Northern Middle Latitudes Research Articles

Studying the Aerosol Effect on Deep Convective Clouds over the Global Oceans by Applying Machine Learning Techniques on Long-Term Satellite Observation

Long-term (1982–2019) satellite climate data records (CDRs) of aerosols and clouds, reanalysis data of meteorological fields, and machine learning techniques are used to study the aerosol effect on deep convective clouds (DCCs) over the global oceans from a climatological perspective. Our analyses are focused on three latitude belts where DCCs appear more frequently in the climatology: the northern middle latitude (NML), tropical latitude (TRL), and southern middle latitude (SML). It was found that the aerosol effect on marine DCCs may be detected only in NML from long-term averaged satellite aerosol and cloud observations. Specifically, cloud particle size is more susceptible to the aerosol effect compared to other cloud micro-physical variables (e.g., cloud optical depth). The signature of the aerosol effect on DCCs can be easily obscured by meteorological covariances for cloud macro-physical variables, such as cloud cover and cloud top temperature (CTT). From a machine learning analysis, we found that the primary aerosol effect (i.e., the aerosol effect without meteorological feedbacks and covariances) can partially explain the aerosol convective invigoration in CTT and that meteorological feedbacks and covariances need to be included to accurately capture the aerosol convective invigoration. From our singular value decomposition (SVD) analysis, we found the aerosol effects in the three leading principal components (PCs) may explain about one third of the variance of satellite-observed cloud variables and significant positive or negative trends are only observed in the lead PC1 of cloud and aerosol variables. The lead PC1 component is an effective mode for detecting the aerosol effect on DCCs. Our results are valuable for the evaluation and improvement of aerosol-cloud interactions in the long-term climate simulations of global climate models.

Characteristics and drivers of vegetation productivity sensitivity to increasing CO2 at Northern Middle and High Latitudes.

Understanding and accurately predicting how the sensitivity of terrestrial vegetation productivity to rising atmospheric CO2 concentration (β) is crucial for assessing carbon sink dynamics. However, the temporal characteristics and driving mechanisms of β remain uncertain. Here, observational and CMIP6 modeling evidence suggest a decreasing trend in β at the Northern Middle and High Latitudes during the historical period of 1982-2015 (-0.082 ± 0.005% 100 ppm-1 year-1). This decreasing trend is projected to persist until the end of the 21st century (-0.082 ± 0.005% 100 ppm-1 year-1 under SSP370 and -0.166 ± 0.006% 100 ppm-1 year-1 under SSP585). The declining β indicates a weakening capacity of vegetation to mitigate warming climates, posing challenges for achieving the temperature goals of the Paris Agreement. The rise in vapor pressure deficit (VPD), that triggers stomata closure and weakens photosynthesis, is considered as the dominated factor contributing to the historical and future decline in β, accounting for 62.3%-75.2% of the effect. Nutrient availability and water availability contribute 15.7%-21.4% and 8.5%-16.3%, respectively. These findings underscore the significant role of VPD in shaping terrestrial carbon sink dynamics, an aspect that is currently insufficiently considered in many climate and ecological models.

Tracing earthquakes through lake sediments for better prediction

<p>Intensive climatic extremes and frequent earthquakes, both in China and other regions, have captured public attention recently. The Tibetan Plateau at middle northern latitudes boasts large glacier and snow coverage, coupled with frequent tectonic activities, exerting profound yet poorly understood influences on zonal atmospheric motion and energy flux during interhemispheric coupling. This study proposes a comprehensive process-based approach involving multi-proxies focused on alpine lakes to illuminate the intricate interactions among tectonics, glacial processes and hydro-climatic variations. This perspective can enhance our understanding of the spatiotemporal diversity of the local-to-global hydro-climatic change pattern, particularly forced by Tibetan Plateau, and suggests to consider its combined summer thermal and tectonic forcing. Additionally, the approach provides unbiased geological data extracted from lakes, contributing to the optimization of hazard and climate models for better predictions of tectonics, abrupt and irreversible climate tipping points with confidence.</p>

