jet structures in the Southern Hemisphere (SH) troposphere is investigated observationally and numerically. First, data analyses are made for 17 winters from 1979 to 1995 using the NCEP reanalysis dataset. Empirical orthogonal functions (EOFs) of the zonal-mean flow are calculated, and a probability density function (PDF) is constructed by the two leading EOF modes. Maxima of the PDF coincide with the two regimes, i.e., the single and double jet regimes, although the latter has relatively large variations in flow patterns. A statistical test indicates marginal significance for the bimodality. Transitions between the two show no marked periodicity. A low-resolution numerical model based on the primitive equation system is constructed, and is found capable of simulating the observed alternation in the single and double jet structures. The dynamical basis of this phenomenon is sought by examining the model's behavior with respect to changes in the magnitude of the horizontal diffusion coefficient. Simulated maxima in PDFs are not so significant for the lower and more realistic values of the diffusion, but become more significant for higher values. Furthermore, it is found that the model has only one attractor corresponding to the single jet regime for sufficiently large diffusivity that makes the system more stable. Near the critical value of the diffusion parameter, the system resides for a very long time in each one of the two flow regimes, and transits irregularly between them. Therefore, it is suggested that the observed variability in the SH jet structure can be understood as chaotic wandering between two distinct flow regimes, whose existence would be clearer if the system were less turbulent.