Low Latitudes Research Articles

Abstract Continental-scale isoscapes of tree-ring oxygen isotopes (δ¹⁸OTR) are crucial for understanding atmospheric circulation dynamics, interpreting climatic significance, and tracing wood provenance. However, continental-scale δ¹⁸OTR isoscapes remain underdeveloped. We compiled 313 multi-year averaged δ¹⁸OTR records across Asia and generated isoscapes using thin-plate spline regression interpolation. Results reveal a “sandwich” pattern: lower values at high (>50°N) and low latitudes (<30°N), higher values at mid-latitudes (30–50°N). This pattern aligns with May–September precipitation oxygen isotopes (δ¹⁸OP) and is primarily driven by δ¹⁸OP, with relative humidity playing a secondary role in arid regions. Continental-scale δ¹⁸OTR-elevation relationships are generally insignificant, except in Indian Summer Monsoon regions showing significant negative correlations (r = –0.69, p < 0.05). These findings suggest δ¹⁸O-based paleoaltimetry reconstructions work best in regions with dominant moisture sources such as Indian Summer Monsoon regions. This study provides Asia’s first continental-scale δ¹⁸OTR isoscapes, establishing a foundation for atmospheric circulation and dendroprovenancing research.

Abstract An ionospheric broadcast model in a low-cost single-frequency GNSS receiver provides ionospheric corrections to improve its positional accuracy. This study evaluates the performance of the Klobuchar, NeQuick-G models, and proposes an optimized ensemble ionospheric model during the minimum (2020) and maximum (2024) phases of Solar Cycle 25 over the Indian low-latitude and equatorial regions. Dual-frequency Total Electron Content (TEC) data from International GNSS Service (IGS) stations over the Indian region, including JMSM, LCK4, JDPR, SHLG, and IISC stations data were analysed to quantify the proposed ionospheric TEC model accuracy under both quiet and disturbed geomagnetic conditions. The year 2024 exhibited strong solar activity, with Dst values reaching −412 nT and Kp index up to 9 during the May 10-11 geomagnetic storm. Model errors were computed as the difference between observed and predicted Vertical TEC (VTEC) values. The proposed optimized ensemble model, based on performance-weighted integration of Klobuchar and NeQuick-G outputs, consistently achieved the lowest RMSE values, ranging from 1.66 TECU (winter 2020) to 6.98 TECU (May 2024 storm), compared with the NeQuick-G (up to 30.07 TECU) and Klobuchar (up to 40.02 TECU). The analysis of proposed optimized ensemble ionospheric model revealed clear seasonal behaviour capture, with VTEC maxima during the equinoxes (50–100 TECU) and minima during solstices (20–40 TECU). Results demonstrate that the optimized model significantly enhances ionospheric prediction accuracy under both quiet and storm-time conditions, offering a robust solution for single-frequency GNSS applications in low-latitude regions strongly affected by ionospheric variability.

Abstract Climate simulations of the industrial era typically start in 1850, using the first fifty years as a baseline for ‘pre-industrial’ climate. However, the period immediately prior to 1850 is of particular interest due to early human influence and heightened volcanic activity, the latter of which led to cooler global temperatures than those observed in the subsequent historical period. In this study, we present a suite of Earth system model simulations (using UKESM1.1) that start in 1750 and span the entire industrial period. We compare these simulations to a new instrumental observation-based dataset, GloSATref, which provides global surface air temperature variations from 1781 onwards. We investigate the climatic changes during the early industrial period, separating the effects of natural and anthropogenic forcings. Model-simulated early-19th-century temperature patterns show substantial cooling relative to the long-term mean, particularly in low latitudes, which agree well with observed patterns. We find significant long-term differences between simulations initialised in 1750 and 1850, with lasting effects well into the 20th century, consistent with differences in vegetation and the substantial ocean cooling driven by high volcanic activity in the 1750 simulations. Our results indicate that an earlier start to historical simulations could lead to more representative climate simulations over the historical period, and deepen our understanding of early anthropogenic warming, natural climate variability, and the climate responses to future volcanic eruptions.

