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Subtropical over the Southwestern South Atlantic: Climatological Aspects and Case Study

LUIZ FELIPPE GOZZO AND ROSMERI P. DA ROCHA Department of Atmospheric Sciences, Universidade de São Paulo, São Paulo, Brazil

MICHELLE S. REBOITA Natural Resources Institute, Universidade de Itajuba, Minas Gerais, Brazil

SHIGETOSHI SUGAHARA Instituto de Pesquisas Meteorologicas e Programa de Pos-Graduacao da Faculdade de Ciencias, UNESP, Bauru, Brazil

(Manuscript received 21 February 2014, in final form 18 August 2014)

ABSTRACT

Hurricane Catarina (2004) and subtropical Anita (2010) called attention to the development of sub- tropical cyclones (SCs) over the South Atlantic basin. Besides strong and organized , a large number of weaker, shallower cyclones with both extratropical and tropical characteristics form in the region, impacting the South American coast. The main focus of this study is to simulate a climatology of subtropical cyclones and their synoptic pattern over the South Atlantic, proposing a broader definition of these systems. In addition, a case study is presented to discuss the main characteristics of one weak SC. The Interim ECMWF Re-Analysis (ERA- Interim) and NCEP–NCAR reanalysis are used to construct the 33-yr (1979–2011) climatology, and a compar- ison between them is established. Both reanalyses show good agreement in the SCs’ intensity, geographical distribution, and seasonal variability, but the interannual variability is poorly correlated. Anomaly composites for austral show that subtropical occurs under a dipole-blocking pattern in upper levels. Up- ward motion is enhanced by the vertical temperature gradient between a midtropospheric cold cutoff low/trough and the intense low-level warm air advection by the South Atlantic subtropical high. Turbulent fluxes in the region are not above average during cyclogenesis, but the subtropical high flow advects great amounts of moisture from distant regions to fuel the convective activity. Although most of the SCs develop during austral summer (December–February), it is in (March–May) that the most ‘‘tropical’’ environment is found (stronger surface fluxes and weaker vertical shear), leading to the most intense episodes.

1. Introduction a tropical or extratropical transition (Ritchie and Elsberry 2001; Hulme and Martin 2009). A (SC) is a low A procedure to identify such cyclones is the cyclone that presents both extratropical and tropical structure phase space (CPS; Hart 2003), an algorithm that describes during its development. These hybrid cyclones are non- the three-dimensional thermal structure of the cyclone by frontal, with a low tropospheric warm core and an upper- its thermal wind profile and horizontal thermal symmetry. level cold core (Hart 2003). They can be formed with such Employing this algorithm, Evans and Guishard (2009) characteristics and maintain them throughout the cyclone studied 18 SCs over the North Atlantic basin, showing that lifetime, or they may originate as an intermediary stage in these systems form by the intrusion of an upper-level trough over an unstable subtropical low [warm (SST) and weak static stability]. The formation of a midtropospheric cutoff low Corresponding author address: Luiz Felippe Gozzo, IAG-USP, Rua do Matao, 1226, Cidade Universitaria, São Paulo, SP, CEP reduces the trough scale and strengthens the interaction of 05508-090, Brazil. upper-level features and the low-level cyclonic perturba- E-mail: [email protected] tion, leading to the development of a hybrid cyclone.

DOI: 10.1175/JCLI-D-14-00149.1

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Guishard et al. (2009) proposed a first unambiguous set region of the basin, in order to better understand the of criteria to define subtropical storms: they should be environment in which they are formed. The paper is diagnosed as hybrid systems (by the CPS parameters) for organized as follows: section 2 describes the data and more than one diurnal cycle and should attain sustained methodology, section 3 presents the results, and section 2 -force (.17 m s 1) during their life cycle. Cli- 4 brings the conclusions. matology based on these criteria for the North Atlantic basin showed an annual mean of four subtropical storms, and the months of September and October as the most 2. Data and methodology active. Their study reported subtropical storms developing a. Data in highly ‘‘unfavorable’’ conditions, in SST as cool as 168C 2 andverticalwindshearsasstrongas40ms 1. This study uses data from ERA-Interim (Dee et al. Evans and Braun (2012) presented the first SC cli- 2011), referred to hereinafter as ERAInt. We have used matology for the South Atlantic basin, reporting a mean data with horizontal resolution of 1.5831.58 and 10 pres- of 1.2 SCs per year and little monthly variability. In this sure levels (from 1000 to 200 hPa), at every 6 h (0000, 0600, study, Rossby wave breaking was pointed out as a gen- 1200, and 1800 UTC). This work will focus on the period esis mechanism for just a few cyclones, while the lee- from 0000 UTC 1 January 1979 to 1800 UTC 31 December cyclogenesis downstream of the Andes and over the 2011. The ERAInt reanalysis presents a four-dimensional warm Brazilian Current accounted for the other cyclo- variational data assimilation system (4D-VAR) and a new genesis mechanisms. moisture analysis, which reduces issues from the previous Over the southwestern South Atlantic, the most or- 40-yr ECMWF Re-Analysis (ERA-40; Hólm 2002). ganized and interesting system recorded was Hurricane A comparison between the climatology constructed Catarina, which began as an extratropical precursor and with ERAInt and the NCEP–National Center for At- presented subtropical structure before the transition mospheric Research (NCAR) reanalysis (Kalnay et al. into a , in March 2004 (McTaggart- 1996), hereafter NCEP1, was carried out to investigate Cowan et al. 2006). It did not develop over high SST, but whether these reanalyses have a similar representation of cold air aloft, associated with a midtropospheric trough, SCs. This dataset was chosen for comparison because it reinforced the upward motion and convective activity, was never used for SC climatology studies before and while a persistent upper-level Rex blocking pattern (Rex spans a large period (available from 1948 onward), being 1950) was responsible for weakening the vertical wind a potential source for studies about the low-frequency shear and the eastward steering of the cyclone (McTaggart- variability of such systems. NCEP1 data used have 2.583 Cowan et al. 2006). In March 2010, the subtropical storm 2.58 horizontal resolution, with pressure levels and times Anita developed in a very similar upper-level condition of identical to ERAInt. dipole blocking, and although the interaction with another SST and sensible and latent surface heat fluxes over the extratropical disturbance prevented the system from un- were obtained from the Woods Hole Oceano- dergoing tropical transition (Dias Pinto et al. 2013), it graphic Institute (WHOI) objective analysis. This dataset acquired all the characteristics of a well-organized SC. is a blend of various satellite data and three atmo- The purpose of this work is to present a climatology of spheric reanalyses (Yu et al. 2008), with the daily fluxes SCs over the southwestern sector of South Atlantic basin, computed from variables estimated by the Coupled including weaker and shallower systems observed mostly – Response Experiment (COARE) in a lower- cyclogenetic region along the South bulk algorithm version 3.0, and available over the globe at American coast, named RG1 by Reboita et al. (2010b). 1.0831.08 resolution, from 1985 to 2011. For this purpose, the definition of SC proposed is slightly The case study was carried out with atmospheric data different of that used by Guishard et al. (2009) and Evans from the ERAInt and oceanic data from the WHOI anal- and Braun (2012). While abundant research has been ysis. The data are from the Tropical Rainfall done to describe the representation of extratropical and Measuring Mission (TRMM)-3B42 daily dataset (Huffman tropical cyclone climatology in several reanalyses, a direct et al. 2007; available at http://mirador.gsfc.nasa.gov/). comparison of SC climatologies has not yet been evalu- Geopotential height and temperature anomaly profile were ated. This will also be addressed in this paper, where the calculated as deviation of a zonal mean from 558 to 358W. Interim European Centre for Medium-Range b. The cyclone tracking algorithm and the cyclone Forecasts(ECMWF)Re-Analysis (ERA-Interim) and Na- phase space tional Centers for Environmental Prediction (NCEP) rean- alyses will be compared. Furthermore, we present The algorithm for identification and tracking of cy- composite analyses for the SCs over the most active clones employed in this study was first developed by

