ABSTRACT:It is generally agreed that models that better simulate historical and current features of climate should also be the ones that more reliably simulate future climate. This article describes the ability of a selection of global climate models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5) to represent the historical and current mean climate and its variability over northeastern Argentina, a region that exhibits frequent extreme events. Two types of simulations are considered: Long-term simulations for in which the models respond to climate forcing (e.g. changes in atmospheric composition and land use) and decadal simulations for 1961-2010 that are initialized from observed climate states. Monthly simulations of precipitation and temperature are statistically evaluated for individual models and their ensembles.Subsets of models that best represent the region's climate are further examined. First, models that have a Nash-Sutcliffe efficiency of at least 0.8 are taken as a subset that best represents the observed temperature fields and the mean annual cycle. Their temperature time series are in phase with observations (r > 0.92), despite systematic errors that if desired can be corrected by statistical methods. Likewise, models that have a precipitation Pearson correlation coefficient of at least 0.6 are considered that best represent regional precipitation features. GCMs are able to reproduce the annual precipitation cycle, although they underestimate precipitation amounts during the austral warm season (September through April) and slightly overestimate the cold season rainfall amounts. The ensembles for the subsets of models achieve the best evaluation metrics, exceeding the performance of the overall ensembles as well as those of the individual models.

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This paper describes the large-scale atmospheric circulation associated with persistent generalized frosts (GFs; at least 75% of the stations report frosts) in the east-central region of Argentina known as the Wet Pampa. The GF events are grouped according to their persistence, and NCEP–NCAR reanalysis data are used to create daily composites of mass and wind field anomalies during the 1961–90 winters. The GFs are caused by an anticyclonic anomaly that enters South America, generating southerly wind anomalies and cold air advection that are strengthened by the meridional layout of a cyclonic anomaly over the South Atlantic Ocean. In the case of the more persistent events the wind anomaly grows during the previous days and becomes quasi-stationary. Also, the study identifies at 250 hPa a double train of eastward-moving Rossby waves along the subtropical and subpolar latitudes, respectively, of the Southern Hemisphere. The layout of both wave trains favors the development of an intense southerly wind anomaly in the entire southern cone of the continent. On the other hand, the propagation pattern during the less persistent GFs shows only one arc-shaped Rossby wave train that reaches South America, and then propagates northeastward. Additionally, there is a subtropical jet entrance/confluence over the western side of the continent that induces a secondary meridional circulation whose subsiding branch facilitates the equatorward displacement of the low-level anticyclone, particularly in the case of the less persistent events. In the case of the more persistent GFs the confluence is located farther east and sustains essentially zonal wave train propagation, so that the surface anticyclone is not able to achieve a major equatorward penetration.

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ABSTRACT:Changes in several temperature-based agroclimatic indices in the central-eastern of Argentina, most of them located within the Pampas region were analysed for 39 meteorological stations. Trends of first (FFD), last (LFD) and number (NFD) of frost days, frost period (FP), start (SGS), end (EGS) and length (LGS) of the growing season, growing degree days (GDD), diurnal temperature range (DTR), chilling hours and lowest annual minimum temperature were computed for two periods, 1940-2007 and 1975-2007. The largest changes were observed for the whole period 1940-2007 and were mostly indicative of a long-term minimum temperature warming throughout the region. During this period, generalized decreases in the NFD and in the FP (i.e., a delayed FFD and an earlier LFD) were found. Although the trends in the growing season indices were not as large as in the frost indices, they were consistent with the overall warming: an earlier SGS and a delayed in the EGS. The trends of the GDD showed a large variability between months with a generalized increased throughout the year. The DTR showed the largest number of stations with statistically significant negative trends from austral late spring (November) to austral early fall (April). For the period 1975-2007, the behaviour changes in all analysed indices: the short-term trends weakened and in some cases reversed sign. The LFD tended to occurred later in the year, particularly for the southern Pampas. The EGS shifted from mostly positive to negative trends, resulting in a shorter LGS. These trend changes were not spatially homogeneous. Although those short-term trends were predominantly non-statistically significant, they could potential affect management decisions and crop yields. In particular, frost is still an important hazard in agricultural activities and within the context of our results short-and long-term characterization of frost risk need to be considered at local and sub-regional scales.

The aim of this paper is to make a synoptic-climatic classification of atmospheric circulation in order to obtain synoptic frost-related patterns in the wet Pampas. Both partial (recorded in 25 to 75% of the meteorological stations) and widespread (recorded in more than 75%) frost events that occurred during the winters (June-July-August) of 1972-83 are included in this study. Frost days are grouped into Neutral (G1), El Niño (G2), and La Niña (G3) years. In addition, the complete dataset, called the total group, is analysed for reference purposes. Each group is analysed using the unrotated and Varimax rotated principal component analysis using the T-mode approach. Six synoptic situations accounted for 94% of the variance associated with frosts in the area studied.In general, the principal component score patterns given by the unrotated and rotated components are similar. The biggest difference between unrotated and rotated solutions was in variance redistribution. After rotation, only one of the two possible situations (direct/inverse) of each pattern represented a real synoptic type associated with frost in the wet Pampas. Persistence and location cause temperature drops in the area studied. The most frequent patterns in rotated results are those termed A, B, C* (in G2) and D (in G1 and G3). They are connected with cold anticyclones, which cause advective and/or radiative frosts. The remaining patterns accounted for about 5% of the variance and represent real, though rare, situations, which are important because of their effect on the wet Pampas.The results obtained for the different groups show that inter-winter variability of the equatorial Pacific signal produces changes in the frequency of frost-connected patterns, rather than different patterns for cold events.