Total Water Column Research Articles

Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

AbstractWe present results from a global inverse marine nitrogen (N) cycle model that include nitrate (NO3−) and nitrite (NO2−) concentrations and their N isotopic compositions as constraints on N cycle process rates in marine oxygen deficient zones (ODZs). NO2− is an important intermediate in the N cycle, particularly in ODZs where it is a substrate in the N loss processes, denitrification, and anammox. Similar to earlier work, our model yields a total water column N loss rate of 61 ± 10 Tg N/year. However, by including NO2− and its N isotopic composition, we are able to assess the relative contributions of denitrification and anammox to N loss and examine some of the potential drivers of that balance. We find that anammox contributes 60% of global water column N loss, dominating N loss along the edges of ODZs, while denitrification is more important in the anoxic ODZ cores. The decoupling of anammox and denitrification is supported by NO2− oxidation, which co‐occurs with NO3− reduction and anammox in ODZs. High rates of NO2− oxidation (up to 400 nM/day), which are tightly coupled to heterotrophic NO3− reduction, are required to match NO3− and NO2− concentration and isotope observations in marine ODZs. Lowering the rate of NO2− oxidation in ODZs by adjusting O2‐sensitive parameters results in higher rates of water column N loss, highlighting the role of NO2− oxidation in maintaining the marine fixed N inventory.

Development of an ANN based corrective algorithm of the operational ECMWF global horizontal irradiation forecasts

This paper proposes a corrective algorithm for improving the accuracy of global horizontal irradiation (GHI) forecasts obtained from the numerical weather prediction (NWP) model of the European Centre for Medium-range Weather Forecast (ECMWF). Firstly, GHI forecasts were compared with experimental values from two ground-based stations located at the south of Portugal (Évora and Sines), and the influence of Sun-Earth geometry and atmospheric variables on the differences between predictions and measurements was analysed in order to identify the most relevant parameters. These differences are shown to be correlated mainly with the clearness index, solar zenith angle, mean air temperature, relative air humidity and total water column. Since the ECMWF model directly or indirectly provides all these variables, it is possible to estimate the bias of the predicted GHI as a function of forecast data taking as reference the measurements, which means that an algorithm based on correlations of such parameters can be used to correct forecasts in an operational time horizon. With that goal, an artificial neural network (ANN) based algorithm was developed in this work to improve GHI predictions, including also as input the global solar irradiation predicted by a reference clear sky model. The internal structure of the ANN was optimised, and a spatial and temporal downscaling procedure was also developed to obtain half-hour irradiation values. This algorithm was tested against the original ECMWF forecasts and a persistence model for four locations with different orography and climate as well as for various sky conditions (overcast, partly cloudy and clear sky), showing that it successfully improves the model predictions. Higher values of a Global Performance Index (GPI) based on seven statistical indicators and Forecast Score (FS) were found for the algorithm simulations, e.g. respectively 1.066 and 0.348 when considering all the locations and cloud cover conditions, while for the original ECMWF predictions the GPI is −1.874 and the FS is 0.282. This algorithm is useful if integrated into energy management tools of solar energy systems, namely low/medium temperature solar thermal and photovoltaic systems.

Assessing Deep-Pelagic Shrimp Biomass to 3000\u2009m in The Atlantic Ocean and Ramifications of Upscaled Global Biomass

