Seasonal Time Scales Research Articles

The influence of climate change on freshwater availability in the Sokoto Rima River Basin, Northwestern Nigeria.

This research entailed the acquisition of long-term (1980-2020) meteorological data (rainfall, temperature, and runoff) which was used to compute the values for the mean annual, monthly, and seasonal hydro-climate variables to establish their trend and behavior. To substantiate if the patterns of runoff are in any way due to the changing climate, the meteorological data were subjected to non-parametric tests-Mann-Kendall (MK), Sen's slope estimator, Kendall tau, and partial correlation. MK trend test for runoff, rainfall, and temperature (on an annual and seasonal basis) revealed significant trends on an annual scale. Nevertheless, rainfall data did not exhibit any, observable trend on a seasonal scale. On both annual and seasonal scales, temperature values showed increasing trends. In the entire period considered (1980-2020), runoff also exhibited a remarkable increase in trend. Sen's estimator values ranged from - 0.04 to 3.23. The results of the Kendall tau correlation showed that runoff is positively correlated to rainfall (tau = 0.222), with a high confidence level (95%). Again, the partial correlation analysis of runoff and climatic variables for annual and seasonal timescale, results show that the runoff is significantly (rxy.z = 0.315) affected by both rainfall and temperature. It is therefore recommended that future research should utilize robust input-output models to determine the amount of water stress in the basin. As a sustainable management approach, artificial surface water reservoirs should be constructed to complement the available water from the Sokoto-Rima river and alleviate the water stress experienced by the population.

Influence of Seasonal Air–Sea Interaction on the Interannual Variation of the NPP of Terrestrial Natural Vegetation in China

Based on Moderate Resolution Imaging Spectoradiometer (MODIS) remote sensing data, meteorological observation data, multisource atmospheric circulation, and sea surface temperature (SST) data from NCEP/NCAR reanalysis, we estimated the net primary productivity (NPP) of terrestrial natural vegetation in China according to the CASA model and analyzed the linear trend and interannual fluctuation of NPP, as well as the spatial distribution characteristics of the annual NPP response to climatic factors. The obtained results revealed the impact of air–sea interaction on interannual NPP variability in key climatic areas. In China, the annual NPP of natural vegetation, linear NPP trend, and interannual NPP fluctuation showed significant regional characteristics. The annual NPP exhibited a significant increasing trend and interannual fluctuation in North China and Northeast China, with spatially consistent responses from NPP to precipitation and temperature. On the seasonal time scale, NPP in the key climatic area (105~135° E, 35~55° N) exhibited a strong response to both summer precipitation and mean temperature. In the summer atmospheric circulation, the circulation anomaly area is mainly distributed in the northeast cold vortex area in the middle- and high-latitude westerlies in East Asia and in the Sea of Okhotsk with dipole circulation. In the SST of the preceding winter and spring, the key SST anomaly area was the Kuroshio region, with an impact of the Kuroshio SST anomaly on the interannual variation in annual NPP in the key climatic area. The cold vortex in Northeast China played a pivotal role in the influence of the SST anomaly in the Kuroshio region on atmospheric circulation anomalies, resulting in abnormal summer precipitation in the key climatic region and affecting the annual accumulation of NPP of natural vegetation.

Climatological Significance of δD‐δ18O Line Slopes From Precipitation, Snow Pits, and Ice Cores at Summit, Greenland

