DYNAMICAL CONDITIONS AND SYNOPTIC TRACKS ASSOCIATED WITH DIFFERENT TYPES OF COLD SURGE OVER TROPICAL SOUTH AMERICA

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 25: (25) Published online in Wiley InterScience ( DOI: 1.12/joc.18 DYNAMICAL CONDITIONS AND SYNOPTIC TRACKS ASSOCIATED WITH DIFFERENT TYPES OF COLD SURGE OVER TROPICAL SOUTH AMERICA ALEXANDRE BERNARDES PEZZA a * and TÉRCIO AMBRIZZI b a School of Earth Sciences, The University of Melbourne, Parkville, Victoria, Australia b Department of Atmospheric Sciences, Institute of Astronomy, Geophysics and Atmospheric Sciences, University of São Paulo, São Paulo, Brazil Received 28 November 23 Revised 2 June 24 Accepted 9 June 24 ABSTRACT A synoptic climatology (every 12 h) is presented of the surface cyclone and anticyclone tracks associated with cold surges in tropical South America, complemented by the corresponding atmospheric circulation for the period of The aim of this study is to provide a new insight into the synoptic paths and the dynamics associated with distinct cold-wave categorization in the tropics. Extreme minimum temperatures and frost occurrence from the University of São Paulo (USP) meteorological station in São Paulo city (Brazil) are used to select cold events with different intensities: extreme (T < C plus frost), strong ( T 2.5 C plus frost), strong without frost ( T 2.5 C without frost) and moderate (T >2.5 C plus frost). Atmospheric variables at low and upper levels derived from the National Centers for Environmental Prediction National Center for Atmospheric Research reanalysis are calculated from day 1 to day with regard to the coldest day in São Paulo. In addition, an automatic tracking scheme is applied to diagnosing and tracking the cyclones and anticyclones at the surface associated with cold surges. Through a superposition technique, climatological clouds showing all tracks on the same map are produced, adding some new insights into the synoptic patterns of propagation and improving the Southern Hemisphere climatology. For all composites the mean cold front crosses the equator, and the extratropical cyclones also play an important role in favouring frost occurrence in São Paulo. The lagged composites indicate that most of the cold events may be tracked up to 9 days before their occurrence, with a persistent upper level signal in the eastern Pacific. Copyright 25 Royal Meteorological Society. KEY WORDS: frost; cold surges; cyclones; anticyclones; tracking schemes 1. INTRODUCTION Polar air outbreaks associated with severe frosts and snowfall are economically important for South America, a continent in which most countries depend strongly on agriculture and primary products. In the literature, this issue has been analysed mainly in terms of case studies affecting the tropical and subtropical areas, known by the Portuguese word friagem (Hamilton and Tarifa, 1978; Fortune and Kousky, 1983; Girardi, 1983; Vera and Vigliarolo, 2; Marengo et al., 1997; Garreaud, 2). Composites of atmospheric fields are also found (Vera and Vigliarolo, 2 and references therein); however, no synoptic climatology of the cyclone and anticyclone tracks related to strong cold surges has been presented yet. Vera and Vigliarolo (2) studied the dynamics of wintertime polar outbreaks from a climatological perspective, emphasizing the differences between cases associated with frosts in the subtropical area and cases confined to mid latitudes. In this study, the principal component analysis (PCA) technique is used to obtain the most relevant physical patterns. At high levels, some Rossby wave dispersion to the northeast was shown, * Correspondence to: Alexandre Bernardes Pezza, School of Earth Sciences, The University of Melbourne, McCoy Building, SE Corner of Swanston and Elgin Streets, Parkville, Victoria 31, Australia; apezza@unimelb.edu.au; ambrizzi@model.iag.usp.br Copyright 25 Royal Meteorological Society

2 216 A. B. PEZZA AND T. AMBRIZZI and at the surface the wave pattern is normally adjusted following the Rossby topographic waves due to the presence of the Andes. Marengo et al. (1997) studied the polar outbreak that occurred in June 1994, when freezing temperatures affected a large part of the subtropical continental area, severely damaging coffee-growing areas and other vegetables. That article addressed the social impacts and the synoptic and dynamic patterns associated with the cold surge, evaluating the contribution of each individual term within the quasi-geostrophic equation. A feedback-like mechanism between the low- and high-level circulation in the Andes region was proposed during the preliminary phase, which could contribute to intensify the trough further to the east, therefore enhancing the cold advection. Recently, Marengo et al. (22) performed a composite of polar waves considering temperature deviations around the coffee growing area in southeastern Brazil (22.5 S, 6 W), showing persistent positive geopotential anomalies to the southwest of South America during the preliminary phase up to 12 days in advance. Krishnamurti et al. (1999) studied the downstream amplification in the Pacific Ocean associated with cold surges over South America, emphasizing the importance of the wave amplitude and the scale interaction among different waves in the overall process. The interaction between baroclinic and barotropic processes was also discussed, and it was shown to be relevant during the preliminary phase. Garreaud and Wallace (1998) performed a composite of summertime outbreaks of mid-latitude air into tropical and subtropical South America, showing a pattern similar to the one described by Marengo et al. (1997), i.e. the presence of a relatively cold migrating anticyclone at the surface accompanied by a welldefined quasi-geostrophic wave pattern above. The importance of the Andes mountains in favouring the cold air advection on its eastern side through an ageostrophic southerly wind was also discussed, complementing previous studies (Gan and Rao, 1994; Seluchi, 1995; Seluchi et al., 1998). Garreaud (1999) simulated an intermediate event that occurred in May 1993, and proposed the vorticity advection aloft as the main process contributing to the low- and high-pressure growth at the surface. The subsidence occurring to the south of the subtropical jet also appeared as a relevant mechanism in intensifying the anticyclone, making the jet meaningful for the process. Finally, Garreaud (2) complemented the previous studies showing the dynamic structure of wintertime cold waves and suggesting that wintertime and summertime cases are very similar but with different amplitudes. Using PCA, Muller et al. (23) studied the synoptic patterns associated with frosts in the Argentine Wet Pampas (central Argentina). They obtained patterns very similar to those previously described, exemplifying the physical grounds of most PCA modes with real synoptic charts related to cold surges. Lupo et al. (21) classified polar outbreaks in South America according to the synoptic pattern responsible for the low-level cold advection and showed that in some cases the extratropical cyclones play a crucial role in pushing the cold air into the Brazilian coast. As a result, strong cold advection could take place in southeastern Brazil even in the absence of an intense surface anticyclone in northern Argentina. This feature is relatively little discussed in other articles. With regard to tracking cyclones and anticyclones associated with cold surges, in the past this process would have been time consuming because of its manual nature, but from the last decade automatic tracking schemes such as the one described in Murray and Simmonds (1991a,b) and Sinclair (1994) made it possible to deal with much information in a shorter time frame. Most automatic procedures can be applied for finding and tracking highs and lows from operational numerical analyses, generating results that can be easily compared. Murray and Simmonds (1991a,b) developed one of the first automatic procedures to find and track surface pressure systems. Jones and Simmonds (1993) analysed the performance of that particular tracking scheme with different datasets and concluded that the algorithm was a very useful tool for meteorological applications. Nowadays, the large number of studies using cyclone/anticyclone tracking schemes have shown their reliability (e.g. Sinclair 1994; Sinclair et al., 1997; Simmonds and Keay, 2a,b; Pezza and Ambrizzi, 23 and references cited therein). In the present study, a synoptic climatology of the particular trajectories associated with cold air outbreaks in subtropical South America is generated through the Murray and Simmonds (MS) automatic scheme. The dynamic differences between cold cases with different extreme minimum temperature thresholds is addressed, showing that different physical processes can take place depending on the intensity of each composite and