We assess the biomass of deep-pelagic shrimps in the Atlantic Ocean using data collected between 40°N and 40°S. Forty-eight stations were sampled in discrete-depth fashion, including epi- (0–200 m), meso- (200–800/1000 m), upper bathy- (800/1000–1500 m), and lower bathypelagic (1500–3000 m) strata. We compared samples collected from the same area on the same night using obliquely towed trawls and large vertically towed nets and found that shrimp catches from the latter were significantly higher. This suggests that vertical nets are more efficient for biomass assessments, and we report these values here. We further compared day and night samples from the same site and found that biomass estimates differed only in the epi- and mesopelagic strata, while estimates from the bathypelagic strata and the total water column were independent of time of day. Maximal shrimp standing stocks occurred in the upper bathypelagic (52–54% of total biomass) and in the mesopelagic (42–43%). We assessed shrimp biomass in three major regions of the Atlantic between 40°N and 40°S, and the first-order extrapolation of these data suggests that the global low-latitude deep-pelagic shrimp biomass (1700 million tons) may lie within the range reported for mesopelagic fishes (estimations between 1000 and 15000 million tons). These data, along with previous fish-biomass estimates, call for the reassessment of the quantity and distribution of nektonic carbon in the deep ocean.

The carbon: 234Thorium ratios of sinking particles in the California current ecosystem 2: Examination of a thorium sorption, desorption, and particle transport model

Thorium-234 (234Th) is a powerful tracer of particle dynamics and the biological pump in the surface ocean; however, variability in carbon: thorium ratios of sinking particles adds substantial uncertainty to estimates of organic carbon export. We coupled a mechanistic thorium sorption and desorption model to a one-dimensional particle sinking model that uses realistic particle settling velocity spectra. The model generates estimates of 238U234Th disequilibrium, particulate organic carbon concentration, and the C:234Th ratio of sinking particles, which are then compared to in situ measurements from quasi-Lagrangian studies conducted on six cruises in the California Current Ecosystem. Broad patterns observed in in situ measurements, including decreasing C:234Th ratios with depth and a strong correlation between sinking C:234Th and the ratio of vertically-integrated particulate organic carbon (POC) to vertically-integrated total water column 234Th, were accurately recovered by models assuming either a power law distribution of sinking speeds or a double log normal distribution of sinking speeds. Simulations suggested that the observed decrease in C:234Th with depth may be driven by preferential remineralization of carbon by particle-attached microbes. However, an alternate model structure featuring complete consumption and/or disaggregation of particles by mesozooplankton (e.g. no preferential remineralization of carbon) was also able to simulate decreasing C:234Th with depth (although the decrease was weaker), driven by 234Th adsorption onto slowly sinking particles. Model results also suggest that during bloom decays C:234Th ratios of sinking particles should be higher than expected (based on contemporaneous water column POC), because high settling velocities minimize carbon remineralization during sinking.

Global joint assimilation of GRACE and SMOS for improved estimation of root-zone soil moisture and vegetation response

Abstract. The lack of direct measurement of root-zone soil moisture poses a challenge to the large-scale prediction of ecosystem response to variation in soil water. Microwave remote sensing capability is limited to measuring moisture content in the uppermost few centimetres of soil. The GRACE (Gravity Recovery and Climate Experiment) mission detected the variability in storage within the total water column. However, root-zone soil moisture cannot be separated from GRACE-observed total water storage anomalies without ancillary information on surface water and groundwater changes. In this study, GRACE total water storage anomalies and SMOS near-surface soil moisture observations were jointly assimilated into a hydrological model globally to better estimate the impact of changes in root-zone soil moisture on vegetation vigour. Overall, the accuracy of root-zone soil moisture estimates through the joint assimilation of surface soil moisture and total water storage retrievals showed improved consistency with ground-based soil moisture measurements and satellite-observed greenness when compared to open-loop estimates (i.e. without assimilation). For example, the correlation between modelled and in situ measurements of root-zone moisture increased by 0.1 (from 0.48 to 0.58) and 0.12 (from 0.53 to 0.65) on average for grasslands and croplands, respectively. Improved correlations were found between vegetation greenness and soil water storage on both seasonal variability and anomalies over water-limited regions. Joint assimilation results show a more severe deficit in soil water anomalies in eastern Australia, southern India and eastern Brazil over the period of 2010 to 2016 than the open-loop, consistent with the satellite-observed vegetation greenness anomalies. The assimilation of satellite-observed water content contributes to more accurate knowledge of soil water availability, providing new insights for monitoring hidden water stress and vegetation conditions.

