[Show abstract][Hide abstract] ABSTRACT: Atlantic Multidecadal Variability (AMV) is known for influencing the mid-latitude climate variability, especially over the European region. This letter assesses the impact of the wintertime AMV in a group of 200-year atmospheric-only numerical experiments, in which the atmosphere is forced with positive and negative AMV-like sea surface temperatures (SSTs) and sea ice concentration patterns. Anomalies are applied separately to the whole North Atlantic ocean, to the extratropics (north of 30° N) and to the tropics (between 0° and 30° N). Results show that AMV anomalies considerably affect the North Atlantic Oscillation (NAO), the jet stream variability and the frequency of atmospheric blocking over the Euro-Atlantic sector, resulting in a negative (positive) NAO during positive (negative) AMV. It is found that the bulk of the signal is originated in the tropics and it is associated with a Gill-like response—an anomalous upper tropospheric streamfunction dipole over the tropical Atlantic driven by the SST anomalies—and with the subsequent structural change of the upper-tropospheric jet, which affects the propagation of Rossby waves in the North Atlantic. Conversely, the NAO response is almost negligible when the AMV anomalies are applied only to the extratropics, suggesting that the relevance of SST anomalies along the North Atlantic frontal zone may be overestimated.
Environmental Research Letters 09/2015; 10(9-9):094010. DOI:10.1088/1748-9326/10/9/094010 · 3.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work analyzes the properties of precipitation in the Hindu-Kush Karakoram Himalaya region as simulated by thirty-two state-of-the-art global climate models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5). We separately consider the Hindu-Kush Karakoram (HKK) in the west and the Himalaya in the east. These two regions are characterized by different precipitation climatologies, which are associated with different circulation patterns. Historical model simulations are compared with the Climate Research Unit (CRU) and Global Precipitation Climatology Centre (GPCC) precipitation data in the period 1901–2005. Future precipitation is analyzed for the two representative concentration pathways (RCP) RCP 4.5 and RCP 8.5 scenarios. We find that the multi-model ensemble mean and most individual models exhibit a wet bias with respect to CRU and GPCC observations in both regions and for all seasons. The models differ greatly in the seasonal climatology of precipitation which they reproduce in the HKK. The CMIP5 models predict wetter future conditions in the Himalaya in summer, with a gradual precipitation increase throughout the 21st century. Wetter summer future conditions are also predicted by most models in the RCP 8.5 scenario for the HKK, while on average no significant change can be detected in winter precipitation for both regions. In general, no single model (or group of models) emerges as that providing the best results for all the statistics considered, and the large spread in the behavior of individual models suggests to consider multi-model ensemble means with extreme care.
[Show abstract][Hide abstract] ABSTRACT: We explore the impact of different resolutions, convective closures, and microphysical parameterizations on the representation of precipitation statistics (climatology, seasonal cycle, and intense events) in 20-yr-long simulations over Europe with the regional climateWeather Research and Forecasting (WRF) Model. The simulations are forced in the period 1979-98, using as boundary conditions the ERA-Interim fields over the European region. Special attention is paid to the representation of precipitation in the Alpine area. We consider spatial resolutions ranging from 0.118 to 0.0378, allowing for an explicit representation of convection at the highest resolution. Our results show that while there is a good overall agreement between observed and modeled precipitation patterns, the model outputs display a positive precipitation bias, particularly in winter. The choice of the microphysics scheme is shown to significantly affect the statistics of intense events. High resolution and explicitly resolved convection help to considerably reduce precipitation biases in summer and the reproduction of precipitation statistics.
Journal of Hydrometeorology 05/2015; 16(4):150508072628008. DOI:10.1175/JHM-D-14-0221.1 · 3.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We quantify climate change hot-spots from observations, taking into account the differences in precipitation and temperature statistics (mean, variability and extremes) between 1981-2010 and 1951-1980. Areas in the Amazon, the Sahel, tropical West Africa, Indonesia and central eastern Asia emerge as primary observed hot-spots. The main contributing factors are the global increase in mean temperatures, the intensification of extreme hot-season occurrence in low-latitude regions and the decrease of precipitation over central Africa. Temperature and precipitation variability have been substantially stable over the past decades, with only a few areas showing significant changes against the background climate variability. The regions identified from the observations are remarkably similar to those defined from projections of global climate models under a “business-as-usual” scenario, indicating that climate change hot-spots are robust and persistent over time. These results provide a useful background to develop global policy decisions on adaptation and mitigation priorities over near-time horizons.
