Low Latitudes Research Articles

Millennial to orbital scale Indian summer monsoon evolution inferred from grain size end-members in Tengchongbeihai wetland, southwestern China

Understanding the Indian summer monsoon (ISM) variabilities under different climatic constraints on various timescales is crucial for socio-economic stabilization within the ongoing warming globe. However, aside from stalagmite records, very few terrestrial paleoclimate archives have been used to explore the last glacial ISM characteristic, which limits our understanding of the ISM natural behaviors and their underlying mechanisms on longer timescales. Here, we present a grain size end-member analysis for Tengchongbeihai wetland sediments from southwestern China to investigate the paleo-hydroclimate changes since the late MIS3. The overall low fine silt content before ∼19 cal ka BP resulted from weak catchment erosion indicating weak ISM during cold glacial while the increasing trend thereafter indicates the ISM strengthened during the last deglaciation and Holocene. Such an orbital ISM evolution resembles to the insolation gradient between 25°N and 25°S during the boreal summer periods, which implies the transequatorial insolation difference could be an alternative explanation for the orbital scale low latitude hydroclimate changes. The remarkable peaks of coarse component around ∼ 9, 12, 19, 26, 32, 37 and 42 cal ka BP were related to significant water level descending indicating the millennial ISM collapse corresponding to the YD and Heinrich events. However, the absence of sawtooth feature as the NGRIP δ18O and the well correspondence to the Indian Ocean sea surface temperatures (SSTs) of the ISM collapses evidence the modulation of low latitude forcing on the North Atlantic climate effects on the millennial scale hydroclimate variabilities.

Holocene biome reconstruction on the Chinese loess plateau based on pollen

Reconstruction of past vegetation changes is critical for sustainable development in the fragile and highly eroded ecosystems on the Chinese Loess Plateau (CLP) under global change, particularly in light of future warming events. In this study, we collected fossil pollen data from 40 sites to reconstruct the Holocene (since 11 ka) vegetation pattern of the CLP using the biomization method. The results demonstrate that the CLP vegetation during the entire Holocene was dominated by grassland landscapes in most areas and periods, whereas the forest landscape only developed in some areas during certain periods. Steppe vegetation rather than forest vegetation was distributed in the central region of the CLP. Six forest biomes were primarily distributed in the southwestern and northeastern plateau high-altitude areas, showing notable expansion toward low latitudes and longitudes during the early to mid-Holocene. The steppe biome showed a rather wide distribution, primarily in the center and south, and especially in the expanded western region during the Early Holocene. Minor variations were observed for the widely distributed steppe biomes that shifted to high longitudes and latitudes during the Holocene. The desert biome was identified in the relatively restricted western and northern regions of China. Reconstructed biome spatial distributions showed a large vegetation gradient on the CLP. This suggests that variations in forest biomes on the CLP were potentially influenced by changes in precipitation. Different topographies within the same area led to different biomes. In addition, increased human activity may cause vegetation destruction.

The impact of the Hunga Tonga–Hunga Ha’apai volcanic eruption on the Peruvian atmosphere: from the sea surface to the ionosphere

