4.1 Atmospheric Patterns
In the three selected years (2006, 2004 and 2008), an anticyclonic circulation at 850hPa, characteristic of the South Atlantic Subtropical High (SASH; Vera et al., 2006; Raia; Cavalcanti, 2008; Marengo et al., 2012) was observed in its most westerly position in September (Fig. 3). As the rainy-season onset approached, the SASH moved away from the coast. The rainy season onset was characterized by the pressure reduction in the SA continent, observed with anomalies of up to -5hPa over the Chaco region (between east Paraguay and north Argentina) in October of the early onset year (Fig. 3b). In the same year, the SASH was weaker compared to the neutral and late onset years (Fig. 3a, b, g, h, m, n). On the other hand, in the late onset year, positive SLP anomalies were observed since July (not shown), configuring a more intense SASH. In addition, in September, between 40–60°S and 60 − 30°W, a ridge and positive SLP anomalies with a magnitude of up to 12hPa were observed (Fig. 3m). It is suggested that this configuration was responsible for the late rainy season onset, which occurred only in November, when the continent’s pressure reduction was verified (Fig. 3o). From November to February (Figure c, d, e, f) of the early onset year, the greatest negative SLP anomalies were concentrated in the central and southern areas of SA, while in the neutral and late onset years, the negative anomalies appeared throughout the SA, being more intense over the southeast, except in January of the late onset year.
In October of the early onset year (Fig. 4b), positive moisture flux magnitude anomalies were observed over the eastern Andes, where the vectors indicate that the northwesterly moisture flux was intensified. This northwesterly flux with magnitudes of up to 30 kg.m− 2.s− 1, was maintained from December to February of the early onset year (Fig. 4d, e, f), and in January (Fig. 4e) the positive moisture flux magnitude anomalies extended from the Amazon Basin towards the Southeast region of Brazil and to the Atlantic Ocean, associated with the SACZ’s configuration (Kodama, 1992).
In September of the late onset year (Fig. 4m), negative moisture flux magnitude anomalies between 40–60°S and 60 − 30°W were observed in an area where positive SLP anomalies were also verified, indicating that the pressure increase was coupled with the moisture flux reduction over this area. In November of the late onset year (Fig. 4o), negative moisture flux magnitude anomalies were observed across SA, but mainly over the Amazon basin and east of the Andes (between Peru and Paraguay), the negative moisture flux anomaly direction was predominantly from southeast, indicating a moisture transport reduction from the Amazon basin to the central and southwestern region of SA. Over these two areas, a moisture flux reduction of up to 30 kg.m− 2.s− 1 was observed in December (Fig. 4p). In the late onset year, it was not possible to identify characteristic moisture flux anomalies over the SACZ’s configuration area, as observed in January of the early and neutral years (Fig. 4e, k). It is suggested that the predominance of moisture flux negative anomalies in the late year, may have contributed to rainfall reduction, in comparison to the other years studied.
In summary, there was a continuous increase in the moisture flux over SA from September to January of the early onset year (Fig. 4a, b, c, d, e, f). However, this pattern was not identified in the late onset year, in which the moisture flux anomalies oscillated from September to December (Fig. 4m, n, o, p, q, r).
From the OLR anomaly analysis accompanied by the 200hPa streamlines (Fig. 5), November to February were characterized by the persistence of negative OLR anomalies over central SA, indicating the presence of cloud vertical development, an indication of the rainy season (Fig. 5c, d, e, f, i, j, k, l, o, p, q, r; Kousky, 1988; Garcia, 2010). In addition, in the 200hPa streamlines, the configuration of an anticyclone, known as Bolivian High (BH; Lenters; Cook, 1997), and of a trough east of the BH, known as Northeast Trough (NET, Lenters; Cook, 1997) or Upper Tropospheric Cyclonic Vortices (UTCN) was observed. This upper tropospheric level circulation pattern is important for the SACZ’s positioning (Kodama, 1992). In the early onset year, BH and NET started to configure in August, and in October, the two systems were more intense compared to the neutral and late onset years (Fig. 5b). However, the circulation at 200hPa was more intense in the late onset year, it means the BH and NET structures were stronger (Fig. 5o, p, q, r). In the late onset year, an area with negative OLR anomalies accompanied by anticyclonic circulation at 200hPa covered SA’s central and northern areas (Fig. 5o, p, q, r), while in the early year it concentrated over the central SA (Fig. 5c, d, e, f).
During the rainy season onset, positive precipitation anomalies were concentrated over SA’s central-eastern portion, in October of the early (Fig. 6b) and neutral onset years (Fig. 6h) and in November of the late year (Fig. 6n). The low-level wind reversal (850hPa), observed when the annual average was removed (Zhou; Lau, 1998; Gan et al., 2004; Silva, 2012), is a rainy season onset feature. Thus, from July to September, the wind vector anomaly was mainly from the south over central-north SA (Figure a, g, m). From October (November) the wind anomaly became mainly from northwest in the early and neutral (late) years (Fig. 6b, h, o).
