Subtropical Basin Research Articles

Lagged response of Tropical Atlantic Ocean to cold and fresh water pulse from Antarctic sea ice melting.

The formation of dense water masses at polar regions has been largely influenced by climate changes arising from global warming. In this context, based on ensemble simulations with a coupled model we evaluate the meridional shift of a climate signal (i.e., a cold and fresh water input pulse generated from melting of positive Antarctic sea ice (ASI) extremes) towards the Tropical Atlantic Ocean (TAO). This oceanic signal propagated from Southern Ocean towards the equator through the upper layers due to an increase in its buoyance. Its northward shift has given by the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) flows, that inject cold and fresh mode/intermediate waters from into subtropical basin. The signal has reached low latitudes through the equatorial upwelling and spreads out southwards, through the upper branch of southern subtropical gyre. We concluded that 10 years of coupled simulations was enough time to propagate the climate signal generated by ASI positive extremes melting, which reached TOA around 2 year later. The oceanic connection between Southern Ocean and TAO is indeed established within the timescale analyzed in the study (10 years). Nonetheless, the period needed to completely dissipate the disturbance generated from ASI seems to be longer.

Flash Drought Characteristics by Different Severities in Humid Subtropical Basins: A Case Study in the Gan River Basin, China

AbstractIt is essential to assess flash drought risk based on a reliable flash drought intensity (severity) index incorporating comprehensive information of the rapid decline (“flash”) in soil moisture toward drought conditions and soil moisture thresholds belonging to the “drought” category. In this study, we used the Gan River basin as an example to define a flash drought intensity index that can be calculated for individual time steps (pentads) during a flash drought period over a given grid (or station). The severity of a complete flash drought event is the sum of the intensity values during the flash drought. We explored the spatial and temporal characteristics of flash droughts with different grades based on their respective severities. The results show that decreases in total cloud cover, precipitation, and relative humidity, as well as increases in 500-hPa geopotential height, convective inhibition, temperature, vapor pressure deficit, and wind speed can create favorable conditions for the occurrence of flash droughts. Although flash droughts are relatively frequent in the central and southern parts of the basin, the severity is relatively high in the northern part of the basin due to longer duration. Flash drought severity shows a slightly downward trend due to decreases in frequency, duration, and intensity from 1961 to 2018. Extreme and exceptional flash droughts decrease significantly while moderate and severe flash droughts trend slightly upward. Flash drought severity appears to be more affected by the interaction between duration and intensity as the grade increases from mild to severe. The frequency and duration of flash droughts are higher in July–October. The southern part of the basin is more prone to moderate and severe flash droughts, while the northern parts of the basin are more vulnerable to extreme and exceptional flash droughts due to longer durations and greater severities than other parts. Moderate, severe, extreme, and exceptional flash droughts occurred at approximately 3–6-, 5–15-, 10–50-, and 30–200-yr intervals, respectively, based on the copula analysis.

Variability of North Atlantic CO<sub>2</sub> fluxes for the 2000–2017 period estimated from atmospheric inverse analyses

Abstract. We present new estimates of the regional North Atlantic (15–80∘ N) CO2 flux for the 2000–2017 period using atmospheric CO2 measurements from the NOAA long-term surface site network in combination with an atmospheric carbon cycle data assimilation system (GEOS-Chem–LETKF, Local Ensemble Transform Kalman Filter). We assess the sensitivity of flux estimates to alternative ocean CO2 prior flux distributions and to the specification of uncertainties associated with ocean fluxes. We present a new scheme to characterize uncertainty in ocean prior fluxes, derived from a set of eight surface pCO2-based ocean flux products, and which reflects uncertainties associated with measurement density and pCO2-interpolation methods. This scheme provides improved model performance in comparison to fixed prior uncertainty schemes, based on metrics of model–observation differences at the network of surface sites. Long-term average posterior flux estimates for the 2000–2017 period from our GEOS-Chem–LETKF analyses are −0.255 ± 0.037 PgC yr−1 for the subtropical basin (15–50∘ N) and −0.203 ± 0.037 PgC yr−1 for the subpolar region (50–80∘ N, eastern boundary at 20∘ E). Our basin-scale estimates of interannual variability (IAV) are 0.036 ± 0.006 and 0.034 ± 0.009 PgC yr−1 for subtropical and subpolar regions, respectively. We find statistically significant trends in carbon uptake for the subtropical and subpolar North Atlantic of −0.064 ± 0.007 and −0.063 ± 0.008 PgC yr−1 decade−1; these trends are of comparable magnitude to estimates from surface ocean pCO2-based flux products, but they are larger, by a factor of 3–4, than trends estimated from global ocean biogeochemistry models.