The eruption of the Hunga Tonga Hunga Ha’apai volcano on 15 January 2022 significantly impacted the lower and upper atmosphere globally. Using multi-instrument observations, we described disturbances from the sea surface to the ionosphere associated with atmospheric waves generated by the volcanic eruption. Perturbations were detected in atmospheric pressure, horizontal magnetic field, equatorial electrojet (EEJ), ionospheric plasma drifts, total electron content (TEC), mesospheric and lower thermospheric (MLT) neutral winds, and ionospheric virtual height measured at low magnetic latitudes in the western South American sector (mainly in Peru). The eastward Lamb wave propagation was observed at the Jicamarca Radio Observatory on the day of the eruption at 13:50 UT and on its way back from the antipodal point (westward) on the next day at 07:05 UT. Perturbations in the horizontal component of the magnetic field (indicative of EEJ variations) were detected between 12:00 and 22:00 UT. During the same period, GNSS-TEC measurements of traveling ionospheric disturbances (TIDs) coincided approximately with the arrival time of Lamb and tsunami waves. On the other hand, a large westward variation of MLT winds occurred near 18:00 UT over Peru. However, MLT perturbations due to possible westward waves from the antipode have not been identified. In addition, daytime vertical plasma drifts showed an unusual downward behavior between 12:00 and 16:00 UT, followed by an upward enhancement between 16:00 and 19:00 UT. Untypical daytime eastward zonal plasma drifts were observed when westward drifts were expected. Variations in the EEJ are highly correlated with perturbations in the vertical plasma drift exhibiting a counter-equatorial electrojet (CEEJ) between 12:00 and 16:00 UT. These observations of plasma drifts and EEJ are, so far, the only ground-based radar measurements of these parameters in the western South American region after the eruption. We attributed the ion drift and EEJ perturbations to large-scale thermospheric wind variations produced by the eruption, which altered the dynamo electric field in the Hall and Pedersen regions. These types of multiple and simultaneous observations can contribute to advancing our understanding of the ionospheric processes associated with natural hazard events and the interaction with lower atmospheric layers.Graphical

Correlation between rate of TEC index and positioning error during solar flares and geomagnetic storms using navigation with Indian constellation receiver measurements

Abstract The real-time position accuracy of the Global Navigation Satellite System (GNSS)/Navigation with Indian Constellation (NavIC) receiver is limited by the dynamic behavior of the ionosphere, particularly in adverse conditions like solar flares and geomagnetic storms. The NavIC satellites broadcast dual coherent radio beacon signals on L5 (1,164.5 MHz) and S (2,472.5 MHz) bands for providing position, velocity, and timing services in all weather conditions. The Total Electron Content (TEC) and Rate of TEC Index (ROTI) are the potential indicators for characterizing the ionosphere and its irregularities. In this research work, the TEC and ROTI are computed from the code and carrier phase observations of the NavIC receiver located at Kurnool low latitude station (15.79° N, 78.07° E) with geomagnetic coordinates (7.30° N, 151.65° E). This paper presents a statistical study of TEC, ROTI, and the correlation between ROTI and NavIC positioning error during highly intense solar flares (X9.3 and X2.2) and geomagnetic storm conditions. Compared to quiet days mean TEC, the enhancement is 3 TECU due to X9.3 flares, and the maximum peak of TEC on storm day (September 8, 2017) is 80.92 TECU. Moreover, the correlation coefficient between ROTI and position error is 0.76 on a quiet day (September 4, 2017), 0.54 on an intense solar flares day (September 6, 2017), and 0.24 on a storm day (September 8, 2017), this indicates positional accuracy degradation on a geomagnetic storm day. The outcome of this research work would be helpful for investigating characteristics of the northern low latitude ionospheric irregularities and, in turn, useful for developing suitable ionospheric nowcasting/prediction models for GNSS applications.

H i galaxy signatures in the sarao MeerKAT galactic plane survey − III. Unveiling the obscured part of the Vela Supercluster

Abstract We conducted a search for $\textrm {H}\, \scriptstyle \mathrm{I}$ emission of the gas-rich galaxies in the Vela region (260○ ≤ ℓ ≤ 290○, −2○ ≤ b ≤ 1○) to explore the Vela Supercluster (VSCL) at Vhel ∼ 18000 $\rm km\, s^{-1}$, largely obscured by Galactic dust. Within the mostly RFI-free band (250 < Vhel < 25000 $\rm km\, s^{-1}$) of MeerKAT, the analysis focuses on 157 hexagonally distributed pointings extracted from the SARAO MeerKAT Galactic Plane Survey located in the Vela region (Vela−SMGPS). These were combined into 10 contiguous mosaics, covering a ∼90 deg2 area. Among the 843 $\textrm {H}\, \scriptstyle \mathrm{I}$ detected sources, 39 were previously discovered in the Parkes HIZOA survey (Vhel < 12000 $\rm km\, s^{-1}$; rms ∼ 6 $\rm mJy\, beam^{-1}$). With the improved rms level of the Vela−SMGPS, i.e., 0.29 − 0.56 $\rm mJy\, beam^{-1}$, our study unveils nearly 12 times more detections (471 candidates) in that same velocity range. We furthermore could identify 187 galaxy candidates with an $\textrm {H}\, \scriptstyle \mathrm{I}$ mass limit reaching $\log (M_{\rm HI}/\rm {\rm M}_{\odot }) = 9.44$ in the VSCL velocity range Vhel ∼ 19500 ± 3500 $\rm km\, s^{-1}$. We find indications of two wall-like overdensities that confirm the original suspicion that these walls intersect at low latitudes around longitudes of ℓ ∼ 272○ − 278○. We also find a strong signature most likely associated with the Hydra/Antlia extension and evidence of a previously unknown narrow filament at Vhel ∼ 12000 $\rm km\, s^{-1}$. This paper demonstrates the efficiency of systematic $\textrm {H}\, \scriptstyle \mathrm{I}$ surveys with the SKA precursor MeerKAT, even in the most obscured part of the Zone of Avoidance (ZOA).