AbstractHydrogen (δD) and oxygen (δ18O) isotopic ratios are strongly correlated in precipitation over time and space, defining the meteoric water line, and the slope of this δD‐δ18O relationship reflects covariations of deuterium excess (d‐excess) with δD or δ18O. This δD‐δ18O line provides a tool for inferring hydrologic processes from the evaporation source to condensation site. Here, we present δD‐δ18O relationships on seasonal and annual timescales for daily precipitation, snow pits, and a 15‐m ice core (Owen) at Summit, Greenland. Seasonally, precipitation δD‐δ18O slopes are less than 8 (summer = 7.70; winter = 7.77), while the annual slope is greater than 8 (8.27). We suggest that intra‐season slopes result primarily from Rayleigh distillation, which, under prevailing conditions, produces slopes less than 8. The summer line has a greater intercept (higher d‐excess) than the winter line. This separation causes annual slopes to be greater than seasonal ones. We attribute high summer d‐excess primarily to contributions of vapor sublimated from the Greenland Ice Sheet and other terrestrial sources. High sublimated moisture proportions result in a large separation between seasonal δD‐δ18O lines, and thus high annual slopes. Inter‐seasonal weighting of precipitation amount also influences annual slopes because slopes are weighed by the number of storms each season. Using snow pit measurements, we demonstrate that precipitation isotopic signals translate to the snowpack. We generate indices to determine Sublimation Proportion Index and Precipitation Weighting Index, and find that annual Owen core δD‐δ18O line slopes are significantly related to these indices, demonstrating that these factors are recorded in ice cores.

Evaluation of the forecast skill of North American Multi‐Model Ensemble for monthly and seasonal precipitation forecasts over Iran

AbstractNorth American Multi‐Model Ensemble (NMME) precipitation forecast skill over Iran is evaluated using Taylor diagrams and ranked probability skill scores (RPSS) as determined over a 29‐year test period (1991–2019). The forecast skill for both monthly (October through June for lead‐times of 0.5–3.5 months) and seasonal (October–December [OND], January–March [JFM], and April–June [AMJ] for lead‐times of 1.5–3.5 months) timescales is evaluated using six NMME models as well as multi‐model ensemble means (MMM). The latest versions of these models for forecasting Iran's precipitation have not been evaluated thus far. The Global Precipitation Climatology Center (GPCC) version 2020 dataset is used to verify the models. Among individual NMME models, Geophysical Fluid Dynamics Laboratory‐Seamless System for Prediction and Earth System Research (GFDL‐SPEAR) has generally the highest forecast skill. Both Taylor diagrams and RPSS of most of the models have indicated that the highest forecast skill is found for the month of November such that the Pearson correlation for both SPEAR and MMM is statistically significant for all lead‐times. For both monthly and seasonal timescales, the temporal Pearson correlation (TPC) between the observed and forecasts of the MMM is higher than the TPC of the individual models. The spatial Pearson correlation (SPC) and normalized centred root mean square error (NCRMSE) of the SPEAR is close to MMM, but the normalized standard deviation (NSD) of the SPEAR is closer to one compared to the MMM for months from November to March and two seasons (OND and JFM seasons). The MMM precipitation forecasts are underestimated over the northern regions and Zagros mountains for JFM and OND seasons for both 1.5‐ and 2.5‐month lead‐times. The degree to which the forecast skill of MMM is dependent on the El Niño–Southern Oscillation (ENSO) connections with precipitation over Iran is examined. Significant Spearman correlations between simultaneous observed Niño3.4 index and Iran precipitation are found for OND, but not for JFM and AMJ. The MMM reproduces the observed ENSO teleconnections to the tropical Pacific in OND, consistent with forecast skill in that season. However, the MMM also produces forecast skill in JFM and AMJ when the ENSO influence is marginal, showing that ENSO is not the only source of skill in the models.