Uncertainty assessment of drought characteristics projections in humid subtropical basins in China based on multiple CMIP5 models and different index definitions

This study presents an assessment of projection and uncertainty of drought characteristics (frequency DF, drought area Da) using three drought indices (Palmer Drought Severity Index, PDSI; Standardized Precipitation Index, SPI; Standardized Precipitation Evapotranspiration Index, SPEI) in the humid subtropical Pearl River basin in southern China during the period 2021–2050. The projection is based on 13 CMIP5 general circulation models (GCMs) under three Representative Concentration Pathway scenarios (RCP2.6, RCP4.5 and RCP8.5). Specifically, the SPI is derived by the precipitation simulations of 13 GCMs, whereas the PDSI and SPEI are computed based on the simulations from the Variable Infiltration Capacity (VIC) model forced by 13 GCMs. The uncertainty of projected drought indices (PDSI, SPI and SPEI) due to various GCMs and RCPs is quantified by the variance-based sensitivity analysis approach. The results indicate that the sign and magnitude of the projected changes in DF and Da are highly dependent on the index definition at the regional scale, and the SPI tends to underestimate the projected changes in DF compared with PDSI and SPEI. There is a large model spread in the projected DF changes (especially for SPEI) under all RCP scenarios, with larger model spread for more extreme drought events. Uncertainty analysis shows that GCM contributes more than 90% of total uncertainty in drought indices projections, while the RCP uncertainty is rather limited (< 10%) compared with GCM. The GCM uncertainty is spatially unevenly distributed and shows large variability at the interannual scale. This study highlights the sensitivity of drought projections to the index definition as well as the large spatial–temporal variability of general sources of uncertainty in drought projections.

Hydrochemical Characteristics and Controlling Factors of Typical Dolomite Karst Basin in Humid Subtropical Zone

The hydrochemical characteristics and variation rules of water in karst areas are important for the basic research of karst field. Because only a few researches focused on the hydrochemical characteristics of humid subtropical dolomite karst basins in the past, the Shanmuhe Basin and its subbasin Huangzhouhe Basin in Shibing Dolomite Karst World Natural Heritage Site in China were selected as the study area. A total of 215 water samples were collected and analyzed. It was found that the hydrochemical type of subtropical dolomite karst basin is HCO3-Ca·Mg type, which is different from that in limestone areas and shale districts, which is HCO3-Ca type for both; EC, Ca2+, Mg2+, HCO3-, Mg2+/Ca2+, and Mg2+/HCO3- have similar spatial variations, which are lower in allogenic water in the upstream in the northwest of the basin and become higher when the water enters the dolomite karst area in the mid and downstream of the basin, and SIC and SID of the allogenic water in the upstream are below zero, while those become saturated after entering the dolomite area in the mid and downstream; the monthly variation of hydrochemistry shows that the seasonal variation of ion concentration is not obvious, and there is a dilution effect in rainy seasons, but the variation of ion concentration is small and relatively stable; higher HCO3- concentration and Mg2+/Ca2+ in the pure dolomite basin are higher than those in the impure dolomite basin; the main influencing factors of the hydrochemical features are geological background especially the lithology, and the water chemistry is mainly controlled by rock weathering and water-rock interaction; rainfall import is low and the low concentrations of K+, Na+, NO3-, Cl-, and SO42- in water indicate that human activities have little effect on the hydrochemistry in the study area.

The unique mean seasonal cycle in the Indian Ocean anchors its various air-sea coupled modes across the basin

The interannual variability of the sea surface temperature (SST) in the Indian Ocean is complex and characterized by various air-sea coupled modes, which occur around El Niño/La Niña's peak phase (i.e. December–January–February, DJF). Indian Ocean Dipole Mode (IODM) develops over the tropical Indian Ocean and peaks in September–October–November (SON), while Ningaloo Niño, Subtropical Indian Ocean Dipole (SIOD) and Indian Ocean Basin Mode (IOBM) occur respectively over northwest off Australia, subtropical and tropical Indian Ocean, during boreal winter to spring. The apparent contrast between their divergent regionality and convergent seasonality around DJF triggers the present study to examine the interaction between the local mean monsoonal cycle and the anomalous forcing from El Niño/La Niña. The diagnosis confirms that the Indian Ocean’s unique complexity, including the monsoonal circulation over the tropics and the trade wind over the subtropical southern Indian Ocean, plays the fundamental role in anchoring the various regional air-sea coupled modes across the basin. The SST anomalies can be readily explained by the wind-evaporation-SST (WES) mechanism, which works together with other more regional-dependent dynamic and thermodynamic mechanisms. This implies that El Niño/La Niña brings much predictability for the Indian Ocean variations.