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between the classes, which allows the existence of cy- clones with mixed characteristics and transformation between types (transitions). This approach evidences the idea of cyclones as a continuum and not strictly separated in classes (Beven 1997) and has been widely used in the study of SCs all over the globe; for the pa- rameters, equations, and application to the Southern Hemisphere, see Dias Pinto and da Rocha (2011) and Dias Pinto et al. (2013). CPS parameters are defined so that for an extra- T T tropical cyclone, B .. 0, 2VL , 0, and 2VU , 0 (the horizontal low-level temperature field is asymmetric, and the cyclone has a cold core extending through- out the troposphere), and for a tropical cyclone B ’ 0, T T 2VL . 0, and 2VU . 0 (implying a thermally symmetric low-level field and a warm core from the surface up to the tropopause). The SCs are nonfrontal, with small values of B (around 10; Evans and Guishard 2009; Guishard et al. FIG. 1. Effective cyclone tracking area (SAO, blue dashed rectangle) 2009), and present a low-level warm core and an upper- T T and cyclogenetic region 1 (RG1, black solid rectangle). level cold core, implying 2VL . 0and2VU , 0. c. Criteria for SCs identification Sugahara (2000), using a methodology similar to that of Sinclair (1994, 1995). It identifies the cyclone trajectory In this study, a cyclone is classified as subtropical if the as a sequence of local minima (in the Southern Hemi- following conditions are met: sphere) of relative vorticity in the gridded horizontal 1) The SC forms between 208 and 408S. In the region of wind field. A detailed description of the tracking is given study, this limitation is important to avoid the inclusion by Reboita et al. (2010b). of polar lows or mesoscale cyclones that can occur The tracking scanned an area encompassing most of throughout the year around the Antarctic continent the southwestern South and a portion of (Carrasco et al. 2003), in the southern portion of the (this region is hereinafter referred to as domain. SAO), as shown in Fig. 1. The cyclones are identified in 2) It presents horizontal thermal symmetry and hybrid 925-hPa horizontal wind fields as relative vorticity nuclei 2 2 structure for more than 36 consecutive hours. Both with less than 21.5 3 10 5 s 1 for more than 24 con- conditions are diagnosed by using the CPS parame- secutive hours, regardless of its extratropical, sub- ters, with B , 25 m, 2VT ,210 and 2VT .250. tropical, or tropical structure. Closed isobars were not U L 3) It attains the required values of B, 2VT, and 2VT a required condition, because even open isobar cyclones U L over the ocean, and within 24 h after its genesis if first may cause significant weather events, especially near the tracked as an extratropical system. coast (Sugahara 2000). The region delimited by the solid black box in Fig. 1, 30.58–218S and 49.58–35.58W, is the These criteria largely follow Evans and Guishard (2009) RG1, in which the cyclones will be analyzed in further and Evans and Braun (2012), but with three main differ- T detail (section 3d). ences. First, the 2VL threshold was relaxed, passing from The algorithm provides the date, time, position (latitude 210 to 250. Because the CPS can sometimes give a very and longitude), and pressure of the cyclones’ low or negative value of this parameter even for pure centers during their lifetimes to the CPS code, which sorts tropical cyclones (Braun 2009), this relaxation ensures out the SCs from the extratropical storms by their thermal that all potential SCs are maintained in the climatology. features. The CPS describes the structure of cyclones Extratropical systems eventually retained by using this based on three parameters: 1) the lower-tropospheric new threshold were rejected upon visual inspection of the thermal symmetry (B), 2) the lower-tropospheric ther- geopotential height anomaly profile (if they presented T mal wind (2VL), and 3) the upper-tropospheric thermal cold and westward tilting cores) and 925-hPa temperature T wind (2VU). These parameters are calculated from the fields (if they had associated frontal zones). three-dimensional geopotential field alone, and according The two most fundamental differences here for pre- to their values the cyclones can be classified as extra- vious definitions are the elimination of the gale-force tropical, subtropical, or tropical. There is no clear division wind threshold and no requirement of an upper closed

