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The development of subseasonal forecasts has seen significant advancements, transforming our ability to predict weather patterns and climate variability on intermediate timescales ranging from 2 weeks to 2 months. Motivated by the need to enhance our understanding of subseasonal precipitation forecasts and their applicability to the hydrology forecast, this study retrospectively analysed precipitation ensemble forecasts from subseasonal prediction models in the Uruguay River basin nearby Salto Grande dam. Three models were considered: two from the S2S project (ECMWF and CNRM) and one from the SubX project (GEFS). Model forecasts were analysed on a weekly time scale using both deterministic and probabilistic approaches. Multimodel probabilistic forecasts combining the three different models were built to increase forecast skill. Individual models have a skill larger than or equal to the climatological forecast until 2 weeks in advance. Particularly, ECMWF shows better skill in both ensemble mean and probabilistic forecast. Multimodel probabilistic forecast improves the skill of the forecast throughout the year, with the skill even surpassing the climatological forecast by up to 4 weeks in advance during the summer. In addition, model skill was analysed considering the state of the El Niño–Southern Oscillation (ENSO) on a weekly and monthly basis. On weekly time scales the ENSO state modifies model skill differently depending on the sub‐basin and season considered. However, the influence of ENSO on forecast skill is more clearly observed on monthly time scales, with largest improvement in the lower basin during springtime. The results of this work suggest that subseasonal models are a promising tool to bridge the gap between weather and climate forecast in the Uruguay River basin and have the potential to be utilized for hydrological forecasting in the study region. [ABSTRACT FROM AUTHOR]

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]

Climate predictions using coupled models in different time scales, from intraseasonal to decadal, are usually affected by initial shocks, drifts, and biases, which reduce the prediction skill. These arise from inconsistencies between different components of the coupled models and from the tendency of the model state to evolve from the prescribed initial conditions toward its own climatology over the course of the prediction. Aiming to provide tools and further insight into the mechanisms responsible for initial shocks, drifts, and biases, this paper presents a novel data set developed within the Long Range Forecast Transient Intercomparison Project, LRFTIP. This data set has been constructed by averaging hindcasts over available prediction years and ensemble members to form a hindcast climatology, that is a function of spatial variables and lead time, and thus results in a useful tool for characterizing and assessing the evolution of errors as well as the physical mechanisms responsible for them. A discussion on such errors at the different time scales is provided along with plausible ways forward in the field of climate predictions. Plain Language Summary: The prediction of climate from weeks to years is usually made using complex numerical models that need to have an appropriate representation of the different components of the climate system (atmosphere, oceans, land areas, etc.,) as well as of their interactions. Most of these simulations, however, are affected by errors that originate in part from the numerical methods and approximations used to solve the system of equations of the atmospheric and oceanic systems, and in part by an incorrect representation of the interaction between components. Naturally, these errors lead to a significant decrease in the skill of the predictions, and therefore, must be taken into account. This study presents a new data set made from climate model predictions that have been manipulated in order to be useful for the assessment and further understanding of the physical mechanisms behind the aforementioned errors. The expected increase in our knowledge in the field will help us obtain more skillful climate predictions in the future. Key Points: Initial shocks, drifts, and biases affect climate predictions at all time scalesAs they usually lead to a substantial reduction of the prediction skill, further efforts toward their understanding are neededThe newly developed data set presented in this article is expected to serve as a useful tool for their study [ABSTRACT FROM AUTHOR]

A total solar eclipse affected southern South America on December 14, 2020. Its occurrence close to the summer solstice in the Southern Hemisphere and around noon brought almost ideal conditions to study the effects of the sudden reduction in incoming solar radiation on different atmospheric variables. Interestingly, the astronomical phenomenon moved over a region that at that time was being affected by a cold front accompanied by significant cloudiness and cold air advection to the area. Therefore, the actual drop in surface temperature over the umbra region resulted from a combination of the radiative effect (i.e., the reduction in solar radiation due to the eclipse itself) and a circulation effect related to the injection of colder air. A quantification of both effects on the full temperature variation is provided, along with a discussion on the underlying mechanisms and the related effects on surface winds derived from the cooling. Results show that temperature drops associated with total solar eclipses in areas far away from the umbra can be as high as (or even higher than) those within the umbra, depending on the prevailing weather conditions. [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]

