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Southeastern South America (SESA) experienced some of the largest positive trends in total and extreme precipitation of the Southern Hemisphere during the last decades. The City of Buenos Aires, the second largest mega‐city of SESA, has been particularly hard‐hit by these trends due to its flat topography and poor natural drainage. Recent studies suggest that these precipitation extremes may exacerbate even further under global warming, but the physical mechanisms responsible for such events have not been documented for this region so far. This study quantifies the relative contributions of dynamics (atmospheric fronts) and thermodynamics (vertical stability) on the observed variations and trends of daily, seasonal and annual precipitation over the City of Buenos Aires between 1981 and 2020 by splitting the precipitation events into convective and stratiform. Results show that the relative contributions from dynamics and thermodynamics depend on the season under consideration: the positive trends in summer precipitation have been favoured by a net increase in vertical instability with a negligible contribution from dynamics, while the increased frequency of fronts in autumn and winter, when no changes in vertical instability have been observed, has contributed to the higher frequency of cold‐season convective events. [ABSTRACT FROM AUTHOR]

The purpose of this study is to assess the ability of a set of large ensembles (LE) of Global Climate Model (GCM) simulations from the Multi‐Model Large Ensemble Archive, Coupled Model Intercomparison Project Phase 6 (CMIP6) and database for Policy Decision making for Future climate change in reproducing the variability and change of the austral summer precipitation observed in South America along the second part of the 20th century and beginning of the 21st. LE show similar biases in the mean austral summer rainfall and interannual variability than those detected in previous model sets, such as the Coupled Model Intercomparison Project Phase 5 (CMIP5). The positive trends in south‐eastern South America (SESA) and negative ones in the southern Andes (SAn), that are the most significant observed regional features, are identified to some extent in LE simulations. The negative trend in SAn is a feature consistently shown among different models. For all models, mean trend is negative in that particular region and larger than the inter‐member dispersion for each large ensemble. Trend magnitudes in the SESA region show a larger dispersion between models than the SAn region. While the mean trend among models is consistent with the observations, models underestimate the observed trend. The multi‐model ensemble of selected models that best reproduce both mean and interannual variability features of rainfall in South America shows an 18% larger positive mean trend in SESA rainfall than those that have the worst performance. However, internal variability uncertainty is still higher than the mean trend for both sets of multi‐model ensembles [5% (18%) for selected (non‐selected) models]. Improvements in the representation of the main features of mean rainfall and variability are needed in order to better describe and reduce the uncertainties associated with the main climate change related signals in the continent. [ABSTRACT FROM AUTHOR]

The main socio-economic activities of the Comahue region in the northern Patagonia in Argentina depend directly on the water availability of the Limay, Neuquén and Negro rivers. In this work, the VIC (Variable Infiltration Capacity) hydrological model was applied to the basins of these rivers in order to study the possible impact climate change will have on the hydrology of the region over the next few decades. It was found that the available observational database is insufficient to carry out an adequate calibration of the model on a daily basis: various corrections to the maximum and minimum temperatures and precipitation were applied separately at a monthly level, obtaining a satisfactory calibration (1999-2009) and validation (2011-2016) at an annual level when reducing the temperatures in the Andes. The projected changes in precipitation and in maximum and minimum temperatures for the remainder of the 21st century under the scenarios with moderate and high radiative forcing suggest a decrease in precipitation in the Andes of the Neuquén province and an increase in temperatures. These conditions would result in a reduction in the mean annual flow for the rivers of the region that would be enhanced by an increase in the concentration of the greenhouse gases. [ABSTRACT FROM AUTHOR]

During the month of June 1967 cold polar air moved from the Antarctic continent into central Argentina, where it brought extremely low minimum temperatures, snow and extensive damage to crops. Fifty years later, this paper revisits such synoptic situation making use of reanalysis data as well as numerical modelling in order to characterize the physical mechanisms at play. In a statistical context, this cold air outbreak was the most intense in the last 50 years in an extensive area covering central Argentina, not only at the surface but also at low and middle levels of the troposphere. The cold air outbreak was associated with the displacement of a cold front, behind which a polar air mass moved from subpolar latitudes onto central and northern Argentina. Although the air mass suffered a noticeable modification while traversing oceanic areas, as seen by the numerical simulations, temperature advection first and radiative cooling afterwards contributed to the extreme minimum temperatures. [ABSTRACT FROM AUTHOR]