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T T T FIG. 2. Cyclone phase diagram of a shallow hybrid cyclone from 0000 UTC 12 Jan to 1800 UTC 13 Jan 2008: (a) B vs 2VL and (b) 2VL vs 2VU. low at 500 hPa. The reasoning behind this is the abun- with no significant upper-level forcings (during all the 2 dant occurrence of shallow cyclonic systems, especially development, the upper jet velocity is below 10 m s 1,and near the southeastern Brazilian coast, that do not reach there are no potential vorticity anomalies; figures not 2 sustained winds of 17 m s 1 but may cause notable shown). The SST was moderately high (258–268C) and the weather events (persistent , floods, etc.). sum of surface latent and sensible heat fluxes reached 2 Thus, we present here a slightly different definition of a peak of 90 W m 2 on 13 January (not shown). SC, simply as a nonfrontal low pressure system with The Geostationary Operational Environmental Satel- a warm core in lower levels and a cold core in upper lite (GOES)-10 infrared image at 0000 UTC 12 January levels. Although it will be clear in the results section that 2008 shows that the surface cyclone formed in a region most SCs indeed reach gale-force winds at some stage of with few (Fig. 3a). There are nuclei of strong their development and/or extend their circulation up to over the continent, but the SACZ is not 500 hPa, the absence of these limitations provided re- configured. At 1200 UTC 12 January and 0000 UTC markable differences in climatology compared to that of 13 January (Figs. 3b,c) the band associated with Evans and Braun (2012). the cold front propagates from the south. In the cyclone region, there are mostly shallow clouds at these times. 3. Results At 1200 UTC 13 January (Fig. 3d) a region of stronger convection is seen to the south of the cyclone center. By a. A shallow SC case study this time, the system is already weakening. This se- A short case study of a shallow SC from 11 to 13 January quence of satellite images shows that the SC was not 2008 is presented to illustrate the kind of SC that is in- associated with deep cumulus convection. cluded in the climatology by the new proposed definition On 11 January, one day before the cyclone begins to of section 2c. This particular cyclone was selected to the be tracked, there is already a precursor cyclonic vorticity study because it develops and stays semistationary near nucleus at 258S, 438W(Fig. 4a). Accumulated rainfall in the southwestern Brazilian coast (a region of particular 24 h occurred over the continent and in a prefrontal re- economic and societal interest) during summer (the most gion over the ocean. The region of the cyclonic precursor active for SCs, as shown later). Also, we have is subject to warm air advection and presents a trough in chosen a cyclone that was not associated with the South the sea level pressure field, near 258S, 458W(Fig. 4c). Atlantic convergence zone (SACZ). Farther south from this region, a cold front extends from The cyclone is detected by the tracking algorithm at the ocean to the north of Argentina, clearly seen in the 0000 UTC 12 January 2008, lasting for more than 36 h cyclonic vorticity and horizontal wind fields. The wind (until 1800 UTC 13 January 2008). During all this time, field also shows a northwesterly flow from the Amazon the cyclone presents small values of B and positive (neg- region toward the South Atlantic, advecting great T T ative) 2VL (2VU) values, placing it in the symmetric amounts of moist air (Figs. 4a,b). Moisture flux converges shallow warm core region of the CPS phase diagram near the São Paulo state coast (Fig. 4b), in a region of (Fig. 2). It forms near the southeastern Brazilian coast, warmer air, creating a suitable unstable environment for

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FIG. 3. GOES-10 IR images at (a) 0000 UTC 12 Jan, (b) 1200 UTC 12 Jan, (c) 0000 UTC 13 Jan, and (d) 1200 UTC 13 Jan 2008. The capital ‘‘L’’ indicates the low pressure center according to the tracking algorithm. the cyclone to develop. The vertical cross section of coming from the oceanic area to the northeast of the de- geopotential height and temperature anomaly from the velopment region (Fig. 5b). The warm advection is weaker zonal mean through 248S shows a wide region of warm in comparison to the day before, and it attains its mini- air from the surface up to 700 hPa. The strongest positive mum sea level pressure (Fig. 5c). By this time the wind 2 temperature anomaly is located between 558 and 508W. reaches its maximum speed (13.4 m s 1), which is below This originated from the warming of the continental the gale-force wind threshold. The warm air bubble from surface. The warm air from the continent advances to- the surface up to 700 hPa detaches from the continental ward the ocean up to 408W(Fig. 4d). During this day, the , and provides the hybrid structure for the 2 wind speed attains around 9 m s 1. cyclone centered on 458W(Fig. 5d). At 1200 UTC 12 January, the cold front displaces The cyclone, still coupled to the cold front, starts to northeastward and is located near the vorticity nucleus weaken at 0000 UTC 13 January (Fig. 6a). Moderate to (Fig. 5a). Moderate rainfall (40 mm in 24h) occurs along heavy persists along the São Paulo state coast and the São Paulo state seashore, likely reinforced by oro- the cold front in the open ocean, regions where there is graphic uplifting of the southeasterly flow. The region of still moisture flux convergence. The cyclonic curvature cyclogenesis still presents moisture flux convergence, and of the moisture transport, seen in Fig. 6b, influences the the vertically integrated moisture flux vectors indicate coastline near the border of São Paulo and Rio de that, unlike the previous day, most of the moisture is Janeiro states; although small, this perturbation may have

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FIG. 4. Synoptic fields at 1200 UTC 11 Jan 2008, from ERAInt. (a) The 925-hPa horizontal wind (vectors, in 2 2 ms 1) and cyclonic relative vorticity (dashed lines, in s 1); daily precipitation (shaded, in mm). (b) Vertically 2 2 2 integrated moisture flux (vectors, in kg m 1 s 1) and its divergence (shaded, in kg s 1). (c) Air temperature ad- 2 vection (shaded, in K day 1) and mean sea level pressure (contours, in hPa). (d) Longitudinal cross section of geopotential height (m) and air temperature (K) anomalies from the 248S zonal mean. an important impact on the weather pattern at the re- included in the Guishard et al. (2009) and Evans and gional scale. The pressure of the SC rises to 1013 hPa and Braun (2012) climatologies because of its weak maximum 2 the maximum wind speed is 9.9 m s 1, while the warm air 925-hPa wind speed and shallow circulation (the geo- advection is negligible in the cyclone area (Fig. 6c). The potential height anomaly associated with the cyclone only low-level warm air core and geopotential height anomaly extends from the surface up to 800 hPa; Figs. 4d, 5d,and also weaken, while the middle levels are colder (Fig. 6d); 6d). However, as these systems are relatively common this hybrid structure persists at least until 1200 UTC and may have significant localized impacts along the 13 January. By this time, the SC continues to weaken. southeastern Brazilian coast, we relaxed the criteria and The warm advection is similar to the previous time, but constructed a broader climatology of hybrid systems. the moisture flux convergence is drastically reduced (not b. Interannual and seasonal variability shown), indicating that the thermodynamic instability arising from this convergence may be an important From January 1979 to December 2011, a total of 238 mechanism to maintain the upward motions and the (233) subtropical cyclogeneses were detected in the cyclonic circulation. ERAInt (NCEP1) database, corresponding to 3.7% Although this SC fulfills the conditions 1–3 of section (4.2%) of the total cyclogeneses tracked in the SAO 2c, and thus can be classified as a SC, it would not be domain (Fig. 1). The mean annual number of SCs and its