Rethinking the Susceptibility‐Based Strategy for Marine Cloud Brightening Climate Intervention: Experiment With CESM2 and Its Implications

AbstractPrevious modeling studies indicate that even though marine cloud brightening under a susceptibility‐based strategy is effective in reducing the global average surface temperature, it triggers a La Niña‐like sea‐surface temperature response with cooling mostly confined within lower latitudes. Here we explore a different cloud seeding strategy involving seeding of regions with low susceptibility, primarily over the ocean in midlatitudes during the winter. Simulations with the Community Earth System Model, version 2 reveal that because the regional forcing is weaker and more widespread, cooling is more evenly distributed over the globe. This new strategy also does not result in the La Niña‐like state seen in the other strategies.

The First Global Insight of Cirrus Clouds Characterized by Hollow Ice Crystals From Space‐Borne Lidar

AbstractCirrus clouds often contain numerous hollow ice crystals, which are distinct in scattering properties from solid ice crystals, and will be challenging to microphysical retrieval and radiative forcing assessment. Currently, hollow ice crystals have not been observed by remote sensing methods, and the estimation of their hollowness is a complex task. To address this issue, the Mixed Modal Hollow Columns (MMHC) model for hollow ice crystals is introduced, and its backscattering properties are computed using the physical optics approximation method. Through comparison with spaceborne lidar observations, we identify a specific type of cirrus associated with the MMHC model for the first time. The visible optical depth of this cirrus is less than or equal to 0.1, and the temperature is between −60 and −40°C. The MMHC characteristic cirrus clouds are prevalent in middle and high latitudes but less comm+on in low latitudes. They exhibit distinct patterns in terms of sea and land distribution as well as seasonal variation.

Paleoclimate changes and ecosystem responses of the Bulgarian Black Sea zone during the last 26000 years

