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Abstract

Antarctic Bottom Water has been warming in recent decades throughout most of the oceans and freshening in regions close to its Indian and Pacific sector sources. We assess warming rates on isobars in the eastern Pacific sector of the Southern Ocean using CTD data collected from shipboard surveys from the early 1990s through the late 2010s together with CTD data collected from Deep Argo floats deployed in the region in January 2023. We show cooling and freshening in the temperature-salinity relation for water colder than ∼0.4°C. We further find a recent acceleration in the regional bottom water warming rate vertically averaged for pressures exceeding 3,700 dbar, with the 2017/18 to 2023/24 trend of 7.5 (±0.9) m°C yr−1 nearly triple the 1992/95 to 2023/24 trend of 2.8 (±0.2) m°C yr−1. The 0.2°C isotherm descent rate for these same time periods nearly quadruples from 7.8 to 28 m yr−1.

Abstract

During February 2023, the quasi-4-day wave (Q4DW) with westward zonal wavenumber 2 (W2) reached its largest amplitude of ∼400 m in the Southern Hemisphere (SH) geopotential height observations since 2004, which occurred simultaneously with an Arctic major sudden stratospheric warming (SSW) with an elevated stratopause (ES). However, the Q4DW-W2 perturbations in the Northern Hemisphere (NH) were unexpectedly suppressed despite the unstable Arctic stratosphere and mesosphere during the 2023 ES-SSW. Diagnostic analysis shows that the westward winds at ∼54°N–70°N in the upper stratosphere of ∼-79 m/s during the 2023 ES-SSW were the strongest during boreal winters over the past two decades, which benefited from the onset of a preceding minor SSW at the end of January. The strongest westward wind generated a wave geometry configuration of full reflection for Q4DW-W2 in the NH, while the Q4DW-W2 enhancement in the SH was induced by the in-situ amplification of the surviving seeding perturbations.

Abstract

Ozone (O3) pollution is a severe air quality issue in China, posing a threat to human health and ecosystems. The climate change will affect O3 levels by directly changing physical and chemical processes of O3 and indirectly changing natural emissions of O3 precursors. In this study, near-surface O3 concentrations in China in 2030 and 2060 are predicted using the process-based interpretable Extreme Gradient Boosting (XGBoost) model integrated with multi-source data. The results show that the climate-driven O3 levels over eastern China are projected to decrease by more than 0.4 ppb in 2060 under the carbon neutral scenario (SSP1-1.9) compared with the high emission scenario (SSP5-8.5). Among this reduction, 80% is attributed to the changes in physical and chemical processes of O3 related to a cooler climate, while the remaining 20% is attributed to the reduced biogenic isoprene emissions.

Abstract

The quantitative relationship between Mesoscale Convective Systems (MCSs) and floods over East Asia has not been established. In this study, MCSs are clustered into four types with Self-Organizing Map approach. Floods in June-August of 2000–2021 are linked with different types of MCS by automated algorithms we constructed. We find that among the major floods (potential flood peak periods), 91% (87%) are related to MCS, 65% (78%) are dominated by MCS, and 38% (20%) are dominated by multi-types of MCS. Types 1 and 2 MCS have higher flood-inducing efficiencies than common MCS (Type-4). Type-1 MCS, characterized by the least number (2% of the total number), the largest precipitation volume, longest lifetime, slowest moving, strongest precipitation, can most efficiently produce floods. Type-2 MCS, characterized by the second largest precipitation volume, more numerous than Type-1 particularly over land, can induce floods not only relatively efficiently but also more frequently than Type-1.

Abstract

An area of ocean centered on 179°E, 46°S has warmed to full depth since 2006, with surface warming around 5 times the global rate. This Subtropical Front area is associated with a confluence of warm, salty, subtropical water from the north carried in a western boundary current and cold, fresh, subantarctic water from the south carried in the northernmost branch of the Antarctic Circumpolar Current. Temperature and salinity changes observed from Argo floats indicate that the Subtropical Frontal Zone has moved west ∼120 km, creating this area of strong warming analogous to changes in extension regions of other western boundary currents. The warming is a result of changes in the local flows of subantarctic water, evident in satellite altimeter data and 1,000 m Argo trajectories, which in turn likely result from changes in meridional ocean heat content and winds. The warming has placed this biologically-significant region in almost perpetual marine heatwave conditions.

