3.2.1 Composites of average and anomalies

In order to identify the main synoptic features associated with SHRE as well as their differences in relation to climatology, composites of average and anomalies were calculated of the variables described in section 2.1. Thirty seven SHRE are considered, which correspond to the total cases identified during the studied period (October-April/2006-2016).

The composites of average and anomalies of the 500 hPa geopotential height and mean sea level pressure, associated with SHRE for D-2, D-1 and D0 are shown in Figure 5. The fields of 500 hPa geopotential height average composites (Fig. 5a) show a slightly amplified frontal trough located near Buenos Aires Province (Argentina) on D-2 that intensifies and amplifies as it moves northeastward and reaches the eastern part of Santa Catarina state on D0. There is another weak trough positioned between northern Argentina and southern Bolivia on D-2, which in the following days connects with the previously mentioned trough. Note that there is a strong geopotential height gradient located to the south of 40ºS, and in this intense baroclinic flow, there are several cyclonic disturbances with a slight amplification that move rapidly eastwards.

Fig. 5 500 hPa geopotential height (gpm in shaded) and mean sea level pressure (hPa in lines) SHRE composites: a) average and b) anomalies. From left to right: D-2, D-1, and D0. CHL= Chaco Low; H=High; L=Low; SASA=South Atlantic Subtropical Anticyclone. 

The mean sea level pressure average composites (Fig. 5a) show a weak cold front located between the Atlantic Ocean and southern Rio Grande do Sul state on D-2, which moves northeastwards in the subsequent days. On this day (D-2), there is a low-pressure system of 1010 hPa related to the Chaco Low, which is located over Paraguay. On D-1, this frontal system is located approximately to the southeast of the Santa Catarina state without reaching the coast of southeastern Brazil on D0. A similar pattern was observed by ^{Andrade and Cavalcanti (2018)} during heavy rainfall summer fronts over southeastern Brazil. Note also that the SASA is located around 30ºS, 20ºW, and slowly moves eastwards over the entire analyzed period. The 500 hPa trough that advances from northern Argentina and southern Bolivia contributes to the formation of a wide low-pressure system of 1012 hPa over the Atlantic Ocean, to the south of RJ during D0. On this day, this cyclone is connected to the cold front and both of these systems con- tribute to the intensification of the moist air mass convergence over the coast of RJ, including the CRJ.

The composite anomalies of 500 hPa geopotential height and sea level pressure are shown in Figure 5b. This combination of levels shows the displacement of a typical baroclinic system, with the 500 hPa wave positioned to the west of the surface wave.

Negative anomalies of mean sea level pressure and geopotential height observed respectively at the surface and the 500 hPa level, allow the identification of the mid-troposphere trough and the cold front at the surface. On the other hand, the positive anoma- lies at both of the levels identify the ridge and the post-frontal anticyclone, respectively at mid- level and at surface. On D-2, there are negative height anomalies with a minimum of -10 gpm at 500 hPa located over northeastern Argentina and Uruguay. This pattern is related to the frontal trough at mid-level that moves northeastwards during the following days. In the South Atlantic Ocean, to the southeast of the Malvinas Islands, there is a minimum of -20 gpm at the 500 hPa level on D-2, associated with a cyclonic disturbance that exhibits a rapid zonal displacement during the analyzed period. The minimum related to the frontal trough intensifies during the subsequent days reaching a minimum value of -40 gpm near 37ºS, 40ºW on D0. Similar behavior is observed at the surface, with slight negative anomalies over RJ of the order of -1 hPa to -2.5 hPa on D-1 and D0, respectively. On SACZ day (D0), a minimum of -5 hPa is observed over the Atlantic Ocean around 42ºS, 30ºW, associated with the low-pressure system related to the cold front mentioned in Figure 5a.

The composites of average and anomalies of streamlines and 850 hPa specific humidity associated with SHRE are shown in Figure 6. The composite of streamlines at 850 hPa (Fig. 6a) show the typical mean circulation pattern over South America during the rainy season in Brazil. There is a clear northerly/northwesterly flow over continental areas during the analyzed period. This circulation pattern at low levels is determined by the trade winds that penetrate to the north of the continent and deviate by the action of the Andes Cordillera, and by the winds coming from the Atlantic Ocean related to the presence of the SASA. On D-2, there is northerly wind over the RJ, determined by the western edge of the SASA since the flow from the Amazon region is directed to the northern part of southern Brazil. On D-1, the flow caused by the SASA persists over the RJ, and the influence of the Amazonian flow is beginning to be observed. During D0, there is a large influence of the Amazonian flow over RJ and a smaller contribution from the wind from the Atlantic Ocean. This change in the atmospheric circulation at the 850 hPa level is related to the advance of a frontal trough that on D-2 is observed approximately in the farther south of Rio Grande do Sul state and on D0 is identified over the Atlantic Ocean, close to São Paulo state. On D-2 the Chaco Low can also be observed in southern Bolivia and western Paraguay, which is connected with the frontal trough mentioned above. On D-1, the frontal trough is near the coast of Santa Catarina state and, simultaneously, another trough can be observed further to the north with its axis extending between southern Bolivia and the far west of Santa Catarina state. On D0, this trough advances northeastwards, extending between southern Mato Grosso state and northern Paraná state, where a low-pressure system forms. With such low-level atmospheric circulation, the RJ is affected by warm and moist air transported from the Amazon Basin.