Historical and registered earthquakes of great magnitude have caused destruction with injuries and deaths in mainland Portugal. This work uses data not yet used to investigate a possible increase of the temperature of the water in the region and possible consequences when materials with different thermal properties are in physical contact. A temporal link between geomagnetic anomalies recorded by Permanent Geomagnetic Observatories and earthquakes detected in the Iberian Peninsula and adjacent regions during the month of February in 1969 was found using data from 3 Observatories located in the Iberian Peninsula. The anomaly recorded on February 27,1969 is due to large alterations in the direction (inclination) of the geomagnetic field and associated variations in the electric field of the atmosphere recorded in Lisbon. The source of the first earthquake filled and registered in the early morning of February 28 was in the Atlantic Ocean. The variations in the geomagnetic inclination, detected in the records, may have led to rotations of the water molecules to align their electric dipoles with the electric field in the region, originating heat release. As consequence, an increase of temperature occurred. The analysis of the arrival times of the first seismic waves at the seismic stations located in Lisbon, Coimbra and Porto (cities located near the Atlantic Ocean at different latitude values) combined with the distances of the cities to the earthquake source, shows different values of the average speed of propagation of the seismic waves. The values found decrease from Porto to Lisbon. Seismic velocity values change with temperature and pressure. As we are talking about the same earthquake, different temperature values found in the last part of the wave paths seems to be responsible for this fact. This means that temperature values at lower latitude values (Lisbon city and earthquake source) are higher. The increase of the temperature values of water contributes to the opening of cracks and faults in the region with the entry of hot water that can reach great depths, increasing the temperature, the volume, and the pressure of the materials. When materials with different values of thermodynamic properties like specific heat, thermal conductivity and thermal expansion coefficient are in physical contact, the heat received from the water can originate perturbations like pressure gradients of thermal origin whose value can increase causing rupture of the materials. The presence of water with elevated temperatures in deep regions can also change the velocity of chemical reactions with heat release, increasing the described effect.

Spatiotemporal variation in foraging ecology and mercury concentrations in ringed seals and bearded seals across a latitudinal gradient in the eastern Canadian Arctic.

Reservoir water-level operations to manage biogeochemical turnover.

The impact of Arctic environments on human cerebral blood flow velocity.

Abstract Precession affects climate through the seasonal redistribution of local insolation. Yet, its effects on climate and its dependence on continental configuration and background climate remain unclear. Here, we use a fully coupled model to conduct a series of simulations across five geological periods (0, 80, 230, 300, and 440 Ma) by altering precession from 270° to 90° (i.e., the boreal summer changes from aphelion to perihelion). The results show that climate changes in certain seasons may dominate the annual-mean changes. Moreover, changes in surface climate show common features especially over land, independent of continental configurations and background climate. The low-latitude continents experience a strong Northern Hemispheric (Southern Hemispheric) cooling (warming) dominated by summer change due to the strengthening (weakening) of the summer monsoon, during which more (less) low and middle clouds reduce (increase) the insolation received at the surface. The increased precipitation further cools the surface by increasing heat loss through enhanced evapotranspiration, and vice versa. Changes over midlatitudinal lands are generally opposite to those at lower latitudes due to opposite cloud changes. However, the changes depend on the background climate; when the background climate is cold, the effect of surface albedo due to land snow changes may overwhelm and reverse the effect of clouds. The climate responses of the polar regions depend on the continental configuration and are opposite between the north and the south. Among all five continental configurations, the present-day (0 Ma) continental configuration has the largest response over the mid–high-latitude regions.

Abstract Vegetation greening, primarily driven by climate change and land management, has been widely reported globally, with China experiencing one of the most pronounced instances of greening in recent decades. Current studies have mainly focused on the temperature effects of changes in land cover types; however, the impacts of vegetation self‐greening, characterized by natural physiological processes such as aging and canopy structure development, on land surface temperature (LST) remain unclear. Here, we employed a space‐for‐time approach to quantify the biophysical effects of vegetation greening on LST (i.e., ΔLST) across China from 2000 to 2018 using multi‐year satellite observations. We found that vegetation greening caused significant daytime cooling (−0.36 K), surpassing that of nighttime (−0.07 K) over the study period. Spatially, pronounced cooling dominated lower latitudes, contrasting with slight warming in mid‐to‐high latitudes, with this warming phenomenon being more evident at nighttime. The latitudinal transition of LST from cooling to warming occurred at around 47°N for daytime, nighttime, and daily mean values. This latitudinal pattern was season‐dependent, shifting to its northernmost extent in summer and southward to around 40°N in winter. Data‐driven methods revealed that albedo decreased by −0.0035 ± 0.009 in total, contributing to warming in Northeastern China, while evapotranspiration (ET) increased by 37.3 ± 33.9 mm across China, driving cooling in other regions. Our findings highlight the importance of vegetation greening in shaping local climate and advance understanding of vegetation feedback on climate, providing valuable insights for developing targeted land management strategies.