Zonally Asymmetric Temperature Trends near the Northern Middle and High Latitude Stratopause during Winter

Zonally Asymmetric Temperature Trends near the Northern Middle and High Latitude Stratopause during Winter

The ‘seed-fern’ Lepidopteris mass-produced the abnormal pollen Ricciisporites during the end-Triassic biotic crisis

The end-Triassic mass extinction (ETE; ~201.6 million years ago) led to dramatic changes in terrestrial ecosystems including the extinction of several seed-plant groups. Among the most intriguing features in the vegetation signal is the dominance of the peculiar pollen, Ricciisporites tuberculatus Lundblad, across large areas of the Northern Hemisphere immediately prior to and during the ETE. The parent plant of this pollen has remained unknown for 70 years. Here, we demonstrate that the ‘seed-fern’ Lepidopteris ottonis (Göppert) Schimper (Peltaspermales) produced R. tuberculatus in permanent tetrads. We show that R. tuberculatus is a large, abnormal form of the small smooth-walled monosulcate pollen traditionally associated with L.ottonis, which disappeared at the ETE, when volcanism induced cold-spells followed by global warming. We argue that the production of aberrant R. tuberculatus resulted from ecological pressure in stressed environments that favoured asexual reproduction in peltasperms. The expansion of dry environments led to the dominance of drought-tolerant plants in the Early Jurassic of northern middle latitudes.

New species of Kuqaia from the Lower Jurassic of Sweden indicates a possible water flea (Crustacea: Branchiopoda) affinity.

The enigmatic acid-resistant mesofossil genus Kuqaia is emended, a new species (Kuqaia scanicus) is instituted, and three established species are described from the Lower Jurassic (lower Pliensbachian) of the Kävlinge BH-928 core, in southern Sweden. Kuqaia has a distribution across the middle northern latitudes of Pangaea and is restricted to Lower to lower Middle Jurassic strata. Morphological characters support Kuqaia being the ephippia (resting egg/embryo cases) of Cladocera (Crustacea: Branchiopoda), and a probable early stem-group taxon of the Daphnia lineage. The paleoecology of the small planktonic crustaceans indicate purely fresh-water environments, such as lakes or ponds, all occurrences being in continental deposits, and the Kuqaia specimens possibly represent dry-season resting eggs. Chemical analyses of these and similar fossils, and of extant invertebrate eggs and egg cases are recommended to improve resolution of the biological affiliations of such mesofossil groups.

Daytime warming strengthened delaying effect of precipitation on end of the vegetation growing season on the Tibetan Plateau

Many studies have reported that daytime warming advances the end of the vegetation growing season (EOS) in arid and semi-arid ecosystems in the northern middle latitudes. This finding, however, seems to contradict the fact that low temperature constrains alpine vegetation activity. Using EOS from 1982 to 2015 retrieved from satellite observations, we show that daytime warming could facilitate a delay in EOS on the Tibetan Plateau, the world's largest and highest alpine region, with a dry and cold climate. Our analysis revealed a positive partial correlation (REOS–Tmax) between EOS and preseason mean daily maximum temperature (Tmax) on 57 % of the Plateau in wetter years, but on only 41 % of it in drier years. At a regional level, REOS–Tmax was 0.69 (P < 0.05, t-test) during wetter years and −0.56 (P = 0.11) during drier years, indicating that daytime warming could directly delay EOS on the Plateau. On the other hand, we found a positive partial correlation (REOS–Prec) between EOS and preseason cumulative precipitation on 62 % of the Plateau during warmer years, but on only 47 % during colder years. At a regional level, REOS–Prec was 0.68 (P < 0.05) during warmer years and − 0.28 (P = 0.46) during colder years. Moreover, REOS–Prec increased on 60 % of the Plateau under increasing Tmax during 1982–2015, suggesting that daytime warming facilitates a delay in EOS on the Tibetan Plateau by regulating the effect of precipitation on EOS. Thus, to improve autumn phenology models in this region, researchers should consider the interactive effects of temperature and precipitation on EOS.