Fig. 6 850 hPa streamlines and 850 hPa specific humidity (gkg^-1 in shaded) SHRE composites: a) average and b) anomalies. From left to right: D-2, D-1, and D0. CHL= Chaco Low. 

The composites of 850 hPa specific humidity (Fig. 6a) show high values over most of the North, Central-West and Southeast regions of Brazil, of the order of 12 to 14 gkg^-1, typical of the rainy season in Brazil. The specific humidity shows an increase over RJ from approximately 10 to 13 gkg^-1 between D-1 and D0. This increase is related to the change in the wind direction from northerly to northwesterly, as previously described. Also, as noted by ^{Moraes et al. (2020)}, when studying mesoscale convective complexes forming in subtropical South America, the moisture increase results from the enhanced convergence between the flows coming from the Amazon Basin and the Atlantic Ocean. It is noteworthy that during the rainy season the specific humidity over the southeast region is very high due to the combination of high temperatures and the persistent northerly flow from the Atlantic Ocean and from the northern Brazil.

The 850 hPa streamline composite anomaly fields (Fig. 6b) show the zonal displacement of cyclonic anomalies that on D-2 are centered over Uruguay and in the following days move over the Atlantic Ocean, approximately south of 40ºS and between 40ºW and 30ºW. This anomalous pattern is mainly associated with the displacement of the cold front described previously (Fig. 6a). The anomalous trough associated with these cyclonic anomalies extends from the southern Amazon Basin to the South Atlantic Ocean, determining a band of negative anomalies that affects much of southeastern Brazil, especially since D-1. A similar configuration was found by ^{Doss-Gollin et al. (2018)} when they analyzed 850 hPa anomaly fields associated with the occurrence of heavy rainfall in the central-southern Paraguay. The authors identified a circulation pattern with cyclonic anomalies over the Atlantic Ocean, southeast of southern Brazil, one day after the occurrence of heavy rainfall in Paraguay.

The specific humidity composite anomaly fields (Fig. 6b) are consistent with the anomalous atmos- pheric circulation pattern described above. There is a large area of positive anomalies of specific humidity located over northeastern Argentina, Paraguay, southern Brazil, most of Mato Grosso do Sul state and the western and southwestern part of São Paulo state during D-2, showing small values (~1 gkg^-1-1.5 gkg^-1). On D-1, these positive anomalies are located to the east of the anomalous trough, affecting most of the states of Paraná, Mato Grosso do Sul, and São Paulo, and the adjacent Atlantic Ocean. Moreover, negative anomalies of specific humidity varying between -1 and -2 gkg^-1 are also observed over Uruguay, northeastern Argentina and central-western Rio Grande do Sul state, related to the advection of dry air produced by the advance of the post-frontal anticyclone (Fig. 4a). On D0, the positive anomalies are located over the São Paulo and RJ states, western and southern Minas Gerais state and in the adjacent Atlantic Ocean. In addition, the negative anomalies are more intense comparing to D-1, affecting Uruguay, northeastern Argentina, most of Paraguay, Rio Grande do Sul state and most of Santa Catarina state.

An interesting feature to mention is the esta- blishment of a “dipole” configuration in the specific humidity anomalies fields during D0. This dipole, also called the seesaw pattern, is determined by the presence of positive anomalies in southeastern Brazil and by the presence of negative anomalies in the South, in agreement with ^{Nogués-Paegle and Mo (1997)}.

The composites of average and anomalies of 200 hPa streamlines and geopotential heights associated with SHRE are shown in Figure 7. Average compo- sites (Fig. 7a) allow identifying a typical upper-level atmospheric circulation pattern of the rainy season in Brazil, with the presence of the Bolivian High and the trough in the Northeast region of Brazil. This pattern is observed in climatology during the rainy season regardless of the occurence of SACZ events because it is associated with strong convective activity in the Amazon Basin (^{Lenters and Cook, 1997}). On D-2, there is a trough with its axis extending from central Argentina to the Atlantic Ocean that intensifies as it propagates eastwards over the following days. On D-1, the axis of this trough is positioned approximately between northeastern Argentina and the Atlantic Ocean and on D0 it reaches southern Brazil, extending between Paraná state and the Atlan- tic Ocean.