A Seasonal to Interannual View of Inorganic and Organic Carbon and pH in Western Lake Superior

AbstractTo investigate the carbon cycle of Lake Superior, Earth's largest freshwater lake by surface area, we performed total organic carbon, dissolved organic carbon, chlorophyll, spectrophotometric pH, total inorganic carbon, and alkalinity measurements on seasonal samples from the western lake. The last three parameters, along with in situ temperature, were used to calculate the partial pressure of carbon dioxide in surface water (pCO2(w)) using the CO2SYS algorithm. There was a strong positive correlation between pH and water temperature and a weaker but significant positive correlation between pH and chlorophyll concentration. Total organic carbon exhibited higher nearshore concentrations (as determined by a negative correlation with total water column depth); such a spatial relationship did not appear in the inorganic carbon parameters (total inorganic carbon, pH, or pCO2(w)). Western Lake Superior exhibited net outgassing in spring, little net gas transfer in summer, and some outgassing in the fall. The pCO2(w) values were negatively correlated with both water temperature and chlorophyll concentration. Seasonal differences in pCO2(w) in Lake Superior appeared more strongly driven by biology and terrestrial inputs as compared to direct effects of temperature on CO2 solubility. Interannual data from the long‐term Great Lakes Environmental Database data set indicated that lake alkalinity has been increasing over the past 20 years, and lake surface water pH appeared relatively stable. Modeling pH change over the same time frame in CO2SYS shows that increases in alkalinity and lake surface water temperature counteract the increase in atmospheric carbon dioxide concentration, leading to a relatively constant pH, consistent with observational data.

The Carbon:234Thorium ratios of sinking particles in the California current ecosystem 1: relationships with plankton ecosystem dynamics

We investigated variability in the C:234Th ratio of sinking particles and its relationship to changing water column characteristics and plankton ecological dynamics during 29 Lagrangian experiments conducted on six cruises of the California Current Ecosystem Long-Term Ecological Research (CCE-LTER) Program. C:234Th ratios of sinking particles collected by a surface-tethered sediment trap (C:234ThST) varied from 2.3 to 20.5 μmol C dpm−1 over a depth range of 47–150 m. C:234ThST was significantly greater (by a factor of 1.8) than C:234Th ratios of suspended >51-μm particles collected in the same water parcels with in situ pumps. C:234Th ratios of large (>200-μm) sinking particles also exceeded those of smaller sinking particles. C:234ThST decreased with depth from the base of the euphotic zone through the upper twilight zone. C:234ThST was positively correlated with several indices of ecosystem productivity including particulate organic carbon (POC) and chlorophyll (Chl) concentrations, mesozooplankton biomass, and the fraction of Chl >20-μm. Principal component analysis and multiple linear regression suggested that decaying phytoplankton blooms exhibited higher C:234ThST than actively growing blooms at similar biomass levels. C:234ThST was positively correlated with indices of the fractional contribution of fecal pellets in sediment traps when the proportion of fecal pellets was low in the traps, likely because of a correlation between mesozooplankton biomass and other indices of ecosystem productivity. However, when fecal pellets were a more important component of sinking material, C:234ThST decreased with increasing fecal pellet content. C:234ThST was also positively correlated with the Si:C ratio of sinking particles. Across the dataset (and across depths) a strong correlation was found between C:234ThST and the ratio of vertically-integrated POC to vertically-integrated total water column 234Th (vC:234Thtot). A mechanistic one-layer, two-box model of thorium sorption and desorption was invoked to explain this correlation. Two empirical models (one using vC:234Thtot; one using depth and vertically-integrated Chl) were developed to predict C:234Th ratios in this coastal upwelling biome. The former regression (log10(C:234ThST) = 0.43 × log10(vC:234Thtot) + 0.53) was found to also be a reasonable predictor for C:234ThST from diverse regions including the Southern Ocean, Sargasso Sea, Subarctic North Pacific, and Eastern Tropical North Pacific.

