1APRIL 2003 NOTES AND CORRESPONDENCE 1075

NOTES AND CORRESPONDENCE

Variability of Southern Hemisphere and Behavior: Further Analysis

ALEXANDRE BERNARDES PEZZA AND TEÂ RCIO AMBRIZZI Department of Atmospheric Sciences, Institute of Astronomy, Geophysics and Atmospheric Sciences, University of SaÄo Paulo, Sao Paulo, Brazil

27 December 2001 and 23 September 2002

ABSTRACT This paper presents some additional results on the use of an automatic scheme for tracking surface and . The Southern Hemisphere (SH) total amount of synoptic tracks (every 12 h) was analyzed for the 1973±96 period using sea level pressure from the National Centers for Environmental Prediction±National Center for Atmospheric Research (NCEP±NCAR) reanalysis. Composites for seven El NinÄo (EN) and La NinÄa (LN) years were constructed in order to analyze the association between the hemispheric cyclone and anticyclone propagation and the phase of the El NinÄo±Southern Oscillation (ENSO) phenomenon. A climatological view of cyclone and anticyclone tracks and orphan centers superposed on the same map is presented and analyzed. A large area with overlapped cyclone and anticyclone tracks is seen between 30Њ and 60ЊS, which is approximately the climatological position of the SH transient activity. To the north of 30ЊS, the subtropical is embedded in a region with just a few cyclone tracks. This feature is not evident for the Paci®c and the Indian Oceans' high. The subtropical cyclones dominate most of the west Paci®c and north of Australia. Orographic and heat lows are well spread over the tropical regions of South America and Africa. Finally, the track region appeared as a very marked feature around the Antarctic continent. In accordance with some previous studies, the total number of the SH cyclones and anticyclones during the austral winter season has shown an overall decline, particularly at the end of the 1970s. Nevertheless, a more complex behavior shows up when the weak systems are eliminated and the intense end of the spectrum is analyzed. For the anticyclone tracks above 1020 hPa, there is still some tendency toward an overall decline, but it is small and not statistically signi®cant. For the stronger anticyclones this tendency rapidly disappears. On the other hand, the cyclone tracks presented a different behavior, since the decreasing trend turned into a signi®cant increase for those stronger than 980 hPa. These results also emphasize how sensitive the tracking scheme is to capturing low and high pressure centers, and it presents another perspective for the interpretation of cyclone and anticyclone trends. The ENSO composites indicated a higher anticyclone concentration near the subtropical South Atlantic high during EN years, while in the subtropical South Paci®c high it occurs during LN years. On the other hand, the cyclone tracks showed a higher variability, with an excess of lows over the subtropical Paci®c, west of South America and southern Argentina during EN years and a more pronounced activity over the subtropical Atlantic and southeastern Australia during LN years. Nevertheless, the trends and the average of the total hemispheric number of cyclones and anticyclones are not signi®cantly affected by the ENSO phase.

1. Introduction ridges are one of the main causes of weather changes all over the and polar regions. Over the decades, operational meteorologists have In the past, the process of monitoring low and high been drawing synoptic charts with a particular concern pressure tracks on a climatic basis demanded a lot of for the precise location of cyclones and anticyclones operational time because of its manual nature. Nowa- near the surface. It is well known by experienced me- days, however, the situation is completely different due teorologists (e.g., Taljaard 1967) that these systems and to recent developments on automatic tracking schemes their movement associated with high-level troughs and like the ones described in Murray and Simmonds (1991a,b) and Sinclair (1994). Automatic procedures can be applied for ®nding and Corresponding author address: Dr. Alexandre Bernardes Pezza, tracking highs and lows from operational numerical Dept. of Atmospheric Sciences, University of SaÄo Paulo, Rua do MataÄo, 1226, CEP 05508-900, SaÄo Paulo SP, Brazil. analyses. The most important advantages are the pos- E-mail: [email protected] sibility of handling a large amount of information in a