Microcystin ELISA in water and animal serum for an integrated environmental monitoring strategy

ABSTRACT Free-range livestock can be exposed to toxic cyanobacteria by drinking water from eutrophic natural sources. This study reports on the analysis of livestock drinking water sources from ranches in an eutrophic subtropical basin of South America (33ºS, 57°W), where frequent Microcystis spp. blooms and occasionally dense scums were found. To minimise monitoring costs, microcystins (MCs) were determined in serum and water with an in-house developed enzyme linked immunosorbent assay (ELISA). The water results were compared with those of liquid chromatography – tandem mass spectrometry (LC-MS/MS) for the most common congeners (MC-LR, MC-RR; MC-YR, MC-LA). Both methods were strongly correlated along a broad range of concentrations. The concentrations of MCs in the livestock drinking water were up to 3,700 µg/L. This is about 1,000 times higher than the Australian and New Zealand livestock MCs guidance values (cattle = 4.2; sheep = 3.9 µg/L) as well as the California-EPA sub-chronic action level (3 µg/L) and implies a high risk of poisoning. The animals exposed to MCs concentrations 70–100 times higher than these guidelines showed no significant alterations in liver enzymes. Nevertheless, serum concentrations of up to 0.11 µg/L MCs were recorded by ELISA. These results reveal the potential of MCs-ELISA in serum as a sensitive, minimally invasive method for risk assessment in integrated environmental monitoring strategies.

Linking forest transition, plant invasion and forest succession theories: socioeconomic drivers and composition of new subtropical andean forests

The patterns and causes of forest transition have been extensively studied, identifying socio-economic drivers of land use deintensification and the associated forest expansion. However, most studies do not take into account the origin of the dominant species of new forests (i.e. native or exotic), which affects biodiversity and the provision of ecosystem services. We develop a framework integrating forest transition, forest succession and biological invasion theories to identify the socio-environmental conditions that facilitate different pathways of spontaneous forest transitions in a subtropical mountain basin. We used Landsat images and Random forest classifier to detect land cover changes over 30 years (1988–2017). We used generalized additive models to identify socioeconomic and biophysical variables associated with expansion of native and exotic-dominated forests. Expanding native forests are scattered throughout the whole basin under a broad spectrum of socioeconomic and environmental conditions. In contrast, the new forests dominated by exotic species were aggregated around their focal introduction areas and their expansion was associated with accessibility and specific land uses (livestock or residential use). Understanding the pathways of alternative forest transition involves the integration of land science, forest succession theory and invasion ecology. Land science explains the availability of sites to be reforested. The species composition of new forests depends on the availability of propagules, dispersal agents and competitive relationships between species (forest succession theory). Invasion ecology explains the role of introduction areas (which are often associated with residential use) of exotic species in the successional process.

Air-Sea interaction over the Gulf Stream in an ensemble of HighResMIP present climate simulations

A dominant paradigm for mid-latitude air-sea interaction identifies the synoptic-scale atmospheric “noise” as the main driver for the observed ocean surface variability. While this conceptual model successfully holds over most of the mid-latitude ocean surface, its soundness over frontal zones (including western boundary currents; WBC) characterized by intense mesoscale activity, has been questioned in a number of studies suggesting a driving role for the small scale ocean dynamics (mesoscale oceanic eddies) in the modulation of air-sea interaction. In this context, climate models provide a powerful experimental device to inspect the emerging scale-dependent nature of mid-latitude air-sea interaction. This study assesses the impact of model resolution on the representation of air-sea interaction over the Gulf Stream region, in a multi-model ensemble of present-climate simulations performed using a common experimental design. Lead-lag correlation and covariance patterns between sea surface temperature (SST) and turbulent heat flux (THF) are diagnosed to identify the leading regimes of air-sea interaction in a region encompassing both the Gulf Stream system and the North Atlantic subtropical basin. Based on these statistical metrics it is found that coupled models based on “laminar” (eddy-parameterised) and eddy-permitting oceans are able to discriminate between an ocean-driven regime, dominating the region controlled by the Gulf Stream dynamics, and an atmosphere-driven regime, typical of the open ocean regions. However, the increase of model resolution leads to a better representation of SST and THF cross-covariance patterns and functional forms, and the major improvements can be largely ascribed to a refinement of the oceanic model component.