Satellite-observed vegetation stability in response to changes in climate and total water storage in Central Asia

Ecosystems in arid and semi-arid regions are vulnerable to climatic and anthropogenic disturbances. However, our understanding of vegetation stability (including resistance and resilience, which are the abilities of ecosystems to resist perturbations and return to pre-disturbance structure or function, respectively) in response to environmental changes in dryland ecosystems remains insufficient, particularly in the absence of large-scale observations of water availability. Here we introduced GRACE monthly total water storage anomaly (TWSA) data into an autoregressive model with remote sensed EVI, air temperature and precipitation to investigate the short-term vegetation stability and its influencing factors in Central Asia (CA) during 2003–2015. The results showed that the grid-level vegetation resilience in CA increased logarithmically as mean annual precipitation (R2 = 0.33, P < 0.05) but decreased linearly with increasing mean annual temperature (R2 = 0.41, P < 0.05). Vegetation resilience was not correlated with TWSA, due to the decoupling of TWSA with precipitation both spatially and temporally in the majority of CA. At the biome level, vegetation resilience also increased as a logarithmical function of aridity index (R2 = 0.80, P < 0.05). Vegetation resistance to TWSA showed minor difference across biomes, while vegetation resistance to precipitation functioned as a parabolic curve along the aridity gradient (R2 = 0.59, P < 0.05). Our results suggest that accounting for the effects of total water column instead of precipitation only is critical in understanding vegetation-water relationships in drylands. The steep decrease in vegetation resilience in areas with high temperature and low water availability implies a high risk of collapse for these water-limited ecosystems if there are severe droughts. Furthermore, reduction in total water storage, induced by, e.g., large-scale extraction of surface runoff or shallow-layer groundwater for irrigation, can result in negative influences to natural biomes in dryland regions.

Phosphorus recycling in deeply oxygenated sediments in Lake Superior controlled by organic matter mineralization

AbstractRemobilization of phosphorus from aquatic sediments has been extensively investigated in systems prone to anoxia, while studies in well‐oxygenated systems have been rare. The recycling efficiency of P in the offshore sediments in the Great Lakes, in particular, is still poorly known. We investigated phosphorus cycling at 13 locations (26–318 m water depth) in oligotrophic Lake Superior where oxygen penetrates into sediments by 2–12 cm. Vertical distributions of iron and phosphorus were measured in porewater and solid fractions, and transformation rates and vertical fluxes were calculated. Whereas a significant fraction of P is bound to ferric Fe in surface sediments, P effluxes into the water column (2.5–7.0 μmol m−2 d−1) are only weakly affected by iron reduction, because Fe : P ratios in surface sediment are high (∼ 40–80 mol : mol), and P sorption capacity is far from its limit. In contrast to organic rich systems where P effluxes are sensitive to redox conditions, phosphate effluxes in organic‐poor well‐oxygenated Lake Superior are controlled by the rates of organic phosphorus mineralization, similar to marine sediments. The efficiency of P recycling in Lake Superior sediments, however, is substantially lower than in marine sediments due to different P biogeochemistry. Only ∼ 12% of deposited P is returned to the water column. While burial into sediments is the dominant sink for P in the lake, sediments still contribute up to 40% of total water column P inputs. Similar behavior should be expected in other well‐oxygenated freshwater systems, such as other large oligotrophic lakes.