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FIG.5.AsinFig. 4, but at 1200 UTC 12 Jan 2008. standard deviation is nearly identical: 7.2 6 2.8 and 7.1 6 (3.2 cyclones in ERAInt, 2.8 cyclones for NCEP1 on 2.8 for ERAInt and NCEP1, respectively. The high average). There are various conditions favoring the oc- number of cyclogenesis cases per year in comparison to currence of SCs at this time of the year such as the results of Evans and Braun (2012) indicates that shallow southward displacement of the subtropical upper-level SCs with moderate winds are relatively common in the jet, the sea level pressure decrease near the southeastern region. coast of Brazil (also influenced by the formation of While the mean number of cyclones is similar in both the South Atlantic convergence zone), and the increased reanalyses, their interannual variability is significantly transport of heat and moisture to the region by the South different (Fig. 7a). The agreement between the annual American low-level jet (Reboita et al. 2012). This sum- number of events is poor for most years, resulting in mer environment promotes more frequent shallow hy- a low Pearson correlation (10.26). Similarly low corre- brid cyclones, by the transport of continental warm air lations were found by Hanson et al. (2004), who also to the ocean (as depicted by the case study of section 3a). used ECMWF and NCEP1 reanalyses and used a similar Because of these systems, the number of cyclones in tracking algorithm for total cyclones in the North At- summer is higher than in autumn, the most active season lantic. They pointed out that this discrepancy is even according to the Evans and Braun (2012) methodology. larger for weak cyclones. In February the number of cyclones starts to decrease, The mean seasonal distribution of SCs is almost but in March they increase again. Especially because of identical for ERAInt and NCEP1 reanalyses. Summer March, autumn is the second most active season of the is the season with highest cyclogenetic activity (Fig. 7b), year in this climatology. The smaller frequency of events January being the most active month of the year occurs in the season, with about 0.3 cyclones for

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FIG.6.AsinFig. 4, but at 0000 UTC 13 Jan 2008.

ERAInt and NCEP1. This frequency during winter is et al. (2010) shows that during summer and autumn, at the similar to the one documented by Evans and Braun (2012), South American coast, there is a maximum number of but in that study winter was the second most active season. cyclones traveling much shorter distances than the mean for the whole Southern Hemisphere; although this was not c. Mean characteristics explored in that work, it is possible that SCs contribute to On average, the SCs’ lifetime in ERAInt (NCEP1) is the reported maximum. 4.1 (4.2) days, during which they travel a distance of Differences in the histograms for lifetime, distance 2 1397.0 (1172.4) km, with a velocity of 4.10 (3.17) m s 1. traveled, and mean velocity are small between the two Thus, their lifetime is similar to extratropical cyclones analyzed datasets, as can be seen in Fig. 8. Most of the (Simmonds and Keay 2000; Tilinina et al. 2013), but they SCs travel 0–500 km in 2–3 days, with a mean velocity 2 are slower and travel a shorter distance than previously less than 2 m s 1, in both reanalyses. The main distinc- observed for all cyclones in SAO (Reboita et al. 2010b; tion in the distributions is the second maximum for 6–7 Krüger et al. 2012). This is expected, since these systems days and 2500–3000 km in ERAInt, which arises because develop usually associated with an upper-level blocking many cyclones in this dataset form as subtropical and pattern that decreases the steering flow. This slow move- later turn into extratropical, with a longer lifetime and ment allows the SCs to interact more deeply with the distance traveled. This is especially true for many near environment, favoring organized convective activity, and coastal shallow cyclones. poses a threat to the coastal regions of South America, as Although some SCs undergo this extratropical tran- it causes prolonged adverse weather conditions. Mendes sition, the peak intensity (minimum cyclonic vorticity

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FIG. 7. (a) Annual frequency and (b) seasonal mean of SC over SAO for ERAInt (blue) and NCEP1 (orange). In (b), the plot is the seasonal mean and the lines represent the seasonal percentage of SC in the total amount of cyclones. and strongest low-level winds) was attained during the Figure 9b presents the distribution of 925-hPa maxi- subtropical stage for most of the cyclones in both data- mum wind speed during subtropical stage. About 20% sets. The cyclones that presented peak intensity during (30%) of SCs in ERAInt (NCEP1) presented maximum late extratropical stage (25.2% in ERAInt and 33.3% in sustained winds below gale force. In NCEP1 occurred NCEP1) were retained in the analysis because they a larger number of SCs with weaker winds (higher 2 formed as SCs and remained with hybrid structure for at frequency between 14 and 16 m s 1) while ERAInt least 36 consecutive hours, and thus must be accounted presented a near-normal distribution with a peak in the 2 for in a subtropical cyclogenesis climatology. Details of 18–20 m s 1 range. the extratropical transition process are beyond the scope According to Guishard et al. (2009), the maximum of this work. wind speed in SCs occurs at a distance of 100 miles To evaluate the SCs’ intensity and representation in (160 km) or more of the cyclone center. For most of the different reanalyses, the frequency distribution of min- South Atlantic SCs, this maximum occurs at a distance imum 925-hPa relative vorticity and maximum value of of 300–450 km during subtropical stage (Fig. 9c). Smaller T 2VL attained by the SCs during its subtropical stage are cyclones, with radius less than 150 km, were few for both compared. Data from cyclones that attained peak in- reanalyses, but were retained in the SC climatology for tensity in extratropical stage were not considered in this this region if meeting the conditions presented in distribution. Relative vorticity is used here because it can section 2c. These usually develop during show differences between the datasets more clearly than summer along the oceanic portion of the South At- sea level pressure or wind speed, since this variable is lantic convergence zone, in an environment of large- more sensitive to changes in horizontal resolution. The scale upward motion and moisture convergence (Quadro T 2VL parameter magnitude is used to assess the intensity 2012). of the low-level warm cores developed in the cyclones. Warm core intensity in SCs during subtropical stage is The minimum relative vorticity distribution for NCEP1 similar in both reanalyses, as shown in the maximum T is strongly concentrated in weak values, while the ERAInt 2VL distribution in Fig. 9d. Most of them attain a max- T 21 distribution is more spread out, with much more intense imum 2VL of 40–60 m s and the more refined grid cyclones (Fig. 9a). This is a common pattern also docu- again allows the representation of more extreme sys- mented by Hodges et al. (2011) for extratropical cyclones tems: the ERAInt overestimates the NCEP1 number of and Strachan et al. (2013) for tropical cyclones, for ex- cyclones for the weaker (below 20) and stronger (above ample. Most SCs reach intensities between 22and21.5 3 100) warm cores. The finer resolution of ERAInt allows, 2 2 10 5 s 1, indicating that these hybrid systems are in gen- in the former case, the tracking of smaller vortices, and eral weaker than the extratropical cyclones in this area in the latter, the identification of smaller regions of high (Sinclair 1995; Reboita et al. 2010b). temperature that are not resolved by NCEP1.

