Antonello Provenzale

Climate warming, ecological mismatch at arrival and population decline in migratory birds

Climate is changing at a fast pace, causing widespread, profound consequences for living organisms. Failure to adjust the timing of life-cycle events to climate may jeopardize populations by causing ecological mismatches to the life cycle of other species and abiotic factors. Population declines of some migratory birds breeding in Europe have been suggested to depend on their inability to adjust migration phenology so as to keep track of advancement of spring events at their breeding grounds. In fact, several migrants have advanced their spring arrival date, but whether such advancement has been sufficient to compensate for temporal shift in spring phenophases or, conversely, birds have become ecologically mismatched, is still an unanswered question, with very few exceptions. We used a novel approach based on accumulated winter and spring temperatures (degree-days) as a proxy for timing of spring biological events to test if the progress of spring at arrival to the breeding areas by 117 European migratory bird species has changed over the past five decades. Migrants, and particularly those wintering in sub-Saharan Africa, now arrive at higher degree-days and may have therefore accumulated a ‘thermal delay’, thus possibly becoming increasingly mismatched to spring phenology. Species with greater ‘thermal delay’ have shown larger population decline, and this evidence was not confounded by concomitant ecological factors or by phylogenetic effects. These findings provide general support to the largely untested hypotheses that migratory birds are becoming ecologically mismatched and that failure to respond to climate change can have severe negative impacts on their populations. The novel approach we adopted can be extended to the analysis of ecological consequences of phenological response to climate change by other taxa.

A comparison of stochastic models for spatial rainfall downscaling

[1]xa0We explore the performance of three types of stochastic models used for spatial rainfall downscaling and assess their ability to reproduce the statistics of precipitation fields observed during the GATE radar experiment. We consider a bounded multifractal cascade, an autoregressive linear process passed through a nonlinear static filter (sometimes called a meta-Gaussian model), and a model based on the presence of individual rainfall cells with power law profile. As test statistics we use the low-order moments of the amplitude distribution, the distribution of generalized fractal dimensions, the generalized scaling exponents, the slope of the power spectrum, and the properties of the spatial autocorrelation. The results of the analysis indicate that all models provide, on average, a satisfactory representation of the statistical properties of the GATE rainfall fields (including the anomalous scaling behavior), with a slightly better performance of the model based on individual rainfall cells. All models, however, display large scatter in the field-to-field comparison with the data. These results indicate that data analysis alone does not allow, at the moment, for preferring one downscaling approach over another.

Hot and cool summers: Multiple equilibria of the continental water cycle

[1]xa0Large variations in soil water reserves and surface temperature over the continents are linked to a positive feedback between precipitation and soil moisture. This mechanism can generate bimodal distributions of soil moisture. Here, we show that bimodality results from the existence of multiple equilibria in the continental water balance, considering the coupled system including the upper soil layer and the atmospheric planetary boundary layer. This mechanism is described with an idealized box model, that includes convergence and divergence of moisture fluxes, convection, precipitation and evapotranspiration. The existence of two equilibria is associated with the variation of precipitation efficiency, which depends on convection intensity. The two regimes correspond to realistic values of climatic variables associated with mean wet or dry summers, and can persist for the whole summer season when forced by a stochastic moisture convergence flux. This suggests that a key role for midlatitude continental summer climate is played by the continental soil water content.

Multiple equilibria on planet Dune: Climate-vegetation dynamics on a sandy planet

ABSTRACT We study the interaction between climate and vegetation on an ideal water-limited planet, focussing on the influence of vegetation on the global water cycle. We introduce a simple mechanistic box model consisting in a two-layer representation of the atmosphere and a two-layer soil scheme. The model includes the dynamics of vegetation cover, and the main physical processes of energy and water exchange among the different components. With a realistic choice of parameters, this model displays three stable equilibria, depending on the initial conditions of soil water and vegetation cover. The system reaches a hot and dry state for low values of initial water content and/or vegetation cover, while we observe a wet, vegetated state with mild surface temperature when the system starts from larger initial values of both variables. The third state is a cold desert, where plants transfer enough water to the atmosphere to start a weaker, evaporation-dominated water cycle before they wilt. These results indicate that in this system vegetation plays a central role in transferring water from the soil to the atmosphere and trigger a hydrologic cycle. The model adopted here can also be used to conceptually illustrate processes and feedbacks affecting the water cycle in water-limited continental areas on Earth. †Now at: International Max Planck Research School on Earth System Modelling, Hamburg, Germany

Red spectra from white and blue noise.