Millions of Boreal Shield Lakes can be used to Probe Archaean Ocean Biogeochemistry

Life originated in Archaean oceans, almost 4 billion years ago, in the absence of oxygen and the presence of high dissolved iron concentrations. Early Earth oxidation is marked globally by extensive banded iron formations but the contributing processes and timing remain controversial. Very few aquatic habitats have been discovered that match key physico-chemical parameters of the early Archaean Ocean. All previous whole ecosystem Archaean analogue studies have been confined to rare, low sulfur, and permanently stratified lakes. Here we provide first evidence that millions of Boreal Shield lakes with natural anoxia offer the opportunity to constrain biogeochemical and microbiological aspects of early Archaean life. Specifically, we combined novel isotopic signatures and nucleic acid sequence data to examine processes in the anoxic zone of stratified boreal lakes that are naturally low in sulfur and rich in ferrous iron, hallmark characteristics predicted for the Archaean Ocean. Anoxygenic photosynthesis was prominent in total water column biogeochemistry, marked by distinctive patterns in natural abundance isotopes of carbon, nitrogen, and iron. These processes are robust, returning reproducibly after water column re-oxygenation following lake turnover. Evidence of coupled iron oxidation, iron reduction, and methane oxidation affect current paradigms of both early Earth and modern aquatic ecosystems.

Erosion of the Alberta badlands produces highly variable and elevated heavy metal concentrations in the Red Deer River, Alberta

Erosion is important in the transport of heavy metals from terrestrial to fluvial environments. In this study, we investigated riverine heavy metal (Cd, Cu, Hg and Pb) dynamics in the Red Deer River (RDR) watershed at sites upstream (n=2) and downstream (n=7) of the Alberta badlands, an area of naturally high erosion. At sites draining the badlands, total water column Cd, Cu, Hg and Pb concentrations frequently exceeded guidelines for the protection of freshwater biota. Furthermore, peak concentrations of total Cd (9.8μgL−1), Cu (212μgL−1), Hg (649ngL−1) and Pb (361μgL−1) were higher than, or comparable to, values reported for rivers and streams heavily impacted by anthropogenic activities. Total suspended solids (TSS) explained a large proportion (r2=0.34–0.83) of the variation in total metal concentrations in the RDR and tributaries and metal fluxes were dominated by the particulate fraction (60–98%). Suspended sediment concentrations (Csed) and metal to aluminum ratios were generally not indicative of substantial sediment enrichment. Rather, the highly variable and elevated metal concentrations in the RDR watershed were a function of the high and variable suspended sediment fluxes which characterize the river system. While the impact of this on aquatic biota requires further investigation, we suggest erosion in the Alberta badlands may be contributing to Hg-based fish consumption advisories in the RDR. Importantly, this highlights a broader need for information on contaminant dynamics in watersheds subject to elevated rates of erosion.

Impact of Madden-Julian Oscillation (MJO) on global distribution of total water vapor and column ozone

The Madden-Julian Oscillation (MJO) is the leading mode of intra-seasonal variability in the tropical troposphere, characterized by an eastward moving ‘pulse’ of cloud and rainfall near the equator. In this study, total precipitable water (TPW) and total column ozone (TCO) datasets from ECMWF ERA-Interim reanalysis were used to analyse the impact of the MJO on the distribution of water vapor and column ozone in the tropics from 1979 to 2013. The results show that seasonal variations of TPW modulated by the MJO are maximized in the tropics of about 10°S-10°N during boreal winter, while the variation in TCO is maximized in the mid-latitudes of about 30°S - 40°N in the same season. The composite analysis shows that MJO modulates TPW and TCO anomalies eastward across the globe. The underlying mechanism of the MJO’s impact on TPW is mainly associated with variation of tropical convection modulated by the MJO, while the underlying mechanism of the MJO’s impact on TCO is mainly associated with an intra-seasonal variability of tropopause height modulated by the MJO activity. This knowledge helps to improve the prediction skill of the intra-seasonal variation of water vapor and column ozone in the tropics during boreal winter.