Abstract

We evaluate average wave intensities at frequencies up to 10 kHz measured by two ground stations in Canada and two others in Finland at auroral and subauroral latitudes over a full year, as well as by the low-altitude DEMETER spacecraft during the years 2004–2010. Lightning location and energy data obtained by the World Wide Lightning Location Network, along with geomagnetic activity characterized by the Kp index, are further used. Latitudinal, diurnal, and annual variations are analyzed, and the global intensities measured on the ground and by the spacecraft are systematically compared for the first time. We show that lightning-generated waves often dominate the measured wave intensities, particularly during the night, in summer, and at higher frequencies. DEMETER observations, supported by ray-tracing analysis, reveal a significant role of nonducted lightning-generated whistler propagation between the hemispheres. Finally, the wave intensity response to geomagnetic activity variations is quite different on the ground compared to in space. While spacecraft-measured wave intensities are considerably larger during periods of enhanced geomagnetic activity, the ground-based intensities are only sporadically enhanced during geomagnetically active periods.

Abstract

We present broadband ionospheric scintillation observations of highly defined symmetric quasi-periodic scintillations (QPS: Maruyama, 1991, https://doi.org/10.1029/91rs00357) caused by plasma structures in the mid-latitude ionosphere using the LOw Frequency ARray (LOFAR: van Haarlem et al., 2013, https://doi.org/10.1051/0004-6361/201220873). Two case studies are shown, one from 15 December 2016, and one from 30 January 2018, in which well-defined main signal fades are observed to be bounded by secondary diffraction fringing. The ionospheric plasma structures effectively behave as a Fresnel obstacle, in which steep plasma gradients at the periphery result in a series of decreasing intensity interference fringes, while the center of the structures largely block the incoming radio signal altogether. In particular, the broadband observing capabilities of LOFAR permit us to see considerable frequency dependent behavior in the QPSs which, to our knowledge, is a new result. We extract some of the clearest examples of scintillation arcs reported in an ionospheric context, from delay-Doppler spectral analysis of these two events. These arcs permit the extraction of propagation velocities for the plasma structures causing the QPSs ranging from 50 to 00 m s−1, depending on the assumed altitude. The spacing between the individual plasma structures ranges between 5 and 20 km. The periodicities of the main signal fades in each event and, in the case of the 2018 data, co-temporal ionosonde data, suggest the propagation of the plasma structures causing the QPSs are in the E-region. Each of the two events is accurately reproduced using a thin screen phase model. Individual signal fades and enhancements were modeled using small variations in total electron content (TEC) amplitudes of order 1 mTECu, demonstrating the sensitivity of LOFAR to very small fluctuations in ionospheric plasma density. To our knowledge these results are among the most detailed observations and modeling of QPSs in the literature.

Abstract

Despite more than half a century of Collisionless shock (CS) research, our understanding of the processes of the shock energy dissipation into the charge particle heating and acceleration remains incomplete. To help to address the problem of the rate of the data analysis on CSs being well below of the rate of the data acquisition, an open-source high-level database of shocks and a centralized source of advanced tools for the purpose of analyzing shock structure and dynamics have been developed. The database is called SHARP shock database by the name of the project SHARP (Shocks: structure, AcceleRation, dissiPation) funded by the European Union's Horizon 2020 program. The SHARP shock database contains shock crossings and corresponding parameters obtained from Cluster and MMS (Magnetospheric Multiscale) missions for terrestrial bow shocks, THEMIS (Time History of Events and Macroscale Interactions during Substorms)/ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) missions for interplanetary shocks, and MAVEN (Mars Atmosphere and Volatile EvolutioN) and VEX (Venus Express) missions for shocks at non-magnetized planets. The SHARP shock database can be accessed via https://sharp.fmi.fi/shock-database/.

