Northern High-latitude Regions Research Articles

Continuous wildfires threaten public and ecosystem health under climate change across continents

Wildfires burn approximately 3%–4% of the global land area annually, resulting in massive emissions of greenhouse gases and air pollutants. Over the past two decades, there has been a declining trend in both global burned area and wildfire emissions. This trend is largely attributed to a decrease in wildfire activity in Africa, which accounts for a substantial portion of the total burned area and emissions. However, the northern high-latitude regions of Asia and North America have witnessed substantial interannual variability in wildfire activity, with several severe events occurring in recent years. Climate plays a pivotal role in influencing wildfire activity and has led to more wildfires in high-latitude regions. These wildfires pose significant threats to climate, ecosystems, and human health. Given recent changes in wildfire patterns and their impacts, it is critical to understand the contributors of wildfires, focus on deteriorating high-latitude areas, and address health risks in poorly managed areas to mitigate wildfire effects.

Assessment of Accuracy of Moderate-Resolution Imaging Spectroradiometer Sea Surface Temperature at High Latitudes Using Saildrone Data

The infrared (IR) satellite remote sensing of sea surface skin temperature (SSTskin) is challenging in the northern high-latitude region, especially in the Arctic because of its extreme environmental conditions, and thus the accuracy of SSTskin retrievals is questionable. Several Saildrone uncrewed surface vehicles were deployed at the Pacific side of the Arctic in 2019, and two of them, SD-1036 and SD-1037, were equipped with a pair of IR pyrometers on the deck, whose measurements have been shown to be useful in the derivation of SSTskin with sufficient accuracy for scientific applications, providing an opportunity to validate satellite SSTskin retrievals. This study aims to assess the accuracy of MODIS-retrieved SSTskin from both Aqua and Terra satellites by comparisons with collocated Saildrone-derived SSTskin data. The mean difference in SSTskin from the SD-1036 and SD-1037 measurements is ~0.4 K, largely resulting from differences in the atmospheric conditions experienced by the two Saildrones. The performance of MODIS on Aqua and Terra in retrieving SSTskin is comparable. Negative brightness temperature (BT) differences between 11 μm and 12 μm channels are identified as being physically based, but are removed from the analyses as they present anomalous conditions for which the atmospheric correction algorithm is not suited. Overall, the MODIS SSTskin retrievals show negative mean biases, −0.234 K for Aqua and −0.295 K for Terra. The variations in the retrieval inaccuracies show an association with diurnal warming events in the upper ocean from long periods of sunlight in the Arctic. Also contributing to inaccuracies in the retrieval is the surface emissivity effect in BT differences characterized by the Emissivity-introduced BT difference (EΔBT) index. This study demonstrates the characteristics of MODIS-retrieved SSTskin in the Arctic, at least at the Pacific side, and underscores that more in situ SSTskin data at high latitudes are needed for further error identification and algorithm development of IR SSTskin.

Reconciling carbon quality with availability predicts temperature sensitivity of global soil carbon mineralization

Soil organic carbon (SOC) mineralization is a key component of the global carbon cycle. Its temperature sensitivity Q10 (which is defined as the factor of change in mineralization with a 10 °C temperature increase) is crucial for understanding the carbon cycle-climate change feedback but remains uncertain. Here, we demonstrate the universal control of carbon quality-availability tradeoffs on Q10. When carbon availability is not limited, Q10 is controlled by carbon quality; otherwise, substrate availability controls Q10. A model driven by such quality-availability tradeoffs explains 97% of the spatiotemporal variability of Q10 in incubations of soils across the globe and predicts a global Q10 of 2.1 ± 0.4 (mean ± one SD) with higher Q10 in northern high-latitude regions. We further reveal that global Q10 is predominantly governed by the mineralization of high-quality carbon. The work provides a foundation for predicting SOC dynamics under climate and land use changes which may alter soil carbon quality and availability.

Carbon and nitrogen-based gas fluxes in subarctic ecosystems under climate warming and increased cloudiness

Increased temperatures coupled with reduced light availability due to increased cloudiness can alter the ecosystem–atmosphere exchange of trace gases in the northern high-latitude regions with important climate feedback implications.