Satellite Observations of the Seasonal Evolution of Total Precipitable Water Vapour over the Mediterranean Sea

This study shows satellite observations and new findings on the time and spatial distribution of the Total Precipitable Water (TPW) column over the Mediterranean Sea throughout the year. Annual evolution and seasonality of the TPW column are shown and compared to the estimated net evaporation over the Mediterranean Sea. Daily spatiotemporal means are in good agreement with previous short-term field campaigns and also corroborate hypothesis and conclusions reached from previous mesoscale modelling studies: (a) from a meteorological point of view, Mediterranean Basin should be considered as two different subbasins (the Western and the Eastern Mediterranean); (b) accumulation processes may affect the radiative balance at regional scale and the summer precipitation regimes. Furthermore, these satellite observations constitute strong empirical evidences that, (a) from late May to early October, contrary to what happens in the Eastern Mediterranean Basin (EMB), there is a net accumulation of TPW on the Western Mediterranean Basin (WMB) that favours the instability of the atmosphere, (b) there is a seasonal anticorrelation between the seasonal variability of the TPW column over the two Mediterranean subbasins, (c) solar radiation can not be the only driver for the annual variability of the TPW column over the Mediterranean Sea, and (d) both previous features are seasonally dependent and, therefore, their effects on the TPW column are attenuated by annual variability.

Primary production in the tropical continental shelf seas bordering northern Australia

Pelagic primary production (14C uptake) was measured 81 times between 1990 and 2013 at sites spanning the broad, shallow Northern Australian Shelf (NAS; 120−145°E) which borders the Australian continent. The mean of all areal production measurements was 1048±109mgCm−2d−1 (mean±95% CI). Estimates of areal primary production were correlated with integral upper-euphotic zone chlorophyll stocks (above the 50% and 20% light penetration depths) accessible to ocean color remote sensing and total water column chlorophyll standing crop, but not surface (0–2m) chlorophyll concentrations. While the NAS is subject to a well characterized monsoonal climate regime (austral summer-NW monsoon -wet: austral winter- SE monsoon -dry), most seasonal differences in means of regional-scale chlorophyll standing crop (11–33mg Chlm−2 for 12 of 15 season-region combinations) and areal primary production (700–1850mgCm−day−1 for 12 of 15 season-region combinations) fell within a 3−fold range. Apart from the shallow waters of the Torres Strait and northern Great Barrier Reef, picoplankton (<2µm size fraction) dominated chlorophyll standing crop and primary production with regional means of picoplankton contributions ranging from 45 to >80%. While the range of our post-1990 areal production estimates overlaps the range of production estimates made in NAS waters during 1960–62, the mean of post-1990 estimates is over 2-fold greater. We regard the difference to be due to improvements in production measurement techniques, particularly regarding the reduction of potential metal toxicity and incubations in more realistic light regimes.

Tracing metals through urban wetland food webs

Constructed wetlands are commonly used to remove contaminants such as heavy metals from stormwater. However, metals can become bioavailable and be transferred into and along wetland food chains. Our study sought to establish whether urbanisation affected concentrations of metals in water, wetland sediments, freshwater crayfish and fish tissues. Samples were taken from constructed wetlands in western Melbourne. There was no relationship between catchment total imperviousness (TI, an index of urbanisation) and water column and sediment concentrations of metals with the exception of Zn, which was higher in more urbanised catchments. Concentrations of metals in fish tissues were highest in benthic species but declined with increasing body size and trophic level. This suggests that metals are not bioaccumulating or biomagnifying along food chains in these systems. Metabolic activity can differ between smaller and larger fish, or smaller fish may also be feeding on different food sources. Our results suggest that there is not a major human health risk associated with accumulation of metals in large-bodied fish in these wetlands, and that most of the metal load is retained in sediments, or the lowest trophic levels.