3 COLD SURGES OVER SOUTH AMERICA 217 the occurrence (or not) of frost. Composites of some low- and mid-level atmospheric fields associated with the mature and preliminary phases (up to 1 days in advance) are also discussed. This paper adds some new insights on the well-known traditional patterns of propagation, complementing the previous studies found in the literature and improving the synoptic climatology of the Southern Hemisphere. 2. DATA AND METHODOLOGY Twice-a-day mean sea-level pressure from the National Centers for Environmental Production (NCEP) National Center for Atmospheric Research (NCAR) reanalysis (Kalnay et al., 1996; Kistler et al., 21) is used from May August 1973 to 2 for applying the MS automatic scheme (Murray and Simmonds, 1991a,b) and generating the wintertime cyclone and anticyclone tracks. In addition, sea-level pressure, 925 hpa wind and specific humidity, and 5 hpa geopotential are used every 6 h from the revised online NCEP reanalysis dataset for the period to produce the composites associated with different cold waves. The period after 1973 was chosen for the synoptic tracks because the sea-level pressure analyses over that period are more accurate for the Southern Hemisphere (Pezza and Ambrizzi, 23). For the period there is an assimilation error involving the use of some of the PAOBS data (Southern Hemisphere surface pressure bogus data; see Vera and Vigliarolo (2) and Garreaud (2 and references cited therein)); however, it was shown that most fields are reliable for climatological applications (Garreaud, 2). In addition, all tracks were checked case by case through a manual comparison with conventional chart data, and no significant differences were found. The composites are based on the corrected online version (no bogus errors). The MS scheme was chosen because previous studies have shown its reliability in capturing the most evident climatic features in the Southern Hemisphere (Murray and Simmonds, 1991a,b; Jones and Simmonds, 1993, 1994; Simmonds et al., 1999; Simmonds and Keay, 2a,b; Pezza and Ambrizzi, 23), and because it deals directly with sea-level pressure, giving a synoptic meaning to the analyses. The main physical principle is that the centre of a closed cyclone (anticyclone) is unequivocally identified with its point of minimum (maximum) pressure; this is normally found within one grid space of the Laplacian maximum (minimum), depending on the degree of symmetry of the system. A cyclone (anticyclone) is deemed to exist at any point at which the pressure is lower (higher) than at any of a small number of surrounding grid points (four or eight). In the second stage of the scheme the path of each system is tracked from the time of its appearance to its dissipation. To make the appropriate decisions, a procedure was developed that does an estimate of the new position of each system, calculates the probability of associations between the predicted and actual positions, and finds the matching of these associations with the highest overall probability. All settings and empirical parameters within the MS automatic scheme were the same as described in Pezza and Ambrizzi (23), who used a sensitive calibration. The synoptic climatology of those particular trajectories associated with cold surges in São Paulo is presented for the period by simply superimposing every track on the same map, creating a cloud of associated paths. Although the MS scheme automatically identifies all cyclones and anticyclones for a given period, it cannot point out which ones were associated with each cold surge. As a result, at least one point of the whole track associated with a given cold surge needs to be located manually. This is made using the synoptic experience based on the classical polar front theory of Bjerknes and Solberg (1922), i.e. considering the geostrophic balance due to the couple cyclone/anticyclone leading to the displacement of the cold surge. After this procedure, a detailed manual check of the resulting trajectories was performed in order to ensure the physical authenticity of the analyses, and an excellent performance was found for all cases. Only for a few anticyclones did the model not capture the passage over the Andes (splitting one track into two), which did not significantly affect the analyses. However, for continuity reasons, those split tracks over the Andes were properly linked to form only one as physically expected. Daily extreme minimum temperatures from the meteorological station of the Institute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (IAG/USP), located in a green area in

4 218 A. B. PEZZA AND T. AMBRIZZI the southern part of São Paulo city (23.7 S, 46.6 W, 799 m), are used for the period. The cold outbreaks are divided according to the intensity of the minimum temperatures and the occurrence of frost. The events are classified into extreme (T < C, all cases with frost), strong ( T 2.5 C with or without frost) and moderate (T >2.5 C with frost). Although rather subjective, these thresholds are based on the long-term climatology and proved robust, since the atmospheric composites indicate a well-developed large-scale associated pattern. A temperature of 2.5 C is commonly used as a frost reference in tropical regions, and corresponds roughly to one standard deviation below the climatological winter long-term daily minimum temperature in São Paulo. Composites of surface and mid-level atmospheric fields associated with the different cold waves are presented for for the day up to day 1, complementing the initial track analysis with a longer period. The composites are independent, and frost occurrence was also supported by in situ observations at the meteorological station during the whole period of analysis. In order to ensure that the days chosen do not mix different synoptic stages, only 1 day per cold front was chosen. One may wonder if only one meteorological station would be good enough to capture the most significant large-scale features associated with cold surges in tropical and subtropical South America. However, the results showed a very strong signal representing the so called friagem mode in South America (Marengo et al., 1997), with strong cold advection taking place from the southern tip of Brazil up to the equator and, hence, confirming the reliability of the procedure. A Student s t test (Wilks, 1995) was applied to all composites presented in this article. The results showed that there is significance above the 95% level around the anomalous centres related to the cold surges in all cases (discussed in the text), giving meaning to the dynamic patterns discussed in the following sections. In fact, maps of the t-statistic showed spatial patterns very similar to the anomaly maps, with maximum t values coinciding with the maximum atmospheric anomalies (figures not shown). 3. RESULTS Table I shows the occurrence of extreme, strong and moderate cold surges at the IAG/USP meteorological station for the period, considering cases with and without frost. From Table I, the most common case is the strong cold surge with frost (15 cases) and the least frequent is the one without frost (only three cases). This is typical of subtropical and tropical areas, where radiative cooling appears as a dominant process (Schwerdtfeger, 1976; Hastenrath, 1985). In addition, frost was seen for all cases with below-zero temperatures and for several cases with temperatures above 2.5 C. Considering all cases, the lowest temperatures were measured during 1955 (2 August: 1.2 C) and 1975 (18 July: 1.1 C), both with severe frost causing significant damage to crops and vegetables in southern Brazil, Argentina, Uruguay and Paraguay (Girardi, 1983). A very recent extreme case was also seen in the year 2, with.2 C and significant economic losses (Marengo et al., 22). In the following sections, the synoptic circulation and the cyclone and anticyclone trajectories associated with each composite are discussed. Some lagged composites are shown in Section 3.6, but for all other cases the figures refer to day, which correspond to the coldest temperatures in São Paulo shown in Table I. The mean circulation analysis for all composites revealed that during day the cold front was found over northeastern Brazil, with southerly wind anomalies spreading well into the western Amazon river basin. This characteristic is observed for every single event shown in Table I, as revealed from a case-by-case synoptic analysis (not shown) Extreme events Figure 1 shows (a) the sea-level pressure and (b) the 5 hpa geopotential anomalies for the extreme case composite. A consistent large-scale pattern is present at the surface and mid levels. The sea-level pressure anomalies (Figure 1(a)) indicate a positive centre above +1 hpa in northern Argentina and southern Paraguay, and a negative centre below 1 hpa over the Atlantic. Almost all the South American