Impacts of the Arctic stratospheric polar vortex changes on the frontogenesis over the northern middle latitudes during winter

The impacts of the Arctic stratospheric polar vortex (SPV) changes on wintertime frontogenesis in the northern middle latitudes are analyzed. Both composite analysis and model simulations reveal that the intensity and frequency of frontogenesis over West Russia, the Mongolian Plateau, the Mediterranean and the southern North Atlantic during weak SPV years are significantly stronger and larger than those during strong SPV years, while the frontogenesis over the northern parts of the North Atlantic and North Pacific Oceans are weaker and less occur during weak SPV years. These features are more noticeable in middle January and February. The contributions of resultant deformation changes to frontogenesis intensity changes over most regions of the middle latitudes are larger than those of horizontal divergence changes, and the contribution of stretching deformation is slightly larger than that of shearing deformation. The changes in frontogenesis intensity are attributed to changes in the tropospheric circulation and temperature gradient associated with the SPV changes. Potential vorticity (PV) anomalies in the upper troposphere and lower stratosphere (UTLS) caused by the weakened and shifted SPV towards Eurasia lead to tropospheric cyclonic flows, favoring more cold-air mass transported towards mid-latitude Eurasia. Meanwhile, more high-PV air towards Eurasia results in steeper tropospheric isentropes during weak SPV years. Consequently, both temperature gradient and frontogenesis over Russia are enhanced. More southward transport of cold-air mass due to the equatorward shift of the polar jet stream induced by the weak SPV enhances the frontogenesis over the southern North Atlantic. Furthermore, the angle between dilatation axis and the isentropes over West Russia, the Mongolian Plateau and the southern North Atlantic is more likely to occur between 0° and 45°, which promotes stronger frontogenesis over these regions during weak SPV years. By contrast, opposite processes occur over the northern North Atlantic, causing weaker frontogenesis there.

How Well Can Current Climate Models Simulate the Connection of the Early Spring Aleutian Low to the Following Winter ENSO?

Abstract A recent study revealed an impact of the intensity of early spring Aleutian low (AL) on the succeeding winter ENSO. This study examines the ability of 41 climate models that participated in CMIP6 in simulating the early spring AL–winter ENSO connection. It is shown that there exists a large diversity among the models in simulating this AL–ENSO linkage. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. Diversity of the AL–ENSO connection is related to the spread in the spatial structure of AL-related atmospheric anomalies over the North Pacific. In the models that capture the observed AL–ENSO connection, weakened AL induces an anomalous anticyclone over the northern middle and high latitudes and an anomalous cyclone over the subtropical North Pacific. The resultant westerly wind anomalies over the tropical western-central Pacific (TWCP) induce an El Niño sea surface temperature (SST) anomaly pattern in the following winter. By contrast, in the models with the AL–ENSO relation opposite to the observations, the AL-associated anomalous anticyclone over the North Pacific extends too southward. As such, the subtropical North Pacific is dominated by northeasterly wind anomalies and SST cooling. The subtropical North Pacific SST cooling induces easterly wind anomalies over the TWCP via wind–evaporation–SST feedback, and leads to a La Niña anomaly pattern in the following winter. The spread in the spatial structure of the AL-associated atmospheric anomalies over the North Pacific is partly due to the diversity in the amplitude of the climatological mean flow. Significance Statement A recent study suggested that variation of the AL intensity in early spring could exert a significant impact on the following winter ENSO. It indicated that inclusion of the early spring AL signal could improve the prediction of ENSO and to some extent help reduce the spring predictability barrier of ENSO. To employ the AL as a predictor in the ENSO prediction and forecast, the current climate model should have the ability to simulate realistically the early spring AL variation as well as the physical process linking the early spring AL with the subsequent winter ENSO. Hence, this study examines the performance of the current coupled climate models that participated in the phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating the linkage between the early spring AL and the following winter ENSO. We show that there exists a large diversity among the CMIP6 models in simulating the early spring AL–winter ENSO connection. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. The factors leading to the spread are further examined. Results of this study would have implications in improving our understanding of the impact of extratropical atmospheric forcing on the ENSO and improving the seasonal forecasting of the ENSO.