Abstract

The OI 630.0 nm airglow emission variability provides salient information on the dynamical changes taking place in the upper atmosphere at around 250 km. The emission rates vary with the changes in the ambient electron densities and the neutral constituents that are associated with these emissions. On several occasions, enhancements in these emissions are observed during post-sunset hours, around 21 local time (LT), as measured from Mt. Abu (24.6°N, 72.7°E, 19°N Mag), a low-latitude location at Indian longitudes. These enhancements occur following the typical monotonic decrease in emission intensity after sunset. The presence of poleward meridional wind preceded by cessation and reversal of equatorward wind at the post-sunset hours was shown to be the cause for such observed emission enhancements in an earlier study. In this study, the cause of such reversal in meridional winds during post-sunset hours has been investigated using the variation in electron densities and meridional winds simulated by the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X), which also shows enhancements in electron densities similar to those observed in the post-sunset OI 630.0 nm nightglow emissions, and simultaneous reversal in meridional winds as well. The amplitudes and phases of different components of tides obtained from WACCM-X meridional winds reveal a significant contribution of higher-order tides, especially, quarter-diurnal tides, to the observed reversal in the meridional winds during post-sunset hours.

Quantitative Sub-Ice and Marine Tracing of Antarctic Sediment Provenance (TASP v1.0)
Jim Marschalek, Edward Gasson, Tina van de Flierdt, Claus-Dieter Hillenbrand, Martin Siegert, and Liam Holder
Geosci. Model Dev. Discuss., https//doi.org/10.5194/gmd-2024-104,2024
Preprint under review for GMD (discussion: open, 0 comments)
Ice sheet models can help predict how Antarctica's ice sheets respond to environmental change, and such models benefit from comparison to geological data. Here, we use an ice sheet model output, plus other data, to predict the erosion of debris and trace its transport to where it is deposited on the ocean floor. This allows the results of ice sheet modelling to be directly and quantitively compared to real-world data, helping to reduce uncertainty regarding Antarctic sea level contribution.

Wind comparisons between meteor radar and Doppler shifts in airglow emissions using field-widened Michelson interferometers
Samuel K. Kristoffersen, William E. Ward, and Chris E. Meek
Atmos. Meas. Tech., 17, 3995–4014, https://doi.org/10.5194/amt-17-3995-2024, 2024
In this paper, the relationship between observations from two instruments, a meteor radar and a field-widened Michelson interferometer (ERWIN) which provide complementary information on this region, is investigated. On average the ratio of ERWIN winds to meteor radar winds is ∼ 0.7. Differences between the wind observations may be caused by variations in the airglow brightness associated with dissipating gravity waves.

Abstract

Carbonate rocks, widely used for quantifying paleolatitude of the Gondwana-derived terranes on the Tibetan Plateau and the geodynamic evolution of the Tethyan Oceans, are prone to remagnetization. However, diagnosing such secondary remanent magnetization is difficult and the mistakes have induced confusion in paleogeographic reconstructions. To evaluate if the Upper Triassic limestones of the Duoburi Formation from the Lhasa terrane carry a primary remanence, we report comprehensive rock magnetic, diffuse reflectance spectroscopic, and petrographic results of these rocks. We discover that magnetic carriers vary systematically from magnetite to magnetite plus minor hematite/goethite to hematite/goethite plus minor magnetite with change of rock color and demagnetization behavior of the specimens. Most magnetite and all hematite/goethite grains have clear authigenic origin and were possibly formed during oxidation of early diagenetic pyrite. Such a process was likely assisted by oxic fluid circulation as shown by omnipresent calcite veins within the rocks. These authigenic iron oxides have widely distributed grain sizes with most of them being superparamagnetic at room temperature. Detrital (titano)magnetite is also recognized in some specimens, but its concentration is much lower than that of the authigenic magnetic grains. Based on these results, we conclude that limestone from the Duoburi Formation was remagnetized due to fluid circulation during late diagenesis. We discuss criteria used for diagnosing remagnetization in carbonate rocks, and suggest that a robust evaluation of the remanence origin should integrate field tests, statistics of the remanence direction, rock magnetic properties, and petrographic observations with the limits of each criterion being carefully considered.

Abstract

We analyze the sources for spread in the response of the Northern Hemisphere wintertime stratospheric polar vortex (SPV) to global warming in Climate Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6) model projections. About half of the intermodel spread in SPV projections by CMIP6 models, but less than a third in CMIP5 models, can be attributed to the intermodel spread in stationary planetary wave driving. In CMIP6, SPV weakening is mostly driven by increased upward wave flux from the troposphere, while SPV strengthening is associated with increased equatorward wave propagation away from the polar stratosphere. We test hypothesized factors contributing to changes in the upward and equatorward planetary wave fluxes and show that an across-model regression using projected global warming rates, strengthening of the subtropical jet and basic state lower stratospheric wind biases as predictors can explain nearly the same fraction in the CMIP6 SPV spread as the planetary wave driving (r = 0.67). The dependence of the SPV spread on the model biases in the basic state winds offers a possible emergent constraint; however, a large uncertainty prevents a substantial reduction of the projected SPV spread. The lack of this dependence in CMIP5 further calls for better understanding of underlying causes. Our results improve understanding of projected SPV uncertainty; however, further narrowing of the uncertainty remains challenging.