᭧ 2003 American Meteorological Society

Unauthenticated | Downloaded 10/02/21 07:59 PM UTC 1076 JOURNAL OF CLIMATE VOLUME 16 shorter time frame and generating results that can be h analyses seem to be suitable for climatological ap- easily compared between them, because most outputs plications. The area of study is the Southern Hemi- obtained from any automatic scheme are reproducible. sphere, and track analyses have been carried out during Furthermore, what constitutes a low or high pressure the SH wintertime, that is June±July±August (JJA). center is precisely speci®ed. Some authors have shown that there are arti®cial pres- Murray and Simmonds (1991a,b) developed one of sure trends in the NCEP±NCAR reanalysis over the the ®rst automatic procedures available nowadays to Southern Hemisphere (Hines et al. 2000; Simmonds and ®nd and track surface pressure systems. Jones and Sim- Keay 2000a). In fact, the SH has experienced a sub- monds (1993, 1994) analyzed the performance of such stantial increase in data coverage over the last few de- a tracking scheme with different datasets and concluded cades, which has affected the quality of the reanalysis that it is a very useful tool for meteorological appli- data (E. Kalnay 2000, personal communication). From cations. They were able to reproduce and complement our own compilations, it was observed that the period some previous results found in the literature. after 1975 experienced a reduction in the extratropical Nowadays, a large number of studies have applied sea level pressure up to 6 hPa in the border of the automatic schemes for tracking cyclones and anticy- Antarctic continent, in comparison to the period be- clones and con®rmed their reliability, adding a new tween 1950 and 1975. Bearing this in mind, only the global perspective for the signature of transient systems last available period was used, in which the coverage (e.g., Sinclair 1994, 1995, 1996; Sinclair et al. 1997; is much better than what it was before. Simmonds and Keay 2000a,b and references therein) The MS scheme was developed by Dr. Ross J. Murray and even tropical systems (Sinclair 2002). Indeed, in a and Dr. Ian Simmonds, from the University of Mel- recent work Sinclair (2002) showed the reliability of his bourne, Australia, in order to identify and track low and automatic scheme in capturing the transition of tropical high pressure centers on a sphere. The main principle cyclones into midlatitude systems over the southwest is that the center of a closed cyclone (anticyclone) is Paci®c. This is just an example of how developed and unequivocally identi®ed with its point of minimum physically consistent (and hence useful) the automatic (maximum) pressure; this is normally found within one schemes can be nowadays. grid space of the Laplacian maximum (minimum), de- In this work, a wintertime Southern Hemisphere (SH) pending on the degree of symmetry of the pressure sys- climatology of cyclone and anticyclone tracks created tem. A cyclone (anticyclone) is deemed to exist at any using the National Centers for Environmental Predic- point at which the pressure is lower (higher) than at any tion±National Center for Atmospheric Research of a small number of grid points (four or eight) sur- (NCEP±NCAR) reanalysis data applied to the Murray rounding it. The dimension problem related to the grid and Simmonds (hereafter MS) automatic scheme is pre- space was partially solved through the use of bicubic- sented for the 1973±96 period. A climatological map splines ®t. The pressure is approximated by a smoothly with the total synoptic tracks superposed was produced varying function, and the pressure Laplacian is used to in order to have a precise hemispheric signature for these ®nd the cyclones and anticyclones. systems. The total amount of tracks and orphan systems The array is ®rst scanned for the sites of ``possible'' were counted in order to analyze possible climatic lows (or highs) by comparing the values at neighboring trends. grid points. To allow for the possibility that a shallow Another perspective for the climatic trend shown by depression (ridge) may not be detected by a local min- Simmonds and Keay (2000b), particularly with relation imization (maximization) of gridpoint values, a less re- to their Fig. 6a will be discussed. Finally, some com- strictive scanning procedure has been implemented. posites of SST anomalies and cyclone and anticyclone This procedure seeks grid points at which the Laplacian track maps overlapping all synoptic trajectories were of the pressure shall be greater (or lower in the case of made for seven El NinÄo (EN) and La NinÄa (LN) years. an anticyclone) than at any of the eight surrounding Possible changes on the hemispheric high and low pres- points and greater than a speci®c threshold. In the case sure belts associated with the El NinÄo±Southern Oscil- of an open depression (ridge), in which no point of lation (ENSO) phase are discussed. minimum (maximum) pressure exists, it was decided that a suitable analog for it would be the in¯exion point in the pressure surface, that is, the point of the minimum 2. Data and methodology (maximum) pressure gradient. Finally, the lows and The MS automatic scheme was used to ®nd and track highs are checked for having the character of a mid- high and low pressure centers near the surface. Mean latitude storm. The method that was found to give good sea level pressure data from the NCEP±NCAR reanal- discrimination is one that requires a minimum average ysis (Kalnay et al. 1996) was used every 12 h from 1973 value of the pressure Laplacian over a speci®ed radius to 1996. Although some studies suggest that the auto- of the cyclone or anticyclone center. matic scheme's performance would increase if the da- In the second stage of the scheme, the path of each taset were used every 6 h, our tests did not show any system is tracked from the time of its appearance to its signi®cant improvement in this case. Therefore, the 12- dissipation. To make the appropriate decisions, a pro-

