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Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958±97

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Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958±97 550 JOURNAL OF CLIMATE VOLUME 13 Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958±97 IAN SIMMONDS AND KEVIN KEAY School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia (Manuscript received 16 November 1998, in ®nal form 31 March 1999) ABSTRACT An analysis of the variability and trends exhibited by many aspects of Southern Hemisphere (SH) mean sea level extratropical cyclones during the period 1958±97 is presented. The investigation is undertaken by applying a state-of-the-art cyclone ®nding and tracking scheme to the 6-hourly reanalyses produced by the National Centers for Environmental Prediction. The outcome of this is arguably the most reliable analysis of SH cyclone variability undertaken to date. Across the 40-yr period the annual and seasonal mean cyclone densities have undergone reductions at most locations south of about 408S (with the greatest reductions near 608S), and increases to the north. This pattern of change resembles the ``high-latitude mode'' identi®ed in many studies of SH circulation features. It is shown that the mean radius of SH extratropical cyclones displays almost everywhere a signi®cant positive trend, and there are also increases in annual mean cyclone ``depth'' (i.e., the pressure difference between the center and the ``edge'' of a cyclone). The annual average number of cyclones per SH analysis rose from the start of the period to a maximum of about 39 in 1972. Since then, the numbers have shown an overall decline, the counts in the 1990s being particularly low. Similar behavior was evident when the count was con®ned to the 308±508S and 508±708S latitude bands. Least squares best ®t to the three time series exhibit signi®cant slopes of 20.58, 20.26, and 20.58 cyclones per analysis per decade, respectively. Between 308 and 708S the annual mean number of cyclones found per analysis assumed a maximum about 1970, but that number has dramatically decreased by about 10% since then. (This analysis suggests that the downward trends in cyclone numbers are associated with a warming Southern Hemisphere.) The overall structure of the time series of annual cyclone per analysis over 308±508S and 508± 708S are similar, but their year-to-year changes are shown to be negatively correlated; hence, there tends to be an interannual compensation of cyclone density between the middle and higher latitudes. The extent to which changes in the semiannual oscillation over the last few decades could be said to have in¯uenced how cyclones are distributed across seasons is brie¯y examined. The results show, in particular, that the interannual relationship between spring and winter cyclone density cannot be explained in terms of a response to a change in the amplitude of the semiannual oscillation. 1. Introduction One of the problems up until now associated with an- alyzing multidecadal variability in the global meteo- Climatologists are increasingly devoting attention to rological analyses that are constructed for operational understanding the modes and mechanisms of natural purposes is that the techniques used to produce these variability in the climate system. The understanding of undergo improvements and upgrades on a regular basis. such variability is one of the major focuses of the Cli- Hence, there is always the doubt that trends or vari- mate Variability and Predictability research program abilities identi®ed in the analyses may be ®ctitious in (WCRP 1995), one of the aims of which is that at least part of the features identi®ed may be as- to describe and understand the physical processes re- sociated with the improvement in the operational prod- sponsible for climate variability and predictability on sea- uct. sonal, interannual, decadal, and centennial time scales, In recent times a way out of this dilemma has been through the collection and analysis of observations and presented by the construction of reanalyses. These are the development and application of models of the coupled performed by assimilating past data into a frozen state- climate system.... of-the-art analysis±forecast model system. The database for this can also be enhanced with many observations that were not available in real time for operations, and the reanalysis product can be regarded as one of the Corresponding author address: Dr. Ian Simmonds, School of Earth most complete, physically consistent meteorological da- Sciences, University of Melbourne, Parkville, Victoria 3052, Austra- lia. tasets. The procedure hence removes any arti®cial var- E-mail: [email protected] iability that might be caused by model changes (but q 2000 American Meteorological Society Unauthenticated | Downloaded 10/01/21 08:24 PM UTC 1FEBRUARY 2000 SIMMONDS AND KEAY 551 obviously cannot address apparent variability associated with temporal changes in data