Multi-proxy analysis (spore-pollen, dinoflagellate cysts, other non-pollen palynomorphs (NPPs), radiocarbon dating and lithology) was performed on marine sediments from three new cores retrieved during the two cruise expeditions on board the Research Vessel “Akademik” in 2009 and 2011. The Varna transect comprises three cores extending from the outer shelf, continental slope and deep-water zone. The record spans the last 26000 years (all ages obtained in this study are given in calendar years BP (cal. yrs BP)). The pollen record reveals the spreading of steppe vegetation dominated by Artemisia and Chenopodiaceae, suggesting cold and dry environments during the Late Pleniglacial – Oldest Dryas (25903–15612 cal. yrs BP). Stands of Pinus and Quercus reflect warming/humidity increase during the melting pulses (19.2–14.5 cal. ka BP) and the Late Glacial interstadials Bølling and Allerød. The Younger Dryas (13257–11788 cal. yrs BP) coldest and driest environments are clearly demonstrated by the maximum relative abundance of Artemisia and Chenopodiaceae. During the Early Holocene (Preboreal and Boreal chronozones, 11788–8004 cal. yrs BP), Quercus appeared as a pioneer species and, along with other temperate deciduous arboreal taxa, formed open deciduous forests as a response to the increased temperature. The rapid expansion of these taxa indicates that they survived in Glacial refugia in the coastal mountains. During the Atlantic chronozone (8004–5483 cal. yrs BP), optimal climate conditions (high humidity and increased mean annual temperatures) stimulated the establishment of species-rich mixed temperate deciduous forests. During the Subboreal chronozone (5483–2837 cal. yrs BP), Carpinus betulus and Fagus expanded simultaneously and became more important components of mixed oak forests and probably also formed separate communities. During the Subatlantic chronozone (2837 cal. yrs BP to pre-industrial time), climate-driven changes (an increase of humidity and a cooling of the climate) appear to be the main drivers of the specific vegetation succession expressed by increased abundance of Alnus, Fraxinus excelsior and Salix along with lianas, suggesting formation of flooded riparian forests (so called ‘Longoz’) lining the river valleys along the Black Sea coast. The first indicators of farming and other human activities have been recorded since 7074 cal. yrs BP. The dinoflagellate cyst (dinocyst) assemblages have been analysed to assess the changes in the Black Sea environment over the last 26000 years in terms of fluctuation in paleoproduction and surface water conditions related to changes in climate, freshwater input and Mediterranean water intrusion. Two major dinocyst assemblages were distinguished: one dominated by stenohaline freshwater/brackish-water species and a successive one dominated by euryhaline marine species. The changes in the composition of the assemblages occurred at 7668 cal. yrs BP. The abrupt decrease of stenohaline freshwater/brackish-water species Pyxidinopsis psilata and Spiniferites cruciformis was followed upwards by a gradual increase in euryhaline marine species, such as Lingulodinium machaerophorum, Spiniferites belerius, S. bentorii and acritarch Cymatiosphaera globulosa. The first occurrence of euryhaline marine species took place synchronously with the onset of sapropel deposition. Modern marine conditions were established after 6417 cal yrs BP when an abundance of Mediterranean-related species, such as Operculodinium centrocarpum and Spiniferites mirabilis, along with other heterotrophic species, occurred. After the stable cold and dry environment during the Last Glacial Maximum, the phytoplankton record of core AKAD 11-17 shows that Pediastrum boryanum var. boryanum has a cyclical abundance associated with the deposition of four red-brown clay layers between 19.2 and 14.5 cal. ka BP. This event is associated with the major melting phase of European Ice drained by the Danube and Dnieper Rivers in response to climate warming observed after the end of the Last Glacial Maximum. During the Early Holocene, P. psilata, characterised by a preference to warmer temperatures, demonstrates its ecological optimum for growth concerning SST reaching maximum relative abundance of 94% between 11072 and 8638 cal. yrs BP. This maximum was interrupted by an abrupt significant short-term decrease in the relative abundance of P. psilata centred between 8500 and 8300 cal. yrs BP reflecting cold conditions similar to those of Younger Dryas. This finding, also confirmed by the rapid significant decrease of arboreal pollen, particularly of Quercus in the same studied core, is considered a regional expression of the well-known ‘8.2 ka cold event’ which is commonly linked to a meltwater-related perturbation of the Atlantic Meridional Overturning Circulation (AMOC) and associated collapse of oceanic northward heat transport. Our fossil pollen and dinocyst data confirm that the high amplitude temperature anomaly associated with ‘the 8.2 ka cold event’ may have also occurred in south-eastern Europe, at lower latitudes of the western Black Sea coastal area, most probably due to atmospheric transition and/or river discharge.

Climate induced declines in maternal size may come at a cost to embryonic investment and larval performance in the American lobster