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21 FIG. 8. Histograms of the (a) lifetime (in days), (b) distance traveled (in km), and (c) mean velocity (in m s ) for SC over SAO in ERAInt (light gray) and NCEP1 (dark gray). d. SC spatial density and the importance of RG1 cyclogenetic area near the southeastern coast of Brazil, cyclogenetic area for both ERAInt and NCEP1 reanalyses (Figs. 10a,b). High density of occurs between 508 and 258W, The spatial distribution of subtropical cyclogenesis and with the densest region coinciding with the genesis re- cyclolysis over the SAO region is examined employing gion, evidencing the semistationary feature of most SCs the spherical kernel method (Hodges 1996), which de- (Figs. 10c,d). A second cyclolysis maximum is seen near picts the density map by superposing areas of influence of 458S, 58W, mainly because at this region the tracking is the cyclogenesis (cyclolysis) point for every cyclone. discontinued (the southeastern corner of the tracking Further information on this technique can be found in domain is located there; Fig. 1). This second nucleus is Bombardi et al. (2014), who used this method to construct much denser in ERAInt, due to a larger number of sub- a density map of total cyclogenesis over the South tropical systems that turn into extratropical ones, then Atlantic. moving up to these regions and beyond. In NCEP1, more Most of the cyclogeneses occur in near-coastal SAO subtropical semistationary lows are present, resulting in regions, as also pointed out by Evans and Braun (2012); the stronger cyclolysis nucleus near the South American here we show that the SCs occur mostly in a well-defined coast and not far from the cyclogenesis region.

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FIG. 9. ERAInt (light gray) and NCEP1 (dark gray) SAO SCs histograms of (a) minimum cyclonic vorticity (in 2 2 10 5 s 1), (b) maximum wind speed attained in the subtropical stage, (c) radius of maximum wind speed during the T subtropical stage, and (d) maximum 2VL.

The denser nucleus of subtropical cyclogenetic activity of SCs over this area (Fig. 10e). The correspondence of in Fig. 10 occur over the RG1 region (Fig. 1, black box) location and seasonal cycle leads us to believe that defined in Reboita et al. (2010b) and also identified by a significant portion of all cyclones that formed in RG1 is Sinclair (1995), Hoskins and Hodges (2005),andKrüger subtropical. Indeed, 34% (22%) of the tracked cyclones et al. (2012). According to these works, RG1 is the genesis in ERAInt (NCEP1) over the RG1 in summer are region of weaker cyclones that for the most part do not subtropical. In and autumn the incidence of these present a closed isobar at the sea level pressure field. That systems is lower, but they still account for about 15% of is why some climatologies based on tracking of cyclones all cyclones. in the sea level pressure field (e.g., GanandRao1991; Based on the larger frequency of cyclogenesis over the Mendes et al. 2010) do not report this cyclogenesis region. RG1, this area is selected to construct storm-centered Regarding all cyclogenesis, Reboita et al. (2010b) composites. The use of cyclones developed only in this show summer (winter) as the most (least) active season in region also contributes to a more precise description, as RG1, and it can also be seen in the seasonal distribution systems that formed farther south or in the northern

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FIG. 10. Subtropical cyclogenesis and cyclolysis (number of cyclones by square radian per day) for (a),(c) NCEP1 and (b),(d) ERAInt. (e) Seasonal mean (bar plot) and percentage of SC in the total amount of cyclones over the RG1 for ERAInt (blue) and NCEP1 (orange).

flank of the subtropical high (as is the case of all winter resolution. The center of the SCs corresponds to the lo- cyclones of ERAInt) may have different characteristics. cation of the relative vorticity nucleus, whose coordinates are given by the tracking algorithm. The horizontal fields e. Cyclone-centered composites during genesis are displayed in an area of 4083408, and the vertical This section presents cyclone-centered composites profile of potential vorticity (PV) and potential temper- around the genesis time, to describe the synoptic envi- ature (u) is shown to the latitude of the cyclone center ronment in which the South Atlantic SCs are formed. For spanning 708 of longitude. Composites are presented this analysis we have used only the ERAInt dataset, as from 24 h prior to the cyclogenesis up to 24 h afterward. the composites derived from the NCEP1 presented sim- A total of 126 SCs that formed over the RG1 from 1979 ilar patterns but with less detailed features, due to lower to 2011 were considered to construct the composites. We

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FIG. 11. Austral summer [December–February (DJF)] SC-relative composites of (a)–(c) sea level pressure (contours, in hPa) and PV at 250 hPa (shaded, in PVU); (d)–(f) vertical cross sections of PV (shaded, in PVU) and potential temperature (contours, in K), for (left) T 5224 h prior to the cyclogenesis, (middle) T 5 0 (1200 UTC on the genesis day), and (right) T 5124 h.