Responses of autumn vegetation phenology to climate change and urbanization at northern middle and high latitudes

• Urbanization influences the vegetation EOS depending on the temperature context. • Urbanization induced advanced vegetation EOS by increasing nighttime temperature. • Strengthened urbanization weakens the effects of temperature on vegetation dormancy. Shifts in vegetation phenology caused by climate change and human activities play a crucial role in regulating global terrestrial ecosystems' nutrients and carbon cycling. However, the mechanism underlying autumn vegetation’s phenological reaction to urbanization remains unexplained. Using normalized difference vegetation index (NDVI) and a nighttime light remote sensing dataset, we examined the dynamics of the end of the growing season (EOS) and responses to climate change and urbanization from 1992 to 2015 across the northern middle and high latitudes (north of 30° N). EOS exhibited an advanced trend with a pace of −0.22 days/yr across the study area. The spatial proportion of advanced EOS was 62.66%, which was more pronounced north of 60° N. In most regions, increased precipitation delayed EOS, whereas daytime temperature (Tmax) and nighttime temperature (Tmin) exhibited the opposite impacts on EOS, which was related to water availability in the study area. In general, we found that higher urbanization could induce earlier EOS in warm regions (south of 46° N) and delay EOS in cold regions (north of 46° N). Urbanization could cause an increase in Tmin and Tmax, and EOS exhibited a significant negative sensitivity to Tmin across the urbanization levels, which decreased from −0.175 days/°C to −0.112 days/°C. Our results indicated that urbanization could advance the EOS by increasing Tmin, however, such a negative effect may be weakened with the development of urbanization. Our findings demonstrated the complicated influence mechanisms of autumn vegetation phenology induced by climate change and urbanization, which provides a unique perspective for exploring the contribution of growing season vegetation to the carbon sink and global carbon balance.

Global Vertical Distribution of Water Vapor on Mars: Results From 3.5Years of ExoMars-TGO/NOMAD Science Operations.

We present water vapor vertical distributions on Mars retrieved from 3.5years of solar occultation measurements by Nadir and Occultation for Mars Discovery onboard the ExoMars Trace Gas Orbiter, which reveal a strong contrast between aphelion and perihelion water climates. In equinox periods, most of water vapor is confined into the low-middle latitudes. In aphelion periods, water vapor sublimated from the northern polar cap is confined into very low altitudes-water vapor mixing ratios observed at the 0-5km lower boundary of measurement decrease by an order of magnitude at the approximate altitudes of 15 and 30km for the latitudes higher than 50°N and 30-50°N, respectively. The vertical confinement of water vapor at northern middle latitudes around aphelion is more pronounced in the morning terminators than evening, perhaps controlled by the diurnal cycle of cloud formation. Water vapor is also observed over the low latitude regions in the aphelion southern hemisphere (0-30°S) mostly below 10-20km, which suggests north-south transport of water still occurs. In perihelion periods, water vapor sublimated from the southern polar cap directly reaches high altitudes (>80km) over high southern latitudes, suggesting more effective transport by the meridional circulation without condensation. We show that heating during perihelion, sporadic global dust storms, and regional dust storms occurring annually around 330° of solar longitude (L S) are the main events to supply water vapor to the upper atmosphere above 70km.