Effects of spatial distance, physical barriers, and habitat on a stream fish metacommunity

Metacommunity structure depends on environmental and spatial factors. Stream fishes are constrained to disperse within dendritic networks and waterfalls and other barriers add resistance to species dispersal. We evaluated the importance of environmental and spatial effects on fish metacommunity composition, adopting different models of spatial distance: overland, watercourse, resistance (physical barriers), cost distance (watercourse resistance), and asymmetrical dispersal (considering flow direction). Fishes were sampled in 31 riffle sites of a small subtropical basin of southern Brazil (~ 250 km2 effective study area). Using redundancy analysis (RDA) and variance partitioning, we found the best model was the one including flow direction weighted by watercourse distance plus environmental variables (overall adj-R2 = 0.42). However, the explanation of fish composition was greatly improved by weighting spatial distance by stream channel sinuosity or height of barriers. Reach slope and width were the two significant environmental variables. Our study supports that dispersal, in addition to environmental factors, affects stream fish metacommunity even in a watershed of small spatial extent. Detecting spatial effects in a small watershed depends on considering realistic watershed features that affect fish dispersal in drainage networks such as waterfalls and other physical barriers, and distance along the watercourse and flow direction.

Coupled carbon and silicon cycling and characteristics in a subtropical watershed, China

Abstract Silicon (Si) and carbon (C) can couple at different timescales. Phytolith carbon fixation and silicate weathering are two key processes of this type of coupling. Silicate rock weathering sequesters CO2 and thus generates a net C sink, which is one way that C is removed from ecosystems. This study selected the Xiangxi River watershed as the research object, which is characteristic of typical watersheds found in subtropical regions, and continuously monitored for Si and C transfers in this basin. We were able to ascertain the Si and C coupled cycling relationship in this basin by analyzing the influence of environmental changes on these two elements. The main findings were as follows: (1) The atmospheric chemical CO2 sequestration rate in this subtropical basin was 14.2 kg hm−2 yr−1, wherein carbonates sequestered atmospheric CO2 at a rate of 5.4 kg hm−2 yr−1 and silicate rock sequestered atmospheric CO2 at a rate of 8.8 kg hm−2 yr−1. (2) Both rainfall and vegetation type had significant effects on Si and C migration. Different characteristics of canopy interception of each tree species had obvious dissociation effects on dissolved organic carbon (DOC), while the absorption or dissociation effect on dissolved silicate (DSi) differed among the various tree species. Basin soil was enriched with a large amount of organic C during the dry season, which was subsequently exported during the rainy season. (3) The Si input into the watershed was less than the Si output (the net accumulation of Si was −27 kg hm−2 yr−1), and the C input was greater than the C output (the net accumulation of C was + 2785 kg hm−2 yr−1). This study determined that the Xiangxi River watershed was an Si “source” and a C “sink”. The results of this study can provide a reference for further study of the coupling relationship between Si and C, and provide support for the study of ecosystem material balance.

Mixing of passive tracers at the ocean surface and its implications for plastic transport modelling

The tracking of virtual particles is one of the main numerical tools to understand the global dispersion of marine plastic debris and has been successful in explaining the global-scale accumulation patterns of surface microplastic, often called ‘garbage patches’. Yet, the inherent inaccuracies in plastic input scenarios and ocean circulation model results produce uncertainties in particle trajectories, which amplify due to the chaotic property of the surface ocean flow. Within this chaotic system, the subtropical ‘garbage patches’ correspond to the attractor. These facts make the large scale surface ocean circulation a mixing dynamical system, which means that the information of a particle’s initial location is lost over time. We use mixing entropy and Markov chain mixing of the transfer operator associated with surface ocean transport to quantify the time scales of mixing for the global surface ocean in each subtropical basin. In the largest parts of all basins we find mixing times in the order of or below 10 years, which is lower than typical simulation times for surface plastic transport simulations. Maximum mixing times of more than 10 years are found in some parts of the North and South Pacific. Our results have important implications for global dispersion modelling of floating materials on the basin scale: precise initial information has little relevance for long term simulations, and there is a temporal limit after which the backtracking of particles is not meaningful any more.

