Reversed Holocene temperature-moisture relationship in the Horn of Africa.

Anthropogenic climate change is predicted to severely impact the global hydrological cycle1, particularly in tropical regions where agriculture-based economies depend on monsoon rainfall2. In the Horn of Africa, more frequent drought conditions in recent decades3,4 contrast with climate models projecting precipitation to increase with rising temperature5. Here we use organic geochemical climate-proxy data from the sediment record of Lake Chala (Kenya and Tanzania) to probe the stability of the link between hydroclimate and temperature over approximately the past 75,000 years, hence encompassing a sufficiently wide range of temperatures to test the 'dry gets drier, wet gets wetter' paradigm6 of anthropogenic climate change in the time domain. We show that the positive relationship between effective moisture and temperature in easternmost Africa during the cooler last glacial period shifted to negative around the onset of the Holocene 11,700 years ago, when the atmospheric carbon dioxide concentration exceeded 250 parts per million and mean annual temperature approached modern-day values. Thus, at that time, the budget between monsoonal precipitation and continental evaporation7 crossed a tipping point such that the positive influence of temperature on evaporation became greater than its positive influence on precipitation. Our results imply that under continued anthropogenic warming, the Horn of Africa will probably experience further drying, and they highlight the need for improved simulation of both dynamic and thermodynamic processes in the tropical hydrological cycle.

The Contribution of Transpiration to Precipitation Over African Watersheds.

The redistribution of biological (transpiration) and non-biological (interception loss, soil evaporation) fluxes of terrestrial evaporation via atmospheric circulation and precipitation is an important Earth system process. In vegetated ecosystems, transpiration dominates terrestrial evaporation and is thought to be crucial for regional moisture recycling and ecosystem functioning. However, the spatial and temporal variability in the dependency of precipitation on transpiration remains understudied, particularly in sparsely sampled regions like Africa. Here, we investigate how biological and non-biological sources of evaporation in Africa contribute to rainfall over the major watersheds in the continent. Our study is based on simulated atmospheric moisture trajectories derived from the Lagrangian model FLEXPART, driven by 1° resolution reanalysis data over 1981-2016. Using daily satellite-based fractions of transpiration over terrestrial evaporation, we isolate the contribution of vegetation to monthly rainfall. Furthermore, we highlight two watersheds (Congo and Senegal) for which we explore intra- and interannual variability of different precipitation sources, and where we find contrasting patterns of vegetation-sourced precipitation within and between years. Overall, our results show that almost 50% of the annual rainfall in Africa originates from transpiration, although the variability between watersheds is large (5%-68%). We conclude that, considering the current and projected patterns of land use change in Africa, a better understanding of the implications for continental-scale water availability is needed.

The influence of soil dry-out on the record-breaking hot 2013/2014 summer in Southeast Brazil.

The 2013/2014 summer in Southeast Brazil was marked by historical unprecedented compound dry and hot (CDH) conditions with profound socio-economic impacts. The synoptic drivers for this event have already been analyzed, and its occurrence within the context of the increasing trend of CDH conditions in the area evaluated. However, so far, the causes for these record temperatures remain poorly understood. Here, a detailed characterization of the 2013/2014 austral summer season over Southeast Brazil is proposed, emphasizing the role played by land-atmosphere interactions in temperature escalation. We demonstrate that a strong soil moisture-temperature coupling regime promoted record-breaking temperatures levels exceeding almost 5 °C over the previous highest record, and played a key role in triggering an outstanding 'mega-heatwave' that lasted for a period of around 20 days. This pronounced soil desiccation occurred within a current climate change trend defined by drier and hotter conditions in the region. The soil dry-out, coupled with strong radiative processes and low entrainment of cooler air masses through mesoscale sea-breeze circulation processes, led to a water-limited regime and to an enhancement of sensible heat fluxes that, ultimately, resulted in a sharp increase of surface temperatures.

On the Use of the Term "Evapotranspiration".

Evaporation is the phenomenon by which a substance is converted from its liquid into its vapor phase, independently of where it lies in nature. However, language is alive, and just like regular speech, scientific terminology changes. Frequently, those changes are grounded on a solid rationale, but sometimes these semantic transitions have a fragile foundation. That is the case with "evapotranspiration." A growing generation of scientists have been educated on using this terminology and are unaware of the historical controversy and physical inconsistency that surrounds it. Here, we present what may appear to some as an esoteric linguistic discussion, yet it was originally triggered by the increasing time some of us have devoted to justifying our word choice to reviewers, editors, and peers. By clarifying our arguments for using the term "evaporation," we also seek to prevent having to revive this discussion every time a new article is submitted, so that we can move directly on to more scientifically relevant matters.

A Precipitation Recycling Network to Assess Freshwater Vulnerability: Challenging the Watershed Convention.

Water resources and water scarcity are usually regarded as local aspects for which a watershed-based management appears adequate. However, precipitation, as a main source of freshwater, may depend on moisture supplied through land evaporation from outside the watershed. This notion of evaporation as a local "green water" supply to precipitation is typically not considered in hydrological water assessments. Here we propose the concept of a watershed precipitation recycling network, which establishes atmospheric pathways and links land surface evaporation as a moisture supply to precipitation, hence contributing to local but also remote freshwater resources. Our results show that up to 74% of summer precipitation over European watersheds depends on moisture supplied from other watersheds, which contradicts the conventional consideration of autarkic watersheds. The proposed network approach illustrates atmospheric pathways and enables the objective assessment of freshwater vulnerability and water scarcity risks under global change. The illustrated watershed interdependence emphasizes the need for global water governance to secure freshwater availability.

Soil Moisture-Temperature Coupling in a Set of Land Surface Models.

The land surface controls the partitioning of water and energy fluxes and therefore plays a crucial role in the climate system. The coupling between soil moisture and air temperature, in particular, has been shown to affect the severity and occurrence of temperature extremes and heat waves. Here we study soil moisture-temperature coupling in five land surface models, focusing on the terrestrial segment of the coupling in the warm season. All models are run off-line over a common period with identical atmospheric forcing data, in order to allow differences in the results to be attributed to the models' partitioning of energy and water fluxes. Coupling is calculated according to two semiempirical metrics, and results are compared to observational flux tower data. Results show that the locations of the global hot spots of soil moisture-temperature coupling are similar across all models and for both metrics. In agreement with previous studies, these areas are located in transitional climate regimes. The magnitude and local patterns of model coupling, however, can vary considerably. Model coupling fields are compared to tower data, bearing in mind the limitations in the geographical distribution of flux towers and the differences in representative area of models and in situ data. Nevertheless, model coupling correlates in space with the tower-based results (r = 0.5-0.7), with the multimodel mean performing similarly to the best-performing model. Intermodel differences are also found in the evaporative fractions and may relate to errors in model parameterizations and ancillary data of soil and vegetation characteristics.