Effects of Arctic ozone on the stratospheric spring onset and its surface impact

Abstract. Ozone in the Arctic stratosphere is subject to large interannual variability, driven by both chemical ozone depletion and dynamical variability. Anomalies in Arctic stratospheric ozone become particularly important in spring, when returning sunlight allows them to alter stratospheric temperatures via shortwave heating, thus modifying atmospheric dynamics. At the same time, the stratospheric circulation undergoes a transition in spring with the final stratospheric warming (FSW), which marks the end of winter. A causal link between stratospheric ozone anomalies and FSWs is plausible and might increase the predictability of stratospheric and tropospheric responses on sub-seasonal to seasonal timescales. However, it remains to be fully understood how ozone influences the timing and evolution of the springtime vortex breakdown. Here, we contrast results from chemistry climate models with and without interactive ozone chemistry to quantify the impact of ozone anomalies on the timing of the FSW and its effects on surface climate. We find that ozone feedbacks increase the variability in the timing of the FSW, especially in the lower stratosphere. In ozone-deficient springs, a persistent strong polar vortex and a delayed FSW in the lower stratosphere are partly due to the lack of heating by ozone in that region. High-ozone anomalies, on the other hand, result in additional shortwave heating in the lower stratosphere, where the FSW therefore occurs earlier. We further show that FSWs in high-ozone springs are predominantly followed by a negative phase of the Arctic Oscillation (AO) with positive sea level pressure anomalies over the Arctic and cold anomalies over Eurasia and Europe. These conditions are to a significant extent (at least 50 %) driven by ozone. In contrast, FSWs in low-ozone springs are not associated with a discernible surface climate response. These results highlight the importance of ozone–circulation coupling in the climate system and the potential value of interactive ozone chemistry for sub-seasonal to seasonal predictability.

Relative Contributions of Open‐Ocean Forcing and Local Wind to Sea Level Variability Along the West Coasts of Ocean Basins

AbstractThis study aims to reconstruct sea level at the coast by quantifying relative contributions of local wind forcing and the penetrated open‐ocean signals, and thus to build a dynamical connection that seamlessly links the ocean basin to the coast. A linear, modified arrested topographic wave (ATW) model is used to examine the transmission of interior sea level across the continental shelf and the role of alongshore wind in determining the coastal sea level. The local wind stress substantially influences the coastal sea level but does not affect the transmission of open‐ocean signals in the linear framework. Increased bottom friction or gentler slope of shelf favors the penetration of open‐ocean signals. This modified ATW model is then applied to the shelf seas of North Pacific and North Atlantic, with model parameters being set according to the realistic features of regional oceanography. The offshore boundary condition is determined by the integrated barotropic and baroclinic ocean responses in the ocean basin and the transmission processes across western boundary currents. Based on the proposed “basin‐to‐coast” dynamical linkage framework, one can estimate the relative contributions of open‐ocean forcing and local wind to the total coastal sea level. According to these partitions, the reconstructed coastal sea levels along the west coasts of North Pacific and North Atlantic on seasonal time scale are in close agreement with tide‐gauge observations. The idea and specific calculations of coastal sea level reconstruction could be readily applied to other coasts of the global ocean.

The influence of increasing sea surface temperature on particulate organic carbon in Indonesian waters based on MODIS Aqua satellite

Both Sea Surface Temperature (SST) and Particulate Organic Carbon (POC) have important roles in the marine ecosystem and carbon cycle. Therefore, we analyze the influence of SST on POC in Indonesian water. We used SST and POC products from the moderate-resolution imaging spectroradiometer (MODIS) Aqua satellite from 2003 to 2021 (nineteen years). Regression and a temporal anomaly correlation approach were applied to assess the effect of SST on POC in Indonesian waters. Our results show that the southeast Indonesian waters, south of Java Island and around the Riau Islands, mostly have a high SST variation. On the other hand, the high variation of POC mostly appeared in Indonesian coastal. During the last nineteen years, the increasing SST anomalies (SSTA) in Indonesian waters have been followed by decreasing POC anomalies (POCA). These results indicate that rising SST reduces the ocean’s ability to absorb carbon in Indonesian waters. A strong negative relation between SSTA on POCA was found from April to August and October to January in a seasonal timescale. In the interannual timescale, our result reveals that the strong negative relationship between SSTA and POCA is likely linked to the El Niño Southern Oscillation, the Indian Ocean Dipole, and global warming.