Using the compound system to synthetically evaluate the enrichment of heavy metal(loid)s in a subtropical basin, China

A compound system involving three matrices (water, sediment, and paddy soil) was conceived to determine the potential sources of metal(loid)s (Ti, Fe, Co, Ni, Cu, Zn, As, Cd, Pb, and U) and synthetically evaluate their pollution levels in the Le’an River basin. The result indicated that the established background values (BVs) of paddy soil and sediment in the compound system were obviously higher than those of the upper continental crust (UCC) and soils from Jiangxi Province, a difference which was especially marked for sediment. The concentrations of Cu, Zn, As, Cd in the system had high coefficients of variation (CVs), and metal(loid)s in sediment showed higher levels than those in paddy soil, except for Pb. Cd and Cu in the system had the highest Ef levels, which probably pose a high risk to organisms and the health of local residents. There were significantly linear relationships between the site rank index (SRI) for water and that for sediment or paddy soil, revealing that matrices in the system interacted with each other. Principal component analysis (PCA) and absolute principal component scores and multiple linear regression model (APCS-MLR) results demonstrated that Cu, Zn, As, Cu, Pb, and U enrichments in the system were mainly affected by mining activities and were predominately deposited in sediment. Point pollution sources rather than non-point pollution sources such as mining activities, contributed most of the anthropogenic metal(loid)s to sediment. Both SRI and Hierarchical cluster analysis (HCA) results visually showed that S5, S8, S9, S10, S11, and S12 severe pollution grouped together and scattered through areas with extensive mining activities, while other sites with moderate pollution were spread along the main stream of the Le’an River.

Contributions of atmospheric and oceanic feedbacks to subtropical northeastern sea surface temperature variability

Previous studies show that the dominant mode of variability in the Northeastern subtropical Pacific and Atlantic are analogous. Most attention has been given to the wind-evaporation-sea surface temperature (WES) feedback, but more recent studies suggest that clouds and ocean play a role. Here, it is shown that, while the mode of variability is similar, the quantitative role of clouds and ocean are different. Using Community Earth System Model, version 1.2, cloud feedbacks and interactive ocean dynamics are disabled separately to diagnose the relative contributions of each to sea surface temperature (SST) variability in subtropical northeastern ocean basins. Results from four experiments show that the relative contributions from WES and cloud radiative feedback depend on the role of the ocean. Positive cloud radiative feedback is evident in both basins but has less impact on SST variance in the Atlantic than in the Pacific. The reason for this is that ocean processes strongly damp SST anomalies in the Pacific and weakly enhance SST anomalies in the Atlantic. When cloud feedbacks are disabled, ocean processes become a larger driver of SST variability in the Atlantic. In line with previous studies, the Northeast Pacific SST variability may be understood as a white-noise-forced linear stochastic system with positive feedback from cloud and damping by latent heat flux and ocean processes, while Atlantic SST is driven partially by variations in ocean circulation and requires vertical mixing for rendition. Between these two regions, different ocean dynamics lead to different roles for atmospheric feedbacks but still result in similar patterns of SST variability.

Isotopic evidence for seasonal and long-term C and N cycling in a subtropical basin of southern China

