Weather regimes in the South American sector and neighbouring oceans during winter Received: 27 September 2002 / Accepted: 27 January 2003 / Published online: 2 April 2003 Ó Springer-Verlag 2003 Abstract We classified 34 years of winter daily 500 hPa geopotential height patterns over the eastern South Pa- cific-South America-South Atlantic region using the K-means clustering method. We found a significant classification into five weather regimes (WRs) defined as the most frequent large-scale circulation anomalies: WR1 (trough centred downstream of the Drake Pas- sage), WR2 (trough over the SW Pacific and ridge downstream), WR3 (ridge over the SE Pacific and NW– SE trough downstream), WR4 (trough over the SE Pacific and NW–SE ridge downstream) and WR5 (weak ridge to the west of southern South America). We also analysed their persistence and temporal evolution, in- cluding transitions between them and development around onsets and breaks of each regime. The preferred transitions, WR1 fi WR3 fi WR2 fi WR4 fi WR1 and also WR1 fi WR3 fi WR2 fi WR1, suggest the progression of a Rossby wave-like pattern in which each of the regimes resemble the Pacific-South America modes. Significant influence of the WRs on local climate over Argentina was found. The preferred transitions WR1 fi WR3 and WR3 fi WR2 induce sustained cold conditions over Patagonia and over northern Argentina, respectively. The most significant change in precipitation frequency is found for WR3, with wetter conditions over all the analysed regions. Finally, the interannual to in- terdecadal significant variations in the occurrence of these regimes were discussed. WR1 and WR3 are more frequent and WR2 is less frequent during El Nin˜o, and WR2 and WR5 are more frequent and WR1 is less frequent during La Nin˜a. A significant decrease in WR2 and increase of WR4 and WR5 during the 1970s and early 1980s were found. 1 Introduction Wintertime extratropical atmospheric flows are charac- terised by certain large-scale patterns that appear recur- rently at fixed locations remaining beyond the lifetime of individual synoptic-scale systems. A large part of the in- traseasonal variability of the Southern Hemisphere (SH) is related to the alternating between such large-scale cir- culation patterns. Previous studies found that the leading modes of both intraseasonal and interannual variability over the SH display a dominance of wave 3 and 4 struc- tures, with fluctuations in similar locations on a broad range of time scales (Kiladis and Mo 1998). The low-frequency variability consists basically of a mid- to high-latitude vacillation in the zonal wind strength (the high-latitude mode, Kidson 1988) and two eastward propagating wave 3 modes in quadrature with each other. These represent Rossby wave trains over the South Pacific characterised by a wave-like pattern extending from the central Pacific arching to Argentina and propagating equatorward into the Atlantic Ocean. They are referred to as the Pacific-South American modes (Ghil and Mo 1991; Mo and Higgins 1998) and represent the main modes of low-frequency oscillation in the SH (nevertheless, it should be mentioned that these are not the only Rossby wave patterns that propagate in these regions). Extratropical atmospheric flow regimes, or weather regimes (WRs), can be described in terms of geographi- cally fixed large-scale circulation anomalies that appear intermittently or episodically. There are several ap- proaches to classify WRs based on their different prop- erties: persistence, recurrence, and quasi-stationarity (see review by Michelangeli et al. 1995; MVL hereafter). Though different techniques exist for classifying WRs, basically those devoted to find the states that maximise the probability density function (e.g. Kimoto and Ghil 1993; Robertson and Ghil 1999), those in which WRs are defined as quasi-stationary states of the atmospheric cir- culation (e.g. Vautard 1990) and those in which recurrent Climate Dynamics (2003) 21: 91–104 DOI 10.1007/s00382-003-0320-x S. A. Solman Æ C. G. Mene´ndez S. A. Solman (&) Æ C. G. Mene´ndez Centro de Investigaciones del Mar y la Atmo´sfera (CIMA- CONICET/UBA) – Dto. Cs. de la Atmo´ sfera y los Oce´anos (UBA) Ciudad Universitaria – Pabello´n II 2° piso (1428) Buenos Aires – Argentina E-mail: solman@at.fcen.uba.ar