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TABLE 1. Summary of the validation of the most important parameters used within the automatic scheme of MS. Different con®gurations applied to highs and lows are indicated in bold. Chosen value Parameter Description Highs Lows Ni Lon projection 73 73 Nj Lat projection 73 73 Rproj Projection radius (2.5Њ) 36 36 Rdiff Horizontal smooth parameter 3.5 3.5 Iopmxc Including or not the open systems (0 ϭ not) 0 0 Nshell Scanning points 8 8 Itmxl Max grid points to be crossed 20 20 D¯tl Min distance between two systems 7.5 7.5 Cmnc1 Min avg for Laplacian pressure 0.1 0.25 Cvrad Radius used for the Laplacian avg 2.0 2.0 Cmncw Min for the Laplacian central value 0.2 0.7 Refdt Time interval, in days 0.5 0.5 Wsteer Weight for the climatological steer on the tracks 0.6 0.6 Fsteer Steering velocity scale factor 2.0 2.0 Wpten Weight for pressure tendency 0.0 0.0 Wmotn If Wmotn ϭ1, the climatological speed will be used for new created systems 1.0 1.0 Rcprob Passage radius for the probability function 12 12 Rpbell Shape of the probability function 0.5 0.5 Qmxnew Max probability for new cyclones 0.75 0.75 Irevmx Max iterations allowed 18 18 cedure was developed which ®rst makes an estimate of unable to distinguish between high and low pressure the new position of each system, then calculates the centers. Since the main interest in this paper is the sig- probability of associations between the predicted and ni®cant extratropical transient activity, which usually realized positions, and ®nally ®nds the matching of these means central pressures outside of the 1010±1020-hPa associations with the highest overall probability. range (Schwerdtfeger 1976), this problem was partially As mentioned at the introduction, previous studies solved applying a pressure selection, forcing the scheme have shown the reliability and ef®ciency of the MS to capture mainly mid- and high-latitude systems. scheme in capturing the most important climatic features Therefore, only anticyclones with central pressure above in the Southern Hemisphere (Murray and Simmonds 1020 hPa and cyclones with central pressure below 1015 1991a,b; Jones and Simmonds 1993, 1994; Simmonds or 1010 hPa, depending on the type of analysis, were et al. 1999; Simmonds and Keay 2000a,b). For more considered. The strong end of the spectrum was also information on the scheme's algorithm, the reader may further analyzed selecting anticyclones above 1035 hPa refer to Murray and Simmonds (1991a,b). and cyclones below 980 hPa. In order to evaluate the MS scheme performance, Murray and Simmonds (1991a,b) have pointed out many different parameters were tested, such as the lat- that any pressure criterion may be considered too sub- itudinal and longitudinal projection, the projection ra- jective and might induce a location bias. Sinclair (1994) dius, the horizontal smoothing parameter, the minimum has also discussed the weakness of any automatic values and radius used for the pressure Laplacian, and scheme based on sea level pressure, not only because some others related to the statistical process. To help the use of pressure minima can induce bias but because other tracking scheme users, Table 1 summarizes the some cyclonic systems without a closed pressure min- values we found were best for each parameter. Param- imum would be missing as well. In his work, Sinclair eters in bold refer to cases in which cyclones and an- pointed out some advantages and disadvantages of using ticyclones have different values. This set of optimal vorticity rather then pressure. He showed that while the parameters was determined after some tests in which a use of vorticity avoids bias and can adequately capture comparison of speci®c automatic and manual trajecto- most of the transient cyclonic activity, it has also some ries of highs and lows applied in two case studies, disadvantages such as its high sensitiveness to analysis proved the scheme robustness and reliability for the errors (which could be a major concern in the SH) and transient extratropical propagation (®gure not shown). a tendency to include elongated geostrophic shear or Although the optimal parameters in Table 1 produced curvature zones not generally thought of as cyclones. very accurate extratropical tracks, a large number of Taking into consideration the proven robustness of the tropical heat and orographic lows were still observed, MS scheme for extratropical systems and considering which is similar to that results obtained by Sinclair that the analyses presented here aim to give a synoptic (1994). Furthermore, there is an increase in the MS perspective through the classic de®nition of cyclones model uncertainty for pressure values ranging between and anticyclones, sea level pressure was used. The pos- 1010 and 1020 hPa, where in some cases the model was sible weaknesses of this kind of scheme should be con-