quality, quantity, and dis- tribution). The two most reliable reanalysis sets that have been disseminated are those of the National Cen- ters for Environmental Prediction±National Center for Atmospheric Research (NCEP±NCAR; Kalnay et al. 1996) and the European Centre for Medium-Range Weather Forecasts (ECMWF; Gibson et al. 1997). It is the NCEP±NCAR (hereafter referred to as NCEP) set we use here. It covers the 40-yr period 1958±97, and the global analyses are available every 6 h. As such, it represents one of the longest records of quality global meteorological analyses and one that may be used to undertake variability studies of up to decadal timescales. Our interest here is in examining the variability in the behavior of Southern Hemisphere (SH) mean sea level extratropical cyclones over the time span of the reanalysis. The problems alluded to above in terms of changes in operational analysis procedures appear to have a great impact on the apparent number and be- havior of these systems. For example, Sinclair (1994) and Simmonds et al. (1998) found a marked increase FIG. 1. Linear trend of the annual average mean sea level pressure in the total number of cyclones present in the ECMWF over the period 1958±97. The contour interval is 0.5 hPa decade21 and negative contours are dashed. Stippling denotes regions over operational analyses after April 1983, which coincided which the trend differs signi®cantly from zero (95% con®dence level). with the date that the ECMWF model was changed from a gridpoint to spectral model with envelope topography. Simmonds and Keay (2000, SK hereafter) have pre- sented a climatology of many SH cyclone statistics av- hPa geopotential) variability (the so-called high-latitude eraged over the 40-yr period covered by the NCEP re- mode), which is apparent on a surprising wide range of analysis. We use their data here and reference should timescales, from about a week to years. In this context be made to that paper, and to Simmonds and Murray we may cite the works of Kidson (1975), Mo and White (1999), and Simmonds et al. (1999), for details con- (1985), Karoly (1990), Ghil and Mo (1991), Mechoso cerning the automatic algorithm with which cyclones et al. (1991), and Sinclair et al. (1997). The pattern and their tracks were identi®ed from the digital analyses. displayed in our ®gure bears considerable resemblance Suf®ce it to say here that the scheme is of one the most to the high-latitude mode shown in these and other stud- sophisticated and reliable automatic cyclone ®nding and ies, indicating that it is an important mode of oscillation tracking schemes available. even on multidecadal timescales. The trends exhibited in the NCEP reanalyses assume larger values at the more southerly latitudes. Most of the region south of 608S 2. Results has experienced negative trends in excess of 1 hPa de- 21 21 a. Geographical distributions of trends cade , and they surpass 2 hPa decade just off the coast in the Haakon VII and Ross Seas, as well as off As a background against which we can interpret the the Amery Ice Shelf, George V Land, and Terre AdeÂlie. variability and changes in SH extratropical cyclone be- The distribution of trends in cyclone system density havior it is important to gain an appreciation of the in the four seasons were also all rather similar and, trends that the mean sea level pressure (MSLP) has hence, closely resemble that of the trend of the annual undergone over the 40-yr reanalysis period. Figure 1 means, which is shown in Fig. 2a. It can be seen that displays the linear trend (least squares best ®t) of the there has been a general reduction, for the most part annual mean MSLP (and stippling indicates where the statistically signi®cant, in cyclone numbers over most trend can be said to be signi®cantly different from zero regions south of 408S, and even farther north in the at the 95% con®dence level, the level adopted through- eastern Paci®c. The greatest reductions occur near the out this paper). (The trends exhibited in the four seasons 608S latitude circle. Immediately on the coast there are are broadly similar to that displayed in the ®gure.) The several ``bull's-eyes'' of increases in the number of cy- pattern displays notable zonal symmetry; negative clones. It is not clear why these appear but there are a trends are found at most locations south of about 408S, few possible explanations. For the period 1979±93 the and increases to the north. A number of authors have sea ice ®eld derived from Scanning Multichannel Mi- referred to, and documented, a seesaw pattern between crowave Radiometer and then from Special Sensor Mi- middle and high southern latitudes in MSLP (and 500- crowave Imagers data were used in the NCEP reanalysis. Unauthenticated | Downloaded 10/01/21 08:24 PM UTC 552 JOURNAL OF CLIMATE VOLUME 13 It has been noticed that sensor differences and drift on these two platforms probably affect the consistency of the series (Zabel and Jezek 1994; Johannessen et al.
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