Impacts of accelerating deployment of offshore windfarms on near-surface climate

The European Union has set the ambitious goal of becoming climate neutral by 2050, which has stimulated renewable energy production and accelerated the deployment of offshore wind energy in the North Sea. Here, a high-resolution regional climate model was used to investigate the impact on the sea surface climate of large-scale offshore wind farms that are proposed for the North Sea. The results show a significant reduction in the air-sea heat fluxes and a local, annual mean net cooling of the lower atmosphere in the wind farm areas down to more than 2.0 Wm−2, due to a decrease in 10 m wind speed and turbulent kinetic energy and an increase in low-level clouds. Mean surface winds decreased by approximately 1 ms−1 downstream of wind farms. Furthermore, an increase of approximately 5% in mean precipitation was found over the wind farm areas. At a seasonal timescale, these differences are higher during winter and autumn than in other seasons. Although the offshore wind farms reduce the heat transport from the ocean to the atmosphere in the region of large wind farms, the atmospheric layers below the hub height show an increase in temperature, which is on the order of up to 10% of the climate change signal at the end of the century, but it is much smaller than the interannual climate variability. In contrast, wind speed changes are larger than projected mean wind speed changes due to climate change. Our results suggest that the impacts of large clustered offshore wind farms should be considered in climate change impact studies. Moreover, the identified offshore windfarm impacts on the sea surface climate and the introduced spatial pattern in atmospheric conditions, in particular the modeled wind speed changes, suggest potential impacts on local ocean dynamics and the structure of the marine ecosystem. This should be considered in future scenarios for the North Sea marine environment and taken into account as a structuring influence in the offshore environment.

Wavelet analysis of hydro-climatic time-series and vegetation trends of the Upper Aragón catchment (Central Spanish Pyrenees)

Water managers and researchers noted with concern a nearly generalized decline in Mediterranean rivers discharge over the last decades. Changes in climatic forces (precipitation and air temperature) and land use and land cover (LULC) changes characterized by re-vegetation and greenness are the two most possible explanations for this discharge decline. The direct impact on river discharge stemming from these changes is difficult to assess and their role is generally studied separately. Here, we use the method of wavelet transformation to interpret the time-scale dependency of catchment discharge concerning the uneven temporal climatic fluctuations and re-vegetation processes. We analyzed the temporal variation of concurrent air temperature, precipitation and river discharge time-series for the Upper Aragón catchment, located in the Central Spanish Pyrenees. A long-term database collected over 60 years (1956–2020) was used. Land cover maps corresponding to different decades were used and the results indicated that the catchment experienced a significant increase in the area covered by mixed and broadleaf forests, mostly as a consequence of land abandonment. We show how temperature slightly increased and precipitation moderately decreased. However, catchment discharge experienced a sharp decline in its magnitude and also changes in its temporal variability dynamics. The relevance of the seasonal time-scales with regard to the available discharge is reduced, which strengthens the importance of the inter-annual time-scales for the catchment discharge dynamics. Furthermore, the catchment storage-discharge cycle at inter-annual time-scales is also reduced. Such changes can mostly be attributed to the changes in plant coverage, with an increasing weight in shaping hydrological processes at catchment scale due to the greenness effect. As such, we conclude that LULC changes have played a dominant role on the river discharge dynamics. Climatic trends, on the contrary, have been small, and they have played a secondary role in the decline of river discharge. Future research can use these observations to constrain the pace of upcoming water demands based on the available water resources at Mediterranean catchment scale.

Global Marine Heatwaves and Cold‐Spells in Present Climate to Future Projections

AbstractMarine heatwave (MHW) characteristics, such as total days, average duration, and mean intensity, have increased in most global oceans. In contrast, marine cold‐spell (MCS) characteristics have a decreasing trend over most regions. However, the difference between MHW and MCS in the mean and trend in the present and future climate is unclear. Here, we provide a comparative global assessment of these changes based on satellite sea surface temperature for 1982–2020 and climate model projections for 2021–2070. Globally, there are clear differences in the mean and trend between MHW and MCS metrics. On seasonal timescales, MHWs mainly occur in summer and autumn, while MCSs generally occur in winter and spring. The occurrence area of MHWs shows an increasing trend during 1982–2020, while the occurrence area of MCSs shows an opposite trend. Furthermore, the increasing trend in MHW average duration led to a near‐permanent MHW status in the future projections. Upper‐ocean warming due to anthropogenic forcing is closely related to increase in MHWs and decrease in MCSs. In addition, the shallower ocean mixed layer depth in the future is favorable for the occurrence of MHWs. In contrast, the projected weakening of the Atlantic meridional overturning circulation facilitates the occurrence of MCSs in the subpolar North Atlantic. These results provide a global baseline for the difference and relationship between MHW and MCS under present climate and future projections.