Synthesis of high-Andean peat cores reveals suite of Holocene climate conditions favorable for peat formation

Peat-core records have a long history of being used for paleoclimate reconstructions in northern high-latitude regions but are less common in the southern tropical zone. In this study, we present a synthesis of published peat-core reconstructions and basal ages from high-elevation (>3000 m a.s.l.) sites in the northern and central Andes of South America. We complement this database by providing a new multi-proxy peat-core-based paleoecological reconstruction from the Alta Murmurani peatland, Cordillera Vilcanota, Peru. This record, with a peat inception age of 8980 cal. yr BP, was analyzed for plant macrofossils and stable isotopes (carbon, oxygen). This review (1) assesses the timing of peat initiation and development across the tropical high-elevation region of South America (8°N to 27°S); (2) assesses the reliability of peat-core paleoclimate reconstructions by comparing the peat-core inferences across sites and against other independent archives (e.g., lake sediments, ice cores, speleothems); and (3) determines which hydroclimatic conditions are favorable for peat formation and expansion across the study area. Our results show that peat initiation occurred throughout the Holocene, progressing from the northern Andes to central Andes, with highest initiation frequencies clustered at ca. 10,700 cal. yr BP and 8300 cal. yr BP, respectively. Our synthesis reveals that peat cores can be used to assess regional paleoclimate trends in the central Andes using a multi-proxy approach combining micro- and macrofossils such as pollen, stable isotopes, as well as organic and inorganic elemental analyses. We also identify three optimal peat-forming time periods, all with optimally wet conditions occurring at transitional hydroclimate phases: (1) following deglaciation, with warming temperatures and increased moisture related to glacial runoff (ca. 11,000 cal. yr BP and ∼8000 cal. yr BP), (2) under moderate moisture regimes related to intensifying ENSO and warmer conditions (∼3500 cal. yr BP), and (3) under conditions related to increased precipitation at the beginning of a cold period (Little Ice Age). We show that either regional climate (e.g., changes in precipitation) or local conditions (e.g., changes in glacial runoff) can promote peat formation across the study region.

Rewilding Risks for Peatland Permafrost

Permafrost thaw is projected to reinforce climate warming by releasing large stocks of stored carbon. Rewilding northern high latitude regions with large herbivores has been proposed as a climate mitigation strategy to protect frozen soils and increase ecosystem resilience to climate warming. We explored the impact of summer reindeer density on subarctic peatlands by comparing 17 peatlands differing in reindeer density in Fennoscandia. We used a combination of high-resolution image analyses and field assessments along 50 transects to assess microtopography, surface water cover, vegetation, summer albedo, permafrost presence, soil temperature, soil nutrients and snow depth. Our results show that high summer reindeer densities fragment the characteristic bumpy topography of the peatlands, reducing the insulating soil properties and the probability of keeping permafrost in elevated hummocks. As a result, waterlogged lawns with surface water increase in size and reduce summer albedo. Furthermore, high reindeer density peatlands were associated with an increase in tall inedible shrubs and thicker snow layers. These changes may favor summer warming and reduce winter cooling of the soil thus accelerating permafrost loss. Our results suggest that high reindeer densities may reduce resilience of the peatland permafrost to climate warming. High densities of large herbivores will likely have different effects in well-drained uplands, but in the lowlands we studied, the complex cascading effects of summer trampling may well offset any climate-protection gained by browsing. Optimal use of wildlife management to mitigate global warming will thus require tuning herbivore densities to different ecosystem types across high northern landscapes.

Global methyl halide emissions from biomass burning during 2003-2021.