Fig. 7 200 hPa streamlines and 200 hPa geopotential height (gpm in shaded) SHRE composites: a) average and b) anomalies. From left to right: D-2, D-1, and D0. BH=Bolivian High. 

This upper-tropospheric trough shows an intense horizontal geopotential height gradient that is associa- ted with the cold front identified at the surface and at the 850-hPa level. On D0, this upper-level trough increases the divergent flow over RJ and southern Minas Gerais state, contributing to the intensification of the convergence flow at the surface. Therefore, with such upper-level atmospheric circulation pa- ttern, the precipitation will intensify over the study area during the SHRE.

The Bolivian High appears practically stationary over Bolivia during the analyzed period; however, its associated ridge changes position particularly from D-1. On D-2, the axis of the ridge extends southward from Bolivia to the northeastern part of Southern Brazil. The ridge moves towards southeastern Brazil on the following days and on D0 it appears approximately over RJ, southern Espírito Santo state and central-southern Minas Gerais state. Simultaneously, the trough in the Northeast region of Brazil moves slightly to the west as the ridge intensifies towards the Southeast region of Brazil. The change in the upper-level atmospheric circulation associated with both of the systems (the ridge and the trough of the Northeast region of Brazil) is related to the advance of the frontal trough from mid-latitudes.

The 200 hPa streamline and geopotential height anomaly fields (Fig. 7b) reveal a well-defined wave train that propagates northeastward, with an anomalous ridge over the South Pacific Ocean and southern South America and an anomalous trough over most southern Brazil. There are positive geopotential height anomalies on D-2 over most southern Brazil and Paraguay and part of Mato Grosso do Sul and São Paulo states, with values varying between 10 and 20 gpm. These positive anomalies increase the intensity and propagates eastwards on D-1, reaching the RJ and the southern part of MG and Espírito Santo states. Anti- cyclonic anomalies are located over the Atlantic Ocean on D0, centered to the southeast of RJ but nevertheless affecting most of RJ, MG and Espírito Santo states. On this day, the highest anomalies are of the order of 20 to 30 gpm over RJ, southern part of Espírito Santo state and the south of Minas Gerais state. These positive anomalies are associated with the intensification of the ridge described in Figure 6a, indicating strong divergence at upper levels and, consequently mantaining moisture convergence at low levels. Thus, such an anomalous upper-level atmospheric circulation pattern is related to the mean position of the band of cloudiness associated with SHRE that contributes to precipitation intensification in the CRJ.

The negative anomalies show a pattern similar to that observed at the 500 hPa level (Fig. 4b). On D-2, there is a band of a cyclonic anomaly of streamline oriented northwest-to-southeast, from northern Argentina to the South Atlantic Ocean. The highest values of the negative geopotential height anomalies are of the order of -10 gpm to -20 gpm, located approximately to the east of the Buenos Aires Province (Argentina) and the adjacent Atlantic Ocean. On D-1, these negative anomalies intensify as they move eastwards and cover northeastern Argentina, most of Uruguay and the Rio Grande do Sul state. On D0, the negative anomalies continue increasing in intensity, showing values of approimately -60 gpm in southeastern Rio Grande do Sul state and the adjacent Atlantic Ocean. On this day, the cyclonic streamline anomalies extend from southern Bolivia to the South Atlantic Ocean. A similar configuration at high levels was found by ^{Nielsen et al. (2019)} when they studied the SACZ episodes during the South American Monsoon period. The authors identified the presence of cyclonic anomalies between southern Brazil and the adjacent Atlantic Ocean associated with the presence of SACZ located in different positions.

In the Tropical Atlantic Ocean east of the Northeast region of Brazil, there are cyclonic streamline anomalies affecting the area where the trough of the Northeast region of Brazil acts during the rainy season. This result is in agreement with ^{Carvalho (1989)} who suggests that the most intense SACZ events are associated with more intense trough of the Northeast region of Brazil.

3.2.2. Synoptic patterns associated with SHRE

The synoptic classification at the surface associated with SHRE, identified seven PPSs that explain 83.8% of the total variance, with the first four PPSs representing 69.8% of that total (Table II). The remaining PPSs were not considered in the analysis because their series of component loadings presented values lower than 0.5. The four PPSs associated with SHRE are shown in Figure 8.

Fig. 8 Principal Pattern Sequences (PPSs) obtained at surface level. a) PPS1, b) PPS2, c) PPS3, d) PPS4. From left to right: D-2, D-1, and D0. CHL= Chaco Low; H=High; L=Low. 