Basin‐wide N2 fixation in the deep waters of the Mediterranean Sea

AbstractRecent findings indicate that N2 fixation is significant in aphotic waters, presumably due to heterotrophic diazotrophs depending on organic matter for their nutrition. However, the relationship between organic matter and heterotrophic N2 fixation remains unknown. Here we explore N2 fixation in the deep chlorophyll maximum and underneath deep waters across the whole Mediterranean Sea and relate it to organic matter composition, characterized by optical and molecular methods. Our N2 fixation rates were in the range of those previously reported for the euphotic zone of the Mediterranean Sea (up to 0.43 nmol N L−1 d−1) and were significantly correlated to the presence of relatively labile organic matter with fluorescence and molecular formula properties representative for peptides and unsaturated aliphatics and associated with the presence of more oxygenated ventilated water masses. Finally, and despite that the aphotic N2 fixation contributes largely to total water column diazotrophic activity (&gt;50%), its contribution to overall nitrogen inputs to the basin is negligible (&lt;0.5%).

Continuous inline mapping of a dissolved methane plume at a blowout site in the Central North Sea UK using a membrane inlet mass spectrometer – Water column stratification impedes immediate methane release into the atmosphere

The dissolved methane (CH4) plume rising from the crater of the blowout well 22/4b in the Central North Sea was mapped during stratified water column conditions. Geochemical surveys were conducted close to the seafloor at 80.3 m water depth, below the thermocline (61.1 m), and in the mixed surface layer (13.2 m) using membrane inlet mass spectrometry (MIMS) in combination with a towed CTD. Seawater was continuously transferred from the respective depth levels of the CTD to the MIMS by using an inline submersible pump. Close to the seafloor a well-defined CH4 plume extended from the bubble release site ∼460 m towards the southwest. Along this distance CH4 concentrations decreased from a maximum of 7872 nmol l−1 to less than 250 nmol l−1. Below the thermocline the well-defined CH4 plume shape encountered at the seafloor was distorted and filaments were observed that extended towards the west and southwest in relation to current direction. Where the core of the bubble plume intersected this depth layer, footprints of high CH4 concentrations of up to 17,900 nmol l−1 were observed. In the mixed surface layer the CH4 distribution with a maximum of up to 3654 nmol l−1 was confined to a small patch of ∼60 m in diameter. The determination of the water column CH4 inventories revealed that CH4 transfer across the thermocline was strongly impeded as only ∼3% of the total water column inventory was located in the mixed surface layer. Best estimate of the CH4 seabed release from the blowout was 1751 tons yr−1. The fate of the trapped CH4 (∼97%) that does not immediately reach the atmosphere remains speculative. In wintertime, when the water column becomes well mixed as well as during storm events newly released CH4 and the trapped CH4 pool can be transported rapidly to the sea surface and emitted into the atmosphere.

Dehydration effects from contrails in a coupled contrail–climate model

Abstract. The uptake of water by contrails in ice-supersaturated air and the release of water after ice particle advection and sedimentation dehydrates the atmosphere at flight levels and redistributes humidity mainly to lower levels. The dehydration is investigated by coupling a plume-scale contrail model with a global aerosol–climate model. The contrail model simulates all the individual contrails forming from global air traffic for meteorological conditions as defined by the climate model. The computed contrail cirrus properties compare reasonably with theoretical concepts and observations. The mass of water in aged contrails may exceed 106 times the mass of water emitted from aircraft. Many of the ice particles sediment and release water in the troposphere, on average 700 m below the mean flight levels. Simulations with and without coupling are compared. The drying at contrail levels causes thinner and longer-lived contrails with about 15 % reduced contrail radiative forcing (RF). The reduced RF from contrails is on the order of 0.06 W m−2, slightly larger than estimated earlier because of higher soot emissions. For normal traffic, the RF from dehydration is small compared to interannual variability. A case with emissions increased by 100 times is used to overcome statistical uncertainty. The contrails impact the entire hydrological cycle in the atmosphere by reducing the total water column and the cover by high- and low-level clouds. For normal traffic, the dehydration changes contrail RF by positive shortwave and negative longwave contributions on the order of 0.04 W m−2, with a small negative net RF. The total net RF from contrails and dehydration remains within the range of previous estimates.