Methyl halides (CH3Cl, CH3Br, and CH3I) are ozone-depleting substances. Biomass burning (BB) is an important source of methyl halides. The temporal variations and global spatial distribution of BB methyl halide emissions are unclear. Thus, global methyl halide emissions from BB during 2003-2021 were estimated based on satellite data. A significant decreasing trend (p<0.01) in global methyl halide emissions from BB was found between 2003 and 2021, with CH3Cl emissions decreasing from 302 to 220Ggyr-1, CH3Br emissions decreasing from 16.5 to 11.7Ggyr-1, and CH3I emissions decreasing from 8.9 to 6.1Ggyr-1. From a latitudinal perspective, the northern high-latitude region (60-90° N) was the only latitude zone with significant increases in BB methyl halide emissions (p<0.01). Based on an analysis of the drivers of BB methyl halide emissions, emissions from cropland, grassland, and shrubland fires were more correlated with the burned area, while BB emissions from forest fires were more correlated with the emissions per unit burned area. The non-BB emissions of CH3Cl increased from 4749Ggyr-1 in 2003 to 4882Ggyr-1 in 2020, while those of CH3Br decreased from 136Ggyr-1 in 2003 to 118Ggyr-1 in 2020 (global total CH3I emissions are not available). The finding indicates that global CH3Cl and CH3Br emissions from sources besides BB increased and decreased during 2003-2020. Based on our findings, not only searching for unknown sources is important, but also re-evaluating known sources is necessary for addressing methyl halide emissions.

Arctic climate-Indian monsoon teleconnection during the last millennium revealed through geochemical proxies from an Arctic fjord

Some of the most visible signs of climate change can be found in the northern high latitude region. Due to dynamic glacial-marine contrast, sedimentary geochemical proxies from western Svalbard fjords may provide a detailed account of paleoenvironmental changes in the region. We constructed a multivariate sedimentary record from a high-Arctic fjord (Kongsfjorden) using the sediment core NP14-FM. The new multi-proxy record includes sedimentary organic matter (OM) concentrations and their carbon and nitrogen isotopic composition, covering a period between AD 1106 and AD 1967. Using an unsupervised learning technique, the multivariate data was partitioned into two groups, helping us identify occurrences of warm and cold Arctic spells at multi-decadal resolution during the last millennium. Individually, the δ15N variability forms a highly coherent pattern with surface temperature records while δ13C and elemental OM concentrations reveal changing mixing proportions between marine and terrestrial sources. Furthermore, the region's interchanging warm and cold episodes, as detected by the multivariate sedimentary record, were linked to stronger and weaker summer monsoon over India, respectively. Modulation of meridional thermal gradients over the Indian monsoon domain amid contrasting climatic conditions in the Arctic region likely facilitated the observed Arctic-Indian monsoon teleconnection during the last millennium.

Deep Clouds on Jupiter

Jupiter’s atmospheric water abundance is a highly important cosmochemical parameter that is linked to processes of planetary formation, weather, and circulation. Remote sensing and in situ measurement attempts still leave room for substantial improvements to our knowledge of Jupiter’s atmospheric water abundance. With the motivation to advance our understanding of water in Jupiter’s atmosphere, we investigate observations and models of deep clouds. We discuss deep clouds in isolated convective storms (including a unique storm site in the North Equatorial Belt that episodically erupted in 2021–2022), cyclonic vortices, and northern high-latitude regions, as seen in Hubble Space Telescope visible/near-infrared imaging data. We evaluate the imaging data in continuum and weak methane band (727 nm) filters by comparison with radiative transfer simulations, 5 micron imaging (Gemini), and 5 micron spectroscopy (Keck), and conclude that the weak methane band imaging approach mostly detects variation in the upper cloud and haze opacity, although sensitivity to deeper cloud layers can be exploited if upper cloud/haze opacity can be separately constrained. The cloud-base water abundance is a function of cloud-base temperature, which must be estimated by extrapolating 0.5-bar observed temperatures downward to the condensation region near 5 bar. For a given cloud base pressure, the largest source of uncertainty on the local water abundance comes from the temperature gradient used for the extrapolation. We conclude that spatially resolved spectra to determine cloud heights—collected simultaneously with spatially-resolved mid-infrared spectra to determine 500-mbar temperatures and with improved lapse rate estimates—would be needed to answer the following very challenging question: Can observations of deep water clouds on Jupiter be used to constrain the atmospheric water abundance?