5 COLD SURGES OVER SOUTH AMERICA 219 Table I. Extreme, strong and moderate cold surges in São Paulo city from 195 to 2, considering cases with and without frost occurrence Year T< Frost Extreme T 2.5 Frost Strong T 2.5 No frost Strong T>2.5 Frost Moderate 6 Jul Jul Jul Aug May Aug Jul Jul Jun Aug Sep Jul Aug May Jul Jul Jun Aug Jun Jul Jul Jun Aug Jun Jul Jul Jun Jul Aug Jun Jul Aug Jun Jul Aug Jul Jul Total continent is dominated by strong positive anomalies extending into the tropics along the eastern flank of the Andes up to 5 N, characterizing the synoptic signature of the friagem (e.g. Marengo et al., 1997; Garreaud, 1999). The positive sea-level pressure anomalies over South America appear to be connected with a strong positive centre to the east of the Antarctic Peninsula and with a secondary centre in the Pacific Ocean. The association of the positive anomalies over the continent and eastern Antarctica and the negative anomalies off the coast of southern Brazil can promote cold advection to the north of 3 S, considering the associated geostrophic wind balance.

6 22 A. B. PEZZA AND T. AMBRIZZI S 2 5S 4 6 6S 7S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W 8 1 b S 3 5S 6S S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W 15 Figure 1. (a) Sea-level pressure (hpa) and (b) 5 hpa geopotencial anomalies (m) for the extreme cold surges in São Paulo, from 195 to 2. Dashed lines indicate negative values. This figure is available in colour online at

7 COLD SURGES OVER SOUTH AMERICA 221 The 5 hpa geopotential anomalies (Figure 1(b)) show an intense wave pattern structure with a +9 m ridge over the Andes and a 18 m trough over the Atlantic Ocean. This is a typical pattern for intense polar outbreak events (e.g. Marengo et al., 1997; Garreaud, 1999; Vera and Vigliarolo, 2; Lupo et al., 21 and references cited therein), and indicates a marked baroclinic structure compared with Figure 1(a). From the comparison between Figure 1(a) and (b) it is also interesting to notice that the maximum sea-level pressure anomalies over Paraguay were located below negative geopotential anomalies, suggesting the presence of a shallow layer of cold air. Figure 2 shows (a) anomalous fields of 925 hpa specific humidity and (b) 925 hpa wind vector and magnitude. An intense area of negative specific humidity anomalies is observed over most of South America, indicating dominant dry and cold air, conditions which approximately coincide with the maximum pressure anomalies in Figure 1(a). The northwest southeast orientation is responding to the frontal-like incursion to the west of the Amazon river basin. Anomalies stronger than 5 g/kg are seen in central south Brazil, northern Argentina, Paraguay and the western Amazon. A slightly positive area appeared over the Atlantic, indicating the passage of a frontal system. The low-level wind anomaly (Figure 2(b)) shows a southeasterly jet with amplitude greater than 1 m/s from Paraguay to the western Amazon. This flow is approximately the reverse of the South American low level jet (SALLJ) shown by Marengo and Soares (22) and Marengo et al. (24). It is interesting to note that the anomalous wind from the south crossed the equator around 55 W, in agreement with previous studies (Parmenter, 1976; Fortune and Kousky, 1983). Over the Atlantic, a pronounced cyclonic circulation is seen, and it generates an anomalous wind parallel to the southeastern Brazilian coast. These features led to cold and dry advection with clear skies, contributing to the frost conditions in the area. On the other hand, anomalous winds from the north were observed in northern Argentina, suggesting post-frontal conditions. At high levels, an intense jet streak is observed near the Brazilian coast, with a core stronger than 55 m/s around 33 S, 3 W, and a deep cyclonic anomaly centred around 33 S, 45 W indicating the presence of a well-developed baroclinic wave in the mature phase (figure not shown). The high-level jet supported the surface pressure anomalies (Figure 1(a)) through the occurrence of subsidence on its southern flank, hence contributing to the spread of the cold surge into the tropics during the mature phase (Garreaud, 1999; Vera and Vigliarolo, 2). The dynamical process associated and the influences on the cyclone and anticyclone trajectories are discussed in Sections 3.7 and Strong events Figure 3 is similar to Figure 1, but for strong events with frost. From Figure 3(a), positive anomalies over Paraguay (+4 hpa) and negative over the South Atlantic ( 9 hpa) are observed. A secondary negative region over the Pacific (37 S, 92 W) extending to southern South America is seen, as well as positive anomalies to the east of the Antarctic Peninsula. From the 5 hpa geopotential anomalies (Figure 3(b)), a negative centre below 12 m is observed over the South Atlantic (36 S, 4 W). In this case, no positive centre was observed over the Andes. However, an anomalous ridge was present in the preliminary phase, with a maximum around day 3 (figure not shown). Figure 4 shows the specific humidity and wind vector anomalies as in Figure 2. From Figure 4(a), a large area of negative anomalies is observed, though less intense than for the extreme composite. It is interesting to note that the similarity in shape between the two composites (extreme and intense) suggests a coherent linear response having an almost coincident spatial pattern but with distinct intensities. Considering the fact that the strong composite had more than twice the number of events and that they are independent, this result suggests that the dynamic mechanisms responsible for the cold surges are very similar. From Figure 4(b), a cyclonic circulation is present over the Atlantic Ocean, though weaker and further south than in Figure 2(b). The southerly winds are also present, but have half the intensity over the continent.

