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Marambio in the Quasi-Conservative Coordinate System

We determine the position of the station as a function of potential temperature and equivalent latitude changing the 3D geographical coordinate system by the 2D quasi-conservative coordinate system. The equivalent latitude is a modified potential vorticity variable defined as the latitude enclosing the same area as the PV contour [McIntyre and Palmer, 1984]. Then the position of the vortex in equivalent latitude is determined. The vortex limit and the width of its edge are calculated in equivalent latitude using the approach of Nash et al. [1996]. In the new coordinates system, the pole corresponds to the position of maximum potential vorticity and coincides with the center of the vortex. We basically identify three regions: inner vortex, characterized by high PV values; outer vortex or ‘surf zone’, by small PV values and the vortex edge, corresponding to the high PV gradient area. The vortex limit corresponds to the maximum gradient of PV weighted by the wind module, as a function of equivalent latitude. The inner and outer borders correspond to the local maximum and minimum of the second PV derivative. The position of Marambio and the vortex borders as a function of equivalent latitude and day at different isentropic level in 1997 is shown in fig. 1. The meteorological data used to compute the equivalent latitude and potential temperature are from the ECMWF (European Center for Median Range Weather Forecasts) analyses. Figure 1 shows that the polar vortex forms earlier in the middle stratosphere in autumn and propagates to the lower levels. The classification method is thus less efficient during this season because of the flatness of the PV fields, especially in the lower levels. The statistical study of the vortex occurrences above Marambio as a function of the isentropic level and season in 1997-1999 period is shown in fig. 2.

Figure 1. Temporal evolution of the Marambio position (dots) in equivalent latitude for autumn-spring period of 1997 at different isentropic levels. The solid lines correspond to the limits and boundaries of the vortex edge.

Figure 2. Percentage of inside (open circles), edge (pluses) and outside (dots) vortex situations of Marambio station as a function of season at different isentropic levels in the 1997-1999 period.

In autumn the outside vortex occurrences are predominant at low altitudes. The number of edge situations increases with altitude. The prevalence of vortex edge situations in 1997 and 1999, and inside situations in 1998 above 600 K is due to the greater vortex area in the middle stratosphere. This feature is most evident in 1998 with ~70% inside vortex occurrences. In winter, the station position alternate between inside and vortex edge situations in more than 40% of the cases at all levels. In 1998 up to ~80% inside vortex situations were encountered. In spring, the inner vortex situations are still predominant.


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