Decadal trends in the release of terrigenous organic carbon to the Mackenzie Delta (Canadian Arctic) using satellite ocean color data (1998–2019)

Arctic rivers operate as integrators of northern high latitude regions, where large stocks of soil organic carbon (OC) are currently experiencing rapid warming. Here we show that tracking total OC in the Mackenzie Delta whose upstream catchment is underlain by permafrost soils is now possible using polar-orbiting satellite ocean color observations. A non-parametric trend analysis that is valid for hydrological data shows a significant increase in dissolved OC (DOC) as well as particulate OC (POC) concentrations in late summer (0.019 and 0.069 g m−3 year−1; p < 0.05 for both). Uncertainties of the satellite estimates of DOC and POC do not influence our results. These concentration increases are not related to changes in river discharge. Parallel increases of independent long-term (1979–2018) in situ measurements of thaw depth of the active layer, as well as meteorological and hydrological patterns suggest that these late summer increases can likely be explained by increasing inputs of permafrost OC. This study shows great promise for remote, large-scale detection of catchment-scale thaw impacts from space.

The impacts of near-bed flow characteristics on river bed sediment transport under ice-covered conditions in 2016–2021

Global climate change has been projected to affect hydrology, the ice-covered flow period and river morphology (including changed sediment transport conditions) in northern high-latitude regions. To understand the impact of the expected shortening of the ice-covered period on bedload transport, one needs to understand the present sediment transport in these high-latitude rivers with annually occurring ice cover. Thus, the aims are (1) to define the impacts of ice cover on near-bed flow characteristics during hydrologically varying years, and (2) to analyse the impacts of these mid-winter flow characteristics on the bed sediment transport potential. The analyses are based on Acoustic Doppler Current Profiler (ADCP: 2016–21) and Acoustic Doppler Velocimeter (ADV: 2020–21) measurements performed in mid-winter ice-covered conditions of the sandy and small (circa 20 m wide) Pulmanki River in northern Finnish Lapland.Despite the ice-covered river conditions in winter, sediment transport occurs even during these harshest mid-winter conditions. The critical velocities and shear velocities of mid-winter conditions were exceeded in winters 2016–2021, and bedload transport occurred according to bedload measurements. Three different situations occurred regarding the bed sediment transport and near-bed velocity conditions: (1) high measured mid-winter discharges indicate high velocities throughout the meander bend; (2) low measured mid-winter discharges cause low near-bed velocities throughout the meander bend; (3) winters having intermediate discharges indicate near-bed velocities and sediment transport potential being higher at the upstream inlet and apex sections of the meander bend but clearly lower downstream of the apex. The confinement by the river ice cover, i.e. bottom-fast ice, explains the velocity variation. The near-bed velocities were the highest at the upstream inlet section of a symmetrical meander bend, where the measurement cross-sections were narrower and shallower. The velocities were the lowest downstream of the apex, where the channel changed from relatively narrow to wider and deeper.

Evaluation of the Landsat-8 Albedo Product across the Circumpolar Domain

Land surface albedo plays an extremely important role in the surface energy budget, by determining the proportion of incoming solar radiation, which is available to drive photosynthesis and surface heating, and that which is reflected directly back to space. In northern high latitude regions, the albedo of snow-covered vegetation and open, leafless forest canopies in winter, is quite high, while the albedo of boreal evergreen conifers can either be quite low (even with extensive snow lying under the canopy) or rather bright depending on the structure and density of the canopy. Here, we present the further development and evaluation of a 30 m Landsat albedo product, including an operational blue-sky albedo product, for application in the circumpolar domain. The surface reflectances from the Landsat satellite constellation are coupled with surface anisotropy information (Bidirectional Reflectance Distribution Function, BRDF) from the MODerate-resolution Imaging Spectroradiometer (MODIS). The product is extensively validated across diverse land cover and conditions and performs well with root mean squared error of 0.0395 and negligible bias when compared to coincident tower-based albedo measurements. The development of this Landsat albedo products allows for better capture of ephemeral, heterogeneous and dynamic surface conditions at the landscape scale across the circumpolar domain.

Response of Carbon Emissions and the Bacterial Community to Freeze-Thaw Cycles in a Permafrost-Affected Forest-Wetland Ecotone in Northeast China.