Global-Scale Interpretable Drought Reconstruction Utilizing Anomalies of Atmospheric Dynamics

Abstract Droughts and associated near-surface temperature anomalies can be attributed to amplified vertical subsidence and anomalous anticyclonic circulations from dynamic perspectives. However, two open and interesting issues remain unknown: 1) whether hydrometeorological situations under droughts can be reproduced directly utilizing variability of atmospheric dynamics and 2) what specific roles atmospheric dynamics play in drought reconstruction. To explore these questions, this study employs three kinds of dynamic features (i.e., vertical velocity, relative vorticity, and horizontal divergence) for hydrometeorological reconstruction (e.g., precipitation and near-surface air temperature) under drought situations through a so-called XGBoost (extreme gradient boosting) ensemble learning method. The study adopts two different reconstruction schemes (i.e., statistically preexisting dynamic–hydrometeorological relationships and interannual variability) and finds dynamically based reconstruction feasible. The three main achievements are as follows. 1) Regarding different hydrometeorological situations reconstructed with preexisting dynamic–hydrometeorological relationships, good reconstruction performance can be captured with the same or different lead times, depending on whether the evolution of dynamic anomalies (e.g., vertical motion and relative vorticity) is temporally asynchronous. 2) Reconstruction on the interannual scale performs relatively well, seemingly regardless of seasonality and drought-inducing mechanisms. 3) More importantly, from interpretable perspectives, global-scale analysis of dynamic contributions helps discover unexpected dynamic drought-inducing roles and associated latitudinal modulation. That is, low-level cyclonic/anticyclonic anomalies contribute to drought development in the northern middle and high latitudes, while upper-level vertical subsidence contributes significantly to tropical near-surface temperature anomalies concurrent with droughts. These achievements could provide guidance for dynamically based drought monitoring and prediction in different geographic regions. Significance Statement It is common sense that severe drought events are physically attributable to amplified vertical subsidence and anomalous anticyclonic circulations. However, the specific contributions of atmospheric dynamics, together with the feasibility of dynamically based drought reconstruction, are crucial components that are seldom investigated. To our knowledge, this manuscript is the first to reconstruct drought utilizing atmospheric dynamics and to interpret quantified dynamic contributions; it also represents a new interdisciplinary attempt to reproduce hydrological variability based on routine atmospheric dynamic variables.

Divergent responses of autumn vegetation phenology to climate extremes over northern middle and high latitudes

AbstractAimCompared with gradual climate change, extreme climatic events have more direct and dramatic impacts on vegetation growth. However, the influence of climate extremes on important phenological periods, such as the end of the growing season (EOS), remains unclear. Here, we investigate the temporal trends of EOS across different biomes and quantify the response of EOS to multiple climate extreme indices (CEIs).LocationNorthern middle and high latitudes.Time period2000–2020.Major taxa studiedPlants.MethodsThree phenology extraction methods were used to compute EOS from satellite, FLUXNET and Pan European Phenology Project PEP725 phenological datasets. Different stress states of cold, hot, dry and wet extremes were represented by 12 CEIs. Partial correlation and ridge regression analysis were used to quantify the response of EOS to climate extremes across latitudinal and biome scales.ResultsOur study showed a delayed EOS in boreal biomes, but a significantly advanced EOS in temperate biomes. The advanced EOS induced by cold stress was observed for c. 80% of the vegetated pixels. The warm‐related CEIs delayed the EOS in high latitudes, and the delayed effect weakened or even reversed with decreasing latitude. In contrast, EOS exhibited opposite response patterns to dry days and wet‐related CEIs. Overall, EOS exhibited higher sensitivity to extreme temperature in boreal biomes than in temperate biomes. Specifically, continuous drought and high heat stress induced an earlier EOS in some temperate forest biomes, whereas moderate heat stress delayed the EOS in most study biomes. In contrast, EOS was not sensitive to extreme drought in water‐restricted biomes.Main conclusionsEOS exhibited divergent responses to various climate extremes with different intensities and frequencies. Moreover, the response of EOS to extreme climate stress was dependent on the biome and latitude. These findings emphasize the importance of incorporating the divergent extreme climate effects into vegetation phenological models and Earth system models.