The roles of atmospheric circulation and sea surface temperature in UK surface climate

AbstractAtmospheric circulation has been identified as a key driver of recent extreme seasons in the UK, while local sea surface temperature (SST) also influences UK surface climate. Here, we investigate the roles of atmospheric circulation and SST in driving UK climate, using atmospheric model simulations with constrained global atmospheric circulation and selected SST boundary conditions. These simulations successfully reproduce observed UK‐mean temperature and precipitation. For UK temperature, circulation has a larger effect than SST for all seasons and timescales. SST influence peaks in summer on seasonal timescales. For precipitation, SST influence is weaker and is modulated by circulation. Nevertheless, SST effects provide skilful forecasts of spring and autumn UK temperature, and also contribute to UK temperature extremes in winter and summer.

Subinertial frequency variations in the axis of the Tsugaru Warm Current east of the Tsugaru Strait

The eastern part of the Tsugaru Strait (located at ~ 41.5° N) is a remarkable region of the coastal flow system around Japan due to the Tsugaru Warm Current (TgWC), an intensive eastward current that flows over its complex topography and exhibits marked tidal variation. The TgWC has a jetlike structure and demonstrates drastic seasonal changes in its path and vertical current structure. Using a high-resolution dataset obtained from a high-frequency radar system (HFR) and a coastal data assimilation experiment (JCOPE-T DA), we focused on the frontal variation, which is shorter than the seasonal timescale, along the axis of the TgWC in relation to the meanders of the jet. We found variations in the axis with multiple timescales between 1 and 14 days, especially in summer and autumn. The ~ 14-day variation, a remarkable occurrence on the west side of the eastern Tsugaru Strait, was consistent with the tidal variation in the strait. The zonal scale of the axis meander estimated by the HFR matched a scale several times larger than the internal deformation radius, especially in summer and autumn. JCOPE-T DA outputs showed that the crest of such a meander at greater depths progresses in relation to that at the surface. The outputs also suggested instabilities, indicating potential and kinetic energy conversion from the mean to the eddy field. The propagation speed of the several-day variation in the axis estimated from the HFR data was consistent with that of the two-layer baroclinic instability. The several-day variation also showed relatively high coherence with wind variation from summer to autumn, during which time the seasonal wind over the strait switched from easterly to westerly. These variations with multiple timescales imply an effect on north–south water mass mixing in the eastern part of the strait, including rapid acidification in the strait and intensification of stratification.

Tropical cyclone impacts on seagrass-associated fishes in a temperate-subtropical estuary.

Major storms can alter coastal ecosystems in several direct and indirect ways including habitat destruction, stormwater-related water quality degradation, and organism mortality. From 2010–2020, ten tropical cyclones impacted coastal North Carolina, providing an opportunity to explore ecosystem responses across multiple storms. Using monthly trawl and contemporaneous seagrass surveys conducted in Back Sound, NC, we evaluated how cyclones may affect the nursery role of shallow-water biogenic habitats by examining seagrass-associated fish responses within a temperate-subtropical estuary. We employed a general before-after-control-impact approach using trawls conducted prior (before) and subsequent (after) to storm arrival and years either without (control) or with (impact) storms. We examined whether effects were apparent over short (within ~three weeks of impact) and seasonal (May-October) timescales, as well as if the magnitude of storm-related shifts varied as a function of storm intensity. Our findings suggest that the ability of these shallow-water habitats to support juvenile fishes was not dramatically altered by hurricanes. The resilience exhibited by fishes was likely underpinned by the relative persistence of the seagrass habitat, which appeared principally undamaged by storms based upon review of available–albeit limited seagrass surveys. Increasing cyclone intensity, however, was correlated with greater declines in catch and may potentially underlie the emigration and return rate of fish after cyclones. Whether estuarine fishes will continue to be resilient to acute storm impacts despite chronic environmental degradation and predicted increases major tropical cyclone frequency and intensity remains a pressing question.

