Changes In Hydrological Cycle Research Articles

Freshening of Antarctic Intermediate Water in the South Atlantic Ocean in 2005–2014

Abstract. Basin-scale freshening of Antarctic Intermediate Water (AAIW) is reported to have occurred in the South Atlantic Ocean during the period from 2005 to 2014, as shown by the gridded monthly means of the Array for Real-time Geostrophic Oceanography (Argo) data. This phenomenon was also revealed by two repeated transects along a section at 30° S, performed during the World Ocean Circulation Experiment Hydrographic Program. Freshening of the AAIW was compensated for by a salinity increase of thermocline water, indicating a hydrological cycle intensification. This was supported by the precipitation-minus-evaporation change in the Southern Hemisphere from 2000 to 2014. Freshwater input from atmosphere to ocean surface increased in the subpolar high-precipitation region and vice versa in the subtropical high-evaporation region. Against the background of hydrological cycle changes, a decrease in the transport of Agulhas Leakage (AL), which was revealed by the simulated velocity field, was proposed to be a contributor to the associated freshening of AAIW. Further calculation showed that such a decrease could account for approximately 53 % of the observed freshening (mean salinity reduction of about 0.012 over the AAIW layer). The estimated variability of AL was inferred from a weakening of wind stress over the South Indian Ocean since the beginning of the 2000s, which would facilitate freshwater input from the source region. The mechanical analysis of wind data here was qualitative, but it is contended that this study would be helpful to validate and test predictably coupled sea–air model simulations.

Lateral groundwater flow and pond interactions during dry and wet years

Groundwater and surface water are tightly coupled elements of the hydrologic cycle that have often been treated as separate entities. Future climate change modelling has predicted that hydrologic cycle changes, namely increasing drought frequency and flood-type events, are likely to occur. These events may directly impact the quality and quantity of both groundwater and surface water. Future water management policies must therefore be based on an understanding of how interactions between groundwater and surface water will change with a warming climate. The aim of this study was to model and analyze the lateral flow of groundwater and its interactions with a nearby pond in a shallow, unconsolidated, unconfined aquifer. Data were collected as part of a larger and ongoing study during the year 2012, a comparatively dry year, and 2013, a comparatively wet year. We first used ArcGIS and Visual MODFLOW Flex to create a conceptual model of the system, its soil layers, monitoring wells, and potential flow patterns. We then analyzed hydraulic head data, and calculated groundwater flow volumes using the Dupuit equation. We found that the groundwater flow direction reversed in the summer of 2012 and continued until the spring of 2013. Additonally, flow rate was greater in 2013 than 2012. The flow reversal was likely caused by higher evaporative demand during the summer months of 2012, drawing substantially more water from the pond than from the soil. The two-year timeframe was not long enough to determine whether this was a typical, yearly pattern, or was primarily due to the fact that 2012 was a particularly dry year.

Hyperspectral Monitoring of Green Roof Vegetation Health State in Sub-Mediterranean Climate: Preliminary Results.

In urban and industrial environments, the constant increase of impermeable surfaces has produced drastic changes in the natural hydrological cycle. Decreasing green areas not only produce negative effects from a hydrological-hydraulic perspective, but also from an energy point of view, modifying the urban microclimate and generating, as shown in the literature, heat islands in our cities. In this context, green infrastructures may represent an environmental compensation action that can be used to re-equilibrate the hydrological and energy balance and reduce the impact of pollutant load on receiving water bodies. To ensure that a green infrastructure will work properly, vegetated areas have to be continuously monitored to verify their health state. This paper presents a ground spectroscopy monitoring survey of a green roof installed at the University of Calabria fulfilled via the acquisition and analysis of hyperspectral data. This study is part of a larger research project financed by European Structural funds aimed at understanding the influence of green roofs on rainwater management and energy consumption for air conditioning in the Mediterranean area. Reflectance values were acquired with a field-portable spectroradiometer that operates in the range of wavelengths 350–2500 nm. The survey was carried out during the time period November 2014–June 2015 and data were acquired weekly. Climatic, thermo-physical, hydrological and hydraulic quantities were acquired as well and related to spectral data. Broadband and narrowband spectral indices, related to chlorophyll content and to chlorophyll–carotenoid ratio, were computed. The two narrowband indices NDVI705 and SIPI turned out to be the most representative indices to detect the plant health status.