Knowledge about the origin and transformation of nutrients at different temporal scales in river ecosystems can provide a better understanding of carbon (C) and nitrogen (N) cycles in rivers and bear important implications for regional and global C and N cycling. In this study, stable isotopes of C and N (δ13C and δ15N) and a radioactive isotope of 210Pb were used to investigate the factors affecting seasonal and long-term changes in N source apportionment and C and N biogeochemical processes in the Beijiang River located in the Pearl River basin, China. Source apportionment by a Bayesian model (stable isotopic analysis in R, SIAR) showed significant seasonal variations. During the flood season, the dominant origins were non-point sources, such as soil N for dissolved nitrogen (DN, accounting for 38%), soil organic matter for particulate organic matter (POM, 58%) and sedimentary organic matter (SOM, 31%), due to intense precipitation. During the non-flood season, fertilizer became the dominant nitrate source (48%), and effluent detritus prevailed in the POM (47%) and SOM (32%) pools. N transformation between the DN, POM and SOM was influenced by seasonally variable hydrology. Our data suggested that low discharge was more favorable for the vertical mixing of the water column and sedimentation, which was supported by similar δ15N values in the DN, POM and SOM in the non-flood season. A sedimentary history record of 65 years (1951–2015) showed that the variation of N in the sediment was mainly affected by human activities. From 1999 to 2005, a decreasing trend in the δ15N values was observed due to the promotion of aquatic plant cultivation after reservoir impoundment. From 2005 to 2011, the enrichment in the δ15N values was caused by increases in manure and sewage due to the development of the tourism industry. From 2011 to 2015, the increase in pollution treatment funds from the government improved the water environment, resulting in δ15N depletion in the sediment. At the 65-year temporal scale, increased temperature and CO2 concentration had less impact on the C and N cycles in river ecosystems, however, high discharge could increase the C and N contents, and flood events might increase the δ15N values in the sediment. This study provided more detailed information regarding the nutrient sources and improved our understanding of the C and N cycling processes at different temporal scales in river ecosystems.

Local and Downstream Relationships between Labrador Sea Water Volume and North Atlantic Meridional Overturning Circulation Variability

Abstract While it has generally been understood that the production of Labrador Sea Water (LSW) impacts the Atlantic meridional overturning circulation (MOC), this relationship has not been explored extensively or validated against observations. To explore this relationship, a suite of global ocean–sea ice models forced by the same interannually varying atmospheric dataset, varying in resolution from non-eddy-permitting to eddy-permitting (1°–1/4°), is analyzed to investigate the local and downstream relationships between LSW formation and the MOC on interannual to decadal time scales. While all models display a strong relationship between changes in the LSW volume and the MOC in the Labrador Sea, this relationship degrades considerably downstream of the Labrador Sea. In particular, there is no consistent pattern among the models in the North Atlantic subtropical basin over interannual to decadal time scales. Furthermore, the strong response of the MOC in the Labrador Sea to LSW volume changes in that basin may be biased by the overproduction of LSW in many models compared to observations. This analysis shows that changes in LSW volume in the Labrador Sea cannot be clearly and consistently linked to a coherent MOC response across latitudes over interannual to decadal time scales in ocean hindcast simulations of the last half century. Similarly, no coherent relationships are identified between the MOC and the Labrador Sea mixed layer depth or the density of newly formed LSW across latitudes or across models over interannual to decadal time scales.

Simulation of the projected climate change impacts on the river flow regimes under CMIP5 RCP scenarios in the westerlies dominated belt, northern Pakistan

Burgeoning hydropower and irrigation demands will wield substantial stresses on available freshwater resources in the Hindukush-Karakoram-Himalayan (HKH) Range. Probable changes in the river flow regimes as a consequence of changes in precipitation, temperature, and snowpack shall further exacerbate this critical situation. However, probable climate change impacts over the Hindukush range's river flow regimes are unexplored. Here, we assessed the potential impacts of projected climate changes on the outflows of a humid subtropical basin which is nested in the westerlies dominated part of the Hindukush Mountains. Downscaled precipitation and temperature projections from six different Global Climate Models (GCMs), under two Representative Concentration Pathways (RCPs 4.5 and 8.5), were forced in a calibrated Soil and Water Assessment Tool (SWAT) model to investigate the climate change impacts on the Swat River flows. Compared to the baseline period (1981–2010), outputs of all selected GCMs indicated a significant increase in the annual average temperature, ranging from 2.22 to 4.18 °C and 4.63 to 8.49 °C under RCP 4.5 and 8.5, respectively, by the end of the 21st century. Outputs of four GCMs suggested a concurrent relative increase in the annual average precipitation, ranging from 1.51–22.52% and 1.05–35.98% under RCP 4.5 and 8.5, respectively. However, two GCMs showed decrease in the precipitation, ranging from −9.74 to −2.99 and – 13.72 to −3.24 under RCP 4.5 and 8.5, respectively. Simulated results of the SWAT model showed a significant impact of projected climate change on the Swat River flows by the end of the century that includes (a) increase in the annual average flow under both RCPs (ranging from 0.3 to 44.4% compared to the annual average flow in the baseline period), (b) an increase in the low and medium flows, and (c) an advancement in the peak flow month (from July to June). We conclude that the projected climate changes could substantially influence the seasonality of the Swat River flows. Findings of this study imply that it is important to consider the climate-induced impacts on freshwater resources for sustainable regional planning and management of hydropower projects as well as for irrigation scheduling in the future.