Variations in Sap Flux Density of Three Urban Tree Species and Their Main Environmental Influencing Factors in Different Timescales in the Beijing Metropolitan Area

Despite the importance of landscape design and water-resources management for urban planning, urban-forest transpiration was seldom estimated in situ. Detailed data on different urban trees’ water resource use and the effect of climatic fluctuations on their transpiration behaviour in different timescales are limited. In this study, we used a thermal dissipation method to measure the sap flux density (Js) of three urban tree species (Pinus tabulaeformis Carrière, Cedrus deodara (Roxb.) G. Don, and Robinia pseudoacacia Linn.) from 1 May 2008 to 30 April 2016 in Beijing Teaching Botanical Garden. The effects of environmental factors on sap flux density (Js) in different timescales were also analyzed. The results showed that there were significant differences in the sap flux density of three trees species in daily, seasonal, and interannual timescales. The hourly, seasonal, and interannual mean sap flux density of Pinus tabulaeformis were higher than that of Cedrus deodara and Robinia pseudoacacia. The seasonal mean Js of Pinus tabulaeformis, Cedrus deodara, and Robinia pseudoacacia in summer were 18.67, 16.19, and 41.62 times that in winter over 2008–2015. The annual mean sap flux density of Pinus tabulaeformis was 1.25–1.72 and 1.26–1.82 times that Cedrus deodara and Robinia pseudoacacia over 2008–2015. The Js responses in three tree species to environmental factors varied differently from daily to interannual timescales. The pattern of day-to-day variation in Js of three urban tree species corresponded closely to air temperature (Ta), soil temperature (Ts), solar radiation (Rs), and vapor pressure deficit (VPD). The Jarvis–Stewart model based on Ta, Rs, and VPD was more suitable for the sap flux density simulation of Pinus tabulaeformis than Cedrus deodara and Robinia pseudoacacia. The main factor affecting the sap flux density of Pinus tabulaeformis and Cedrus deodara was Ta in seasonal timescales. However, the main factor affecting the sap flux density of Robinia pseudoacacia was Ts. The interannual variations in the Js of Pinus tabulaeformis and Robinia pseudoacacia were mainly influenced by wind speed (w) and soil water content (SWC), respectively. The selected environmental factors could not explain the variation in the sap flux density of Cedrus deodara in an interannual timescale. The findings of the present study could provide theoretical support for predicting the water consumption of plant transpiration under the background of climate change in the future.

Varying Responses of Vegetation Greenness to the Diurnal Warming across the Global.

The distribution of global warming has been varying both diurnally and seasonally. Little is known about the spatiotemporal variations in the relationships between vegetation greenness and day- and night-time warming during the last decades. We investigated the global inter- and intra-annual responses of vegetation greenness to the diurnal asymmetric warming during the period of 1982–2015, using the normalized different vegetation index (NDVI, a robust proxy for vegetation greenness) obtained from the NOAA/AVHRR NDVI GIMMS3g dataset and the monthly average daily maximum (Tmax) and minimum temperature (Tmin) obtained from the gridded Climate Research Unit, University of East Anglia. Several findings were obtained: (1) The strength of the relationship between vegetation greenness and the diurnal temperature varied on inter-annual and seasonal timescales, indicating generally weakening warming effects on the vegetation activity across the global. (2) The decline in vegetation response to Tmax occurred mainly in the mid-latitudes of the world and in the high latitudes of the northern hemisphere, whereas the decline in the vegetation response to Tmin primarily concentrated in low latitudes. The percentage of areas with a significantly negative trend in the partial correlation coefficient between vegetation greenness and diurnal temperature was greater than that of the areas showing the significant positive trend. (3) The trends in the correlation between vegetation greenness and diurnal warming showed a complex spatial pattern: the majority of the study areas had undergone a significant declining strength in the vegetation greenness response to Tmax in all seasons and to Tmin in seasons except autumn. These findings are expected to have important implications for studying the diurnal asymmetry warming and its effect on the terrestrial ecosystem.