A total precipitable water retrieval method over land using the combination of passive microwave and optical remote sensing

Atmospheric water vapor plays an important role in hydrologic cycle and climate change of the Earth. A number of studies have focused on retrieval of the total precipitable water (TPW) using microwave or optical remote sensing. In this paper, the global quarter-degree gridded TPW over land was retrieved using water vapor sensitivity parameter ∆Tb18.7/∆Tb23.8 based on the combination of AMSR-E and MODIS observations. There are two major improvements in the retrieval algorithm, including optimization of the estimation model of surface emissivity ∆ε18.7/∆ε23.8 and correction of the terrain influence to the retrieval of TPW using DEM. To obtain a high resolution TPW, we also developed an algorithm to downscale the retrieved quarter-degree gridded TPW to a fine scale of 0.05°×0.05° using DEM and NDVI. In addition, the downscaled TPW was further calibrated using high precision TPW from MODIS in the clear-sky condition to improve its accuracy. Finally, both quarter-degree and 0.05°×0.05° gridded TPW were validated against SuomiNet GPS retrieved TPW on a global scale. The RMSE for the retrieved quarter-degree gridded global TPW is 3.45mm, with a correlation coefficient of 0.95. In addition, the RMSE for the downscaled 0.05°×0.05° gridded global TPW is 4.18mm, with a correlation coefficient of 0.95. An obvious advantage of our algorithm compared with MODIS TPW product is that it can retrieve TPW under cloudy sky condition over land. The algorithm developed in this study can be easily transferred to AMSR2 on board GCOM-W1 and provides the long-term global daily TPW over land since the launch of Aqua to present day to support hydrologic cycle and climate change studies.

Parameter sensitivity analysis and optimization for a satellite‐based evapotranspiration model across multiple sites using Moderate Resolution Imaging Spectroradiometer and flux data

AbstractGlobal and regional estimates of daily evapotranspiration are essential to our understanding of the hydrologic cycle and climate change. In this study, we selected the radiation‐based Priestly‐Taylor Jet Propulsion Laboratory (PT‐JPL) model and assessed it at a daily time scale by using 44 flux towers. These towers distributed in a wide range of ecological systems: croplands, deciduous broadleaf forest, evergreen broadleaf forest, evergreen needleleaf forest, grasslands, mixed forests, savannas, and shrublands. A regional land surface evapotranspiration model with a relatively simple structure, the PT‐JPL model largely uses ecophysiologically‐based formulation and parameters to relate potential evapotranspiration to actual evapotranspiration. The results using the original model indicate that the model always overestimates evapotranspiration in arid regions. This likely results from the misrepresentation of water limitation and energy partition in the model. By analyzing physiological processes and determining the sensitive parameters, we identified a series of parameter sets that can increase model performance. The model with optimized parameters showed better performance (R2 = 0.2–0.87; Nash‐Sutcliffe efficiency (NSE) = 0.1–0.87) at each site than the original model (R2 = 0.19–0.87; NSE = −12.14–0.85). The results of the optimization indicated that the parameter β (water control of soil evaporation) was much lower in arid regions than in relatively humid regions. Furthermore, the optimized value of parameter m1 (plant control of canopy transpiration) was mostly between 1 to 1.3, slightly lower than the original value. Also, the optimized parameter Topt correlated well to the actual environmental temperature at each site. We suggest that using optimized parameters with the PT‐JPL model could provide an efficient way to improve the model performance.