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si®es years into neutral, El NinÄo, or La NinÄa. After the selection of the years, the SST anomalies for the SH wintertime period were checked, only keeping the cases in which consistent anomalies were present. The SST dataset used was obtained from the Comprehensive Ocean±Atmosphere Data Set (COADS; Woodruff et al. 1998) with a resolution of 1.0Њϫ1.0Њ latitude±longi- tude. All the cases selected here are included in the Trenberth (1997) list. The total amount of synoptic cyclone and anticyclone trajectories was superposed on the same map. This pro- cedure created a ``climatic cloud'' containing many dif- ferent signatures and a variety of paths from individual synoptic propagation systems. As a result, an easy gen- eral recognition of the hemispheric patterns was pos- sible. In order to distinguish between the geographical re- gions dominated only by cyclones or by anticyclones and the regions overlapped by both systems, a graphical treatment based on colors was used. The same procedure was applied to distinguish the paths that only occurred in LN and EN years. For each feature of interest it was attributed a different color in such a way that all the trajectories could be visualized in the same map. Dif- ferent colors were used to determine the intersection regions where common areas were removed in order to emphasize other characteristics. (This methodology was applied to Figs. 3a±c). Further details are given in sec- tion 3b.

3. Results a. Time series of cyclones and anticyclone counts and climatic trends Figure 1 shows the Southern Hemisphere winter (JJA) total number of cyclone and anticyclone tracks with no restriction to pressure ranges (Fig. 1a), for cyclones be- low 1010 hPa and anticyclones above 1020 hPa (Fig. FIG. 1. Number of cyclone and anticyclone tracks every 12 h during 1b), and for cyclones below 980 hPa and anticyclones JJA (1973±96), according to the automatic scheme of MS for the SH above 1035 hPa (Fig. 1c) according to the MS automatic (a) without pressure restriction, (b) for cyclones of central pressure scheme using data every 12 h for the 1973±96 period. below 1010 hPa and anticyclones of central pressure above 1020 hPa, and (c) for cyclones of central pressure below 980 hPa and anticy- The linear regression lines and the squared linear cor- clones of central pressure above 1035 hPa. Tracks consisting of only relation coef®cients calculated according to the least one point (orphan systems) were also counted. squares method (Wilks 1995) are shown in each case. The con®dence level according to the Student's t test (Wilks 1995) is above 99% for the regressions in Figs. sidered when interpreting the results though, specially 1a and 1c, excluding the anticyclones in Fig. 1c that those expressed in Fig. 1. presented no tendency with a near-zero squared linear In order to study the tracking process variability and correlation coef®cient, and slightly above 95% for Fig. its relation with ENSO events, some El NinÄo and La 1b. Tracks with one point, that is, those that disappeared NinÄa years were chosen within the data period used here: in the following analysis (de®ned here as orphan sys- namely; 1976, 1982, 1983, 1987, 1991, 1992, and 1993 tems) were included as well. for EN and 1973, 1984, 1985, 1988, 1989, 1995, and It is known that any pressure criterion may be rather 1996 for LN. The criterion used to select the events was subjective and might induce a location bias (e.g., Sin- based on the NCEP seasonal list de®nition from 1950 clair 1994) but, as discussed in section 2, the MS to 2000, available at its Web site (www.cpc.ncep. scheme's accuracy is signi®cantly higher for stronger noaa.gov). The NCEP criterion presented on its site clas- transient systems. From a synoptic point of view Fig.