From the OLR (Fig. 5) and precipitation (Fig. 6) anomalies analysis, it was possible to identify the SA’s rainy season onset conceptual model, developed by Nieto-Ferreira and Rickenbach (2011). Under this model, the rainy season onset’s first stage occurs when precipitation persists over SA’s northwest region and gradually extends to the southeast (October 18–22, Fig. 6d). The second stage is characterized by the SACZ’s configuration (October 28 to November 1, Figs. 5 and 6c, i, o). The third stage involves the monsoon arrival (wind anomaly reversal) at the Amazon River mouth between the North and Northeast regions of Brazil (between February and March, Figs. 5 and 6f, l, r).
In the early onset year, larger positive precipitation anomalies were observed between the southeast of the Amazon Basin and the southeast region of Brazil, where the SACZ’s pattern was identified from November to January (Fig. 6c, d, e). In February (Fig. 6f), positive anomalies up to 250 mm (monthly cumulative) were concentrated over the central and eastern portion of SA including the Northeast and North regions of Brazil (the third stage rainy season onset area). The size of the wind vector anomalies at 850hPa (indicating the intensity) was bigger in the early onset year compared to the neutral and late years, mainly in February, when the precipitation anomaly magnitude was also greater (Fig. 6f). In the late onset year, precipitation positive anomalies concentrated between central SA, the Southeast region of Brazil and the adjacent Atlantic Ocean, in November (Fig. 6o). From December to February, positive precipitation anomalies extended to the North and Northeast regions of Brazil (Fig. 6p, q, r).
To summarise the analysis of atmospheric patterns in each onset year (early, neutral and late), the main large-scale patterns configuration associated with SAMS were consistent with previous studies (Zhou; Lau, 1998; Gan et al., 2004; Vera et al., 2006; Marengo et al., 2012) are shown in Table 3.
The main differences in the atmospheric pattern between the early and late onset years are shown in Fig. 7. In the early year, a more intense SLP reduction was observed over central-east SA (blue area), where the northwesterly moisture flux was also more intense (red arrow), enhancing the precipitation and the SACZ’s configuration. In addition, the SASH’s performance was less intense compared to the neutral and late onset years, so it is suggested that it favoured the advance of cold fronts over the SA’s east coast, and thus contributed to the earlier rainy season onset. These same configurations were observed by Raia and Cavalcanti (2008) in the early rainy season onset, in which the advance of an intense frontal system contributed to the soil and atmospheric moisture increase, in addition, the northwesterly flux was also more intense while the SASH’s configuration and the east flux towards the continent were weakened.
In the late onset year, precipitation was enhanced over northwest SA, and the SACZ was positioned further north compared to the early onset year. The BH and NET configuration were also more intense. In addition, it is suggested that the configuration of a high-pressure system in September between 40–60°S and 60 − 30°W, suppressed the advance of cold fronts, and consequently contributed to the onset delay over central-east SA. For the late onset year, Raia and Cavalcanti (2008) found that the SLP was higher over central-east SA and the first precipitation episodes were associated with a frontal system, which, despite persisting for a few days and even resulting in a false onset, were not sufficient to promote the rainy season onset’s necessary conditions.
4.4 Surface Patterns
This section focusses on the surface pattern analysis of the energy fluxes (sensible and latent heat) for each of the early (2006), neutral (2004) and late (2008) onset years. From September to October of the early onset year, negative sensible heat flux (H) anomalies were observed between northwest SA and southeast Brazil (Fig. 10a, b), indicating that the atmosphere was less warm compared to the other years. From November of the early and neutral onset years, positive H anomalies appeared throughout the SA (Fig. 10c, i) and gradually reduced in December and January, when they concentrated over south, northeast and northwest SA (Fig. 10e, f, k, l). The persistence of positive H anomalies after the rainy season onset in the neutral and early years seems important for the atmospheric instability maintenance, also verified by Silva (2012) and Garcia (2010). The H increase, when associated with the atmospheric warming, promotes pressure reduction and, consequently, mass convergence at surface. Convergence and cloud formation generate precipitation, characterizing the rainy season onset. In February of the early and neutral years, negative H anomalies were observed over central SA, being a possible effect of both precipitation and OLR negative anomalies persistence. It contributes to incident shortwave radiation reduction and, consequently, for the atmospheric cooling (Fig. 10f, l).
In the late onset year, positive H anomalies were observed from September to November concentrated over central-east SA, indicating a warmer condition before the onset (Fig. 10m, n, o). By promoting the atmospheric instability, this condition favours convection, however, the reduced moisture flux limited the convective process. From December to January, negative H anomalies appeared over central-east SA (Fig. 10p, q), a pattern that appeared only in February of the early and neutral onset years (Fig. 10f, l). It is suggested that the atmospheric cooling in December of the late onset year, has suppressed convection, by promoting a more stable atmospheric condition (warming reduction).