Timeslice experiments for understanding regional climate projections: applications to the tropical hydrological cycle and European winter circulation

A set of atmosphere-only timeslice experiments are described, designed to examine the processes that cause regional climate change and inter-model uncertainty in coupled climate model responses to \(CO_2\) forcing. The timeslice experiments are able to reproduce the pattern of regional climate change in the coupled models, and are applied here to two cases where inter-model uncertainty in future projections is large: the tropical hydrological cycle, and European winter circulation. In tropical forest regions, the plant physiological effect is the largest cause of hydrological cycle change in the two models that represent this process. This suggests that the CMIP5 ensemble mean may be underestimating the magnitude of water cycle change in these regions, due to the inclusion of models without the plant effect. SST pattern change is the dominant cause of precipitation and circulation change over the tropical oceans, and also appears to contribute to inter-model uncertainty in precipitation change over tropical land regions. Over Europe and the North Atlantic, uniform SST increases drive a poleward shift of the storm-track. However this does not consistently translate into an overall polewards storm-track shift, due to large circulation responses to SST pattern change, which varies across the models. Coupled model SST biases influence regional rainfall projections in regions such as the Maritime Continent, and so projections in these regions should be treated with caution.

SEASONAL CHANGES IN PRECIPITATION EXTREMES IN RUSSIA FOR THE LAST SEVERAL DECADES AND THEIR IMPACT ON VITAL ACTIVITIES OF THE HUMAN POPULATION

Seasonal regional features of the daily precipitation extremes were studied based on Russian meteorological stations datasets for the period of 1991- 2013 compared to the 1961-1990 climate baseline conditions. Precipitation extreme changes were assessed for the most vulnerable regions of Russia with high population density, where precipitation extremes result in negative impacts on the environment and human activities. It was found that the frequency of precipitation extremes in winter and in spring for the period 1991-2013 significantly increased, by 20-40% at the average, in most parts of the case study area. Due to positive trends in daily precipitation extremes changes which was revealed for the winter and spring periods (not exceeding on average 0.2 mm/day/decade), the risks of catastrophic spring floods have been analysed, especially in the areas with a higher recurrence rate of dangerous floods, i.e. - the South Urals region and the Altai region. Strong positive trends of extreme precipitation changes were observed in the Russian Far East region. It indicates higher risk of summer rain floods in the Amur River basin. A significant impact on human activities and in particular population health is associated with revealed trends in hydrological cycle changes that are not relevant to typical meteorological and hydrological regimes. The significant increase of the frequency of extreme summer precipitation events in the Central Chernozem region of European Russia in the period of 1961-2013 was accompanied by the leptospirosis disease incidences.

Large scale climate and rainfall seasonality in a Mediterranean Area: Insights from a non‐homogeneous Markov model applied to the Agro‐Pontino plain

AbstractIn the context of climate change and variability, there is considerable interest in how large scale climate indicators influence regional precipitation occurrence and its seasonality. Seasonal and longer climate projections from coupled ocean–atmosphere models need to be downscaled to regional levels for hydrologic applications, and the identification of appropriate state variables from such models that can best inform this process is also of direct interest. Here, a Non‐Homogeneous Hidden Markov Model (NHMM) for downscaling daily rainfall is developed for the Agro‐Pontino Plain, a coastal reclamation region very vulnerable to changes of hydrological cycle. The NHMM, through a set of atmospheric predictors, provides the link between large scale meteorological features and local rainfall patterns. Atmospheric data from the NCEP/NCAR archive and 56‐years record (1951–2004) of daily rainfall measurements from 7 stations in Agro‐Pontino Plain are analyzed. A number of validation tests are carried out, in order to: 1) identify the best set of atmospheric predictors to model local rainfall; 2) evaluate the model performance to capture realistically relevant rainfall attributes as the inter‐annual and seasonal variability, as well as average and extreme rainfall patterns.Validation tests show that the best set of atmospheric predictors are the following: mean sea level pressure, temperature at 1000 hPa, meridional and zonal wind at 850 hPa and precipitable water, from 20°N to 80°N of latitude and from 80°W to 60°E of longitude. Furthermore, the validation tests show that the rainfall attributes are simulated realistically and accurately. The capability of the NHMM to be used as a forecasting tool to quantify changes of rainfall patterns forced by alteration of atmospheric circulation under climate change and variability scenarios is discussed.