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1b expresses the most important transient activity, where coef®cient. Nevertheless, no tendency has been ob- the spurious and weak systems were partially eliminat- served for the intense anticyclones with central pres- ed, and Fig. 1c only shows the systems located in the sures above 1035 hPa, with a near-zero coef®cient. most intense end of the spectrum. As commented before, Figure 1a bears some resemblance to Fig. 6a from the MS scheme uncertainty may be high when one con- Simmonds and Keay (2000b), who suggested a negative siders the whole spectrum as in Fig. 1a but its accuracy trend in the number of transient systems in the Southern improves considerably for the selections in Figs. 1b and Hemisphere. Their ®gure showed the time series of the 1c. total annual number of cyclone tracks and only included According to Fig. 1a, when no pressure restriction is cyclones that lasted at least 24 h. In our case, we limited applied, the number of anticyclones is higher than the our analysis to just one season (JJA) and the anticyclone number of cyclones, showing 583 tracks of highs and series and ``orphan'' systems were included as well. 529 tracks of lows, on average. On the other hand Fig. Furthermore, from Fig. 5 of Simmonds and Keay's 1b, which considers anticyclones with central pressure work, which shows the time series of seasonal averages above 1020 hPa and cyclones with central pressure be- of the number of cyclones per analysis for the SH, a low 1010 hPa, shows an average of 340 anticyclone marked decline of cyclones during the austral winter is tracks and 358 cyclone tracks, which indicates a cyclone also noticed. count slightly higher. In percentage terms, it means a From Fig. 1b, it is clear that when the pressure se- reduction of about 42% and 32% for the anticyclones lection was applied, the difference between the total and cyclones, respectively, when this ``synoptic'' pres- number of cyclones and anticyclones, as well as the sure restriction was considered. It is worth noting that trends, almost disappeared. Based on this result, one these reductions mostly assured not only the elimination may wonder if the previous trend is related to how the of weak extratropical systems but some that might have automatic scheme is adjusted in a weak pressure gra- also been falsely identi®ed by the automatic scheme dient environment rather than to possible climate chang- because of the associated weak pressure gradients. The es. Nevertheless, Simmonds and Keay (2000b) also doc- fact that the reduction was more effective for the an- umented an increase in the radius and depth of the sys- ticyclones probably suggests that they were more sus- tems. This would suggest fewer but more intense sys- ceptible to misidenti®cation in a weak-gradient envi- tems, and hence, if one progressively eliminates the ronment. weaker systems, an apparent downward trend could Figure 1c shows cyclones below 980 hPa and anti- even turn into an increase. cyclones above 1035 hPa, which corresponds to the Figure 1c con®rms the hypothesis above, that is, the most intense 25% and 10% of tracks, respectively, in intense cyclonic systems are becoming more frequent. relation to the total count. These last pressure ranges Some tests considering further variations in the pressure only show the systems at the intense ends of the spec- range used to select the intense systems in Fig. 1c were trum, and a signi®cant additional reduction is observed. made, but no signi®cant change of pattern was obtained. Anticyclones above 1035 hPa are usually seen over con- Considering only the cyclones between 1000 and 1010 tinents in association with strong polar air masses, and hPa, an overall decline similar to the one shown in Fig. cyclones below 980 hPa only occur in mid- and high 1a is also veri®ed (®gure not shown). Furthermore, these latitudes and are usually associated with strong winds observations agree and con®rm the suggestions of Sim- and precipitation. These strong cyclones are frequently monds and Keay (2000b) that there would be fewer but seen in association with polar air masses when passing more intense cyclones. over the southern tip of South America (Ambrizzi and Other studies have also pointed out a reduction in the Pezza 1999). overall cyclonic activity in different regions of the From Fig. 1a, the regression indicates a signi®cant Southern Hemisphere (Leighton and Deslandes 1991; overall decline in the number of cyclones and anticy- Leighton 1997; Leighton et al. 1997; Simmonds et al. clones during the analysis period. Nevertheless, the per- 1998; Key and Chan 1999). However, Sinclair et al. centage of the total variance explained by the ®t, which (1997) showed an increase for cyclone number over the is given by the squared correlation coef®cients, is about Southern Ocean during the 1980s, using a different au- 55% for the anticyclones and 44% for the cyclones, tomatic scheme applied to the European Centre for Me- indicating a high variability. The trends are less notic- dium-Range Weather Forecasts (ECMWF) analyses. able in Fig. 1b, where the variability is much higher These considerations raise the question about how (only 18% and 19% of the total variance are explained tracking schemes are sensitive to capturing low and high by the linear ®t for the anticyclones and cyclones, re- pressure centers when different restrictive pressure cri- spectively). Finally, Fig. 1c shows a different pattern, teria are applied. Different datasets, different automatic with an overall increase of cyclones with central pres- procedures, and how the cyclones are exactly de®ned, sures below 980 hPa, which indicates an opposite be- that is, via vorticity or sea level pressure, are factors havior at the intense end of the spectrum. About 46% that may also play an important role on the ®nal results of the total variance is explained by the linear regression (see also discussions in Carnell et al. 1996; Sickmoller in this case, as shown by the squared linear regression et al. 2000). The possibility of a pressure bias, as dis-

