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NOTES and CORRESPONDENCE Variability of Southern Hemisphere Cyclone and Anticyclone Behavior: Further Analysis

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NOTES and CORRESPONDENCE Variability of Southern Hemisphere Cyclone and Anticyclone Behavior: Further Analysis 1APRIL 2003 NOTES AND CORRESPONDENCE 1075 NOTES AND CORRESPONDENCE Variability of Southern Hemisphere Cyclone and Anticyclone 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 cyclones and anticyclones. 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 308 and 608S, which is approximately the climatological position of the SH transient activity. To the north of 308S, the subtropical South Atlantic high 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 storm 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 subtropics 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 q 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.
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