Geophysical Research Letters 04/2015; 42(9). DOI:10.1002/2015GL063891 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show that the absence of the Greenland ice sheet would have important consequences on the North Atlantic Ocean circulation, even without taking into account the effect of the freshwater input to the ocean from ice melting. These effects are investigated in a 600-year long coupled ocean–atmosphere simulation with the high-resolution global climate model EC-Earth 3.0.1. Once a new equilibrium is established, a cooling of Eurasia and of the North Atlantic and a poleward shift of the subtropical jet are observed. These hemispheric changes are ascribed to a weakening of the Atlantic Meridional Overturning Circulation (AMOC) by about 12%. We attribute this slowdown to a reduction in salinity of the Arctic basin and to the related change of the mass and salt transport through the Fram Strait – a consequence of the new surface wind pattern over the lower orography. This idealized experiment illustrates the sensitivity of the AMOC to local surface winds.
Geophysical Research Letters 02/2015; 42(3). DOI:10.1002/2014GL062668 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We simulate three-dimensional, horizontally periodic Rayleigh-B\'enard
convection between free-slip horizontal plates, rotating about a horizontal
axis. When both the temperature difference between the plates and the rotation
rate are sufficiently large, a strong horizontal wind is generated that is
perpendicular to both the rotation vector and the gravity vector. The wind is
turbulent, large-scale, and vertically sheared. Horizontal anisotropy,
engendered here by rotation, appears necessary for such wind generation. Most
of the kinetic energy of the flow resides in the wind, and the vertical
turbulent heat flux is much lower on average than when there is no wind.
[Show abstract][Hide abstract] ABSTRACT: The mountain regions of the Hindu Kush, Karakoram and Himalaya (HKKH) are the ‘‘third
pole’’ of our planet, and the glaciers in this area are ‘‘water towers’’ of Asia. Recent studies
postulated the existence of a ‘‘Karakoram anomaly’’, i.e. with substantially unchanged ice
cover during the last decade, against noticeable area loss worldwide. Yet, recent major floods
occurring in Pakistan and the Karakoram area, may represent an effect of modified climate in
the area, carrying heavier precipitation in the Monsoon season, and possibly faster ice
melting. We present here the results of the SHARE-Paprika project of the EvK2CNR
Committee of Italy, aiming at evaluating the impact of climate change on the hydrology of
the upper Indus river. We focus here on a particular watershed, the Shigar river closed in
Shigar, with an area of about 7,000 km2 and fed by seasonal melt from two major glaciers
(Baltoro and Biafo). Based upon data gathered during three field campaigns we set up a semidistributed,
altitude belt based hydrological model, providing acceptable depiction of in
stream flows, and snow and ice cover dynamics. We then project the future (until 2,100)
hydrological cycle in the area by feeding the hydrological model with future precipitation and
temperature (plus downscaling, whenever necessary) from a number of climate models,
under different RCP scenarios.
01/2015: pages 43-49; Springer International Publishing., ISBN: 3319092995
[Show abstract][Hide abstract] ABSTRACT: In what follows, we present the study of different configurations of the Weather Research and Forecast model restricted to the EURO-CORDEX area at different spatial resolutions from 0.44°× 0.44° down to 0.04° × 0.04°. Our numerical model is forced at the boundaries by ERA-interim re-analysis data. Monthly and daily statistics for the year 1979 are compared in order to set the best configuration for rainfall predictions. Precipitation climatologies are then derived and particular attention is paid to the Alpine region. Our results show that local precipitation patterns are well reproduced by the regional model while highlighting the need of grid-resolved convection to avoid artificial additive precipitation and to reduce the rainfall rate bias especially of regions with complex orography.
[Show abstract][Hide abstract] ABSTRACT: The Hindu Kush, Karakoram, and Himalaya (HKKH) mountain ranges feed the most important Asian river systems, providing water to about 1.5 billion people. As a consequence, changes in snow dynamics in this area could severely impact water availability for downstream populations. Despite their importance, the amount, spatial distribution, and seasonality of snow in the HKKH region are still poorly known, owing to the limited availability of surface observations in this remote and high-elevation area. This work considers global climate models (GCM) participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) and analyzes how they represent current and future snowpack in the HKKH region in terms of snow depth and snow water equivalent. It is found that models with high spatial resolution (up to 1.25 degrees) simulate a spatial pattern of the winter snowpack in greater agreement with each other, with observations, with reanalysis datasets, and with the orographic features of the region, compared to most lower-resolution models. The seasonal cycle of snow depth displays a unimodal regime, with a maximum in February-March and almost complete melting in summer. The models generally indicate thicker [in Hindu Kush-Karakoram (HKK)] or comparable (in the Himalayas) snow depth and higher snow water equivalent compared to the reanalyses for the control period 1980-2005. Future projections, evaluated in terms of the ensemble mean of GCM simulations, indicate a significant reduction in the spatial average of snow depth over the HKK and an even stronger decrease in the Himalayas, where a reduction between 25% and 50% is expected by the end of the twenty-first century.