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FIG. 2. SST SH anomaly composites for JJA, based on (a) seven EN (1976, 1982, 1983, 1987, 1991, 1992, and 1993) and (b) seven LN (1973, 1984, 1985, 1988, 1989, 1995, and 1996) years. Values between Ϫ0.2Њ and ϩ0.2ЊC are not shown, and negative values appear as dashed lines. Contours are plotted every 0.2ЊC. cussed in section 2, is also one point to be considered For clarity, those tracks inside of common areas, that when comparing Figs. 1a±c. Any physical mechanism is, those that occurred during both ENSO phases, were that would reasonably explain an environment with few- removed from Figs. 3b and 3c, according to the meth- er but more intense cyclones and a static number of odology discussed in section 2. One should note that intense anticyclones is rather complex and dif®cult to the cyclone de®nition considered in this case (below address. This issue needs further analyses in light of the 1015 hPa) differs from the selection used in Fig. 1b. possible climate changes that may have occurred in the The idea in this section is to present a general climatic SH. view that allows for the consideration of weaker sys- tems. The most signi®cant Southern Hemisphere climatic b. Cyclone and anticyclone track climatology and features related to cyclone and anticyclone activity are ENSO-related tracks plotted in Fig. 3a. The purple color on the subtropical Figure 2 shows the sea surface temperature anomalies belt indicates a high anticyclone concentration over the associated with the EN (Fig. 2a) and LN (Fig. 2b) com- oceans corresponding to the well-known pressure cen- posites during the SH wintertime. This ®gure provides ters of action (Hastenrath 1985; Machel et al. 1998; an overall view of the hemispheric forcing that may Kapala et al. 1998), particularly in the subtropical At- have impacted the cyclone and anticyclone propagation lantic high. The subtropical Paci®c high and the sub- during the composed years. From Fig. 2a, a large pos- tropical Indian high are not as evident because they are itive anomaly area is observed from the date line to the embedded in regions of high cyclone activity (transient west coast of South and Central America, with values region showed in yellow color). A high concentration above ϩ1.2ЊC around the equator and eastward of of orographic anticyclones, especially over the Andes 120ЊW. For the LN composite (Fig. 2b), anomalies ex- and over the Antarctic Plateau is also observed. tending from the date line to the west coast of South Some migratory extratropical anticyclones appear in America with amplitudes up to Ϫ1ЊC are observed. Al- the center of Australia and southeastern South America, though the anomalies' magnitude is not very intense, but most of the transient hemispheric activity between the anomalies are signi®cant because they represent a 30Њ and 60ЊS is characterized by the passage of both composite of 21 wintertime months for each ENSO cyclones and anticyclones represented by the yellow phase. color in the map. The pink color shows a marked cy- Figure 3a shows a superposition of systems indicating clone concentration around the Antarctic continent that the geographical track regions of anticyclones (tracks approximates the position of the SH storm tracks doc- and ``orphan systems'') with central pressure above umented earlier (Wallace et al. 1988; Trenberth 1991). 1020 hPa (violet) and cyclones (tracks and orphan sys- Several heat and orographic lows (and highs) can be tems) with central pressure below 1015 hPa (pink color), observed over the continents as well. and the regions overlapped by both systems (yellow), Figures 3b and 3c show the total amount of tracks according to the automatic MS scheme for the 1973± and orphan systems that occurred in the EN (red) and 96 JJA period. Fig. 3b shows the same for anticyclones LN (blue) years. Comparing Fig. 3a with Figs. 3b and and Fig. 3c shows the same for cyclones that occurred 3c one can see that, depending on the ENSO phase, during the El NinÄo (in red) and La NinÄa (in blue) com- there are differences on the tracks concentration. Fur- posites alone. Orphan tracks are indicated by crosses. thermore, the cyclone and anticyclone gap areas be-

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tween the subtropics and the subpolar belt shown in Figs. 3b and 3c correspond to a baroclinic region where the transients are active in all seasons. Indeed, Sim- monds and Keay (2000a), in their study of the SH ex- tratropical cyclone behavior, have already shown that a large number of cyclonic systems are found between 50Њ and 70ЊS during all seasons. The idea of showing those tracks and points that only appeared during EN or LN years (Figs. 3b and 3c), eliminating the overlap- ping areas, contributes to emphasize the main features associated with ENSO and the differences between its phases. Two pronounced bands of anticyclone tracks are seen in Fig. 3b. The ®rst one is about 20ЊS and the second is located around 50ЊS. For the EN composites, the sub- tropical band shows a higher anticyclone concentration over the . It indicates that during warm ENSO events the Atlantic subtropical high is enhanced. This pattern is in agreement with some works, which showed that during the EN austral winters there is a tendency for above-normal frontal systems blocking over southern Brazil in association with a stronger than normal subtropical high-level jet over the region (see Kousky et al. 1984; Nobre et al. 1986; Ambrizzi 1994). A higher anticyclone concentration during EN years is also seen over the South Atlantic Ocean, to the south of the Australian continent and to the west of the Ant- arctic Peninsula. On the other hand, the subtropical east- ern Paci®c seems to favor the anticyclone concentration during LN years, which is also true for the Antarctic Peninsula and its eastern part. The cyclone tracks found in the EN and LN years are concentrated around 30ЊS in a broader band (Fig. 3c). During LN years, there are more tracks over the sub- tropical Atlantic and southeastern Australia. The cy- clone tracks during the EN years are more pronounced over the subtropical Paci®c, southern Argentina, the west coast of South America, and over the Indian Ocean. The ENSO phase is well known for its important social and economic impacts, especially with regard to precipitation anomalies all over the globe. The differ- ences in the cyclone and anticyclone tracks obtained here are re¯ecting possible changes in the upper-level wave patterns, since surface pressure is linked with the circulation above. The high cyclone concentration in southern Argentina (Fig. 3c) and the enhanced subtrop- ical Atlantic high (Fig. 3b), for instance, are physically

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FIG. 3. (a) Superposition of systems indicating the geographical track regions of anticyclones (tracks and orphan systems) with central pressure above 1020 hPa (violet) and cyclones (tracks and orphan systems) with central pressure below 1015 hPa (pink color), and the regions overlapped by both systems (yellow), based on applying the automatic MS scheme for the 1973±96 JJA period; (b) the same for just anticyclones; and (c) for just cyclones that occurred during the EN (in red) and LN (in blue) composites alone. Orphan tracks are indicated by crosses. See text for further details.

