A prolonged SALLJ event that occurred in conjunction with biomass burning took place from 23 to 28 August. The low-level jet had an important latitudinal extent and strength with a pattern that varied according to a baroclinic synoptic environment.

3.1.1 Meteorological environment and SALLJ features

Figure 7 depicts the 1000 hPa geopotential height and the 500/1000 hPa thickness fields for selected days during the event. On 23 August, the western branch of the SASH was over an important extension of SA and the low-level flow was from the N as far as 40° S. In the southernmost edge of SA, a baroclinic region — oriented NW to SE — was present and deep low-pressure systems were moving south-eastward. During the following day, a geopotential trough developed over central Argentina. The thickness field showed the associated maximum depth. There was a persistent N-NW flow over south-eastern SA. On 25 August, a further deepening of the trough over central Argentina occurred. The baroclinic region related to the cold front was located between 30° S and 40° S and moved towards the northeast. The low-pressure system behind the cold front weakened. There was a strong channelling of the low-level flow between the trough and the western region of the SASH. Twenty-four hours later, the baroclinic zone approached the southern region of Buenos Aires. A deep thickness trough was present over the eastern Pacific Ocean. Central

Argentina was still with the minimum geopotential. The flow from the north at low levels persisted. On 27 August, the situation was almost similar, with the new system strengthening and moving eastward and starting to surpass the Andes barrier. The northern low-level flow was still present over south-eastern SA and the low pressure further deepened over central Argentina. On 28 August, the system was able to reach eastern Argentina. The associated cold front presented a nearly north-south orientation and moved eastward towards the Atlantic Ocean. The low-pressure system over Argentina deepened and the low-level north-western flow persisted. During 29 August the cold sector of the front moved past Buenos Aires and Uruguay and the related surface cyclone, centred near 40° S and 55° W, deepened. The near-surface airflow over south-eastern SA was from the N — NE sector and from the S in Buenos Aires. On 30 August, the baroclinic region in the 500/1000 hPa thickness field was located at 30° S, with zonal orientation. The surface low — pressure system had its maximum depth at 0600 UTC and then started to fill while travelling to the east over the Atlantic Ocean. Central Argentina had relatively higher surface pressure. The near surface flow was from the S over northern Argentina. During the final day of the study period (31 August) the baroclinic region was in southern Brazil, co­located with a surface col region. Argentina had near surface southerly winds. The surface cyclone was in the occlusion stage at 1800 UTC.

Bolivia to north-eastern Argentina, part of Uruguay and southern Brazil. The jet core was located over northern Paraguay. The related flow was from the N-NW sector. During the following day, the SALLJ had increased strength and vertical shear, as well as more spatial extension. The southernmost edge was near 40° S. On 25 August, the NW-SE region associated with the SALLJ had a greater latitudinal extension and the low-level flow was from the northwest and stronger due to the westward displacement of the Atlantic anticyclone. During the next day, the SALLJ had a smaller southward penetration and reached only 35° S. This was due to the advance of the cold front that was located past 40° S at the 850-hPa level over the ocean. The flow was more northerly oriented. The jet core was over western Paraguay and northern Argentina.

On 27 August, the SALLJ was present over northern and central Argentina. The flow was from the N-NE sector mostly governed by the western region of the anticyclone centred near 32.5° S and 40° W over the Atlantic Ocean. During 28 August, the jet strengthened and spread, reaching the latitudes near 45° S and extending from 65° W to 40° W. The SALLJ reinforced due to the new cold front that was located near 60° W at 1200 UTC with north — south orientation. On 29 August, the front reached Paraguay and south-eastern Brazil. The wind field at 850 hPa shows clearly the northwest wind ahead of the front whereas the winds behind were strong, from the southwest. The region spanned by the strongest winds

has the typical shape of the frontal zone but the wind field did not verify the Bonner’s criteria. During 30 August, the north-western edge of the frontal zone was over Sao Paulo, with the southerly winds blowing clear and dry air over South America up to 15° S. The convergence in the airflow is related to the surface cold front and the baroclinic region near Sao Paulo. The situation persisted on 31 August. SALLJ did not occur either. During this particular event, an important southward penetration of the low-level jet occurred and the associated moisture convergence at the exit region of the current favoured the development of convective systems south of 40° S, which strengthened mostly over the Atlantic Ocean. The interaction with the cold front further contributed to the convection.