Abstract Climate models require boundary condition information, such as vegetation and soil distributions because they influence the mean state climate, and feedbacks can significantly influence regional climate and climate sensitivity to CO 2 forcing. Information about past distributions comes primarily from the paleobotanical record, which is often supplemented by a vegetation model to fill data gaps. For recent past periods such as the Pliocene, a quantitative suitability assessment of these vegetation model simulations is sufficient. However, the Miocene Climate Optimum spanning 16.9–14.7 Ma was the warmest period on Earth over the last ∼25 million years and models struggle to reproduce those conditions for the range of paleogeographies and CO 2 concentrations tested, particularly at high latitudes. Here we bring together the Miocene modeling and data communities to update previous vegetation reconstructions used for climate modeling with a new regional approach that relaxes the requirement for a single model simulation to be used, blending instead simulations forced by different paleogeographies and CO 2 concentrations. This ensures the simulated vegetation is first, and foremost, consistent with the paleorecord and provides a baseline for future comparisons. The reconstruction shows global increases in forest cover at all latitudes as compared to today and extensive C 3 grasslands across the high northern latitudes. Data gaps at high latitudes are filled with vegetation models forced by higher CO 2 concentrations than were required at lower latitudes consistent with the inability of current models to simulate Miocene high latitude warmth.

We investigate the excitation and variability of migrating and non-migrating diurnal and semi-diurnal tides in the mesosphere and lower thermosphere (MLT) during the 2021 Northern Hemisphere sudden stratospheric warming (SSW). Zonal wind data from MERRA-2 reanalysis are decomposed into tidal components using a two-dimensional least-squares harmonic fitting technique. The migrating diurnal tide (DW1) strengthens at low latitudes following the SSW onset, whereas the migrating semi-diurnal tide (SW2) intensifies at high latitudes. Non-migrating diurnal tides (D0, DW2, DW3) arise from nonlinear interactions between DW1 and stationary planetary waves (SPWs), while non-migrating semi-diurnal tides (SW1, SW3) are modulated by stratospheric ozone variability linked to planetary-wave activity. The zonally symmetric semi-diurnal tide (S0) responds primarily to dynamical perturbations associated with the SSW. Eastward non-migrating diurnal tides (DE2, DE3) correlate strongly with total precipitable water vapor (TPWV), indicating tropospheric latent-heat forcing, whereas DE1 exhibits weak coupling. These results reveal distinct, latitude-dependent excitation pathways connecting stratospheric and tropospheric dynamics to tidal variability in the MLT during major SSW events.

Secondary metabolites are critical to plant defenses and frequently exhibit variation among populations associated with heterogeneous climatic factors and/or herbivore communities. We examined genetic-based geographic and ontogenetic variation in phenylpropanoid glycoside (PPG) concentrations and arsenals in Mimulus moschatus, a perennial monkeyflower native to western North America. We conducted a greenhouse common garden with maternal lines collected from three populations sampled along a latitudinal gradient paralleling spatial sampling from a previous study in the closely related congener, Mimulus guttatus. We assessed PPG arsenals as well as concentrations of individual and total PPGs within leaf tissue from two developmental stages. Populations differed in total concentration and arsenal composition of PPGs, with the populations from intermediate latitudes exhibiting higher concentrations than higher and lower latitude populations. Leaves from earlier developmental stages had higher overall PPG concentrations than leaves from later developmental stages across all populations, suggesting juvenile tissue is better defended than more mature tissue. Spatial patterns in relative PPG concentrations and arsenal composition tightly paralleled those in the closely related M. guttatus across space despite consistently lower total PPG concentrations in M. moschatus. Because we did not measure herbivory and our sample size is limited, these results should be interpreted cautiously. Nonetheless, they provide baseline data for understanding geographic and developmental variation in chemical defenses and generate hypotheses about environmental influences and potential convergence in defenses among closely related species.

Abstract The China Meteorological Administration convection‐permitting ensemble prediction system (CMA‐CPEPS) adopted a singular vector (SV) downscaling from the global ensemble prediction system because of its simplicity and applicability. The key to this method is reasonably describing the initial perturbation structures and growth characteristics, with targeted areas being crucial factors. However, previous studies using fixed targeted areas failed to capture perturbation structures at low latitudes and those associated with convection, leading to mismatches with convective weather development, insufficient ensemble spreads, and low precipitation predictability. To address these issues, we constructed convective adjustment targeted areas using composite parameters of convective available potential energy and shear. The results show that (a) the energy norms of SV for experiments with targeted areas across China exhibit a two‐peaked structure, effectively capturing perturbation structures at low and middle levels, similar to the largest areas for experiments with convective adjustment targeted areas. Both experiments have larger potential energy norms than kinetic energy norms at the initial time. (b) The new method largely improves the spread‐skill relationships and slightly enhances the probabilistic forecasting abilities for both precipitation and nonprecipitation variables, effectively describing uncertainties associated with heavy precipitation. (c) The new method yields more broadly distributed initial perturbation structures and larger perturbation magnitudes, particularly those associated with moist convection, thereby enhancing ensemble prediction skills in the CMA‐CPEPS. Moist convection is the primary physical mechanism driving the perturbation growth. Overall, these findings provide a foundation for improving the SV dynamical downscaling method in CMA‐CPEPS.