ABSTRACT There are different methodologies to compute daily mean temperatures ( DMT), including averaging the 24-hourly temperature values, readings at specific times throughout the day or simply averaging the minimum and maximum daily temperatures. This study provides an intercomparison of some of such methods applied to six meteorological stations in Argentina with continuous hourly measurements for a period of more than 24 years. Results show that differences arising from the various methodologies are largely dependent on the local weather conditions, particularly those related to cloud cover and wind intensity, while the role of air moisture is less important. Furthermore, trends derived from DMT estimates using different methodologies are found to be highly sensitive to the chosen method. In fact, statistically insignificant trends could be significant should other methodologies to calculate DMT had been used. This result could be of importance for diverse scientific areas such as agriculture or climate warming studies. [ABSTRACT FROM AUTHOR]

ABSTRACT Southern South America ( SSA), considered as the continental region south of 20°S, has experienced significant precipitation variability and trends in the last decades. This article uses monthly quality-controlled precipitation data from rainfall stations with continuous observations during at least 100 years to quantify long-term trends as well as interannual-to-centennial variability. Several statistical methods are applied to the data, primarily to detect jumps and look for changes due to relocation of the gauge stations, as well as to identify significant trends. Most of the regions have registered an increase in annual rainfall, largely attributable to changes in the warm season. On the other hand, during winter most stations in Argentina and Brazil do not have significant trends, although eastern Patagonia registered an increase in precipitation and Chile, a marked decrease in rainfall. In order to look into the physical mechanisms behind the observed variability, the changes in mean sea level pressure and precipitable water are quantified for different sub-periods. Also explored is the variability related to the Hadley cell width and strength over the region around SSA. Results show that the Hadley cell has shrunk and shifted towards the equator in winter over the area, which has caused an enhancement of the sinking motion over much of Argentina, Chile and Brazil, while likely increasing the baroclinicity (and associated precipitation) over Patagonia. In summer, the strength of the subsidence decreased and this was associated with an increase of the low-level moisture advection, favouring more rainfall. The observational evidence presented here suggests that the zonal asymmetry in the change of the Hadley cell position over SSA could be linked to the presence of the Andes Cordillera. [ABSTRACT FROM AUTHOR]

The Iberó region is the second largest wetland in the world and hosts hundreds of plants and animal species. Water availability in the region is the result of the balance between precipitation, evapotranspiration and runoff, and the region is highly sensitive to changes in this balance. In this paper the VIC hydrologic distributed model is calibrated over the Iberá region using observed temperature, precipitation and surface wind data. This study also examines the ability of the RCA3-E regional climate model to simulate the present climate and the skill of VIC to represent the hydrologic cycle of the wetland when forced by RCA3-E output data. It was found that errors in the climate model significantly affect hydrological simulations and this can be improve if a correction scheme for systematic errors is applied. Finally, in order to determine a potential future scenario of water availability in the Iberáa region, the hydrologic model is forced with simulations provided by the RCA3- E by the end of the present century for an intermediate greenhouse gases emissions scenario after correcting the identified systematic errors. The results show that, although the regional climate model projects an increase in both precipitation and mean temperature, rainfall changes could compensate the rise in evapotranspiration given the increase in temperatures and this would lead to larger streamflows of the Ibera region's main river. [ABSTRACT FROM AUTHOR]

Positive trends in annual rainfall in the La Plata Basin (LPB), south of 20°S observed in the last four decades of the twentieth century were not reversed and became more statistically significant when calculated until 2005. These trends were part of a more general change in the monthly precipitation distribution including extreme precipitation. Precipitation in dry and extremely dry months (below the 35th percentile) has been decreasing in the whole LPB region south of 22°S. On the contrary, precipitation in the above normal (between the 65th and 90th percentile) and the extremely high rainfall (above the 90th percentile) ranges has been increasing accounting for most of the annual precipitation trends. More than a steady trend, there has been an abrupt change in extreme monthly precipitation concentrated between 1977 and 1983. The analysis of intensity and frequency of extreme events was done fitting Generalized Extreme Values (GEV) and Poisson distributions. Each distribution was fitted with and without trends in the location parameter and tested to determine the best fit in each case. The regions where GEV with a positive trend was the best fit coincide with areas affected by extensive floods during the last decades. Spatially aggregated results highlight the signal of change towards higher maximum monthly precipitations for a wide span of return periods. The atmospheric circulation associated with cases where extreme monthly precipitation was observed in most of the stations was studied through the integrated water vapour transport in the lower troposphere and its associated divergence. During warm months, an intense northern low-level water vapour flow with two convergence nuclei, one over eastern Argentina, southern Brazil and Uruguay, and the other over western Argentina, along with a weakened south Atlantic Convergence Zone was associated with the more extreme precipitation months favouring the occurrence of Mesoscale Convective Systems. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]