Many crustacean taxa mature at a smaller size in warmer thermal regimes. Over the past four decades, female lobsters American lobster (Homarus americanus) in the rapidly warming Gulf of Maine have also been reported to be maturing at significantly smaller sizes over time. Smaller females have lower fecundity, but whether they also produce lower quality eggs and larvae has been unclear. Here we examine the hypothesis that smaller females invest less energy per larva than larger females, thereby compromising post-hatch performance and survival. This study used a combination of laboratory and field studies to assess the maternal size effect on embryos and larvae. In a space-for-time comparison of lobsters from contrasting thermal regimes along the coast of New England, we also investigated how lobster collected from populations in cooler northern regions may respond to future warming. We found smaller females not only are less fecund, but their embryos are smaller and generally less well invested with lipids than those from larger females. Furthermore, stage I larvae from smaller females are smaller and have less energy reserves needed to survive starvation. Our geographic comparison also revealed that female lobsters living in the warmer thermal regime of southern New England are more fecund and produced larger eggs than females of the same size from colder regimes in the Gulf of Maine, and we cautiously interpret this novel finding as evidence of a counter-gradient adaptation that could compensate for smaller maturation size at lower latitudes, a phenomenon requiring further investigation. These results help to reveal the cascading effects warming can have on important reproductive and larval survival traits on an economically important species.

Energetic particle precipitation influences global secondary ozone distribution

The secondary ozone layer is a global peak in ozone abundance in the upper mesosphere-lower thermosphere (UMLT) around 90-95 km. The effect of energetic particle precipitation (EPP) from geomagnetic processes on this UMLT ozone remains largely unexplored. In this research we investigated how the secondary ozone responds to EPP using satellite observations. In addition, the residual Mean Meridional Circulation (MMC) derived from model simulations and the atomic oxygen [O], atomic hydrogen [H], temperature measurements from satellite observations were used to characterise the residual circulation changes during EPP events. We report regions of secondary ozone enhancement or deficit across low, mid and high latitudes as a result of global circulation and transport changes induced by EPP. The results are supported by a sensitivity test using an empirical model.

Aerosols in the central Arctic cryosphere: satellite and model integrated insights during Arctic spring and summer

Abstract. The central Arctic cryosphere is influenced by the Arctic amplification (AA) and is warming faster than the lower latitudes. AA affects the formation, loss, and transport of aerosols. Efforts to assess the underlying processes determining aerosol variability are currently limited due to the lack of ground-based and space-borne aerosol observations with high spatial coverage in this region. This study addresses the observational gap by making use of total aerosol optical depth (AOD) datasets retrieved by the AEROSNOW algorithm over the vast cryospheric region of the central Arctic during Arctic spring and summer. GEOS-Chem (GC) simulations combined with AEROSNOW-retrieved data are used to investigate the processes controlling aerosol loading and distribution at different temporal and spatial scales. For the first time, an integrated study of AOD over the Arctic cryosphere during sunlight conditions was possible with the AEROSNOW retrieval and GC simulations. The results show that the spatial patterns observed by AEROSNOW differ from those simulated by GC. During spring, which is characterized by long-range transport of anthropogenic aerosols in the Arctic, GC underestimates the AOD in the vicinity of Alaska in comparison with AEROSNOW retrieval. At the same time, it overestimates the AOD along the Bering Strait, northern Europe, and the Siberian central Arctic sea-ice regions, with differences of −12.3 % and 21.7 %, respectively. By contrast, GC consistently underestimates AOD compared with AEROSNOW in summer, when transport from lower latitudes is insignificant and local natural processes are the dominant source of aerosol, especially north of 70° N. This underestimation is particularly pronounced over the central Arctic sea-ice region, where it is −10.6 %. Conversely, GC tends to overestimate AOD along the Siberian and Greenland marginal sea-ice zones by 19.5 % but underestimates AOD along the Canadian Archipelago by −9.3 %. The differences in summer AOD between AEROSNOW data products and GC-simulated AOD highlight the need to integrate improved knowledge of the summer aerosol process into existing models in order to constrain its effects on cloud condensation nuclei, on ice nucleating particles, and on the radiation budget over the central Arctic sea ice during the developing AA period.