What are the most fire-dangerous atmospheric circulations in the Eastern-Mediterranean? Analysis of the synoptic wildfire climatology

Wildfire management is closely linked to robust forecasts of changes in wildfire risk related to meteorological conditions. This link can be bridged either through fire weather indices or through statistical techniques that directly relate atmospheric patterns to wildfire activity. In the present work the COST-733 classification schemes are applied in order to link wildfires in Greece with synoptic circulation patterns. The analysis reveals that the majority of wildfire events can be explained by a small number of specific synoptic circulations, hence reflecting the synoptic climatology of wildfires. All 8 classification schemes used, prove that the most fire-dangerous conditions in Greece are characterized by a combination of high atmospheric pressure systems located N to NW of Greece, coupled with lower pressures located over the very Eastern part of the Mediterranean, an atmospheric pressure pattern closely linked to the local Etesian winds over the Aegean Sea. During these events, the atmospheric pressure has been reported to be anomalously high, while anomalously low 500hPa geopotential heights and negative total water column anomalies were also observed. Among the various classification schemes used, the 2 Principal Component Analysis-based classifications, namely the PCT and the PXE, as well as the Leader Algorithm classification LND proved to be the best options, in terms of being capable to isolate the vast amount of fire events in a small number of classes with increased frequency of occurrence. It is estimated that these 3 schemes, in combination with medium-range to seasonal climate forecasts, could be used by wildfire risk managers to provide increased wildfire prediction accuracy.

Autonomous profiling float observations of the high-biomass plume downstream of the Kerguelen Plateau in the Southern Ocean

Abstract. Natural iron fertilisation from Southern Ocean islands results in high primary production and phytoplankton biomass accumulations readily visible in satellite ocean colour observations. These images reveal great spatial complexity with highly varying concentrations of chlorophyll, presumably reflecting both variations in iron supply and conditions favouring phytoplankton accumulation. To examine the second aspect, in particular the influences of variations in temperature and mixed layer depth, we deployed four autonomous profiling floats in the Antarctic Circumpolar Current near the Kerguelen Plateau in the Indian sector of the Southern Ocean. Each "bio-profiler" measured more than 250 profiles of temperature (T), salinity (S), dissolved oxygen, chlorophyll a (Chl a) fluorescence, and particulate backscattering (bbp) in the top 300 m of the water column, sampling up to 5 profiles per day along meandering trajectories extending up to 1000 km. Comparison of surface Chl a estimates (analogous to values from satellite images) with total water column inventories revealed largely linear relationships, suggesting that these images provide credible information on total and not just surface biomass spatial distributions. However, they also showed that physical mixed layer depths are often not a reliable guide to biomass distributions. Regions of very high Chl a accumulation (1.5–10 μg L−1) were associated predominantly with a narrow T–S class of surface waters. In contrast, waters with only moderate Chl a enrichments (0.5–1.5 μg L−1) displayed no clear correlation with specific water properties, including no dependence on mixed layer depth or the intensity of stratification. Geostrophic trajectory analysis suggests that both these observations can be explained if the main determinant of biomass in a given water parcel is the time since leaving the Kerguelen Plateau. One float became trapped in a cyclonic eddy, allowing temporal evaluation of the water column in early autumn. During this period, decreasing surface Chl a inventories corresponded with decreases in oxygen inventories on sub-mixed-layer density surfaces, consistent with significant export of organic matter (~35%) and its respiration and storage as dissolved inorganic carbon in the ocean interior. These results are encouraging for the expanded use of autonomous observing platforms to study biogeochemical, carbon cycle, and ecological problems, although the complex blend of Lagrangian and Eulerian sampling achieved by the floats suggests that arrays rather than single floats will often be required, and that frequent profiling offers important benefits in terms of resolving the role of mesoscale structures on biomass accumulation.