Abstract

Anthropogenic CO2 emission reduction strategies depend on how well we track emission enhancements at the urban scale. The estimation system, based on inverse modeling, relies on our knowledge of atmospheric transport and prior flux distributions. Hence, the analysis framework must account for uncertainties associated with each component in order to interpret the variations in observed CO2. Using an ensemble of simulations, we quantify the uncertainties in simulating anthropogenic CO2 mixing ratio enhancements at 15 locations in India. Differences in the representation of transport mechanisms and prior emission in the forward model induce a consistently large model spread of 62.2% and 41.9%, respectively, in the simulated mixing ratio over cities. The analysis reports an average uncertainty of 2.2 ppm with a maximum of 8 ppm for representing diurnally averaged anthropogenic CO2 enhancement. Diurnal variations in emissions and transport induce a rectification effect in those enhancements. The outcome of this study can thus inform future atmospheric CO2 inversion modeling at an urban scale on the expected forward model uncertainties, which are the essential components in the Bayesian inversion framework, typically lacking in the Indian region. The first-order inversion experiments show that the change in the transport model induces significant uncertainty (up to 84.9%) in anthropogenic CO2 flux estimation at the national scale. Hence, the confidence level of inverse-based emission estimation in India depends considerably on the accuracy of atmospheric transport modeling.

Abstract

Stable Auroral Red (SAR) arcs are enhanced OI 630.0 nm emissions formed due to an increased electron temperature (Te) near the equatorward wall of mid-latitude trough during geomagnetic disturbances. The Te enhancement associated with SAR arcs is driven by electron heating through heat flux precipitation from near plasmapause region to ionospheric F-region via heat conduction. Although Te enhancements have been reported by radar/satellite measurements along with increased 630.0 nm brightness during SAR arc events, measurements of corresponding heat flux are sparse, and almost none in the daytime. This work presents the results on the estimation of electron heat flux incident during SAR arcs formed during daytime obtained by a comprehensive suite of measurements, and forward modeling. We present observations of several SAR arc events when the ground-based OI 630.0 nm emissions were larger than the model values during disturbed periods and were found to be existing in conjunction with increased Te at the altitude of DMSP (∼840 km). Forward modeling was carried out to determine the values of Te that would cause an enhancement in these emissions during daytime at much lower altitudes (∼400–500 km). These values of Te were used to estimate the required electron heat flux varying in the range of ∼1.0–4.6 × 1010 eV-cm−2-s−1. These results present the first estimates of F-region heat flux enabled using ground-based OI 630.0 nm emissions and open a new approach in the investigations of energy released into the ionosphere through heat conduction for daytime conditions during geomagnetically disturbed periods.

Abstract

A statistical study has been made of dynamic small-scale auroral events in order to understand the drivers of the large variability in the electrodynamics of auroral arcs at fine scales. We used the Auroral Structure and Kinetics (ASK) instrument, located on Svalbard, in order to measure various electrodynamic properties of fine scale auroral arcs. We performed Spearman and Kendall statistical tests and found two significant correlations. The first is between the mean precipitation flux and the variability of flux, which we assume is because of the dynamic and bursty nature of the acceleration mechanism and its dependence on Alfvén waves. The second correlation is between the variability of the precipitating electron flux and the variability of the tangential component of the electric field close to the arc and perpendicular to the magnetic field. We propose that both variabilities occur because of the variability of the upward (field-aligned) current sheet in and around the arc, which is dynamic and non-uniform. The correlation between the two variabilities can therefore be explained by their common source.

Science, Volume 385, Issue 6704, July 2024.

Science, Volume 385, Issue 6704, July 2024.

Science, Volume 385, Issue 6704, July 2024.

Science, Volume 385, Issue 6704, July 2024.