Previous: The performed simulations Next:Results and Remarks Up: Ext. Abst.

Diagnostic Tools.

In order to better understand the mechanism involved in the establishment and maintenance of those circulation we use two diagnostic tools which are presented here: E-P flux and balance of vorticity analysis.

Eliassen-Palm (E-P) flux diagrams

A concise diagnostic for the propagation characteristic and mean flow forcing of the large scale planetary waves is provided by E-P flux cross sections (Edmon et al. 1980; Andrews et al. 1987). We follow here Section 3.5, from Andrews et al. 1987 and partially Section 3 (particularly c) from Randel and Newman, 1997.

Estimates of the planetary wave EP flux and its divergence are calculated here for the "Control" simulation (Fig. 10) as indication of the October "climatological" (simulated by the model) mean flow capability of the large scales wave to drive meridional residual circulations and/or changes in the zonal mean wind.

We show in Fig. 10 EP flux diagram and its associated wave forcing (), (F is EP flux vector, as defined in Randel and Newman 1997), for the "Control" simulation, based on the sigma levels used by the model. We can conclude that most of the more relevant features of the SH springtime phenomena are present in the October background circulation simulated by our experiments. Particularly, important values of the springtime EP flux are obtained for the SH, specially in low stratospheric levels, which is a very important feature if we want to be able to adequately simulate driving of the stratospheric circulation through tropospheric anomalous circulations (as those imposed when a strong EN events develops in the tropical Pacific, as in our experiment).

Vorticity Analysis

We study the mechanisms involved in the polar vortex wave forcing by making a vorticity analysis.

A vorticity equation for the monthly means (noted with overbars), can be derived from the basic momentum equation solved by the model, and we obtain:

(1)

( in this case primes indicate deviation from the overbared magnitude; a complete deduction of this equation can be read in Cazes and Pisciottano 1998).

Equation 1 is written in vertical sigma coordinate, sigma dot is the respective vertical velocity and p is adequately defined.

The first three terms in the right side of (Eq. 1) are the usual Rossby wave mechanisms: the advection of planetary vorticity (beta-effect), the advection of the relative vorticity and the induction of vorticity by divergence, respectively. The fourth term represents the forcing (to the monthly variables) due to (intramonthly) transients. Figures 13 and 14 show different terms of the equation of balance of vorticity for the "Anomaly" simulation, representing the features associated to this strong EN event.