Journal of Hydrometeorology 12/2014; 15(6):2293-2313. DOI:10.1175/JHM-D-13-0196.1 · 3.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Winter precipitation over the Hindu Kush-Karakoram (HKK) range in the western Himalayas is generated by westerly perturbations whose dynamics is affected by the North Atlantic Oscillation (NAG). Larger precipitation is typically recorded during the positive NAO phase. In this work, the relationship between the NAO and winter precipitation in the HKK is explored further, using an ensemble of precipitation datasets and the 40-yr ECMWF Re-Analysis (ERA-40) and Twentieth Century Reanalysis (20CR) data. The mechanisms underlying this relationship are discussed, with a focus on the secular variations that occurred in the last century. The NAO exerts its control on HKK precipitation by altering the intensity of westerly winds in the region of the Middle East jet stream (MEJS). Results indicate that evaporation from the Persian Gulf, the northern Arabian Sea, and the Red Sea plays an important role. During positive NAO phases, westerlies are strengthened and enhanced evaporation occurs from these basins owing to higher surface wind speed. The extra moisture combines with stronger westerlies and results in enhanced moisture transport toward the HKK. Precipitation datasets covering the twentieth century show an alternation of periods of strong and weaker influence of the NAO on precipitation in the HKK. It is found that these variations are associated with changes in the spatial pattern of the NAO: the relative position of the two centers of action of the NAO determines to what extent it can modulate the MEJS, affecting precipitation in the HKK.
Journal of Climate 10/2014; 27(20):7890-7902. DOI:10.1175/JCLI-D-14-00286.1 · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fingering convection is a convective instability that occurs in fluids where two buoyancy-changing scalars with different diffusivities have a competing effect on density. The peculiarity of this form of convection is that, although the transport of each individual scalar occurs down-gradient, the net density transport is up-gradient. In a suitable range of non-dimensional parameters, solutions characterized by constant vertical gradients of the horizontally averaged fields may undergo a further instability, which results in the alternation of layers where density is roughly homogeneous with layers where there are steep vertical density gradients, a pattern known as “doubly-diffusive staircases”. This instability has been interpreted in terms of an effective negative diffusivity, but simplistic parameterizations based on this idea, obviously, lead to ill-posed equations. Here we propose a mathematical model that describes the dynamics of the horizontally-averaged scalar fields and the staircase-forming instability. The model allows for unstable constant-gradient solutions, but it is free from the ultraviolet catastrophe that characterizes diffusive processes with a negative diffusivity.
[Show abstract][Hide abstract] ABSTRACT: We analyse the observed climate-driven changes in summer wildfires and their future evolution in a typical Mediterranean environment (NE Spain). By analysing observed climate and fire data from 1970 to 2007, we estimate the response of fire number (NF) and burned area (BA) to climate trends, disentangling the drivers responsible for long-term and interannual changes by means of a parsimonious Multi Linear Regression model (MLR). In the last forty years, the observed NF trend was negative. Here we show that, if improvements in fire management were not taken into account, the warming climate forcing alone would have led to a positive trend in NF. On the other hand, for BA, higher fuel flammability is counterbalanced by the indirect climate effects on fuel structure (i.e. less favourable conditions for fine-fuel availability and fuel connectivity), leading to a slightly negative trend. Driving the fire model with A1B climate change scenarios based on a set of Regional Climate Models from the ENSEMBLES project indicates that increasing temperatures promote a positive trend in NF if no further improvements in fire management are introduced.
[Show abstract][Hide abstract] ABSTRACT: The mysterious ‘fairy circles’ are vegetation‐free discs that cover vast areas along the pro‐Namib Desert. Despite 30 yr of research their origin remains unknown. Here we adopt a novel approach that focuses on analysis of the spatial patterns of fairy circles obtained from representative 25‐ha aerial images of north‐west Namibia. We use spatial point pattern analysis to quantify different features of their spatial structures and then critically inspect existing hypotheses with respect to their ability to generate the observed circle patterns. Our working hypothesis is that fairy circles are a self‐organized vegetation pattern. Finally, we test if an existing partial‐differential‐equation model, that was designed to describe vegetation pattern formation, is able to reproduce the characteristic features of the observed fairy circle patterns. The model is based on key‐processes in arid areas such as plant competition for water and local resource‐biomass feedbacks.