8 222 A. B. PEZZA AND T. AMBRIZZI S.1 5S.2.3 6S 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W.4.5 4S 5S 6S 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W Figure 2. (a) 925 hpa specific humidity (kg kg 1 ) and (b) 925 hpa vector wind and >5 m/s magnitude anomalies for the extreme cold surges in São Paulo from 195 to 2. Dashed lines in (a) indicate negative values. This figure is available in colour online at

9 COLD SURGES OVER SOUTH AMERICA S 2 5S 6S 7S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W S 3 5S 6 9 6S 12 7S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W 15 Figure 3. As Figure 1, but for the strong cold surges in São Paulo. This figure is available in colour online at wiley.com/ijoc

10 224 A. B. PEZZA AND T. AMBRIZZI S.1 5S.2.3 6S.4 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W.5 4S 5S 6S 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W Figure 4. As Figure 2, but for the strong cold surges in São Paulo. This figure is available in colour online at wiley.com/ijoc

11 COLD SURGES OVER SOUTH AMERICA 225 An approximately 5 m/s equatorward flux suggests the occurrence of the friagem over the same area shown in Figure 2(b). At high levels, the jet stream was weaker and slightly further north in this case, with a core around 27 S, 35 W (figure not shown) Strong events without frost From Table I, one can see that strong events without frost are relatively rare (only three cases). The reason is that for temperatures between and 2.5 C at 1.5 m either a very wet or eventually dry atmosphere are necessary in order to prevent ice deposition on the ground. In the particular case of São Paulo, it was observed that wet conditions prevented frost occurrence in this composite, characterizing a different large-scale pattern when compared with the other composites. The events were independent and presented a similar synoptic pattern when analysed case by case, with the anomalous centres being statistically significant at the 95% confidence level, as in the other composites. Figure 5(a) shows an anomalous high-pressure area dominating most of South America associated with a very strong blocking-like high (+18 hpa at the surface with a barotropic structure) at 62 S, 9 W that was also present in all cases. Over the Atlantic one can see a weak negative anomaly around 4 S, 29 W. These patterns are considerably different from Figures 1(a) and 3(a), and suggest a distinct large-scale forcing. Furthermore, they indicate that local controls are not the only factors responsible for triggering frost occurrence in the region of São Paulo. The fact that the negative anomalies over the Atlantic were much weaker in this case suggests that the absence of the cyclone over the Atlantic contributed to the absence of frost (see comments in Section 3.4). This point was recently shown by Lupo et al. (21) through one of the leading propagation modes. The circulation in the mid-troposphere shown by Figure 5(b) is also very different from the previous patterns. An elongated ridge is seen from the southwest of South America towards the Atlantic, confirming the presence of a blocking-like high. On the other hand, the negative anomalies near the Brazilian coast are much weaker than the previous pattern, with a core of approximately 6 m. The specific humidity anomalies are shown in Figure 6(a). Although a region of negative anomalies is still observed in central South America, both the area and the magnitude are weaker compared with the strong events with frost. As the temperature threshold used in the present composite is the same as used before, it is to be expected that the weaker anomalies are reflecting rather wetter than warmer conditions. Another distinct feature is the presence of positive anomalies in central Argentina and northeast Brazil, indicating wetter than normal conditions in the boundaries of the cold air mass, which was not seen in the previous case. Despite the weaker specific humidity anomalies seen before, southerly wind anomalies (Figure 6(b)) are observed from eastern Paraguay to the equator (7 m/s), indicating that a friagem event is also present. The anomalous circulation in the central Amazon is stronger in this case, suggesting an eastern shift compared with the previous cases. On the other hand, the cyclonic circulation over the Atlantic appears much weaker than was shown in Figure 4(b), being almost absent and with winds towards the southern Brazilian coast rather than parallel to it. It seems that the predominant maritime air over southeastern South America can explain the frost absence in the present case. At high levels, a different pattern is also observed, with a polar jet core of over 5 m/s around 6 W, and a weaker subtropical jet near 25 S, 48 W, suggesting the relevance of the blocking system to the southwest of South America in modulating the large-scale circulation (figure not shown) Moderate events Table I shows the occurrence of 13 events of moderate cold surges (T > 2.5 C) with frost in São Paulo, being the second most frequent pattern during the winter. Figure 7(a) shows sea-level pressure anomalies with a large negative region over the Atlantic ( 6 hpa) centred on 35 S, 35 W. Although slightly weaker, this pattern is similar to the strong events with frost, indicating that, despite the inclusion of warmer temperatures, the synoptic circulation presented little change. As a result, the physical mechanisms leading to frost are similar, but at the same time distinct from the non-frost composite, i.e. the presence of a deep cyclonic barotropic circulation near the southern Brazilian

12 226 A. B. PEZZA AND T. AMBRIZZI S 2 5S 4 6 6S 8 7S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W S S 6S 7S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W Figure 5. As Figure 1, but for the strong cold surges without frost in São Paulo. This figure is available in colour online at

13 COLD SURGES OVER SOUTH AMERICA S.1 5S.2.3 6S.4 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W.5 4S 5S 6S 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W Figure 6. As Figure 2, but for the strong cold surges without frost in São Paulo. This figure is available in colour online at

14 228 A. B. PEZZA AND T. AMBRIZZI S 2 5S 4 6 6S 8 7S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W S 3 5S 6S S 11W 1W 9W 8W 7W 6W 5W 4W 3W 2W 1W 15 Figure 7. As Figure 1, but for the moderate cold surges in São Paulo. This figure is available in colour online at wiley.com/ijoc