Responses of landscape pattern of China’s two largest freshwater lakes to early dry season after the impoundment of Three-Gorges Dam

The effects of hydrologic cycle change (caused by human activity and global climate change) on ecosystems attract the increasing attention around the world. As a result of impounding of the Three Gorges Dam (TGD), climate change and sand mining, the dry season of Poyang Lake and Dongting Lake (China’s two largest freshwater lakes) came early after the TGD impoundment. It was the primary cause of the increasing need for sluice/dam construction to store water in the Lakes and attracted increasing attention. In this paper, we compared the landscape pattern between three hydrologic years with early dry season (EY) and three normal hydrologic years (NY) of each lake by remote sensing technology, to reveal the effect of early dry season on landscape pattern. The results showed that early dry season caused expanding of Phalaris to mudflat zone in Poyang Lake, while caused expanding of Carex to Phalaris zone and expanding of Phalaris to mudflat zone in Dongting Lake. In landscape level, there was no significant difference in landscape grain size, landscape grain shape, habitat connectivity and landscape diversity between EY and NY in the two lakes. While in habitat class level, there were significant changes in area of mudflat and Phalaris and grain size of mudflat in Poyang Lake, and in area of Carex, grain size of Phalaris and grain shape of Carex and Phalaris in Dongting Lake. These changes will impact migrating birds of East Asian and migratory fishes of Yangtze River.

Hemispherically asymmetric volcanic forcing of tropical hydroclimate during the last millennium

Abstract. Volcanic aerosols exert the most important natural radiative forcing of the last millennium. State-of-the-art paleoclimate simulations of this interval are typically forced with diverse spatial patterns of volcanic forcing, leading to different responses in tropical hydroclimate. Recently, theoretical considerations relating the intertropical convergence zone (ITCZ) position to the demands of global energy balance have emerged in the literature, allowing for a connection to be made between the paleoclimate simulations and recent developments in the understanding of ITCZ dynamics. These energetic considerations aid in explaining the well-known historical, paleoclimatic, and modeling evidence that the ITCZ migrates away from the hemisphere that is energetically deficient in response to asymmetric forcing.Here we use two separate general circulation model (GCM) suites of experiments for the last millennium to relate the ITCZ position to asymmetries in prescribed volcanic sulfate aerosols in the stratosphere and related asymmetric radiative forcing. We discuss the ITCZ shift in the context of atmospheric energetics and discuss the ramifications of transient ITCZ migrations for other sensitive indicators of changes in the tropical hydrologic cycle, including global streamflow. For the first time, we also offer insight into the large-scale fingerprint of water isotopologues in precipitation (δ18Op) in response to asymmetries in radiative forcing. The ITCZ shifts away from the hemisphere with greater volcanic forcing. Since the isotopic composition of precipitation in the ITCZ is relatively depleted compared to areas outside this zone, this meridional precipitation migration results in a large-scale enrichment (depletion) in the isotopic composition of tropical precipitation in regions the ITCZ moves away from (toward). Our results highlight the need for careful consideration of the spatial structure of volcanic forcing for interpreting volcanic signals in proxy records and therefore in evaluating the skill of Common Era climate model output.

Impacts of agricultural water saving practice on regional groundwater and water consumption in an arid region with shallow groundwater

Irrigation is important to agricultural production and can induce changes in the regional hydrological cycle. In particular, in areas with shallow groundwater, fluctuation of groundwater levels will be present with irrigation events and groundwater evaporation. To deal with the shortage of water resources, agricultural water saving (AWS) has been conducted in arid and semiarid area. The Jiefangzha irrigation district was selected as the study area to investigate groundwater dynamics in response to AWS. With the abundant various data, the trend of groundwater level change from 1980 to 2013 was evaluated based on geographic information systems. Groundwater net consumption (ETng) and regional evapotranspiration (ET) during crop growth period were estimated by water fluctuation method and the water balance method. The results indicate that the groundwater depth was relatively stable before 2002, remaining at around 1.7 m. Due to reduction of the water diversion from the Yellow River and the implementation of water saving measures (WSMs), the canal seepage and irrigation infiltration declined, leading to groundwater levels declining by 0.4 m in recent 10 years. Groundwater net consumption of crop growth period was estimated since 1990 and increased from 50 mm/year in 1990 to 110 mm/year in 2013 with the reduction of water diversion. At the same time, regional evapotranspiration has shown a slightly decreasing trend. Thus, the contribution of groundwater to regional evapotranspiration has an obvious increasing trend, accounting for 20 % in 2013, which is doubled compared with that in 1980. Groundwater is very important to sustain crop production and healthy ecology, and it is quite essential to consider groundwater response to regional WSMs and water management.