The value of maps of the interval in modelling population dynamics has recently been called into question because temporal variations from such maps have blue or white power spectra, whereas many observations of real populations show time–series with red spectra. One way to deal with this discrepancy is to introduce chaotic or stochastic fluctuations in the parameters of the map. This leads to on–off intermittency and can markedly redden the spectrum produced by a model that does not by itself have a red spectrum. The parameter fluctuations need not themselves have a red spectrum in order to achieve this effect. Because the power spectrum is not invariant under a change of variable, another way to redden the spectrum is by a suitable transformation of the variables used. The question this poses is whether spectra are the best means of characterizing a fluctuating variable.

Tree‐grass competition for soil water in arid and semiarid savannas: The role of rainfall intermittency

Arid and semiarid savannas are characterized by the coexistence of trees and grasses in water limited conditions. As in all dry lands, also in these savannas rainfall is highly intermittent. In this work, we develop and use a simple implicit-space model to conceptually explore how precipitation intermittency influences tree-grass competition and savanna occurrence. The model explicitly includes soil moisture dynamics, and life-stage structure of the trees. Assuming that water availability affects the ability of both plant functional types to colonize new space and that grasses outcompete tree seedlings, the model is able to predict the expected sequence of grassland, savanna, and forest along a range of mean annual rainfall. In addition, rainfall intermittency allows for tree-grass coexistence at lower mean annual rainfall values than for constant precipitation. Comparison with observations indicates that the model, albeit very simple, is able to capture some of the essential dynamical processes of natural savannas. The results suggest that precipitation intermittency affects savanna occurrence and structure, indicating a new point of view for reanalyzing observational data from the literature.

Assessing Climate Change Risks Under Uncertain Conditions

Climate and environmental change is expected to affect hydrometeorological hazard and ecosystem functioning, with possible threats to human societies due to increased probability of extreme events and loss of ecosystem services. In mountain regions, the environmental response could be even larger. For this reason, it is important to obtain estimates of the expected modifications in natural hazards associated with climate and environmental change, to develop appropriate adaptation and risk mitigation strategies. This goal, however, is made difficult by the scale mismatch between climate model projections and land surface response, which requires the use of appropriate climate downscaling procedures. To complicate the picture, one should also cope with the chain of uncertainties which affect climate and risk projections, from the wide range of global climate model estimates for the water cycle variables, to the uncertainties in regional climate response, to the uncertainties in the hydrological and/or ecosystem models themselves. Precipitation data used to validate the models, on the other hand, are also affected by severe uncertainties, especially in mountain regions. This leads to the general problem of assessing natural hazards for different climate and environmental change scenarios under uncertain conditions.

A hierarchy of coupled maps.

A large number of logistic maps are coupled together as a mathematical metaphor for complex natural systems with hierarchical organization. The elementary maps are first collected into globally coupled lattices. These lattices are then coupled together in a hierarchical way to form a system with many degrees of freedom. We summarize the behavior of the individual blocks, and then explore the dynamics of the hierarchy. We offer some ideas that guide our understanding of this type of system. (c) 2002 American Institute of Physics.

Impact of Microphysics and Convective Parameterizations on Dynamical Downscaling for the European Domain

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.

Water in the climate system

Water is an essential element of climate, owing to its dynamical and thermodynamical effects. Water is, in addition, one of the main fluids active in the climate system: oceans, ice sheets and glaciers, snow and atmospheric water (in vapor, liquid or solid form) and the whole hydrological cycle are at the heart of the workings of the Earth System. In these short notes, we shall describe just a few of the many ways water enters the fluid dynamics of climate.