Climate warming can affect freeze–thaw cycle (FTCs) patterns in northern high-latitude regions and may affect permafrost carbon emissions. The response of carbon release and microbial communities to FTCs has not been well characterized. Here, we conducted laboratory incubation experiments to investigate the relationships among carbon emissions, bacterial community, and soil variables in a permafrost-affected forest–wetland ecotone in Northeast China. The emission rates of CO2 and CH4 increased during the FTCs. FTC amplitude, FTC frequency, and patch type had significant effects on carbon emissions. FTCs increased the contents of soil DOC, NH4+-N, and NO3−-N but reduced bacterial alpha diversity. CO2 emissions were mainly affected by bacterial alpha diversity and composition, and the inorganic nitrogen content was the important factor affecting CH4 emissions. Our findings indicated that FTCs could significantly regulate CO2 and CH4 emissions by reducing bacterial community diversity and increasing the concentration of available soil substrates. Our findings shed new light on the microorganism-substrate mechanisms regulating the response patterns of the soil carbon cycle to FTCs in permafrost regions.

Respiratory loss during late-growing season determines the net carbon dioxide sink in northern permafrost regions.

Warming of northern high latitude regions (NHL, > 50 °N) has increased both photosynthesis and respiration which results in considerable uncertainty regarding the net carbon dioxide (CO2) balance of NHL ecosystems. Using estimates constrained from atmospheric observations from 1980 to 2017, we find that the increasing trends of net CO2 uptake in the early-growing season are of similar magnitude across the tree cover gradient in the NHL. However, the trend of respiratory CO2 loss during late-growing season increases significantly with increasing tree cover, offsetting a larger fraction of photosynthetic CO2 uptake, and thus resulting in a slower rate of increasing annual net CO2 uptake in areas with higher tree cover, especially in central and southern boreal forest regions. The magnitude of this seasonal compensation effect explains the difference in net CO2 uptake trends along the NHL vegetation- permafrost gradient. Such seasonal compensation dynamics are not captured by dynamic global vegetation models, which simulate weaker respiration control on carbon exchange during the late-growing season, and thus calls into question projections of increasing net CO2 uptake as high latitude ecosystems respond to warming climate conditions.

Spatial distributions of &amp;lt;i&amp;gt;X&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;&amp;lt;/sub&amp;gt; seasonal cycle amplitude and phase over northern high-latitude regions

Abstract. Satellite-based observations of atmospheric carbon dioxide (CO2) provide measurements in remote regions, such as the biologically sensitive but undersampled northern high latitudes, and are progressing toward true global data coverage. Recent improvements in satellite retrievals of total column-averaged dry air mole fractions of CO2 (XCO2) from the NASA Orbiting Carbon Observatory 2 (OCO-2) have allowed for unprecedented data coverage of northern high-latitude regions, while maintaining acceptable accuracy and consistency relative to ground-based observations, and finally providing sufficient data in spring and autumn for analysis of satellite-observed XCO2 seasonal cycles across a majority of terrestrial northern high-latitude regions. Here, we present an analysis of XCO2 seasonal cycles calculated from OCO-2 data for temperate, boreal, and tundra regions, subdivided into 5∘ latitude by 20∘ longitude zones. We quantify the seasonal cycle amplitudes (SCAs) and the annual half drawdown day (HDD). OCO-2 SCAs are in good agreement with ground-based observations at five high-latitude sites, and OCO-2 SCAs show very close agreement with SCAs calculated for model estimates of XCO2 from the Copernicus Atmosphere Monitoring Services (CAMS) global inversion-optimized greenhouse gas flux model v19r1 and the CarbonTracker2019 model (CT2019B). Model estimates of XCO2 from the GEOS-Chem CO2 simulation version 12.7.2 with underlying biospheric fluxes from CarbonTracker2019 (GC-CT2019) yield SCAs of larger magnitude and spread over a larger range than those from CAMS, CT2019B, or OCO-2; however, GC-CT2019 SCAs still exhibit a very similar spatial distribution across northern high-latitude regions to that from CAMS, CT2019B, and OCO-2. Zones in the Asian boreal forest were found to have exceptionally large SCA and early HDD, and both OCO-2 data and model estimates yield a distinct longitudinal gradient of increasing SCA from west to east across the Eurasian continent. In northern high-latitude regions, spanning latitudes from 47 to 72∘ N, longitudinal gradients in both SCA and HDD are at least as pronounced as latitudinal gradients, suggesting a role for global atmospheric transport patterns in defining spatial distributions of XCO2 seasonality across these regions. GEOS-Chem surface contact tracers show that the largest XCO2 SCAs occur in areas with the greatest contact with land surfaces, integrated over 15–30 d. The correlation of XCO2 SCA with these land surface contact tracers is stronger than the correlation of XCO2 SCA with the SCA of CO2 fluxes or the total annual CO2 flux within each 5∘ latitude by 20∘ longitude zone. This indicates that accumulation of terrestrial CO2 flux during atmospheric transport is a major driver of regional variations in XCO2 SCA.