[Show abstract][Hide abstract] ABSTRACT: Precipitation extremes and small-scale variability are essential drivers in many climate change impact studies. However, the spatial resolution currently achieved by global climate models (GCMs) and regional climate models (RCMs) is still insufficient to correctly identify the fine structure of precipitation intensity fields. In the absence of a proper physically based representation, this scale gap can be at least temporarily bridged by adopting a stochastic rainfall downscaling technique. In this work, a precipitation downscaling chain is introduced where the global 40-yr ECMWF Re-Analysis (ERA-40) (at about 120-km resolution) is dynamically downscaled using the Protheus RCM at 30-km resolution. The RCM precipitation is then further downscaled using a stochastic downscaling technique, the Rainfall Filtered Autoregressive Model (RainFARM), which has been extended for application to long climate simulations. The application of the stochastic downscaling technique directly to the larger-scale reanalysis field at about 120-km resolution is also discussed. To assess the ability of this approach in reproducing the main statistical properties of precipitation, the downscaled model results are compared with the precipitation data provided by a dense network of 122 rain gauges in northwestern Italy, in the time period from 1958 to 2001. The high-resolution precipitation fields obtained by stochastically downscaling the RCM outputs reproduce well the seasonality and amplitude distribution of the observed precipitation during most of the year, including extreme events and variance. In addition, the RainFARM outputs compare more favorably to observations when the procedure is applied to the RCM output rather than to the global reanalyses, highlighting the added value of reaching high enough resolution with a dynamical model.
Journal of Hydrometeorology 03/2014; 15(2). DOI:10.1175/JHM-D-13-096.1 · 3.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In arid and semi-arid ecosystems, local vegetation-soil moisture feedback mechanisms are important for vegetation self-organization. Vegetation spatial patterns, such as spots, stripes and gaps, are frequently observed. At larger scales vegetation exerts a direct role on water and energy fluxes, which influence directly local precipitation recycling, particularly in drylands. To evaluate the interactions between vegetation self organization, on the small scale, and land-atmosphere feedbacks, on larger scales, it is crucial to model accurately the evapotranspiration fluxes. In this contribution, we discuss a new explicit-space model for vegetation dynamics in water-limited ecosystems, including two soil layers. The model is suitable for evaluating how evapotranspiration fluxes are modified by the presence of patterns, and to establish the climatological importance of related modifications in local soil-vegetation feedbacks.
Advances in Water Resources 09/2013; 53:131-138. · 3.42 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using a spatially explicit mathematical model for water-limited vegetation we show that spatial instabilities of uniform states can lead to species coexistence under conditions where uniformly distributed species competitively exclude one another. Coexistence is made possible when water-rich patches formed by a pattern forming species provide habitats for a highly dispersive species that is a better competitor in uniform settings.
[Show abstract][Hide abstract] ABSTRACT: We studied hydrochemistry and plankton dynamics in two remote Alpine lakes: lake Nivolet superiore (2530 m asl) and lake Trebecchi superiore (2729 m asl) in the Gran Paradiso National Park (Western Italian Alps) in summer 2009. The aim of this study was to enhance the understanding of natural ecological dynamics in the pelagic habitat of Alpine lakes by enlarging the number of biotic and abiotic variables usually considered to this end and by increasing the frequency of samplings, generally low in remote lakes. During the eight samplings performed in 2009, chemical and physical variables were measured both in situ and in the laboratory. We also followed the dynamics of all the compartments of the naturally simplified trophic chain of the two lakes from pico-prokaryotes to phytoplankton and zooplankton. Our results confirm the oligotrophic, close-to-pristine state of lake Nivolet and lake Trebecchi as they are not affected by hydromorphological alterations, they are naturally fishless and are not sensitive to acidification risk and acidity pulses. On the other hand, the two lakes have distinct abiotic conditions due to their glacial origin and to the lithological composition of their watersheds. We found some differences in the spatial variation of pico-prokaryotes, phytoplankton and zooplankton due to the different mixing regimes and maximum depth of the two lakes. Conversely, temporal patterns were similar in both lakes, related to ecological interactions and to changes in the abiotic conditions. The rapid succession of events in extreme ecosystems, such as the Alpine lakes studied here, confirm the predominant role of external environmental factors (e.g., the duration of the ice-free season) and of ecological interactions among different trophic compartments. This research underlines the importance of seasonal niche partitioning among organisms with different size.
Journal of limnology 05/2013; 72(3):2. DOI:10.4081/jlimnol.2013.e33 · 1.18 Impact Factor