Research on Polar Operational Limit Assessment Risk Indexing System for Ships Operating in Seasonal Sea-Ice Covered Waters

The Polar Operational Limit Assessment Risk Indexing System (POLARIS) has been established as a viable framework for assessing operational capabilities and associated risks in polar waters. Despite its inherent suitability for high-latitude territories, ships navigating through seasonal ice-infested waters at lower latitudes also encounter critical safety, environmental, and economic issues exacerbated by the presence of ice. This necessitates a reliable and adaptable methodology that can serve as a reference for devising effective countermeasures. This study evaluated the use of POLARIS in the intricate ice conditions prevalent in the northern navigable waters (channels and anchorages) within Liaodong Bay of the Bohai Sea, located at relatively low latitudes. Using GF-4 satellite imagery, ice conditions were collected, and the POLARIS methodology was employed to calculate Risk Index Outcome (RIO) values for non-ice-strengthened vessels during the winter season of 2021–2022. The results showed that sectors 3, 4, 5, 7, 9, 10, and 11 within the northern part of Liaodong Bay exhibited a higher risk, with sectors 5 and 10 exhibiting the most significant risk, while sectors 1 and 2 demonstrated relatively lower risk levels. The concurrence of these findings with acknowledged ice patterns and local maritime practices confirms the applicability of the POLARIS methodology in saline, seasonally ice-covered seas. Notably, the combination of POLARIS with high-resolution satellite imagery facilitated a more precise and rapid assessment of ice risk, thereby enhancing situational awareness and informing decision-making processes in maritime operations under icy conditions. In addition, this study provides preliminary evidence that POLARIS is suitable for fine-scale scenarios, in addition to being applicable to sparse-scale scenarios, such as polar waters, especially with high-resolution ice data. At the same time, this study highlights the potential of POLARIS as a disaster prevention strategy and a tool for the maritime industry to address ice challenges.

Analysis of the temporal and spatial characteristics of lunar reconnaissance orbiter’s orbit error based on multi-coverage narrow angle camera images

ABSTRACT Photogrammetric mapping of large area of the lunar surface using high-resolution orbiter images is very challenging due to increasing spatial resolution and lack of a comprehensive understanding of orbit error characteristics. The Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) images are currently one of the highest-resolution images of the Moon available. The study of LRO orbit errors from the perspective of mapping applications is important for high accuracy mapping of large areas using NAC images. This paper proposes a method for orbit error estimation using multi-coverage NAC images. First, available NAC images covering the study regions are orthorectified using ISIS. Then, the impact crater feature matching algorithm is used to match the multi-coverage images. Finally, estimation of the orbit error is achieved through statistical analysis of a large number of homologous points obtained from the matching. The orbit error analyses of nine representative regions on the Moon show that in most cases the refined ephemeris provided by the LOLA/GRAIL team can significantly reduce the orbit error. The orbit errors show a sawtooth variation with a period of about one year, which have been stable for a long time and tend to increase slightly in recent years. There is a slight variation of the orbit error in space, which is even less obvious when using the refined ephemeris. The general tendency is for the far side to be larger than the near side at the same latitude, and for the higher latitudes to be larger than the lower latitudes. The proposed method and analysis results provide a reference for selection and processing of multi-coverage LROC NAC images for high-resolution large-area mapping.

Exploring Mechanisms for Model‐Dependency of the Stratospheric Response to Arctic Warming

AbstractThe Arctic is estimated to have warmed up to four times faster than the rest of the globe since the 1980s. There is significant interest in understanding the mechanisms by which such warming may impact weather and climate at lower latitudes. One such mechanism is the “stratospheric pathway”; Arctic warming is proposed to induce a wave‐driven weakening of the stratospheric polar vortex, which may subsequently impact large‐scale tropospheric circulation. However, recent comprehensive model studies have found systematic differences in both the magnitude and sign of the stratospheric response to Arctic warming. Using a series of idealized model simulations, we show that this response is sensitive to characteristics of the warming and mean polar vortex strength. In all simulations, imposed polar warming amplifies upward wave propagation from the troposphere, consistent with comprehensive models. However, as polar warming strength and depth increases, the region through which waves can propagate is narrowed, inducing wave breaking and deceleration of the flow in the lower stratosphere. Thus, the mid‐stratosphere is less affected, with reduced sudden stratospheric warming frequency for stronger and deeper warming compared to weaker and shallower warming. We also find that the sign of the stratospheric response depends on the mean strength of the vortex, and that the stratospheric response in turn plays a role in the magnitude of the tropospheric jet response. Our results help explain the spread across multimodel ensembles of comprehensive climate models.