first present the results for summer (68 cases), and then were not considered for the climatology construction. the main differences between the summer SCs and the Figures 11d–f indicate that during the whole analyzed ones that developed in autumn and spring (39 and 19 period the surface cyclone lies to the east of an equa- cases, respectively). torward extension of PV, in a region where the favored upward motions help the cyclone to intensify (Hoskins 1) COMPOSITES FOR SUMMER CYCLOGENESIS et al. 1985). It is interesting to notice that, on a climato- The composites for sea level pressure field show that logical perspective, the summer hybrid cyclones in South the summer SCs develop in a broad area of low pressure America are not associated with a detached PV core that (by the geographical position of the RG1, it is possibly originated via Rossby wave break [as in the SC of Evans the thermal low over the South American continent) and Guishard (2009)], nor are they cradled by two PV extending eastward, toward the South Atlantic Ocean, regions as in the case study of Braun (2009).Thecom- 24 h prior to the cyclogenesis (Fig. 11a). At this time, posites of Fig. 11, with an upper-level cyclonic PV anomaly there is already a 1011-hPa closed isobar associated with located westward of the cyclone, and high positive PV air the incipient cyclone. On the genesis day (t 5 0), the to the east (associated with an outflow ), cyclone maintains basically the same configuration, while is qualitatively similar to the ‘‘favorable superposition’’ the South Atlantic subtropical high (SASH) slightly pattern for tropical cyclone intensification described in strengthens (Fig. 11b). The cyclone attains 1010 hPa at Hanley et al. (2001): the intensification arises from the t 5124 h, and the pressure gradient between the low and reduction of vertical and the establishment of the SASH is intensified (Fig. 11c). The central closed a divergent flow downshear from the cyclone. As shown by isobar in the sea level field presents a diameter of 58 and these authors, the upper-level cyclonic PV does not cross the whole circulation associated with the cyclone has the center; instead, it is eroded by horizontal PV advection an approximate diameter of 158. These features in the and interaction with the region of strongest diabatic composite analysis indicate that most of the summer SCs heating (Fig. 11c). present closed isobars and horizontal scale larger than The upper-level PV anomaly can be seen in a cross sec- mesocyclones, although as discussed above these criteria tion through the cyclone center (Figs. 11d–f), extending

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FIG. 12. Austral summer (DJF) SC-relative anomaly composites of (a)–(c) 250-hPa flow (streamlines) and 250-hPa magnitude of 2 horizontal wind (shaded, in m s 1); (d)–(f) 500-hPa geopotential height (contours, in m) and sea level pressure (shaded, in hPa). Anomaly fields are the difference between days of cyclogenesis and the DJF mean. (left) T 5224 h, (middle) T 5 0, (right) T 5124 h. down to 300 hPa, with a magnitude of 20.6 to 20.9 PVU. surface low (Figs. 12a–c). Poleward of the SC, an anti- At t 5 0, two regions of negative PV develop in the cyclonic circulation develops, and both structures form central axis of the cyclone, induced by temperature a Rex blocking pattern with slow zonal movement, anomalies: the one between 900 and 700 hPa is associated decreasing the vertical shear and favoring organized with the low-level warm core characteristic of hybrid convection. systems, and the anomaly between 700 and 450 hPa arises The middle troposphere over the cyclone is cold, as as a response to cloud condensation (Fig. 11e). As the SC indicated by the negative geopotential height anomaly in develops, these nuclei get stronger, but they do not form Figs. 12d–f. This anomaly represents a trough to the west a single vorticity tube, resembling the composites of of the region where the cyclone will develop (Fig. 12d), Evans and Guishard (2009). The weaker PV concentra- and in later stages a stronger trough or (in most cases) tion near the cyclone center is one of the reasons why a cutoff low detached from the westerly flow (Figs. 12e,f). transitions to a tropical storm are less frequent in this area At t 5124 h, this feature is almost vertically stacked of the globe (Braun 2009). with the surface low (less than 58 to the west), decreasing The composite of 250-hPa horizontal flow difference the vertical wind shear over the cyclone (not shown) and between cyclogenesis days and the seasonal [December– favoring strong destabilization of the low troposphere, February (DJF)] mean shows a cyclonic circulation linked by quasigeostrophic and thermodynamic ascent. At the to the PV anomaly, from 24 h prior to the genesis up to surface, the cyclone is intensified by the scale reduced 24 h after, always located around 58 to the west of the upper-level low (Fig. 12f), and an anomalous high pressure