Climate Change, Agriculture and Water Resources in the Southwestern United States

AbstractIn February 2014 the USDA established regional climate hubs across the United States to assist farmers, ranchers and foresters in adapting to the effects of climate change. The Southwest (SW) region encompasses six states which provide highly diverse agricultural crops including cotton, stone fruit and grapes. Here we report on the hydrologic context within which SW working landowners operate, with focus on regional water resources and likely impacts of climate change. Water is a critical component of agricultural vulnerability in the SW, where high agricultural production can occur with sufficient irrigation. Since 1978, crop yield declines were reported on 11–21% of total irrigated acres, mostly due to surface water shortage. Southwestern agriculture relies on groundwater, using it to supply at least one‐third of the agricultural water demand across the region since 1955. Regional groundwater use varies over time, with a decline in the agricultural groundwater fraction in Arizona, but an increase in the fraction in Nevada and Utah. Observed and predicted changes in the southwestern hydrologic cycle can impact regional agriculture. Observed records show an increase in the fraction of precipitation falling as rain, which is expected to continue with future warming, and decrease the natural high‐mountain storage reservoir provided by snowpack. Warming causes the snowmelt to peak earlier in the season than observed in historical records, and can reduce water available to crops during the summer months without additional water storage. Observed records indicate streamflow has shifted earlier in the year, most notably in snow dominated watersheds. A continuation of this trend may challenge regional agriculture by further limiting water supplies.

Addressing the local aspects of global change impacts on stream metabolism using frequency analysis tools

Global change, as a combination of climate change, human activities on watersheds and the river flow regulation, causes intense changes in hydrological cycles and, consequently, threatens the good ecological status of freshwater biological communities. This study addresses how and whether the combination of climatic drivers and local human impacts may alter the metabolism of freshwater communities.We identified a few factors modulating the natural water flow and quality in 25 point spread within the Ebro river Basin: waste water spills, industrial spills, reservoir discharges, water withdrawals, agricultural use, and the presence of riparian forests. We assessed their impacts on the freshwater metabolism as changes in the annual cycle of both gross primary production—GPP – and ecosystem respiration—ER –. For this purpose, daily data series were analyzed by continuous wavelet transformation, allowing for the assessment of the metabolic ecosystem Frequency Spectrum Patterns (FSPs).Changes in the behavior of ecosystem metabolism were strongly associated with local characteristics at each sampling point, however in 20 out of 25 studied points, changes in metabolic ecosystem FSP were related to climatic change events (the driest period of the last 140years). The changes in FSP indicate that severe impacts on how biological communities use carbon sources occur as a result of the human water management – too much focus on human needs – during intense climatic events.Results show that local factors, and specially the flow regulation, may modulate the impact of global change. As example those points exposed to a more intense anthropization showed a clear disruption – and even disappearance – of the annual FSP. This information may help managers to understand the action mechanisms of non-climatic factors at ecosystem level, leading to better management policies based on the promotion of ecosystem resilience. The method here presented may help on improving the calculation of ecological flows to maintain the river metabolic annual cycles as close as possible to the natural ones.