Amplification of regional tides in response to sea level

The study investigates the temporal change in tides under the background of rapid sea level rise, for a highly vulnerable coastal region. It highlights the existence of non-astronomic tidal variability at seasonal and secular time-scales in the Ganga-Brahmaputra-Meghna (GBM) delta, along with increasing low and high tidal levels. The observed variability in semi-diurnal tides was found to be coherent to the mean sea level changes. M2 tide can be a proxy for sea level changes as they show a direct relationship. Steric changes were introduced in the barotropic ADCIRC (ADvanced CIRCulation) model and the results proved that change in water depth modulates the energetics of the tidal wave thereby causing amplification. A regionally varying tidal response was noticed, such that the regions associated with the GBM delta showed maximum amplification. The factors that can cause tidal amplification are region-dependent and coastal geomorphology plays an important role in it. Tidal prediction based on changing sea level was able to capture the observed seasonal modulation of tides and increasing trend in low and high tidal levels. Incorporating the effect of changing tides can improve the accuracy of tidal predictions and will help advance studies regarding regional sea level change, coastal flooding and tide-surge interaction.

Supraglacial lake expansion, intensified lake drainage frequency, and first observation of coupled lake drainage, during 1985–2020 at Ryder Glacier, Northern Greenland

Along the Greenland Ice Sheet margin, supraglacial lakes store and redistribute ice sheet surface run off, and comprise an important potential hydrological link between the ice surface and the base, with ramifications for subglacial drainage systems and ice flow. As a consequence of increasing global mean surface air temperatures, these lakes have been predicted to expand further inland and to affect larger areas of the ice sheet. However, as contemporary dynamics of such supraglacial lake expansion are not well studied, any assessment of their future implications remains afflicted with uncertainty. Here, recent changes in supraglacial lake distribution and expansion, and in their drainage behavior and frequency, are presented for Ryder Glacier, Northern Greenland, as concluded from a remote sensing based analysis. The 35-year time span covered in the analysis allows for the detection of trends in lake processes and ice velocity, which otherwise were found to exhibit large inter-annual variability. It also reveals the first occurrence of a coupled lake drainage event in 2002. By linking supraglacial lake expansion, drainage modes, and drainage frequency to the efficiency of the subglacial drainage system and ice flow on seasonal and decadal timescales, a contribution is made to better understand the complexity of coupled glacio-hydrological processes, and to help reduce uncertainties in predictions of future mass loss from the Greenland Ice Sheet.

Seasonal variability of CO2, CH4, and N2O content and fluxes in small agricultural reservoirs of the northern Great Plains

Inland waters are important global sources, and occasional sinks, of CO2, CH4, and N2O to the atmosphere, but relatively little is known about the contribution of GHGs of constructed waterbodies, particularly small sites in agricultural regions that receive large amounts of nutrients (carbon, nitrogen, phosphorus). Here, we quantify the magnitude and controls of diffusive CO2, CH4, and N2O fluxes from 20 agricultural reservoirs on seasonal and diel timescales. All gases exhibited consistent seasonal trends, with CO2 concentrations highest in spring and fall and lowest in mid-summer, CH4 highest in mid-summer, and N2O elevated in spring following ice-off. No discernible diel trends were observed for GHG content. Analyses of GHG covariance with potential regulatory factors were conducted using generalized additive models (GAMs) that revealed CO2 concentrations were affected primarily by factors related to benthic respiration, including dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), dissolved organic carbon (DOC), stratification strength, and water source (as δ18Owater). In contrast, variation in CH4 content was correlated positively with factors that favoured methanogenesis, and so varied inversely with DO, soluble reactive phosphorus (SRP), and conductivity (a proxy for sulfate content), and positively with DIN, DOC, and temperature. Finally, N2O concentrations were driven mainly by variation in reservoir mixing (as buoyancy frequency), and were correlated positively with DO, SRP, and DIN levels and negatively with pH and stratification strength. Estimates of mean CO2-eq flux during the open-water period ranged from 5,520 mmol m−2 year1 (using GAM-predictions) to 10,445 mmol m−2 year−1 (using interpolations of seasonal data) reflecting how extreme values were extrapolated, with true annual flux rates likely falling between these two estimates.

