Synoptic–dynamic indicators associated with blocking events over the Southeastern Pacific and South Atlantic oceans.

The presence of atmospheric blocking over the Southern Hemisphere causes a change in the zonal flow, resulting in the interruption of the propagation of synoptic systems. This behavior significantly affects the weather on the South American continent and adjacent oceanic areas. This work analyzes the synoptic–dynamic features of blocking, considering the atmospheric conditions that favor onset and decay days of blockings. Blocking events are identified using two meridional gradients of the geopotential height at 500 hPa, over the Southeastern Pacific (SEP) and South Atlantic (SAT) areas, for the period from 1979 to 2015. In general, during the 37 years, blocking events in SAT (SEP) are dominated by the Omega (dipole)-type pattern. Positive anomalies of potential vorticity at 200 hPa, relative vorticity at 200–850 hPa and geopotential height at 500 hPa are found near the blocking regions, two days before onset ([t = to-2]). On onset day ([t = to]), these positive anomalies intensify and expand, affecting much of the blocking region. In addition, negative (positive) GH anomalies at 500 hPa appear in phase with potential vorticity (PV) anomalies at 200 hPa and relative vorticity (RV) at 850–200 hPa. The positive PV anomalies, at high levels, result from the incursion of PV from mid-latitudes towards the pole. On decay day ([t = td]), positive anomalies of GH, PV and RV are still found in the blocking region, but with lower amplitude. After the decay day, the anomalous anticyclonic center (positive GH, PV, and RV anomalies) gradually propagates westward (southeastward) in SEP (SAT). Finally, the RV anomalies, in the blocking region, extend from low to high levels of the troposphere, characterizing the barotropic structure for consecutive days. [ABSTRACT FROM AUTHOR]