Eukaryotic microalgae are key organisms in marine environments, contributing to essential nutrient and biogeochemical cycles. Climate change is already causing shifts in the composition and diversity of microalgal assemblages, with coastal ecosystems experiencing increased pressure. Molecular techniques, such as environmental DNA (eDNA) metabarcoding, have allowed eukaryotic microalgal communities (EMCs) to be characterised more rapidly than traditional methods, addressing some data gaps. Many geographic regions still lack comprehensive baseline assessments of entire EMCs, often focusing on specific taxonomic or functional groups. In this study, we used eDNA metabarcoding of the 18S V9 gene region to characterise EMCs across a latitudinal gradient spanning five ecoregions from the South Pacific to the Ross Sea. Temperate sites had the highest alpha diversity and the most distinct EMCs across all ecoregions. Community composition showed clear latitudinal gradients, with a shift from dinoflagellate-dominant EMCs at lower latitudes to diatom-dominant EMCs at higher latitudes. The formation of EMCs was influenced by similar habitats/conditions (homogeneous selection) and proximity (homogenising dispersal) within ecoregions, while variable conditions (variable selection) and limited interconnectivity (dispersal limitation) were more prevalent between ecoregions. This snapshot of coastal EMC diversity across five ecoregions highlights distinct latitudinal patterns in community composition, with temperate regions showing higher diversity compared to polar areas. These findings provide a useful baseline for future monitoring and can help guide assessments of potential changes in microalgal communities due to climate change.

In this work, we present two individual cases showing that a negative solar wind dynamic pressure pulse can lead to magnetospheric decompression and generate a clockwise vortex in the duskside magnetosphere. In response to this decompression, a magnetospheric vortex in the duskside sector was observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. Minimum variance analysis (MVA) of the mass flux, together with the corresponding equivalent ionospheric currents (EICs), indicates that the magnetospheric vortex rotated clockwise which is opposite to the typical direction of vortices induced by positive solar wind dynamic pressure pulses. The associated clockwise EICs vortex suggests that the magnetospheric vortex was connected to the ionospheric vortex via downward field-aligned currents (FACs). Time series analysis of the EICs further reveals that the vortex propagated eastward, consistent with the tailward motion of the magnetospheric vortex. Moreover, the occurrence of the clockwise EIC vortex at lower latitudes, together with the THEMIS satellite location, indicates that the magnetospheric vortices associated with traveling convection vortices (TCVs) are generated not at the magnetopause, but within the nightside magnetosphere.

Abstract. Previous research has shown that (1) treelines are shifting upward in elevation on high mountain peaks worldwide, and (2) the rate of the upward shift appears to have increased markedly in recent decades, at least in a few cases that have been studied in detail. Because treeline elevational shift is a process manifested over broad scales of space and time, a particular challenge has been that of obtaining a broad enough view of patterns of treeline shift to permit inferences about geographic and environmental patterns. What is more, intensive studies of treelines have been concentrated in north temperate regions such that little information is available about treeline shift patterns at lower latitudes. We attempted to address this challenge by analyzing long time series of vegetation indices derived from Landsat imagery obtained and prepared via Google Earth Engine from the 1980s to the present. We sampled vegetation indices at points spaced every 100 m along 100 km transects radiating out in eight directions from 115 high peaks across western North America (Canada to Central America), which means that we are sampling approximately every second or third pixel in the corresponding Landsat images. Considerable data preparation was necessary, including ending transects <2 km into closed forest, identifying current treelines via reference to Google Earth imagery, and consideration only of up to <1 km above the treeline. Patterns that emerged were – as is well known – that treelines are generally higher at lower latitudes but – previously unknown – that the magnitude of treeline shifts is nonrandomly distributed with respect to latitude, longitude, and their interaction. This analysis, via a broad-scale view of treeline shifts over almost 40 years and a geographic span of more than 40° of latitude, demonstrates that climate change effects and consequent treeline shifts are most dramatic in tropical regions where few or no detailed treeline studies have been or are being conducted.