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21 FIG. 13. Austral summer (DJF) SC-relative composite of air temperature advection (shaded, in K day ) and upward vertical velocity at 2 700 hPa (contours, in Pa s 1) at times (a) T 5224 h, (b) T 5 0, and (c) T 5124 h. develops downstream; this whole pattern resembles the of the SASH advects not only warm but very moist air midlevel and surface composites for SCs of the North toward the region where SCs will develop. The inspection Atlantic (Evans and Guishard 2009). of moisture flux integrated in the troposphere and its The thermodynamic ascent caused by anomalous divergence field (Figs. 14d–f) shows that the northeasterly cooling of the middle troposphere is enhanced by the flow transports great amounts of water vapor from this intense low-level warm air advection that further in- region toward the eastern side of the cyclone. It con- creases the . From t 5248 h the region where verges with a second northwesterly moisture flow from the cyclone will develop is subject to warm air advection the continent, in a region of strong ascent, conducive to (not shown), and 24 h before the genesis upward motion powerful convective development. starts to occur (Fig. 13a). As the cyclone deepens, the 2) MAIN DIFFERENCES BETWEEN SUMMER, pressure gradient with the SASH increases, strength- SPRING, AND AUTUMN CYCLOGENESIS ening the warm air advection eastward of the low due to ENVIRONMENTS more intense northerly winds (Figs. 13b,c). In the same 2 region, the upward motion intensifies, reaching 20.4 Pa s 1, The main characteristics and the environment where in part because of the enhanced thermodynamic de- theSCsareinsertedinspringarealmostthesameasin stabilization and also by being located just below a re- summer. An incipient surface low is deepened by a west- gion of upper-level diffluence in the eastern flank of the ward PV anomaly in upper levels. Whereas in summer 250-hPa cyclonic circulation (Figs. 12b,c). and autumn most SCs had a cutoff low at 500 hPa, the Evans and Guishard (2009) and Evans and Braun majority of spring cyclones counted on a shortwave trough (2012) already mentioned the warm air advection as forcing quasigeostrophic upward motions. Warm and a fundamental mechanism to increase the atmospheric moist air advection occur in a similar spatial pattern and convective potential in hybrid cyclones that formed over magnitude compared to summer; the only difference is relatively cold waters. This is true for the systems under that in spring the moisture contribution almost entirely study here that form over waters around 248Candsurface comes from the northeasterly SASH flow. The turbulent 2 turbulent fluxes that do not exceed 90 W m 2 (Fig. 14a). fluxes in the cyclone formation area are relatively weak This value does not differ much from the climatological again, but they are stronger in the northern flank of SASH 2 mean for this region (Reboita et al. 2010a). At t 5 0and (up to 150 W m 2). 2 t 5124 h, the fluxes are even weaker (below 60 W m 2) The midtropospheric geopotential height field in au- due to cloudiness, precipitation, and the oceanic vertical tumn presents a stronger positive anomaly to the south of mixing promoted by the cyclone (Figs. 14b,c). The sum- the cyclone, prior to and on the day of cyclogenesis mer SCs are not associated with very strong local surface (Figs. 15a,b). At the surface, the high pressure anomaly heat fluxes, but there is a broad permanent region of in- poleward of the autumn SCs arises due to transient anti- 2 tense heat fluxes (above 120 W m 2) and high SST to the cyclones (in other it is an extension of the SASH). northeast of the system, just over the northern flank of the As this anticyclone displaces to the south/southeast, the SASH. By its position relative to the RG1, the circulation SCs also move farther south, placing the surface low under

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FIG. 14. Austral summer (DJF) SC-relative composites of (a)–(c) SST (contours, in 8C) and sensible 1 latent surface heat fluxes 2 2 2 2 (shaded, in W m 2); (d)–(f) vertically integrated moisture flux (vectors, in kg m 1 s 1) and its divergence (shaded, in kg s 1). (left) T 5 224 h, (middle) T 5 0, (right) T 5124 h. a pronounced easterly wind anomaly at 500 hPa (not and organized SCs of the basin (, and shown). This results in a weaker vertical wind shear subtropical storms Anita and Arani) occurred during compared to summer or spring. autumn. Another remarkable difference of the autumn SCs is the configuration of the surface fluxes: they are more 4. Discussion and conclusions intense than in summer and spring over the whole South Atlantic basin, and the region of strongest fluxes is no A climatology of subtropical cyclones in the south- longer northeastward of the SC, but southward of it western South Atlantic basin from the ERA-Interim (Figs. 15d,e). This area of stronger fluxes happens in and NCEP1 reanalyses is presented. The cyclone response to the drier air and finer weather associated classification methodology follows mostly Evans and with high pressure systems. In this scenario, most of the Guishard (2009) and Evans and Braun (2012),but turbulent latent and sensible heat fluxes fueling the cy- here a relaxation in the maximum 925-hPa wind and clone may come from local contributions and from vertical depth conditions is proposed to account for nu- southern regions, by the southeasterly flow associated merous shallow and weaker hybrid systems developing with the transient . along the southeastern coast of South America. The environment where SCs develop in autumn is the A case study of such a shallow SC shows that the most conducive to tropical development, with strongest low-level warm core originated from low-level warm local turbulent fluxes and weakest vertical wind shear of air masses advected from the continent to the sea. It all seasons. It is likely the reason why the most intense does not develop over anomalously warm waters, but

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FIG. 15. Austral autumn [March–May (MAM)] SC-relative anomaly composites of (a)–(c) 500-hPa geopotential height (contours, in m) 2 and sea level pressure (shaded, in hPa); (d)–(f) SST (contours, in 8C) and sensible 1 latent surface heat fluxes (shaded, in W m 2). (left) T 5224 h, (middle) T 5 0, (right) for T 5124 h. the horizontal moisture convergence seems to be the During the 1979–2011 period, the annual mean num- most important factor to sustain the low pressure re- ber of SCs is almost identical for ERAInt and NCEP1 gion: once this convergence vanishes, the low weakens (7.2 6 2.8 and 7.1 6 2.8 cyclones per year, respectively). and disappears. This convergence may contribute to This mean is much greater than presented by Evans and increase convection and latent heat release in the at- Braun (2012) for the same region, due to the inclusion mospheric column, deepening the surface cyclone. of SCs that do not extend to the midtroposphere and This mechanism described in numerous studies for systems that do not attain gale-force winds. However, extratropical cyclones (Gyakum 1983; Nuss and Anthes each dataset presents distinct interannual variability, 1987, and others) is also fundamental for SC development. as indicated by the low correlation between the time Sardie and Warner (1985) showed that the development series. of polar lows is more efficient when the latent heat Regarding the complete lifetime of all tracked SCs, release takes place in the lower troposphere, inducing they have smaller traveled distance and slower velocity greater convergence of warm and moist air near the as compared to the extratropical ones, allowing them to surface. Satellite images show that the analyzed cyclone interact more efficiently with the unstable environment was not associated to deep convection, indicating in which they develop and to have an even greater im- that low tropospheric heating was a more important pact on the South American coastal climate. Analysis of mechanism to maintain this SC than strong cumulus the complete lifetime indicates that extratropical tran- convection. sition takes place for some South Atlantic SCs.