Recent intensification of short‐term concurrent hot and dry extremes over the Pearl River basin, China

AbstractOngoing climate warming is largely affecting the frequency and magnitude of short‐term climatic extremes in many regions of the world, causing significant damage to the environment and society. Here, based on the long‐term (1980–2010) meteorological observations and reanalysis products, we investigated the spatio‐temporal evolutions of short‐term concurrent hot and dry extreme (SCHDE) events over the Pearl River basin, a typical humid and subtropical basin in southern China. We also explored the physical mechanisms of SCHDE and its relationship with a series of hydrometeorological variables and large‐scale climate indices. The results indicate an intensification of SCHDE over the study basin during the study period, especially for the period 1997–2010. SCHDE occurs mostly in the northeastern and western regions and shows a statistically significant increasing trend mainly in the midwestern and southeast regions, which is mainly associated with an increased temperature (Tair), drying trend in soil moisture (SM), and a sharp increasing trend in evapotranspiration (ET). The contribution of Tair variability to the SCHDE variability is largest (57.27%), followed by SM (28.43%) and ET (14.30%). In addition to Tair, SM deficit before the onset also plays an important role in the SCHDE occurrence. Furthermore, the SCHDE variability tends to be more sensitive to the SM deficits in the high‐altitude areas than in the low‐altitude areas. Besides, wavelet transform analysis results indicate that the cold phase of ENSO tends to trigger a higher risk of SCHDE events, while El Niño Modoki events have different influences on the variations of SCHDE during different warm/cold phases over the Pearl River basin.

Attribution Analysis for Runoff Change on Multiple Scales in a Humid Subtropical Basin Dominated by Forest, East China

Attributing runoff change to different drivers is vital in order to better understand how and why runoff varies, and to further support decision makers on water resources planning and management. Most previous works attributed runoff change in the arid or semi-arid areas to climate variability and human activity on an annual scale. However, attribution results may differ greatly according to different climatic zones, decades, temporal scales, and different contributors. This study aims to quantitatively attribute runoff change in a humid subtropical basin (the Qingliu River basin, East China) to climate variability, land-use change, and human activity on multiple scales over different periods by using the Soil and Water Assessment Tool (SWAT) model. The results show that runoff increased during 1960–2012 with an abrupt change occurring in 1984. Annual runoff in the post-change period (1985–2012) increased by 16.05% (38.05 mm) relative to the pre-change period (1960–1984), most of which occurred in the winter and early spring (March). On the annual scale, climate variability, human activity, and land-use change (mainly for forest cover decrease) contributed 95.36%, 4.64%, and 12.23% to runoff increase during 1985–2012, respectively. On the seasonal scale, human activity dominated runoff change (accounting for 72.11%) in the dry season during 1985–2012, while climate variability contributed the most to runoff change in the wet season. On the monthly scale, human activity was the dominant contributor to runoff variation in all of the months except for January, May, July, and August during 1985–2012. Impacts of climate variability and human activity on runoff during 2001–2012 both became stronger than those during 1985–2000, but counteracted each other. The findings should help understandings of runoff behavior in the Qingliu River and provide scientific support for local water resources management.

Microbialite-like structures in Cladophora sp. (Ulvophyceae) mats from a subtropical Andean basin: ecological implications

The solubility equilibrium of calcite is influenced by physicochemical, climatic and biological factors. Annual cycles of exceptionally prolonged drought, in conjunction with naturally occurring diffuse organic pollution, generate the unique conditions for the precipitation of lithified carbonate structures (microbialites). The aim of this article is to analyze the possible implications of calcite precipitation produced in mats of Cladophora sp. in an Andean subtropical basin, considering it is the first time this phenomenon is described for the region. We collected samples from selected sites at the Lules River Basin, in four sampling dates between the years 2003 and 2004, within a monitoring work of 15 years. Samples were analyzed using an electron microscope and X-ray diffraction analysis. We found that Gomphonema sp. attached to Cladophora sp. contributes to precipitation of calcite and formation of microbialite like structures, in the studied area. This work presents an initial discussion of the discovery of microbialites-like structures attached to Cladophora sp. mats in a subtropical Andean stream and the environmental conditions that lead to their production, as well as the possible ecological implications of these microbialites.