Environmental controls and phenology of sea ice algal growth in a future Arctic

The future of Arctic sea ice algae is examined using a regional ocean and sea ice biogeochemical model, with a simulation from 1980 to 2085, considering a future scenario with strong warming. To analyze the impacts of climate change, we computed key dates in the development of sympagic blooms, corresponding to the occurrence of specific growth conditions, and designed diagnostics of ice algal phenology to estimate the onset and peak of blooms. These diagnostics help understand how the timing of light and nutrient availability governs the growth of ice algae and how environmental controls will be altered by climate change across regions. With thinner ice, photosynthetically active radiation in bottom ice will reach levels sufficient for growth earlier, resulting in a better synchrony of high levels of light and nutrients. Increases in snow cover can potentially offset the effect of thinner ice, leading to shorter periods of favorable growth conditions in certain regions. The loss of sea ice cover before the late 21st century only impacts sympagic blooms at lower latitudes, as the timing of sea ice break-up shows little change relative to other key dates at higher latitudes. In response to climate change, the model simulates a modified spatial distribution of blooms, with the emergence of highly productive areas and the loss of blooms in other regions. However, the changes in the timing of growth conditions do not substantially alter the timing of blooms, and both onset and peak ice algae see little change. The simulated lack of sensitivity of bloom onset is attributed to the delay in sea ice freeze-up projected by the model, causing a reduction of overwintering ice algae. The resulting lower initial biomass at the beginning of spring then causes a delay in the development of blooms, offsetting earlier light from thinner ice.

Periodic Oscillation of VLF Transmitter Signals Measured in Low and Middle Latitude Regions

AbstractMeasurements of Very Low Frequency (VLF) signals from navy transmitters carry direct information about the D‐region ionosphere and have been widely utilized for detecting the electron density at D‐region altitudes, but not frequently for the atmospheric waves therein. Atmospheric waves have been extensively studied using the total ionospheric electron content, but if and how they are correlated with the D‐region ionosphere and VLF measurements still remains poorly investigated. In this study, we have conducted a comprehensive analysis using 7‐year measurements (2016–2022) of VLF signals from the JJI, NWC, and VTX transmitters as being recorded in Suizhou, China. These three transmitter‐receiver paths are representative and the corresponding observations constitute a valuable data set to investigate the periodicities of VLF data. Different from previous studies, we have utilized the ensemble empirical mode decomposition and Lomb‐Scargle methods to determine the periodicities of these data. By contrasting these paths, prominent periodicities ranging from 2 to 730 days have been found, with clear diurnal variation and suggestive latitudinal/longitudinal dependence. Moreover, we have found that the mesospheric temperature is closely related with the annual oscillation of VLF measurements, while this oscillation has a low correlation with solar Lyman‐α fluxes or geomagnetic activity. The oscillations with relatively shorter periods are likely atmospheric waves such as gravity waves, planetary waves, or harmonics of these waves. Our results suggest that, in addition to the electron density, the subionospheric VLF technique can be potentially utilized to remotely sense atmospheric waves that propagate up to or through the D‐region ionosphere.

New Discovery of Calamitaceae from the Cisuralian in Northwest China: Morphological Evolution of Strobilus.

Calamitaceae was the predominant arborescent element of the Late Paleozoic plant assemblages. However, there is currently controversy surrounding the evolutionary relationships of their four reproductive organs, and there is no unified understanding of the geographical distribution and species richness characteristics. This paper is based on the detailed description of the macro- and microstructures of Calamites and Macrostachya discovered in the Shanxi Formation of the Cisuralian in the eastern Hexi Corridor of northwestern China, and it discusses the evolutionary patterns of calamitean reproductive organs. The results indicate that the current specimens can be identified as the Calamites cistii and Macrostachya sp., and the in situ spores should exhibit distinct trilete marks, indicating that they belong to the Punctatisporites punctatus. The abundant fossil records suggest that the "Calamostachys" type should be considered as the ancestral type of strobili. Additionally, Calamitaceae was widely distributed in mid to low latitudes globally from the Pennsylvanian to the Cisuralian and coupled with paleogeographic distribution characteristics.