PPS1 (Fig. 8a) and PPS2 (Fig. 8b) show a sequence pattern associated with the displacement of a cold front that arrives on the coast of RJ on D0. PPS1 explains 21.4% of the total variance and is associated with a shorter and more oceanic baroclinic system than PPS2. It is noted that the cold front is unable to reach the continent because on D0 the frontal system appears connected with a low-pressure system loca- ted over the Atlantic Ocean, close to the coast of RJ. PPS1 also shows the Chaco Low, located between northern Argentina, central-western Paraguay, and southern Bolivia. This synoptic pattern at the surface is similar to the average of the mean sea level pressure associated with SHRE, obtained through the composite method (Fig. 5a). This result was expected due to the fact that the PPSs that explain the higher percentage of the total variance is related to the most frequent synoptic pattern, which coincides frequently with the average field. The PPS2 sequence pattern (Fig. 8b) explains 20.0% of the total variance and shows on D0 the cold front over RJ with a more intense and wider post-frontal anticyclone compared to PPS1.

PPS3 (19.0%) (Fig. 8c) also shows an intense and wide post-frontal anticyclone with a slow displacement during the analyzed period. On D-2, the frontal system is already over São Paulo state, reaching RJ during D-1. On D-1 and D0, the post-frontal anticyclone is practically stationary over the Atlantic Ocean around 35ºS, 45ºW, and acquiring blocking charac- teristics. With such surface atmospheric circulation pattern, the RJ is affected by persistent southeast winds which favor increased moisture convergence and, consequently, heavy rain intensification in the CRJ.

The patterns PPS2 and PPS3 are similar to those identified by ^{Escobar (2019)} related to a synoptic climatology during the Brazilian rainy season. The last sequence pattern (PPS4) (Fig. 8d) explains 9.4% of the total variance and reveals the presence of a low-pressure system located over the Atlantic Ocean to the south of RJ, which remains practically stationary during the analyzed period. This low-pressure system on D-2 is observed connected with a cold front located over the South Atlantic Ocean, close to 40ºS, 35ºW. On D-1 and D0, the cold front moves eastwards quickly whilst the low-pressure system remains practically in the same position, approximately at 25ºS, 45ºW. In general, these cyclones have a weak baroclinic structure whose associated cold fronts affect the coasts of the São Paulo and RJ states. On several occasions, these cyclones are not frontal and show subtropical features (^{Evans and Braun, 2012}; ^{Escobar, 2014}; ^{Gozzo et al., 2014}; ^{Brasiliense et al., 2018}; ^{Silva et al. 2019}). In both cases (extratropical or subtropical cyclones), this cyclone contributes to increase moist and warm air convergence towards southeastern Brazil. Moreover, surface southerly and southeasterly winds also favor rainfall intensification over the coastal areas of RJ.

While the low sample size (37 cases) is a statistical limitation, pattern PPS3 related to a blocking configuration, was the synoptic pattern of SHRE that showed the longest duration. The rest of the patterns did not show significant differences associated with the duration of SHRE events.

Similar results were obtained asking how these four primary patterns varied throughout the October April rainy season. There were no significant differences associated with the intraseasonal variability of the four PPS of SHRE.

Despite the small number of cases, pattern PPS2 associated with the incursion of cold fronts, was more common in spring (November) and autumn (March) than in summer. This result is similar to ^{Escobar (2019)} of the synoptic climatology during the Brasilian rainy season.

Differences in the accumulated precipitation were evaluated for each of the four patterns associated with SHRE. Despite the few cases of SHRE events, the synoptic composite maps (figures not shown) did not show significant differences between the patterns (PPSs).

Figure 9 shows the synoptic surface charts highly correlated with the four PPSs associated with SHRE. The synoptic surface charts associated with the patterns PPS1 (Fig. 9a) and PPS2 (Fig. 9b) show a cold front over the Atlantic Ocean, favoring mass and moisture convergence towards the continent. The synoptic surface chart related to the pattern PPS3 (Fig. 9c) is associated with a wide high-pressure system located over the Atlantic Ocean and the stationary front close to the coast of RJ state. Such surface atmospheric circulation is similar to a blocking configuration pattern, with southeasterlies over the São Paulo and RJ state’s coast and favoring the intensification of heavy rain in the CRJ. The surface synoptic chart associated with PPS4 (Fig. 9d) shows the presence of the subtropical cyclone “Bapo” located in the Atlantic Ocean, approximately to the southeast of Santa Catarina state. This low-pressure system can persist for several days maintaining the convergence of a warm and moist air mass from the Amazon region to the Southeast region of Brazil and, consequently contributing to the intensification of the rain in the CRJ.

Fig. 9 Surface synoptic charts elaborated by CPTEC/INPE at 1200 UTC associated with SHRE in the CRJ. a) January 14, 2016. b) March 12, 2016. c) February 17, 2014. d) February 6, 2015. Sea level pressure (hPa) in continuous yellow lines and 1000-500 hPa thickness in red dashed lines. Synoptic symbols as in conventional mode.