15 COLD SURGES OVER SOUTH AMERICA 229 coast for the cases with frost and the occurrence of a blocking-like structure over the Pacific for the nonfrost cases. Another interesting feature is the positive anomalous area found from southern Brazil to the western Amazon, also suggesting an extratropical influence at low latitudes. On the other hand, some positive anomalies at high latitudes found to the southwest of South America are similar to the non-frost composite, though much weaker and not reflecting an intense blocking high. The geopotential anomalies show a large negative region over the Atlantic (Figure 7(b)). This pattern is stronger than the case without frost, resembling the two previous cases with frost occurrence (extreme and strong cases). In addition, a ridge over the southeast Pacific near the southern Chilean coast confirmed the presence of a weak barotropic structure somewhat similar to that in Figure 5(b), though much weaker. Figure 8 is similar to Figures 2, 4 and 6, but for the moderate events with frost. From Figure 8(a) it is noticed that the presence of negative specific humidity anomalies dominate most of central South America, having a spatial pattern very similar to the previous cases. However, the pattern is more similar to the other cases with frost if one considers the absence of positive anomalies over the continent. Whereas for the other composites the maximum anomalies are measured in western Brazil, Paraguay and Bolivia, in the present case they take place near the coast (including the São Paulo area), with values stronger than 3.5 g kg. This suggests a cold tongue near the state of São Paulo, which could explain the occurrence of frost even with relatively warmer temperatures, because of the enhanced radiative cooling at the surface. From Figure 8(b), the 925 hpa wind anomalies confirm a strong cyclonic circulation over the Atlantic Ocean similar to the other composites with frost. This pattern seems to be a key point in favouring the cold and dry advection to the São Paulo area, as suggested by the strong winds parallel and slightly off the coast of southeastern Brazil. Such circulation is present in all composites with the exception of the events without frost. Figure 8(b) also shows weak southerly wind anomalies in the Amazon region and central Brazil, suggesting a milder friagem in this case. This is in accordance with a possible linear response of the wind anomalies with regard to the minimum temperatures in São Paulo, i.e. weaker anomalies associated with warmer temperatures. Finally, at high levels the pattern was similar to the other two cases with frost occurrence, showing a welldeveloped subtropical jet stream in association with a strong cyclonic region over the subtropical Atlantic and an absent polar jet Synoptic indexes In order to compare some additional parameters (mean and anomalous) related to the composites, Table II contains information about the positioning and intensity of the mean surface and upper level systems. From Table II, The South Pacific subtropical high (SPSH) is present for all composites with the exception of the strong waves without frost, indicating that the non-frost composite has a different synoptic circulation. The SPSH was somewhat stronger and closer to the continent for the extreme events, with a central pressure of 122 hpa. The South Atlantic subtropical high (SASH) is present in every case, with central pressures ranging from 125 hpa for the moderate event to 13 hpa for the strong event without frost. It is reasonable that the most intense SASH has occurred for the non-frost composite, as a result of a weaker trough near the coast. The extreme events present the stronger and also more continental migratory cold high (MCH). In addition, the moderate cases show a weaker MCH, suggesting an approximately linear response in relation to the temperature. The strong case without frost has an MCH almost as strong as the extreme events but shifted 4 to the east, reflecting a less continental but a still strong anticyclone, and being dynamically consistent with the idea of wetter conditions preventing frost occurrence in the São Paulo area. The cyclonic vortex at the surface (CVS), i.e. the mean cyclone directly associated with the passing cold fronts, appears with close isobars only in the extreme case, as shown by Figure 1(a). For the remaining events the CVS is seen as a trough in the mean fields but with a closed anomalous centre (ACVS) as shown by Figures 3(a), 5(a) and 7(a). The presence of this system is important in favouring the occurrence of cold advection towards lower latitudes (i.e. cold advection westward of the trough), and it is meaningful that it is significantly weaker and at least 5 shifted to the east in the case without frost.

16 23 A. B. PEZZA AND T. AMBRIZZI S.1 5S.2.3 6S.4 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W.5 4S 5S 6S 7S 9W 8W 7W 6W 5W 4W 3W 2W 1W Figure 8. As Figure 2, but for the moderate cold surges in São Paulo. This figure is available in colour online at wiley.com/ijoc

17 COLD SURGES OVER SOUTH AMERICA 231 Table II. Comparison of some meteorological systems related to the cold surges, indicating the strength and positioning of the SPSH (South Pacific subtropical high), SASH (South Atlantic subtropical high), MCH (migratory cold high), CVS (cyclonic vortex at the surface), ACVS (anomalous cyclonic vortex at the surface), SAPDS (South American pressure dipole at the surface), ACV5 (anomalous cyclonic vortex at 5 hpa), ACV2 (anomalous cyclonic vortex at 2 hpa), ARID5 (anomalous ridge at 5 hpa), ARID2 (anomalous ridge at 2 hpa), 1/5DSB (1 5 hpa depth in southern Brazil), MINSHNA (minimum specific humidity in northern Argentina), JC2 (jet core at 2 hpa) and the MWFRI (mean 925 hpa wind associated with the friagem) Meteorological system T< with frost T 2.5 with frost T 2.5 without frost T>2.5 with frost SPSH (hpa) 3 S, 85 W S, 92 W 12 Indefinite 28 S, 98 W 12 SASH (hpa) 3 S, 3 W S, 2 W S, W S, 4 W 125 MCH (hpa) 27 S, 55 W S, 52 W S, 51 W S, 52 W 12 CVS (hpa) 4 S, 35 W 16 Meridional trough 35 W Tilted trough 3 W Meridional trough 38 W ACVS (hpa) 38 S, 37 W 1 38 S, 35 W 9 4 S, 29 W 3 35 S, 38 W 6 SAPD (hpa) ACV5 (m) 34 S, 4 W S, 39 W S, 38 W 5 34 S, 38 W 7 ACV2 (m) 34 S, 42 W S, 4 W S, 42 W 1 33 S, 38 W 1 ARID5 (m) 37 S, 78 W +9 Indefinite 57 S, 88 W S, 89 W +6 ARID2 (m) 4 S, 75 W +75 Indefinite 57 S, 88 W S, 9 W +6 1/5TSB (m) MINEHNA (g/kg) JS2 (m/s) 33 S, 3 W >55 27 S, 35 W >5 24 S, 48 W >4 26 S, 3 W >45 MWFRI (m/s) The South American pressure dipole (SAPD) at the surface, which is the difference (in the anomalous fields) between the MCH and the CVS, gives an idea of how strong the cold advection was for each case. An approximately linear response is found with regard to the temperature threshold, i.e. the lower the temperature the higher the index. The SAPD for the moderate case is less than half of the value found for the extreme case. The indexes ACV5 (anomalous cyclonic vortex at 5 hpa), ACV2 (anomalous cyclonic vortex at 2 hpa), ARID5 (anomalous ridge at 5 hpa) and ARID2 (anomalous ridge at 2 hpa) indicate the upper level forcing associated with the wave patterns, showing a stronger pattern for the extreme events in all cases, and weaker anomalies associated with the moderate cold surges. The strong case without frost is an exception, since the ARID5 and ARID2 indexes had the highest values. This behaviour is due to a blocking ridge in the Pacific Ocean to the southwest of South America, reflecting a distinct synoptic condition. In addition, ARID5 and ARID2 are indefinite for the strong events with frost. The 1 5 hpa thickness in southern Brazil (1/5TSB) is estimated for a region around 3 S, 51 W, which is typically used by local experts as a prognostic tool in order to help in the estimation of snowfall probabilities for the mountainous regions in southern Brazil. It is interesting to notice a linear response with regard to the temperature threshold, with the lowest thickness associated with the colder cases, and the same thickness for the strong events with frost or without frost. The minimum specific humidity in northern Argentina (MINEHNA) indicates drier conditions for the extreme and the strong events with frost, whereas the strong events without frost and the moderate events present higher values. For São Paulo, the non-frost composite presents the wettest conditions. The jet streak at 2 hpa (JS2) is stronger and further south (55 m/s at 33 S) for the extreme events and weaker and further north (4 m/s at 24 S) for the strong events without frost. Therefore, the jet is approximately over São Paulo for the non-frost events, suggesting that the absence of frost was also related to a dynamic influence. In addition, the existence of a different large-scale pattern associated with the non-frost composite is reinforced. Finally, the mean wind associated with the friagem (MWFRI), measured in eastern Bolivia, presents a reasonable linear response to temperature, showing a higher value for the coldest composite and the same values for the strong events with or without frost, though presenting a shift as mentioned before.