This paper quantifies the potential benefit of limiting the global warming to 1.5°C above the preindustrial level on the high‐latitude climate of the Southern Hemisphere compared to 2.0°C and above. A set of Coupled Model Intercomparison Project Phase 5 global climate model experiments is used in order to determine the mean projected changes in surface air temperature, sea level pressure, surface winds and sea ice distribution over and around Antarctica. Alongside, changes in selected variables related to extremes (such as the annual number of extremely cold mornings and extremely cold days) are also quantified as a function of the Representative Concentration Pathway (RCP) considered reaching the given warming threshold. Physical mechanisms for the projected variations in sea ice are proposed, including surface air temperature, changes in low‐level air temperature advection and meridional winds. Results show that there would be significant differences in the atmospheric and sea ice variable changes depending on the warming threshold and RCP considered, and that indeed keeping the global warming under 1.5°C could have a noticeable impact particularly on the distribution of sea ice over the Southern Ocean. [ABSTRACT FROM AUTHOR]

The South Pacific Ocean is a key driver of climate variability within the Southern Hemisphere at different time scales. Previous studies have characterized the main mode of interannual sea surface temperature (SST) variability in that region as a dipolar pattern of SST anomalies that cover subtropical and extratropical latitudes (the South Pacific Ocean Dipole, or SPOD), which is related to precipitation and temperature anomalies over several regions throughout the Southern Hemisphere. Using that relationship and the reported low predictive skill of precipitation anomalies over the Southern Hemisphere, this work explores the predictability and prediction skill of the SPOD in near-term climate hindcasts using a set of state-of-the-art forecast systems. Results show that predictability greatly benefits from initializing the hindcasts beyond the prescribed radiative forcing, and is modulated by known modes of climate variability, namely El Niño-Southern Oscillation and the Interdecadal Pacific Oscillation. Furthermore, the models are capable of simulating the spatial pattern of the observed SPOD even without initialization, which suggests that the key dynamical processes are properly represented. However, the hindcast of the actual phase of the mode is only achieved when the forecast systems are initialized, pointing at SPOD variability to not be radiatively forced but probably internally generated. The comparison with the performance of an empirical prediction based on persistence suggests that initialization may provide skillful information for SST anomalies, outperforming damping processes, up to 2–3 years into the future. [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]

The assessment of regional climate and hydrological impacts at different levels of global‐mean temperature increase became vital to guide decision makers and water management planners after the adoption of the Paris Agreement in 2015. With current emission trends close to a pathway of 3 °C warming by the end of the present century, this study investigates the projected regional hydrology impacts over La Plata basin (LPB) for the warming thresholds of 1.5 and 2 °C established in Paris and for 3 °C above the pre‐industrial level. We also explore the consequences from following a medium or high‐emission representative concentration pathway (RCP) to achieve the different warming targets. In order to determine the possible changes in precipitation, evapotranspiration, runoff and river discharges, we use the variable infiltration capacity (VIC)‐distributed hydrology model in combination of bias‐corrected GCM outputs from the Inter‐Sectorial Impact Model Intercomparison Project phase 2a (ISIMIP). Because the behaviour of the hydrometeorological variables is not homogeneous over LPB, we analysed impacts of the different changes in mean global temperature and RCPs over four sub‐basins: Paraguay, Paraná, Iguazú and Uruguay. Overall, most of the changes over the sub‐basins suggest moister conditions with increasing temperatures. The comparison of the RCPs indicates that increases in precipitation, evapotranspiration and runoff would be larger under the medium‐emission scenario. When the different responses of the various components of the terrestrial water cycle were integrated, results show that variation of annual mean streamflow in all sub‐basins ranges between ±20%. However, in most cases, the sign of the changes highly depends on the RCP chosen to achieve a warming level. [ABSTRACT FROM AUTHOR]