Trends in the tropospheric general circulation from 1979 to 2022

Abstract. Atmospheric general circulation changes from March 1979 to February 2022 are examined using the ERA5 reanalysis. Maps of linear trends and time series for specific areas are presented. Attention is concentrated on monthly, seasonal and annual means, but shorter-timescale variability is also considered, including extremes. Changes in near-tropopause winds are the main focus, but related changes in temperature, wind and other variables throughout the troposphere are discussed. Middle- and upper-tropospheric warming is larger in the subtropics and outer tropics than in the deep tropics, except over the Pacific. This is linked with a strengthening and meridional expansion of the tropical easterlies that has received little previous attention. The change occurs predominantly over the first half of the period. Warming over several mid-latitude and subtropical land areas comes close to matching the large warming of the Arctic, in some seasons at least. Westerly upper-level winds in general weaken over the Arctic in winter but strengthen in northern middle latitudes, contrary to arguments based on circulation changes due solely to amplified Arctic warming. The jet-stream region over the eastern North Atlantic and western Europe shifts southward. Westerlies strengthen in a band stretching south-eastwards from the tropical western Pacific to southern Australia, as well as in the polar-jet-stream region that surrounds Antarctica. Extreme jet-stream winds increase over the North Atlantic. Net kinetic energy also increases, mostly associated with sub-monthly variability along the mid-latitude storm tracks and over the tropical Pacific. Available potential energy changes less. Geopotential height shows a distinct pattern of change in stationary long-wave structures. There are increases in surface pressure over the North Pacific and southern mid-latitudes and decreases over the Arctic Ocean and offshore of Antarctica. Several comparisons are made between ERA5 and the JRA-55 reanalysis and between ERA5 and the observations it assimilated. They show reassuring agreement, but some regional differences require further investigation.

Evaluation of remote sensing-based evapotranspiration products at low-latitude eddy covariance sites

Remote sensing-based evapotranspiration (ET) products have been evaluated primarily using data from northern middle latitudes; therefore, little is known about their performance at low latitudes. To address this bias, an evaluation dataset was compiled using eddy covariance data from 40 sites between latitudes 30° S and 30° N. The flux data were obtained from the emerging network in Mexico (MexFlux) and from openly available databases of FLUXNET, AsiaFlux, and OzFlux. This unique reference dataset was then used to evaluate remote sensing-based ET products in environments that have been underrepresented in earlier studies. The evaluated products were: MODIS ET (MOD16, both the discontinued collection 5 (C5) and the latest collection (C6)), Global Land Evaporation Amsterdam Model (GLEAM) ET, and Atmosphere-Land Exchange Inverse (ALEXI) ET. Products were compared with unadjusted fluxes (ETorig) and with fluxes corrected for the lack of energy balance closure (ETebc). Three common statistical metrics were used: coefficient of determination (R2), root mean square error (RMSE), and percent bias (PBIAS). The effect of a vegetation mismatch between pixel and site on product evaluation results was investigated by examining the relationship between the statistical metrics and product-specific vegetation match indexes. Evaluation results of this study and those published in the literature were used to examine the performance of the products across latitudes. Differences between the MOD16 collection 5 and 6 datasets were generally smaller than differences with the other products. Performance and ranking of the evaluated products depended on whether ETorig or ETebc was used. When using ETorig, GLEAM generally had the highest R2, smallest PBIAS, and best RMSE values across the studied land cover types and climate zones. Neither MOD16 nor ALEXI performed consistently better than the other. When using ETebc, none of the products stood out in terms of both low bias and strong correlations. The use of ETebc instead of ETorig affected the biases more than the correlations. The product evaluation results showed no significant relationship with the degree of match between the vegetation at the pixel and site scale. The latitudinal comparison showed tendencies of lower R2 (all products) but better PBIAS and normalized RMSE values (MOD16 and GLEAM) for forests at low latitudes than for forests at northern middle latitudes. For non-forest vegetation, the products showed no clear latitudinal differences in performance.