Significant Seasonal N2O Dynamics Revealed by Multi‐Year Observations in the Northern South China Sea

AbstractThe coastal ocean and marginal sea play a disproportionally important role in the release of nitrous oxide (N2O) into the atmosphere. The spatial and temporal distribution of N2O in these important source regions remains highly uncertain due to the scarcity of N2O measurements. Here we present a large data set of N2O concentrations and fluxes obtained from 10 cruises covering four seasons in the Northern South China Sea (NSCS). The study area is overall a net source of atmospheric N2O with an annual flux of 1.9 ± 1.2 × 108, 0.8 ± 0.5 × 108 and 1.2 ± 0.7 × 108 mol N2O yr−1 in the shelf, slope and basin regions, respectively. In terms of global warming potentials, the N2O emissions offset 27.8% of the CO2 sink on the shelf, and are equivalent to 3.5 and 0.2‐fold of the CO2 emission in the slope and basin of the NSCS. On the seasonal time scale, N2O flux was significantly higher in autumn and winter than in the warm seasons. The spatial variability was contrastingly less pronounced. The seasonality of N2O distribution in the shelf region was modulated by the riverine discharge, while intrusion of the Kuroshio Current exerted profound control on N2O distribution in the open waters of the NSCS. The variable relationships between N2O excess, apparent oxygen utilization, and nitrate in the shelf, NSCS basin and the Luzon Strait indicated a regional difference in N2O cycling pathways along with the impact of water mass mixing. Our study establishes a robust baseline to understand N2O distribution and flux in the NSCS.

Revisiting Michael Bonell's work on humid tropical rainforest catchments: Isotope tracers reveal seasonal shifts in catchment hydrology

It has been almost 50 years since the foundational work at the Babinda catchments in North Queensland kickstarted the field of tropical hydrology globally. To expand upon this work and build a more generalized hydrological understanding of steep rainforest catchments, we studied the seasonal evolution of hydrological response from two catchments with broadly similar characteristics to the Babinda catchments. Both hydrometric and water stable isotope data were collected at relatively high frequencies during one wet season (Thompson Creek) and a 3-year period (Atika Creek). The longer dataset spans a wide range of environmental conditions experienced in the humid tropics, including events that cover the wetting-up transitional period of the wet season and tropical cyclones (TC). Both catchments displayed a fast streamflow response to rainfall with the shallow upper soil profile responding quickly to rainfall at Atika Creek. New findings from this study include the importance of pre-event water (>50% using the two component hydrograph separation technique) for overall event flows, especially when the catchment was wet. Rainfall, surface runoff and groundwater isotope and specific electrical conductivity (SEC) compositions varied between rainfall events with the most complex bivariate mixing plots observed for multi-peak events that occurred at the start of the wet season and after a dry period within the wet season. Two-tracer, 3 component hydrograph separations did not provide satisfactory results in identifying source water contributions to streamflow. These results highlighted the time-variant and non-conservative behaviour of the rainfall, surface runoff and shallow groundwater source waters over the seasonal timescale, with soil water being an important unidentified source contributor. Our findings highlight the need for high frequency multi-source sampling to accurately interpret catchment behaviour and the importance of soil water contributions to streamflow. We propose a framework to describe the seasonal evolution of streamflow response in steep tropical rainforest catchments experiencing seasonal rainfall activity.