The South Pacific (SP) region exerts large control on the climate of the Southern Hemisphere at many times scales. This paper identifies the main modes of interannual sea surface temperature (SST) variability in the SP which consist of a tropical-driven mode related to a horseshoe structure of positive/negative SST anomalies within midlatitudes and highly correlated to ENSO and Interdecadal Pacific Oscillation (IPO) variability, and another mode mostly confined to extratropical latitudes which is characterized by zonal propagation of SST anomalies within the South Pacific Gyre. Both modes are associated with temperature and rainfall anomalies over the continental regions of the Southern Hemisphere. Besides the leading mode which is related to well known warmer/cooler and drier/moister conditions due to its relationship with ENSO and the IPO, an inspection of the extratropical mode indicates that it is associated with distinct patterns of sea level pressure and surface temperature advection. These relationships are used here as plausible and partial explanations to the observed warming trend observed within the Southern Hemisphere during the last decades. [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 Iberá wetlands, located in La Plata Basin, is a fragile ecosystem habitat of several species of flora and fauna and it also constitutes one of the largest inland freshwater of the world. In this study, the hydroclimatologic response to projected climatic changes in the Iberá wetlands is assessed. Bias corrected temperature and precipitation data from four Regional Climate Models (RCMs) developed for the CLARIS-LPB project were used to drive the calibrated variable infiltration capacity (VIC) hydrological model for different time slices. Derived future scenarios consist on changes in temperature, precipitation and water level of the Iberá Lake for the periods 2021–2040 and 2071–2090 with respect to present. All RCMs are consistent in predicting a warming for the near future (0–2°C) and also to the end of the century (1.5–4.5°C) in the study region, but differ in the sign and percentage of precipitation changes. VIC modelling results suggest that the Iberá Lake level could increase in the twenty-first century and that this increment would be higher in the summer months. Nevertheless, the projected 10 cm of water-level increase could be not so relevant as it is of the same order of magnitude than the observed interdecadal variability of the system. [ABSTRACT FROM PUBLISHER]

There is an increasing demand for future climate scenarios, particularly for impact studies. In this study, simulation outputs taken from a set of three regional climate models (RCMs) are used to force a hydrologic model to derive future streamflow scenarios for La Plata Basin. As RCMs have biases in their mean precipitation and temperature fields, a statistical scheme is previously used to remove the systematic part of the bias. Future hydrologic scenarios were derived considering two future periods: 2021–2040 (near future) and 2071–2090 (far future). In terms of climate projections, RCMs predict warmer conditions in almost the whole basin, while precipitation variations are not uniform in sign across the region but overall tend to be positive over the southern part of the basin. Nevertheless, a trend towards a gradual increase in streamflow was found for the majority of the rivers in the basin particularly for the near future followed by less uniform variations towards the end of the present century. Future changes in the largest monthly streamflow are similar to those in the mean values, with also some differences among RCMs and on the period and sub-basin considered. [ABSTRACT FROM PUBLISHER]

Hydrological modelling with climate scenario data are used to develop projections of changes in frequency and duration of flood events in the margins of the lower sections of the Paraná and Uruguay Rivers in La Plata Basin for the twenty-first century. Discharges were simulated with the Variable Infiltration Capacity hydrologic model considering the statistically bias corrected daily maximum and minimum temperatures and rainfall outputs from five regional climate models and different emission scenarios. Results show that although it is expected that compared to the current conditions the temperature would rise and precipitation would have a slight increase in La Plata Basin during the present century, more frequent and lasting fluvial flooding events in the lower Paraná and Uruguay basins could be expected. However, the range of results derived from different climate models though consistent in sign, indicate that the uncertainty is large. [ABSTRACT FROM PUBLISHER]

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]