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The 925-hPa minimum relative vorticity and maxi- 58S). During the summer months, an important sec- mum strength of the low-level warm core attained dur- ondary moisture source comes from the continent, ing the SCs’ subtropical stage are similar in NCEP1 and transporting moisture from the Northern Hemisphere ERAInt. The latter reanalysis is more skilled to detect tropical Atlantic and from the Amazon region to the the weakest and strongest cyclones, due to the finer RG1. These transports favor an unstable stratification of resolution. the atmosphere over the RG1, leading to an enhanced The difference between ERAInt and NCEP1 cyclo- convective potential for the SC. In autumn, more intense genesis annual cycle might be due to differences in the local turbulent fluxes and a weaker vertical wind shear data horizontal resolution (as SCs have a smaller hori- promote the most ‘‘tropical’’ environment of all seasons. zontal scale than extratropical ones, and the CPS values This most favorable environment tends to produce are sensitive to changes in resolution). However, the fact stronger and more organized SCs in autumn than in that ERAInt does not systematically present a higher summer, although most of the cyclones develop in the number of annual cyclones indicates that this may not be latter season. This raises the question of what season is the only source of discrepancy. The area under study has then more likely to cause damages and monetary losses in few conventional data, making reanalyses more de- the region. To quantify this information, we have used pendent on their numerical models and data assimila- the accumulated cyclone energy (ACE) index. ACE is tion techniques. The difference in these features obtained by summing the square of 6-hourly maximum between ERAInt and NCEP1 may impact the SC for- wind speed for all subtropical cyclones while they present mation throughout the years. It is important to notice, intensity greater than tropical storm, merging information though, that once these systems are formed, their kine- on the frequency, intensity, and lifetime of the systems matic characteristics, strength, cyclogenesis and cyclolysis (Bell et al. 2000; Camargo and Sobel 2005). Summer regions, and seasonal variability are consistently similar presented a higher ACE than autumn for both reanalyses 2 in both reanalyses. (the seasonal values were 5.3 3 105 m2 s 2 and 2.2 3 2 The coastal region near southwestern Brazil (RG1) is 105 m2 s 2, respectively, in ERAInt). These values in- the main subtropical cyclogenetic area in the South dicate that summer is the most active season not only in Atlantic basin. In fact, during the most active season frequency, but also in the combined cyclones’ intensity. (summer) more than 1/3 of the cyclones tracked over Along with the fact that in this season the SCs present RG1 are of hybrid nature. a more stationary development along the coastal south- Composite analyses from a set of 126 SCs initiated western Brazil (near the largest port of Latin America over the RG1 showed that they form to the east of a PV and over a concentration of many oil platforms), it is minimum at 250 hPa, in a region of reduced vertical during summer that these systems can cause more fi- wind shear and upper-level mass divergence. For most nancial and societal impacts. of them, a Rex blocking pattern develops in the upper The composites presented in this study were con- and middle troposphere, further reducing the vertical structed for both deep and shallow SCs, because com- shear and also preventing the cyclones from being posites of shallow SCs (not shown) do not differ much steered. This circulation anomaly in summer SC genesis from the deep ones. The overall structure of the cyclone, days, with an anticyclone to the south/southwest of a cy- as well as the heat and moisture horizontal advections clone placed over the surface low, is similar to the pattern and turbulent surface fluxes patterns, is very similar in presented in the composites of Evans and Braun (2012) both cases. The main difference is observed in the for the same season. magnitude of the fields. Notably, shallow cases present Turbulent sensible and latent heat fluxes over the less pronounced upper-level PV anomalies and weaker cyclogenesis region are close to their climatological horizontal moisture flux convergence. This is verified in mean value in summer [in agreement with the summer all three analyzed seasons. weak SST anomaly field of Evans and Braun (2012)] and Results presented in this study add to the incipient spring. These fluxes tend to further decrease as the investigation of SCs over the South Atlantic basin, subtropical cyclone develops due to cloud coverage, assessing their seasonal variability, how two different precipitation, and vertical mixing of the upper ocean datasets represent the cyclogenesis climatology, and layers. To destabilize the environment and allow the SCs the main synoptic features. It is particularly interesting to develop, low-level warm and moist air advection how these systems develop and sustain themselves over takes place during all the cyclone lifetime (and even 24 h a region of relatively weak turbulent heat fluxes. The before genesis), mainly due to the SASH flow bringing moisture that is locally provided by the ocean to the a great amounts of moisture originating in the tropical development of hybrid cyclones in other regions of South Atlantic (northeastward of the cyclone, around the globe seems to be reduced in the South Atlantic SCs.

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However, the moisture advection from distant regions ——, and A. Braun, 2012: A climatology of subtropical cyclones in may balance this weak local surface flux, offering the the South Atlantic. J. Climate, 25, 7328–7340, doi:10.1175/ necessary moisture to fuel convection and warm the low JCLI-D-11-00212.1. Gan, M. A., and V. B. Rao, 1991: Surface cyclogenesis over South troposphere by latent heat release. This mechanism America. Mon. Wea. Rev., 119, 1293–1302, doi:10.1175/ maintains the SC. A more detailed study of these non- 1520-0493(1991)119,1293:SCOSA.2.0.CO;2. local moisture sources, applying a Lagrangian particle Guishard, M. P., J. L. Evans, and R. E. Hart, 2009: Atlantic sub- dispersion model, is ongoing research and will be pre- tropical storms. Part II: Climatology. J. Climate, 22, 3574– sented in a future paper. 3594, doi:10.1175/2008JCLI2346.1. Gyakum, J. R., 1983: On the evolution of the QE II storm: Dynamic and thermodynamic structure. Mon. Wea. Rev., Acknowledgments. We thank Jean Peres and Livia 111, 1156–1173, doi:10.1175/1520-0493(1983)111,1156: Dutra for the help in automating the cyclone tracking and OTEOTI.2.0.CO;2. sorting algorithm. Thanks to Rodrigo Bombardi for pro- Hanley, D., J. Molinari, and D. Keyser, 2001: A composite study of the interactions between tropical cyclones and upper- viding assistance with the spherical kernel algorithm. Also tropospheric troughs. Mon. Wea. Rev., 129, 2570–2584, doi:10.1175/ we appreciate the helpful comments from the anonymous 1520-0493(2001)129,2570:ACSOTI.2.0.CO;2. reviewers. This work was supported by CAPES/PROEX Hanson, C. E., J. P. Palutikof, and T. D. Davies, 2004: Objective and the Conselho Nacional de Desenvolvimento Cienti- cyclone climatologies of the North Atlantic—A comparison fico e Tecnologico (CNPq) Grants 558121/2009-8, between the ECMWF and NCEP reanalyses. Climate Dyn., 22, 757–769, doi:10.1007/s00382-004-0415-z. 307202/2011-9, 140839/2011-9, and 481942/2013-0. Hart, R. 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