Atmospheric residence times from transpiration and evaporation to precipitation: An age‐weighted regional evaporation tagging approach

AbstractThe atmospheric water residence time is a fundamental descriptor that provides information on the timescales of evaporation and precipitation. In this study, a regional climate model‐based evaporation tagging algorithm is extended with an age tracer approach to calculate moisture residence times, defined as time between the original evaporation and the returning of water masses to the land surface as precipitation. Our case study addresses how long this time is for the transpired and for the direct evaporated moisture. Our study region is the Poyang Lake region in Southeast China, the largest freshwater lake in the country. We perform simulations covering the period from October 2004 to December 2005. In 2005, 11% of direct evaporated water (10% of transpired water) precipitates locally. Direct evaporated water accounts for 64% and transpired water for 36% of the total tagged moisture with a mean age of around 36 h for both. Considering precipitation, a large proportion (69%) originates from direct evaporated water with a mean atmospheric residence time of 6.6 h and a smaller amount from transpired water with a longer residence time of 10.7 h. Modulated by the East Asian monsoon, the variation of the meteorological conditions, the magnitude of the partitioned moisture, and the corresponding residence time patterns change seasonally and spatially and reveal the different fate of transpired and direct evaporated water in the atmospheric hydrological cycle. We conclude that our methodological approach has the potential to be used for addressing how timescales of the hydrological cycle changes regionally under global warming.

Interannual variability in the summertime hydrological cycle over European regions

AbstractA variety of observations‐based hydrological variables from different data sets are used to investigate interannual variability and changes in the summertime hydrological cycle over four European regions—Iberian Peninsula (IP), British Isles (BI), Central Europe (CE), and European Russia (ER). An analysis performed on seasonal means (June, July, and August) suggests that soil moisture variability is impacted almost equally by precipitation and air temperature in BI and ER regions. However, stronger links between soil moisture and precipitation are revealed for CE region and between soil moisture and air temperature for IP region. In all except IP regions summertime interannual variability of column‐integrated water vapor is strongly linked to air temperature consistent with the dominating influence of the Clausius‐Clapeyron equation. In BI, CE, and ER interannual variability of regional precipitation is driven by variations in atmospheric moisture transport into these regions. In IP the link between precipitation and moisture transport is relatively weak. Based on monthly data, analysis of the lag‐lead correlations revealed specific regional relationships between different hydrological variables. In particular, it is shown that in some regions (and months) interannual variability of soil moisture is linked more strongly to precipitation and air temperature anomalies in the previous month, rather than in the coinciding month. An analysis of the vertical structure of regional atmospheric moisture transport has revealed that the more continental the climate of the region is, the larger deviation from the mean (i.e., climatological) profile might be observed during anomalously dry/wet summers.

A critical review of groundwater utilization and management in China's inland water shortage areas

Groundwater, as an important store of freshwater, plays a more critical role in sustaining the ecosystem and enhancing human adaptation to changing climate than surface water. In particular, it can store large volumes of water to naturally buffer the pressure of water shortage against seasonal changes in rainfall. However, groundwater itself is also vulnerable to climate change, showing a great change in hydrologic cycle. Therefore, effective groundwater management has a strategic importance for China's water security. At present, China is facing a groundwater crisis because of the dual effects of natural and anthropogenic factors. Many new ideas and solutions have been given in previous studies on groundwater utilization and management. This paper vividly captures these studies. The paper summarizes groundwater properties and the situation of groundwater development and utilization. The paper also reports challenges, strategies and policies in groundwater sustainability.

Arctic Vortex changes alter the sources and isotopic values of precipitation in northeastern US

Altered atmospheric circulation, reductions in Arctic sea ice, ocean warming, and changes in evaporation and transpiration are driving changes in the global hydrologic cycle. Precipitation isotopic (δ18O and δ2H) measurements can help provide a mechanistic understanding of hydrologic change at global and regional scales. To study the changing water cycle in the northeastern US, we examined the longest (1968–2010) record of precipitation isotope values, collected at the Hubbard Brook Experimental Forest in New Hampshire, US (43o56′N, 71o45′W). We found a significant reduction in δ18O and δ2H values over the 43-year record, coupled with a significant increase in d-excess values. This gradual reduction in δ18O and δ2H values unexpectedly occurred during a period of regional warming. We provide evidence that these changes are governed by the interactions among the Atlantic Multidecadal Oscillation, loss of Arctic sea ice, the fluctuating jet stream, and regular incursions of polar air into the northeastern US.

Highlighting Uncertainty and Recommendations for Improvement of Black Carbon Biomass Fuel-Based Emission Inventories in the Indo-Gangetic Plain Region.