18 232 A. B. PEZZA AND T. AMBRIZZI 3.6. Lagged composites A lagged composition technique was also performed in order to verify previous atmospheric conditions associated with cold surges, which could help weather forecasters to predict extreme events in South America. Considering the implications of chaotic atmospheric behaviour (Lorenz, 1963) and the fact that a composite for too many days in advance could also mix up previous cold surges with the targeted systems, we decided to work with the period ranging from D-1 to D-1. The lagged analyses are statistically significant (above 95%) up to day 1 for the most relevant anomalous wave patterns related to the cold surges (figure not shown). Although a very coherent lagged wave pattern is shown, the anomalous fields have not been filtered to retain only the synoptic-scale variability; and even considering that the physical interpretation seems robust, it is possible that some variability not related to the synoptic scale of interest could be present here. Figure 9 shows the 5 hpa geopotencial anomalies for (a) D-9, (b) D-7, (c) D-5 and (d) D-3 in relation to the extreme cold surges in São Paulo. Figure 9(a) shows a large area of positive anomalies from the south of New Zealand to the southwest of South America, with positive anomalies dominating the western part of the Antarctic continent as well. The southeastern part of South America is dominated by negative anomalies. The mass balance between the positive anomalous centres (marked with crosses) and the negative centre (marked with a minus) suggests strong cold-air advection towards the southern tip of the continent. In addition, the temperature field indicates a persistence of negative anomalies at high and mid latitudes during the preliminary phase of the cold outbreak (figure not shown), suggesting that this mechanism may have a role in triggering a major cold outbreak at low latitudes (i.e. the friagem). From Figure 9(b), we observe that during day 7 the positive anomalies that were initially over western Antarctica seem to have played an important role in intensifying the pre-existing ridge to the west of South America, contributing to strengthening a pattern favourable to cold advection over most of Argentina. In accordance with this amplification, a well-defined anomalous wave pattern started to develop over South America, with intensifying positive anomalies to the west and negative anomalies from the central Andes towards the east. During day 5, the positive anomalies persisted over the Pacific Ocean and the negative anomalies were intensified and shifted to the east, with a maximum near eastern Uruguay and southern Brazil (Figure 9(c)). It is interesting that the anomalies acquired a more pronounced zonal (and hence baroclinic) orientation, if compared with Figure 9(b), hence contributing to an even stronger cold advection over mid latitudes and the growing phase of a major cold surge. Finally, Figure 9(d) shows a significant enhancement of the wave pattern over South America, with a well-developed anomalous dipole indicating the previous upper level support responsible for the onset of the extreme event at lower latitudes. This wave pattern is well known from the literature (e.g. Marengo et al., 1997, 22; Krishnamurti et al., 1999; Garreaud 1999, 2; Vera and Vigliarolo, 2; Lupo et al., 21). The animated field showed that the anomalous dipole propagated towards the northeast from day 3 to day, and originated as the mature pattern seen in Figure 1(b). The association suggested in Figure 9(a), linking the southeast of Australia with the southwest of South America through a polar connection, was not found in the previous studies in the literature. However, some articles dealing with cold surges in South America pointed out that the central and western Pacific Ocean are key areas in favouring the wave amplification and dispersion towards South America, e.g. the downstream amplification discussed by Krishnamurti et al. (1999) or the possible connection with the tropical convection near Australia discussed in Marengo et al. (22). Other articles also suggested that tropical extratropical wave interaction (subtropical waves versus mid to high latitude waves, as described in Krishnamurti et al. (1999) and Vera and Vigliarolo (2)) and a jet interaction (subtropical versus subpolar) could help to explain the observed strengthening of the pressure dipole near South America during the preliminary phase of the event; thus, it is believed that a subpolar waveguide like the one suggested in Figure 9(a) is an additional physical mechanism leading to a stronger ridge to the west of the Andes. Therefore, it could be used as a complementary tool in order to forecast extreme cold events over South America. Another feature shown in Figure 9, which can also be seen very clearly in the animated field day by day (not shown), is the stationary characteristic of the positive anomalies to the west of South America, present

19 COLD SURGES OVER SOUTH AMERICA Figure 9. 5 hpa geopotential anomalies (m) for (a) 9, (b) 7, (c) 5 and (d) 3 days in relation to the coldest day of the extreme events in São Paulo, from 195 to 2. Dashed lines indicate negative values, and the heavy curves show the approximate positions of the wave trains responsible for the cold surge (see text for further details). This figure is available in colour online at at the same position from day 9 today 3, i.e. around 9 W, and being intensified on days 7 and 3. This stationary feature is typical of large-amplitude waves and was also observed in previous frost studies for subtropical South America (Marengo et al., 1997, 22; Krishnamurti et al., 1999 and references cited therein; Vera and Vigliarolo, 2). The other frost composites show very similar patterns. Both the strong and the moderate events present a stationary anomalous ridge near 9 W from day 9 to day 3, indicating a very similar upper level development when compared with the extreme case (figures not shown). This result suggests that the frost composites had a similar development, with the most significant differences appearing in the magnitude of the anomalies. This result is not a surprise, considering the similarity for the frost composites discussed in Sections The strong events without frost also present a stationary anomalous ridge in the southeastern Pacific Ocean during the preliminary phase, but significantly shifted to the southwest compared with the other composites, indicating a distinct upper level development in association with the blocking system discussed in Section 3.3.