Analysis of Spatial Differentiation of NDVI and Climate Factors on the Upper Limit of Montane Deciduous Broad-Leaved Forests in the East Monsoon Region of China

The vertical transition zone of mountain vegetation is characterized by high species diversity, and the width of the transition zone may serve as an indirect indicator of climate change. However, research into the differential characteristics of vegetation response to climate changes at the boundary of vertical transition zones has been limited. This study employs MODIS and climate data spanning 2001 to 2018 to investigate spatiotemporal trends in precipitation (PRE), temperature (TMP), radiation (RAD), and Normalized Difference Vegetation Index (NDVI) across nine montane deciduous broad-leaved forests in the eastern monsoon region of China. It explores the time-lag and -accumulation effects of climatic variables on NDVI, quantifying their relative contributions to both its short-term and interannual variations. Results show that, notably, with the Qinling-Daba Mountains as a demarcation, northern regions exhibit significant increases in RAD (0.874–2.047 W m−2/a), whereas southern regions demonstrate notable rises in TMP (0.59–0.73 °C/10a). Areas of lower annual PRE correspond to the most rapid increases in annual average NDVI (5.045 × 10−3/a). NDVI’s lag time and cumulative duration responses to TMP are the shortest (0 and 2~4 periods), while its correlation with RAD is the strongest (0.815–0.975), generally decreasing from higher to lower latitudes. TMP significantly affects NDVI variations, impacting both short-term and interannual trends, with PRE driving short-term fluctuations and RAD dictating long-term shifts. This research provides critical data and a theoretical framework that enhances our understanding of how regional vegetation’s vertical zonation responds to climate change, thereby making a substantial contribution to the study of mountain vegetation’s diverse adaptability to climatic variations.

Wind Shear Driven Double Layer Structures of E‐Region Irregularities at Low Latitudes

AbstractPrevious theoretical simulations showed the generation of double‐layer structures of E‐region field‐aligned irregularities (FAIs). In this study, we report the double‐layer structures of E‐region FAIs observed by an all‐sky radar at low latitude Ledong (18.4°N, 109°E), China. These FAIs appeared at the altitudes ∼90 and 110 km respectively. Both layers displayed quasi‐periodic patterns with synchronized spatial features, that is, being manifested as spatially separated patches or wavelike structures over similar longitudes at the two layers simultaneously. The neutral wind observations revealed the existence of wind shears at two separated altitudes where the double FAI layers occurred. The two wind shears created two vertically separated sporadic E layers and possibly drove the generation of the double‐layer FAIs via gradient drift instability. The synchronized spatial features of the FAIs at the two layers could be modulated by gravity waves.

The global pattern of epiphytic liverwort disparity: insights from Frullania

The geographical and ecological patterns of morphological disparity are crucial to understand how species are assembled within communities in the context of the evolutionary history, morphological evolution and ecological interactions. However, with limited exceptions, rather few studies have been conducted on the global pattern of disparity, particularly in early land plants. Here we explored the spatial accumulation of disparity in a morphologically variable and species rich liverwort genus Frullania in order to test the hypothesis of latitude disparity gradient. We compiled a morphological data set consisting of eight continuous traits for 244 currently accepted species, and scored the species distribution into 19 floristic regions worldwide. By reconstructing the morphospace of all defined regions and comparisons, we identified a general Gondwana-Laurasia pattern of disparity in Frullania. This likely results from an increase of ecological opportunities and / or relaxed constraints towards low latitudes. The lowest disparity occurred in arid tropical regions, largely due to a high extinction rate as a consequence of paleoaridification. There was weak correlation between species diversity and disparity at different spatial scales. Furthermore, long-distance dispersal may have partially shaped the present-day distribution of Frullania disparity, given its frequency and the great contribution of widely distributed species to local morphospace. This study not only highlighted the crucial roles of paleoenvironmental changes, ecological opportunities, and efficient dispersal on the global pattern of plant disparity, but also implied its dependence on the ecological and physiological function of traits.