Changes in different land cover areas and NDVI values in northern latitudes from 1982 to 2015

Climate warming leads to vast changes in the land cover types and plant biomass in the northern high-latitude regions. The overall trend is of shrubland and tree lines moving northwards, while changes in different land cover types and vegetation growth in response to climate change are largely unknown. Here, we selected land areas with latitudes higher than 50°N as the study area. We compared the land cover type changes and explored relationships between the normalized difference vegetation index (NDVI) values of different land cover types, air temperature, and precipitation during 1982–2015 based on dynamic grid. The results indicated that forest and shrubland areas increased as a large area of grassland shifted to forest and shrubland. The snow/ice, tundra and grassland largely have decreased from 1982 to 2015. Although approximately 277.3 × 103 km2 of barren land (6.2% of the total barren land area in 1982) changed to tundra, the tundra area still decreased because some tundra shifted to forest and grassland. The NDVI values of tundra significantly increased, but the shrubland showed a decreasing trend. Temperature in the growing season (June to September) showed the largest positive correlation coefficients with the NDVI values of forest, tundra, grassland, and cropland. However, due to shrubification processes and plant mortality in shrubland areas, the shrubland NDVI showed negative relationship with annual temperature but positively correlated with monthly t. Taken together, although there is large room for improvement of the land cover type data accuracy, our results suggested that the land cover types in high-latitude regions changed significantly, while the NDVI values of the different land cover types showed different responses to climate change.

Winter CH4 oxidation in cold-temperate grassland soils of northern Japan: 222Rn as a proxy for the validation of CH4 diffusivity

Amidst changing seasonal snow-covered periods and extents in northern high latitude regions, winter CH4 oxidation is susceptible to drastic climate change, all of which modulates an accurate assessment of regional carbon balance. The importance of winter CH4 oxidation to the soils in these regions has previously been recognized; in this study, concentrations of CH4 and 222Rn were observed in the snowpack and underlying cold temperate soils during the winters of 1995/96 and 1996/97. These observations were carried out in order to 1) validate the 222Rn tracer as a proxy for variations in CH4 diffusivity within a snowpack containing many seasonal ice lenses, and to 2) estimate winter CH4 oxidation through the snowpack to the soils. Winter CH4 oxidations estimated by snow density (method 1); using the 222Rn tracer under steady (method 2) and non-steady (method 3) conditions; and using a static chamber (method 4) during the winter of 1996/97 yielded 16.7 ± 6.3; 8.2 ± 2.9; 9.8 ± 7.4; and 4.5 ± 2.5 mg CH4–C m−2, respectively. These results suggest that contributions from average CH4 oxidation represent 23.3% (range: 12.7–35.1%) of the annual CH4 oxidation into cold temperate soils. Wintertime CH4 oxidation significantly reflects temporal variation in the snowpack, demonstrating that soil temperature depends on snow cover, and that a threshold snow depth of 40 cm does not prevent, but rather promotes, atmospheric CH4 oxidation through snowpack to the soils. These findings demonstrate that the 222Rn tracer was useful for estimating winter CH4 diffusivity and oxidation, using a one-dimensional vertical diffusion box model in cold temperate soils in Japan during multiple snow-covered seasons.