20 234 A. B. PEZZA AND T. AMBRIZZI Figure 1 shows the 5 hpa geopotential anomalies for day 3 with regard to the strong events without frost. From Figure 1, a negative centre with a magnitude around 15 m can be seen at the eastern tip of Argentina, which is very similar to the one found in Figure 9(d). On the other hand, an anomalous ridge in excess of 15 m found over the South Pacific from 17 W to the South American sector indicates a different pattern if compared to the one obtained in Figure 9(d). Another important difference is seen in the Antarctic Peninsula, with intense negative anomalies in Figure 1 and positive anomalies in Figure 9(d) Synoptic climatology (1973 2) of cyclone and anticyclone tracks associated with the different cold surges The synoptic climatology of the tracks associated with cold surges in South America is described next. Figure 11 shows the anticyclone and cyclone tracks associated with (a) the extreme cold surges in São Paulo, (b) the strong events, (c) the strong events without frost and (d) the moderate events. The tracks were found every 12 h for For all cases, the trajectories initiate to the west and finish to the east, following Figure 1. 5 hpa geopotential anomalies (m) for the day 3 with regard to the strong events without frost in São Paulo, from 195 to 2. Dashed lines indicate negative values, and the heavy curve shows the approximate position of the wave train responsible for the cold surge (see text for further details). This figure is available in colour online at

21 COLD SURGES OVER SOUTH AMERICA 235 Figure 11. Cyclone (red) and anticyclone (blue) tracks directly associated with the cold surges in South America, according to the automatic scheme of MS, for (a) the extreme events, (b) the strong events, (c) the strong events without frost and (d) the moderate events. The crosses indicate the centre positions every 12 h. Mean pressure values and the trajectory lengths are indicated at the bottom left corner, and the spots show the positions of the cyclones and anticyclones at 12UTC of the coldest day the typical westerlies associated with the baroclinic development as predicted from quasi-geostrophic theory (Bluestein, 1993a,b). The crosses indicate the position of the high- and low-pressure centres every 12 h, and the spots indicate the approximate positions of the cyclone and anticyclone centres at 12UTC during the coldest day in São Paulo. The mean pressures and lengths (in days) of the trajectories are also indicated for the anticyclones (A) and cyclones (C) in the bottom left corners of each picture. From Figure 11, coupled trajectories showing anticyclones are observed to the northwest (southern cone of South America and southeastern Pacific) and cyclones to the southeast (over the South Atlantic, from the eastern shore of South America to the eastern Antarctic coast). This coupling forms the basis of the physical mechanisms leading to cold surges, with strong cold advection during the early stages plus the radiative cooling that will generate a frosty environment at low latitudes during the mature phase. The strong events without frost (Figure 11(c)) present a distinct behaviour, with weak and very short cyclone tracks and more intense zonally oriented anticyclone tracks. Figure 11(a) shows that most anticyclone tracks started over the southeastern Pacific Ocean between 4 and 5 S (anticyclogenetic area), crossing the Andes between 35 and 5 S and having a pronounced meridional

22 236 A. B. PEZZA AND T. AMBRIZZI displacement, reaching northern Argentina and Paraguay and going through the continent towards the Atlantic at latitudes ranging from 25 S (São Paulo area) to 35 S. As pointed out in many previous studies (e.g. Marengo et al., 1997; Garreaud and Wallace, 1998; Seluchi et al., 1998; Garreaud, 1999, 2; Vera and Vigliarolo, 2; Lupo et al., 21), the northward displacement of the anticyclone cell is due to the dynamic influence of the Andes, being one of the most important physical mechanisms leading to the occurrence of a strong cold surge at low latitudes in South America. Over the Atlantic, there is an anticyclolysis area and some trajectories die out next to the climatological position of the SASH, i.e. near the Greenwich Meridian (Kapala et al., 1998; Machel et al., 1998), in agreement with previous studies that suggest migratory highs can reinforce the semi-permanent systems. For the cyclones, Figure 11(a) shows a well-defined preferred path, with most trajectories starting over the South Atlantic near the Patagonian coast (cyclogenetic area), going first to the northeast (not in all cases), then towards the southeast following the climatological track previously shown by Sinclair (1994) and Simmonds and Keay (2a,b), and finally disappearing next to the Antarctic coast (cyclolytic area). It is interesting to note that only one cyclone track originated over the Pacific Ocean, where such a pattern is rather infrequent because that area is typically anticyclogenetic. This feature can be easily explained from a dynamic point of view using Ertel s potential vorticity conservation, i.e. an increase in the relative vorticity for a westerly flow approaching the mountains is to be expected. The spots mark the positions of the cyclone and anticyclone centres at 12 UTC for the frost day in São Paulo. It is interesting to see that for the extreme events the mornings with frost saw a persistence of anticyclones ranging from northeastern Argentina to Santa Catarina and Parana states in Brazil (to the south of São Paulo), which is coincident with the position of the MCH shown in Table II, thus suggesting a good agreement between the analyses for the mean (and anomalous) field discussed in Sections and the present synoptic climatology. For the cyclones, Figure 11(a) shows that their positioning for the coldest day was highly variable, appearing either near the South American coast (local cyclogenesis) or at high latitudes over the Atlantic, suggesting an indirect influence on the frost mechanism in this latter case. The coupled mechanism consisting of intense radiative cooling associated with the passing anticyclones near southern Brazil, plus the previous cold advection due to the strong pressure gradient is the main physical reason for frost in the São Paulo area. The average pressure for the trajectories is 128 hpa for the anticyclones, with a mean length of 9 days, and 987 hpa for the cyclones, with a mean length of 11 days (Figure 11(a)). If we consider that these pressure values were averaged every 12 h over the whole paths, the systems can be considered very intense, i.e. well above the climatological average of the transient activity in South America (Schwerdtfeger, 1976). The synoptic climatology for the strong cases is seen in Figure 11(b), where a pattern similar to the previous one can be observed, though with regional differences, i.e. with anticyclone tracks typically longer and shifted towards lower latitudes in the Pacific and cyclones with relatively shorter trajectories around Antarctica, with more cases of local cyclogenesis near the Brazilian coast. The average pressure in this case is 127 hpa for the anticyclones, with a mean length of 13 days, and 994 hpa for the cyclones, with a mean length of 5 days, where both cyclones and anticyclones are slightly weaker than in the previous case. The spots also show a similar pattern when compared with the extreme case, with a superposition of anticyclones around São Paulo state and very variable cyclones in the Atlantic. It is interesting to notice that some very long anticyclone tracks occurred in this case, with a particular path being originated near the western New Zealand coast roughly 25 days before the frost in São Paulo. One may wonder what the synoptic meaning of such a long track is, since the typical synoptic (and frontal) time scale is around 7 days. Comparing the trajectories with the reanalysis fields day by day and also with satellite pictures (figures not shown), it was observed that the surface cold fronts responsible for the cold surges in South America were in most cases formed locally, triggered by the passage of the extratropical cyclones off the Patagonian coast (Figure 11(b)). Hence, the persistence of the anticyclone tracks from the western Pacific with many days in advance is not directly associated with the synoptic field leading to the frost in São Paulo, but is responding to the previous upper level signal. The strong case without frost can be seen in Figure 11(c). In this case, the cyclones describe significantly shorter paths and the anticyclones have more zonally oriented tracks. For this composite, the anticyclones