Black carbon (BC) is a major contributor to hydrological cycle change and glacial retreat within the Indo-Gangetic Plain (IGP) and surrounding region. However, significant variability exists for estimates of BC regional concentration. Existing inventories within the IGP suffer from limited representation of rural sources, reliance on idealized point source estimates (e.g., utilization of emission factors or fuel-use estimates for cooking along with demographic information), and difficulty in distinguishing sources. Inventory development utilizes two approaches, termed top down and bottom up, which rely on various sources including transport models, emission factors, and remote sensing applications. Large discrepancies exist for BC source attribution throughout the IGP depending on the approach utilized. Cooking with biomass fuels, a major contributor to BC production has great source apportionment variability. Areas requiring attention tied to research of cookstove and biomass fuel use that have been recognized to improve emission inventory estimates include emission factors, particulate matter speciation, and better quantification of regional/economic sectors. However, limited attention has been given towards understanding ambient small-scale spatial variation of BC between cooking and non-cooking periods in low-resource environments. Understanding the indoor to outdoor relationship of BC emissions due to cooking at a local level is a top priority to improve emission inventories as many health and climate applications rely upon utilization of accurate emission inventories.

Contrasting Short- and Long-Term Projections of the Hydrological Cycle in the Southern Extratropics

Abstract Analysis of model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that, in the zonal mean, the near-term projections of summertime changes of precipitation in the Southern Hemisphere (SH) subtropics are very widely scattered among the models. As a consequence, over the next 50 years, the CMIP5 multimodel mean projects no statistically significant trends in the SH subtropics in summer. This appears to be at odds with the widely reported, and robust, poleward expansion of the subtropical dry zones by the end of the twenty-first century. This discrepancy between the shorter- and longer-term projections in SH summer, as shown here, rests in the recovery of the ozone hole in the coming decades, as a consequence of the Montreal Protocol. This is explicitly demonstrated by analyzing model experiments with the Whole Atmosphere Community Climate Model, version 4 (WACCM4), a high-top model with interactive stratospheric chemistry, and coupled to land, ocean, and sea ice components. Contrasting WACCM4 integrations of the representative concentration pathway 4.5 with and without trends in surface concentrations of ozone-depleting substances allows for demonstrating that stratospheric ozone recovery will largely offset the induced “wet gets wetter and dry gets drier” projections and the accompanying poleward expansion of the subtropical dry zone in the SH. The lack of near-term statistically significant zonal-mean changes in the SH hydrological cycle during summer is of obvious practical importance for many parts of the world, and it might also have implications for the Southern Ocean and the Antarctic continent.

The impact on nutrient cycles from tropical forest to pasture conversion in Costa Rica

Changes in nutrient and hydrological cycles caused by land disturbance typically lead to detrimental changes to ecosystems. This study utilized a paired, small-catchment approach to examine the effect of deforestation on soils and streams of the tropical Osa Peninsula, Costa Rica. The first catchment had been cleared for pasture and the second consisted of undisturbed tropical wet forest. Soil concentrations of organic matter, total and soil-available phosphorus (P) were higher in the forested catchment with reductions of >33 % of each in the deforested catchment. The effect of deforestation on stream discharge was a 59 % increase in flow during the wet season and a higher Q5:Q95 (percentile flow) ratio showing that the deforested stream yielded shorter duration, higher magnitude flood peaks. The deforested catchment loss of dissolved inorganic nitrogen increased 95 % over the forested catchment. Soluble reactive phosphorus showed a 43 % higher load in the deforested catchment compared to the forested catchment. The molar N:P ratios were low and both streams were well below the level at which N limitation of lotic algal growth has been reported. It, therefore, appears that N is the limiting nutrient in streams in the study area. Soil nutrient depletion in the deforested catchment, accelerated by a changed hydrological regime, is the likely trajectory of soil–water interactions in this tropical ecosystem. Loss of nutrients and organic matter from terrestrial ecosystems will likely lead to long-term impacts on lowland tropical communities. Should deforestation become widespread along this stretch of the Pacific coastline possible eutrophication of receiving transitional and coastal waters may occur.