We investigate the dynamics of the multi-scale processes involved in the ionospheric response to a minor geomagnetic storm occurred on 14-15 January, 2022. During this storm, the LOw Frequency ARray (LOFAR), an European distributed radio telescope array, provided ionospheric measurements from the radio source Cassiopeia A, that we complemented with the ionospheric information provided by the Global Navigation Satellite System (GNSS) receivers covering the European sector. LOFAR operates in the HF/VHF band (between 30 and 250 MHz), while GNSS signals are in the L-band ([[EQUATION]] 1 Ghz), translating into the possibility to investigate the ionospheric irregularities formed in response to the storm at different spatial scales. The combined use of data from these two instruments gave us the opportunity to observe three distinct phenomena: (i) the increment of direct particles precipitation in the auroral oval, (ii) the steepening of the equator-ward edge of the ionospheric trough, and (iii) the propagation towards lower latitudes of wave-like structures having scale sizes of few km and velocity of hundreds of meters per seconds.

Abstract This study investigates how the relationship between the preceding winter El Niño–Southern Oscillation (ENSO) and the East Asian summer monsoon (EASM) varies with the phases of the Pacific decadal oscillation (PDO) and the Victoria mode (VM), considering whether ENSO was El Niño or La Niña. Results show that the strongest El Niño–EASM relationship occurs during positive PDO and negative VM. This combination features a dipolar sea surface temperature (SST) anomaly pattern characterized by tropical western North Pacific (TWNP) cooling and north Indian Ocean (NIO) warming, which sustains an anomalous western North Pacific anticyclone (WNPAAC) through the Indo-Western Pacific Ocean capacitor (IPOC) effect. In spring, El Niño–induced northeasterly anomalies intensify the prevailing trades, enhancing TWNP cooling via wind–evaporation–SST (WES) feedback. This cooling suppresses convection and triggers a westward-propagating Rossby wave, reinforcing the WNPAAC. The TWNP cooling persists into summer and couples with NIO warming under the westerly monsoon regime, further intensifying the WNPAAC via Matsuno–Gill-type response. During positive PDO and positive VM, however, significant TWNP cooling is lacking and only NIO warming is prominent. The WNPAAC still forms but is significant mostly in low latitudes, resulting in a weaker El Niño–EASM relationship. This relationship further weakens during negative PDO and negative VM, where NIO warming is significant only in spring and TWNP cooling is absent. The weakest El Niño–EASM relationship occurs during negative PDO and positive VM, where both NIO warming and TWNP cooling are lacking. Meanwhile, the La Niña–EASM relationship is generally insignificant across all PDO–VM combinations, as the relatively weaker and slower-decaying La Niña induces less pronounced anomalous SST and atmospheric responses than El Niño. Significance Statement The preceding winter El Niño–Southern Oscillation (ENSO) is a crucial predictor of the East Asian summer monsoon (EASM), but their relationship is nonstationarity, challenging accurate EASM prediction. The Pacific decadal oscillation (PDO) and the Victoria mode (VM) may modulate the ENSO–EASM relationship. Similar to the PDO, the VM has recently gained attention for its role in connecting extratropical North Pacific dynamics to tropical climate variability. By utilizing observations and coupled general circulation model ensemble simulations, this study examines how the ENSO–EASM relationship changes depending on different phases of the PDO and VM. This not only improves our understanding of the ENSO–EASM relationship and its underlying mechanisms but also provides insights that can enhance seasonal EASM predictions.

Explosive volcanism occurred on Mars during its early history (Noachian–Hesperian; ~4.1–3.0 Ga). Because of Mars’ cold atmospheric temperatures, water released from explosive eruptions may precipitate as ice or ice-ash aggregates. This process may have supplied ice to equatorial regions, which contain high excess hydrogen and potential buried ice deposits. We simulate explosive volcanic eruptions using the Laboratoire de Météorologie Dynamique Generic Planetary Climate Model and find that up to ~5 meters of ice is delivered to the surface in only one high-magnitude eruptive event. This ice can persist for long periods if preserved by widespread cooling from volcanic sulfuric acid or by burial under dust or pyroclasts. Here we show that over time, explosive eruptions may have served as a recurring mechanism for delivering ice to the equator, explaining elevated ice content at low latitudes independent of obliquity.

Case studies of the unseasonal ionospheric irregularities in China’s low latitude on July 5 and July 17, 2013