23 COLD SURGES OVER SOUTH AMERICA 237 average 13 hpa, with a mean length of about 11 days, and the cyclones average 13 hpa, with only 2 days, indicating stronger anticyclones (even when compared with the extreme events) and weaker cyclones. Furthermore, the absence of spots over the Atlantic shows that no influence regarding cyclones is occurring during the coldest day in São Paulo, and the anticyclones appear slightly shifted to the south when compared with the other cases. These results confirm the previous discussions about the importance of the cyclones in triggering frost in São Paulo, and show that a different pressure behaviour is present for the nonfrost composite. Finally, Figure 11(d) shows the tracks associated with the moderate case, describing long anticyclone and cyclone paths similar to the ones associated with the strong cases (Figure 11(b)). For this case, the anticyclones average 125 hpa (11 days) and the cyclones 995 hpa (1 days). These are the weakest values among the frost composites confirming a relatively linear response in the intensities of both the highs and lows with regard to the minimum temperature, as shown by the SAPDS index in Table II. In relation to the positioning during the coldest day, the spots show that most anticyclones were located directly over São Paulo or slightly to the east, whereas the cyclones showed considerable variability over the Atlantic, being similar to the other frost composites. This result suggests that the frost cases present a similar synoptic pattern, with regional differences dependent on the temperature threshold. The synoptic climatology of all trajectories associated with cold surges in South America (superposition of all cases seen in Figure 11) is plotted in Figure 12. The mean central pressure, the track s length in days and Figure 12. Synoptic climatology of the overall trajectories associated with cold surges in São Paulo from 1973 to 2. Anticyclone tracks are shown in blue and cyclones in red, and the average and extreme pressures are indicated at the bottom. The spots show the positions of the strongest systems

24 238 A. B. PEZZA AND T. AMBRIZZI the positions of the most intense cyclone and anticyclone (spots) are indicated. From this figure, two dense clouds of paths are seen, with anticyclones to the northwest and cyclones to the southeast. It is clear that most anticyclone tracks started between 9 and 12 W and formed a closed area of high density to the west of South America, indicating an anticyclogenetic region associated with the dynamic reinforcement due to the Andes mountains (e.g. Bluestein, 1993a,b; Gan and Rao, 1994; Seluchi et al., 1998) and also with the SPSH to the north (Schwerdtfeger, 1976). Over South America, a very pronounced meridional shift is observed, as discussed for the independent composites, with most tracks going northwards through the eastern flank of the Andes, then moving to the Atlantic by two preferable paths, one over the eastern tip of Argentina and the other over southern Brazil. As discussed before, and as is well known from the literature (see Section 1), the shape of the Andes strongly influences the paths from a dynamical point of view, leading to a large meridional displacement and, hence, producing the friagem in the western Amazon, with the cold front reaching the equator (see also Section 4). Figure 12 also shows a large cloud summarizing the high variability of the cyclone tracks, indicating the importance of the South American Atlantic sector as a cyclogenetic region from the Brazilian coast to the southern tip of Patagonia. Considering all tracks, the anticyclones have an average intensity of 127 hpa with a length of 11 days, and the cyclones show an average of 994 hpa with a length of 8 days. If we consider that these values were averaged over the whole tracks every 12 h, then they are very intense. The most intense anticyclone from 1973 to 2 reached a central value of 15 hpa near the city of Bariloche, in Argentina (41 S), and the most intense cyclone measured 944 hpa near the eastern Antarctic coast, as shown by the marks on the map. 4. FINAL DISCUSSION It is shown that distinct cold waves affecting tropical and subtropical South America are associated with welldefined large-scale wave patterns that can be monitored over the Pacific Ocean up to several days in advance, suggesting a polar connection linking the southeast of Australia with southwestern South America during the preliminary stage (9 days in advance). The patterns associated with frost in São Paulo are qualitatively similar when considering the different temperature thresholds, pointing out the fundamental role of the cyclone off the Brazilian coast as a dynamic mechanism. On the other hand, the non-frost pattern is characterized by a much weaker trough in the Atlantic and an intense blocking-like pattern over the southeastern Pacific Ocean. Figure 13 is a conceptual model for the cold surge problem in South America. It shows the most frequent cyclone and anticyclone paths superposed on a topographic map of South America and the adjacent oceans and continents, summarizing the physical properties described in this article. The thick arrows indicate the anticyclone and cyclone tracks derived from Figure 12, and other frequent paths are indicated by the dashed arrows. The crosses indicate the climatological position of the Pacific high (PH) and the Atlantic high (AH). The cyclogenetic (cyclolytic) and anticyclogenetic (anticyclolytic) areas and the typical sea-level pressure values are also plotted. The cold front line approximately shows the furthest displacement of the cold advection, as derived from Figures 1 to 8 and from the previous studies found in the literature. This figure adds a new insight to the friagem physical mechanism proposed in the literature. A simplified conceptual model of the dynamical influences of the Andes in association with the migratory cyclones and anticyclones can be summarized as follows: (1) At the southern tip of South America previous southerly wind anomalies occur as a result of the geostrophic balance between the developing migratory anticyclone near the southern Chilean coast and the extratropical cyclone over the Atlantic. These wind anomalies are accompanied by cold advection, and hence pressure starts to increase quickly over the continent. (2) When pressure becomes very high at the southern tip of the continent, a strong meridional pressure gradient is established and, as a consequence, the blocking effect of the Andes produces mass accumulation to the northwest of the high-pressure cell. (3) As a result, the wind speed is slowed, reducing the Coriolis effect and generating an ageostrophic component from the south (driven by the pressure gradient), therefore advecting cold air towards lower latitudes at the eastern side of the Andes. (4) The anticyclone centre tends to move to the north towards the region of maximum cold advection, with subsidence and anticyclonic

25 COLD SURGES OVER SOUTH AMERICA 239 Figure 13. Topographic map of South America and the adjacent oceans and continents, depicting the synoptic climatology of cyclone (in red) and anticyclone (in blue) tracks associated with polar air outbreaks in subtropical South America. Regions of higher track densities are indicated by the heavy coloured areas and arrows. Other frequent paths are indicated by dashed arrows. The crosses indicate the climatological position of the Pacific (PH) and Atlantic (AH) semi-permanent high-pressure centres and the cold front line approximately shows the northern boundary of the cold advection. Typical cyclogenetic and anticyclogenetic regions, and sea-level pressure values (hpa) are also plotted vorticity advection increasing aloft. (5) When the cold air reaches latitudes near 18 S, the blocking effect of the Andes is diminished because of its shape (see Figure 13) and also due to the fact that the geostrophic adjustment is very slow at tropical latitudes (Garreaud, 1999); hence, the remaining cold air is inertially advected to the western Amazon river basin, characterizing the mature phase of the friagem event. There are other possible interactions, both at the surface and at upper levels, that could play a role in this process, such as the possible associations with the sea ice around Antarctica, the remote influences of the tropical convection over the western Pacific (Marengo et al., 22) and the El Niño southern oscillation, which is well known for its influences on the subtropical jet stream over South America. Nevertheless, it is believed that the most significant features associated with the synoptic climatology of cold surges in South America are discussed here.