Global Terrestrial Ecosystem Carbon Flux Inferred from TanSat XCO 2 Retrievals

TanSat is China’s first greenhouse gases observing satellite. In recent years, substantial progresses have been achieved on retrieving column-averaged CO 2 dry air mole fraction (XCO 2 ). However, relatively few attempts have been made to estimate terrestrial net ecosystem exchange (NEE) using TanSat XCO 2 retrievals. In this study, based on the GEOS-Chem 4D-Var data assimilation system, we infer the global NEE from April 2017 to March 2018 using TanSat XCO 2 . The inversion estimates global NEE at −3.46 PgC yr -1 , evidently higher than prior estimate and giving rise to an improved estimate of global atmospheric CO 2 growth rate. Regionally, our inversion greatly increases the carbon uptakes in northern mid-to-high latitudes and significantly enhances the carbon releases in tropical and southern lands, especially in Africa and India peninsula. The increase of posterior sinks in northern lands is mainly attributed to the decreased carbon release during the nongrowing season, and the decrease of carbon uptakes in tropical and southern lands basically occurs throughout the year. Evaluations against independent CO 2 observations and comparison with previous estimates indicate that although the land sinks in the northern middle latitudes and southern temperate regions are improved to a certain extent, they are obviously overestimated in northern high latitudes and underestimated in tropical lands (mainly northern Africa), respectively. These results suggest that TanSat XCO 2 retrievals may have systematic negative biases in northern high latitudes and large positive biases over northern Africa, and further efforts are required to remove bias in these regions for better estimates of global and regional NEE.

Greater temperature sensitivity of vegetation greenup onset date in areas with weaker temperature seasonality across the Northern Hemisphere

The temperature sensitivity (ST) of the vegetation greenup onset date (VGD) is critical for the assessment and prediction of phenological response to climate warming. Spatial variations in ST, though well documented based on satellite observations in recent studies, have remained largely under-explained. We determined VGD from satellite observation of the normalized difference vegetation index over the period 2000–2018 and showed that ST is substantially smaller in areas with stronger temperature seasonality (i.e. the multiyear mean of standard deviations of monthly mean temperature over a 12-month period). Spatially, ST averaged over each 2 °C bin of temperature seasonality increased from about −10.0 days K−1 in areas with temperature seasonality of about 3.5 °C to about −3.3 to −2.3 days K−1 in areas with temperature seasonality between 15.0 and 22.3 °C. Statistical analyses suggest that this pattern was not related with spatial variations in the length of chilling exposure, spring frost risk, inter-annual variability of spring temperature, or photoperiod. Rather, it may occur because areas with stronger temperature seasonality have stronger pre-VGD constraints because of a lower temperature and a faster temperature increase from the onset of greenup to summer, which could result in faster leaf development rates and thus higher speeds of completion of leaf development. Our results also show that the greater ST associated with higher mean annual temperature and multiyear mean VGD across the northern middle and high latitudes observed in previous studies is probably caused by temperature seasonality. This study deepens our understanding of the spatial variation in the temperature sensitivity of vegetation spring leafing phenology in the Northern Hemisphere.

Nonclimatic and extrabasinal processes controlled pedogenesis in paleosols of the Paleocene Nacimiento Formation, New Mexico, USA