The influence of the Madden-Julian oscillation on high-latitude surface air temperature during boreal winter

By analyzing NCEP-NCAR reanalysis daily data for 1979–2016, the modulation by Madden-Julian Oscillation (MJO) of the wintertime surface air temperature (SAT) over high latitude is examined. The real-time multivariate MJO (RMM) index, which divides the MJO into eight phases, is used. It is found that a significantly negative SAT anomaly over the northern high latitude region of (180°–60 °W, 60°–90 °N) lags the MJO convection for 1∼2 weeks in phase 3, in which the enhanced convective activity exists over the Indian Ocean. While a significantly positive SAT anomaly appears over the same region following the MJO phase 7, as the tropical heating shows an opposite sign. Analysis of the anomalous circulation indicates that the observed SAT signal is probably a result of the northeastward propagating Rossby wave train triggered by MJO-related tropical forcing through Rossby wave energy dispersion. By using an anomalous atmospheric general circulation model (AGCM), the significant effect of tropical forcing on organizing the extratropical circulation anomaly is confirmed. Analysis of a temperature tendency equation further reveals that the intraseasonal SAT anomaly is primarily attributed to the advection of the mean temperature by the wind anomaly associated with the anomalous circulation of the MJO-related variability.

EUV-dependence of Venusian dayside ionopause altitude: VEX and PVO observations

The Venusian dayside ionosphere, similar to other planetary ionospheres, is produced primarily by ionization of its neutral upper atmosphere due to solar extreme ultraviolet (EUV) radiation. It has become clear that the expansion of the ionosphere may be strongly controlled by the EUV level, as exhibited in data collected by the Pioneer Venus Orbiter (PVO) during one solar cycle (1978–1992). However, the EUV-dependence of the Venusian dayside ionopause altitude, which defines the outer boundary of the ionosphere, remains obscure because the PVO crossed the dayside ionopause only during the solar maximum; its periapsis lifted too high during the solar minimum. Recently, during the period 2006–2014, which included the longest and quietest solar minimum of the past several decades, Venus Express (VEX) provided measurements of the photoelectron boundary (PEB) over the northern high-latitude region. Since the photoelectron boundary is closely related to the ionopause, we have an opportunity to analyze the EUV effect on the dayside ionopause by combining PVO and VEX observations. We have evaluated and then reduced the orbit bias effect in data from both PVO and VEX, and then used the results to derive a relationship between solar EUV level and the dayside ionopause altitude. We find that the dayside ionopause altitude increases as the solar EUV level increases, which is consistent with theoretical expectations.

Dissolved Organic Carbon Turnover in Permafrost-Influenced Watersheds of Interior Alaska: Molecular Insights and the Priming Effect

Increased permafrost thaw due to climate change in northern high-latitudes has prompted concern over impacts on soil and stream biogeochemistry that affect the fate of dissolved organic carbon (DOC). Few studies to-date have examined the link between molecular composition and biolability of dissolved organic matter (DOM) mobilized from different soil horizons despite its importance in understanding carbon turnover in aquatic systems. Additionally, the effect of mixed DOM sources on microbial metabolism (e.g., priming) is not well understood. No studies to-date have addressed potential priming effects in northern high-latitude or permafrost-influenced aquatic ecosystems, yet these ecosystems may be hot spots of priming where biolabile, ancient permafrost DOC mixes with relatively stable, modern stream DOC. To assess biodegradability and priming of DOC in permafrost-influenced streams, we conducted 28 day bioincubation experiments utilizing a suite of stream samples and leachates of fresh vegetation and different soil horizons, including permafrost, from Interior Alaska. The molecular composition of unamended DOM samples at initial and final time points was determined by ultrahigh resolution mass spectrometry. Initial molecular composition was correlated to DOC biodegradability, particularly the contribution of energy-rich aliphatic compounds, and stream microbial communities utilized 50-56% of aliphatics in permafrost-derived DOM within 28 days. Biodegradability of DOC followed a continuum from relatively stable stream DOC to relatively biolabile DOC derived from permafrost, active layer organic soil, and vegetation leachates. Microbial utilization of DOC was ~3-11% for stream bioincubations and ranged from 9% (active layer mineral soil-derived) to 66% (vegetation-derived) for leachate bioincubations. To investigate the presence or absence of a priming effect, bioincubation experiments included treatments amended with 1% relative carbon concentrations of simple, biolabile organic carbon substrates (i.e., primers). The amount of DOC consumed in primed treatments was not significantly different from the control in any of the bioincubation experiments after 28 days, making it apparent that the addition of biolabile permafrost-derived DOC to aquatic ecosystems will likely not enhance the biodegradation of relatively modern, stable DOC sources. Thus, future projections of carbon turnover in northern high-latitude region streams may not have to account for a priming effect.