This event extended from 17 to 21 October and was characterised by a variable low level flow pattern, which had a short SALLJ episode and a changing meteorological scenario, with transient perturbations of short duration.

3.1.2 Meteorological environment and SALLJ features

On 17 October, the 1000 hPa height shows the dominance of a post-frontal high pressure system over central Argentina (Figure 12). The surface front is located over central South America. On the south-western region of Argentina, the 500/1000 hPa depths show a baroclinic zone associated with a new frontal system.

During the following day, the anticyclone moved to the Atlantic Ocean, centred about 40° W and 35° S. Behind the baroclinic zone, a low pressure system located near 65° W and 47° S, developed. A thickness through oriented from the NW to the SE, is observed over the Pacific Ocean associated to an upper air through. The low level flow over north-eastern Argentina was from the north. On 19 October, the surface low pressure region had a fast displacement towards the SE. On the other hand, an anticyclonic system moved eastward covering the southern region of Argentina. North of 30° S, central South America showed relatively lower pressures. By 20 October, the thickness through axis was over Los Andes Mountains and then moved eastward. The low pressure system on central-northern Argentina displaced to the east and accordingly, the flow near the surface turned and blew from the east over Buenos Aires. On 21 October, a low pressure system developed and evolved in agreement with the displacement of the pattern at upper levels. It is located around 40° S and 50° W. Argentina was under the influence of an extended anticyclone. The near surface flow was from the south.

Figure 13 illustrates the 850 hPa flow and SALLJ features. On 17 October the low level flow associated to the post-frontal anticyclone centred over Buenos Aires is clearly shown. A very weak SALLJ is evident in the 850-700 layer, between Los Andes and the west of an anticyclone. The smaller wind intensities are observed over the biomass burning source regions. By 18 October, the low level flow strengthened and organized in a northerly current due to the approach from the southwest of the new cold front and the presence of the anticyclone now centred at 45° W and 35° S over the Atlantic Ocean. The 850 hPa winds did not satisfy the Bonner criteria. The north-western edge of the cold front is located near 35° S and 65° W. On 19 October the SALLJ spanned from central Bolivia to Paraguay and northern Argentina. The wind was from the north. Buenos Aires was behind the cold front. Another region with low level jet occurrence is over the Atlantic Ocean centred at 15° S. South of 30° S, the flow turned counter clockwise and acquired a north-western orientation ahead of the cold front. On 20 October, a SALLJ occurred, with its southern edge near 30° S. The front remained stationary over central Argentina. A low pressure system developed in the central region of Argentina whereas the exit region of the SALLJ was on southern Brazil. During the next day, there is a clear evidence of a strengthening and rapid displacement of the cold front that is oriented NW to SE. The low-level flow was from the south up to 20° S.