Unauthenticated | Downloaded 10/02/21 07:59 PM UTC 1082 JOURNAL OF CLIMATE VOLUME 16 consistent with the observed subtropical high-level jet 4. Conclusions enhancement over southern Brazil during austral EN winters, as discussed before. This pattern is related to This study presents further analysis of the variability heavy rains over southern Brazil, Uruguay, and north- of the Southern Hemisphere cyclone and anticyclone east Argentina (e.g., Grimm et al. 2000 and references behavior using the University of Melbourne's automatic therein). scheme. The NCEP±NCAR reanalysis data were used One important point related to the geographical re- for the austral wintertime period of 1973±96. We have gions shown in Figs. 3b and 3c is that the observed also discussed some of the results found by Simmonds accumulated systems are not the result of chance. Since and Keay (2000b) who showed a possible climatic trend the tracks have been analyzed every 12 h during a 14- related to the total annual number of cyclone tracks over yr period (7 EN and 7 LN years), there are enough the Southern Hemisphere using the same automatic systems to assure that the ENSO signal is higher than scheme and data source, but with a slightly different the sample noise. Another aspect to be considered is methodology. Our results suggest another perspective for the down- that there may be a built-in bias to position as a result ward trends in the cyclone numbers proposed by Sim- of the problems applying automatic schemes to sea level monds and Keay (2000b). It is shown that when a ``syn- pressure data, but it seems that the most relevant weath- optic'' selection is applied to the central pressure in er-producer systems and their position according to the order to prevent the inclusion of weak systems, the ENSO phase were correctly captured. trends change considerably. The synoptic pressure cri- Some of the above results are also in good agreement terion used here was an attempt to eliminate most of with those shown by Sinclair et al. (1997) who found the weak and tropical systems, focusing on the midlat- a coherent cyclone response to ENSO using an auto- itude and extratropical weather systems, since the MS matic scheme based on vorticity. Furthermore, they ar- automatic scheme skill is signi®cantly improved for rel- gued that the LN cyclone patterns are almost exactly atively stronger pressure gradients. However, analyses the reverse of the EN ones, suggesting a predominantly at both ends of the spectrum show a lower total number linear response. This feature was somewhat observed in of cyclones with a decrease in the number of weak sys- our results, though it showed a high variability. For tems and an increase in the number of intense systems, instance, over the subtropical Atlantic the high anti- particularly for those with central pressure below 980 cyclone concentration during EN years is replaced by hPa. For the anticyclones, there is also an overall de- a high cyclone concentration during LN years, and over crease in their number due to a decrease in the number much of the subtropical Paci®c, the high concentration of weak systems, but the strong ones are not undergoing of anticyclones during LN is replaced by cyclones dur- any change. These results are complementary to the ing EN. Simmons and Keay study, indicating how sensitive the One may wonder if ENSO events would affect the tracking scheme is to capturing low and high pressure total hemispheric number of surface cyclones and an- centers, where the observed differences may depend on ticyclones and the trends as well. If a plot similar to the pressure selection criteria used. Fig. 1 is prepared considering the EN and LN years A climatological analysis highlighted the most im- alone (®gure not shown), the same patterns discussed portant signatures associated with transient and semis- before are present, that is, the trends and the average tationary activity over all the Southern Hemisphere. The total hemispheric count are very similar to those pre- superposition of cyclonic and anticyclonic paths indi- viously discussed. This indicates that the trends are not cated the transition regions and the areas crossed by associated with the ENSO phase. cyclones and anticyclones alone. These results are in Furthermore, the fact that the average of the total accordance with previous studies found in the literature, hemispheric count is not signi®cantly affected by ENSO emphasizing the climatological signatures of transient is physically reasonable and it is in agreement with Figs. (Wallace et al. 1988; Trenberth 1991) and semiperma- 3b and 3c, because one would expect that the regions nent patterns (Hastenrath 1985). where there is an increase would compensate for the The analysis of the EN and LN track composites regions of systems where there is a decrease, resulting showed that there are some signi®cant differences in the in an approximate equal number, on a hemispheric basis. extratropical propagation patterns when the ENSO phas- The work of Sinclair et al. (1997), for instance, showed es are compared. For instance, there appears to be a a very similar pattern. In their Figs. 11c and 11d, which higher anticyclone concentration near the subtropical show the winter track density anomaly composite for a South Atlantic high during EN years, while for the sub- Southern Oscillation index (SOI) below minus 1.5 (Fig. tropical South Paci®c high it occurs during LN years. 11c) and above 0.5 (Fig. 11d), one can see that on a On the other hand, the cyclone tracks showed a higher hemispheric basis the areas of negative correlation can variability, with an excess of lows over the subtropical approximately compensate for the areas of positive cor- Paci®c west of South America, southern Argentina, and relation, suggesting the possibility of a hemispheric the Indian Ocean during EN years, and a more pro- equilibrium. nounced band over the subtropical Atlantic and south-