AbstractPaleosols can represent intervals of nondeposition in sedimentary packages and are used with increasing frequency as proxies for paleoenvironmental conditions during basin filling. However, the complexities of factors both internal and external to pedogenesis require consideration of paleosols in any basin or sedimentary package in a broader context than is often assumed. With this in mind, we measured and analyzed pedogenic features, stratigraphic position, geochemical composition, and petrography of paleosols in the Nacimiento Formation to gain insight into the paleoenvironmental conditions of the early Paleocene in the San Juan Basin. During this interval, the San Juan Basin was located in middle northern latitudes (~40°N) and saw rapid terrestrial siliciclastic sedimentation related to Laramide tectonism. Evidence from earlier researchers suggests that prevailing climate conditions during Nacimiento Formation deposition were warm and humid. We used morphological properties of paleosols to categorize paleosols into pedotypes indicative of distinct pedogenic conditions. The general stratigraphic distribution of these pedotypes shows an up-section increase in soil drainage conditions through the Nacimiento Formation that cannot be correlated with known climate changes. We suggest that fluvial system evolution was the major control on pedogenic conditions. We investigated Nacimiento Formation paleosols with widely used paleosol geochemical climate analyses, which provided paleoclimate estimates that are in disaccord with independently derived estimates. We show that in alluvial depositional systems with source areas in weathered sedimentary rocks, these analyses can be difficult to interpret and likely lead to estimates that do not reflect true pedogenic conditions during the postdepositional near-surface alteration of sediments. Petrographic analysis of Nacimiento Formation paleosols showed that some likely formed under semiarid to subhumid conditions that allowed pedogenic accumulation and illuviation of smectite clays yet did not substantially chemically alter primary detrital plagioclase feldspar grains in paleosol B horizons. The paleosols of the Nacimiento Formation, when analyzed at the basin scale, show that sedimentary aggradational processes can overpower climate processes in creation of a sequence of paleosols in a stratigraphic section. In addition, the incorporation of clays from sedimentary rocks in a basin's source area can drastically skew the geochemical signatures and therefore interpretations of paleosols in that basin.

Leveraging observed soil heterotrophic respiration fluxes as a novel constraint on global-scale models.

Microbially explicit models may improve understanding and projections of carbon dynamics in response to future climate change, but their fidelity in simulating global-scale soil heterotrophic respiration (RH ), a stringent test for soil biogeochemical models, has never been evaluated. We used statistical global RH products, as well as 7821 daily site-scale RH measurements, to evaluate the spatiotemporal performance of one first-order decay model (CASA-CNP) and two microbially explicit biogeochemical models (CORPSE and MIMICS) that were forced by two different input datasets. CORPSE and MIMICS did not provide any measurable performance improvement; instead, the models were highly sensitive to the input data used to drive them. Spatial variability in RH fluxes was generally well simulated except in the northern middle latitudes (~50°N) and arid regions; models captured the seasonal variability of RH well, but showed more divergence in tropic and arctic regions. Our results demonstrate that the next generation of biogeochemical models shows promise but also needs to be improved for realistic spatiotemporal variability of RH . Finally, we emphasize the importance of net primary production, soil moisture, and soil temperature inputs, and that jointly evaluating soil models for their spatial (global scale) and temporal (site scale) performance provides crucial benchmarks for improving biogeochemical models.

Climatology of Interhemispheric Mesopause Temperatures Using the High‐Latitude and Middle‐Latitude Meteor Radars

AbstractWe present the climatology of mesopause temperatures using high‐latitude and middle‐latitude meteor radars. The daily mesopause temperatures are estimated using ambipolar diffusion coefficient data from the meteor radars at Davis Station (68.6°S, 77.9°E), in Antarctica, Svalbard (78.3°N, 16°E), Tromsø (69.6°N, 19.2°E) in the Arctic, and Mohe (53.5°N, 122.3°E) and Beijing (40.3°N, 116.2°E) in the northern middle latitudes. The seasonal variations in the meteor radar‐derived temperatures are in good agreement with the temperatures from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the TIMED satellite. Interhemispheric observations indicate that the mesopause temperatures over the southern and northern polar regions show a clear seasonal asymmetry. The seasonal variations in the Davis Station meteor radar temperatures in the southern polar mesopause are dominated by an annual oscillation (AO) with a relatively weak semiannual oscillation (SAO), which show a clear minimum during summer and a maximum during winter. The mesopause temperatures in the northern high and middle latitudes observed by the Svalbard, Tromsø, Mohe, and Beijing meteor radars mainly show an AO, with a maximum during winter and a minimum during summer. The AO in the northern polar regions is stronger than that in the southern polar regions, while the SAO in the southern polar regions is relatively strong compared to that in the northern polar regions.