In September, positive latent heat flux (LE) anomalies of up to 80 W.m− 2 were observed in northwest SA, in the three onset years (Fig. 11a, g, m). These positive anomalies were associated with the rainy season onset’s first stage, involving the precipitation intensification over southeast Amazon. Also in September, negative LE anomalies were observed, mainly over central-east SA, which were associated with warming before onset, and the H increase was important to the atmospheric instability in this area, as verified in the H analysis (Fig. 11a, g, m). From October to December, the positive LE anomalies extended from the northwest to the southeast of SA, indicating the increase in moisture from the second stage of the rainy season onset that involves the SACZ’s configuration (Fig. 11b, c, d, h, i, j, n, o, p). From January onwards, the magnitude of positive LE anomalies began to decrease throughout SA, mainly in the late onset year (Fig. 11q). It means that in the late onset year there was less moisture transfer from the surface to the atmosphere, which is important to maintain the rainy season convection. According to Silva (2012), LE increases after the precipitation starts and consequently the soil moisture increases, which in turn increases evaporation and therefore acts to maintain convection after the onset. In general, the differences observed in LE anomalies between the studied years were smaller compared to the differences in H anomalies. It means, that the LE contribution to each onset year was smaller than the H contribution.
Figure 12 shows the differences in 2m temperature (Fig. 12a, e), soil water (Fig. 12b, f), H (Fig. 12c, g) and LE (Fig. 12d, h) anomalies between the early and late onset years for SON and DJF. In SON, negative temperature differences were observed over most of SA but mainly in the central-east, indicating that the temperature of the late onset year was up to 2.5°C higher than of the early onset year. This increase of temperature in the late onset year was also verified in the negative H differences that reached values of up to 50 W.m− 2 over central-east and south SA in the early onset year (Fig. 12c). Regarding the soil water content in SON, positive differences were observed, indicating that the water content between the Northeast and Midwest regions of Brazil was higher in the early year compared to the late onset year. In northwest SA, negative soil water anomalies indicated more moisture in the late year compared to the early onset year (Fig. 12b). This differences pattern was also observed in LE, where positive differences were observed over the Northeast and Midwest regions of Brazil in SON (Fig. 12d), while negative LE differences were observed in northwest SA. The differences magnitude observed in soil water content and LE were smaller compared to the differences observed in temperature and H. Furthermore, the soil water and LE differences patterns seem to have been a consequence of the positive precipitation anomalies pattern, in which more precipitation occurred over the central-east (northwest) SA in the early (late) onset year.
In DJF, the temperature differences between the early and late onset years, shows that the temperature in the late onset year was up to 2.5°C higher compared to the early year, especially in the northern region of SA (Fig. 12e). In this area, in the late onset year, an increase in H (Fig. 12g), soil water content (Fig. 12f) and LE (Fig. 12h) was verified. This increase is characteristic of the rainy season during the austral summer, in which more energy is available at the surface to energy fluxes partitioning.
The differences observed between early and late onset years in surface variables are summarized in Fig. 13, especially for SON, where the red (blue) areas represent the soil water content reduction (increase) and the up (down) arrows indicate increase (decrease) of latent and sensible heat fluxes, Leaf Area Index (LAI) and temperature over each indicated area.
4.5 Surface Water Budget Analysis
From the surface water budget analysis obtained for the area between 10–20°S and 50–60°W (Fig. 14a), it was observed that from July to October, the three years (2004, 2006, 2008) had water budgets of the same mm.day− 1 order. However, the water budget in the early onset year was higher compared to the neutral and late onset years, except in August. In November, the budget reduced compared to October, mainly in the late onset year, becoming negative. From December to February, the early onset year water budget remained between 20 and 25 mm.day− 1, while in the neutral year it varied from 5 to 20 mm.day− 1 from December to January, becoming negative in February. In the late onset year, the negative budget in December turned positive (5 mm.day− 1) in January, and in February reached 20 mm.day− 1.
The analysis of water budget components allowed us to verify each individual contribution. The moisture advection pattern and its magnitude (Fig. 14b) were very similar to the water budget pattern (Fig. 14a), therefore, the moisture advection was the most important component for the differences between the onset years, while the other water budget components contribution were secondary.
In the precipitation component analysis (Fig. 14c), between the months of July and September, low rainfall was verified in the dry season. It is noteworthy that in September of the early onset year, precipitation was higher than the other two years, suggesting that this higher moisture condition may be associated with frontal systems advance, which favoured the early onset (Raia; Cavalcanti, 2008). From October to February, precipitation increased in the three years, more precipitation in the early onset year was observed in comparison to the late year. For the evapotranspiration component (Fig. 14d) it was observed that the differences between the onset years were subtler, and the evapotranspiration increase from October onwards was due to the rainy season precipitation increase. Finally, the runoff component (Fig. 14e) was also dependent on the precipitation increase, being observed from September in the early onset year and from October in the neutral and late onset years. In the early onset year runoff was also higher compared to the other years.