Unauthenticated | Downloaded 10/02/21 07:59 PM UTC 1APRIL 2003 NOTES AND CORRESPONDENCE 1083 eastern Australia during LN years. Nevertheless, the cy- Leighton, R. M., 1997: Variations in annual cyclonicity across the clone and anticyclone trends previously discussed do Australasian region during the 29-year period 1965±1993. Pre- prints, Fifth Int. Conf. on Southern Hemisphere Meteorology and not seem to be related to the ENSO phenomenon, and Oceanography, Pretoria, South Africa, Amer. Meteor. Soc., 362± on a hemispheric basis the regional differences are ap- 363. proximately compensated, producing similar count ÐÐ, and R. Deslandes, 1991: Monthly anticyclonicity and cyclon- numbers for EN and LN years. icity in the Australasian region: Averages for January, April, July, and October. Aust. Meteor. Mag., 39, 149±154. A complete understanding of the physics associated ÐÐ, K. Keay, and I. Simmonds, 1997: Variations in annual cy- with cyclone and anticyclone tracks may be of great clonicity across the Australasian region during the 29-year period interest and very useful as a forecast tool, speci®cally 1965±1993 and relationships with annual Australian rainfall. Cli- with regard to the occurrence of major and polar mate Prediction for Agricultural and Resources Management, R. K. Munro and L. M. Leslie, Eds., Bureau of Resource Sci- wave propagation (Ambrizzi and Pezza 1999). Further- ences, 257±267. more, the well-known global in¯uences associated with Machel, H., A. Kapala, and H. Flohn, 1998: Behavior of the centers of the ENSO phases can be better understood considering action above the Atlantic since 1881. Part I: Characteristics of sea- their impacts on the transient activity over the hemi- sonal and interannual variability. Int. J. Climatol., 18, 1±22. Murray, R. J., and I. Simmonds, 1991a: A numerical scheme for sphere. These issues need further analysis. tracking cyclone centres from digital data. Part I: Development and operation of the scheme. Aust. Meteor. Mag., 39, 155±166. Acknowledgments. This research was funded by Fun- ÐÐ, and ÐÐ, 1991b: A numerical scheme for tracking cyclone dacËaÄo de Amparo aÁ Pesquisa do Estado de SaÄo Paulo centres from digital data. Part II: Application to January and (FAPESP) under Grants 99/04105-2 and 98/12976-0. July general circulation model simulations. Aust. Meteor. Mag., 39, 167±180. TA has also received support from CNPq (301111/93- Nobre, C. A., A. S. Oliveira, and E. K. Neves, 1986: Precipitation 6) and the Inter-American Institute for Global Change and Circulation Anomalies in South America and the 82/83 El Research (IAI) through the project IAI-CRN-055. The NinÄo/Southern Oscillation Episode. Vol. 1, Fourth Brazilian Me- authors would like to thank Dr. Ian Simmonds and an teorological Conf., Brasilia, Brazil, Brazilian Meteorological So- ciety, 339±345. anonymous reviewer for their very helpful comments Schwerdtfeger, W., 1976: Climates of Central and South America. in the early version of the paper. They would also like Vol. 12, World Survey of Climatology, Elsevier, 532 pp. to thank Dr. Ross Murray and Dr. Ian Simmonds, from Sickmoller, M., R. Blender, and K. Fraedrich, 2000: Observed winter the University of Melbourne (Australia), for providing cyclone tracks in the northern hemisphere in re-analyzed the automatic scheme. ECMWF-data. Quart. J. Roy. Meteor. Soc., 126, 591±620. Simmonds, I., and K. Keay, 2000a: Mean Southern Hemisphere ex- tratropical cyclone behavior in the 40-year NCEP±NCAR Re- analysis. J. Climate, 13, 873±885. 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