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SummaryBedrock geology from Antarctica remains largely unknown since it is hidden beneath thick ice sheets. Geophysical methods such as gravity and magnetic inverse modelling provide a framework to infer crustal rock properties indirectly in Antarctica. However, due to limited availability of rock samples, validation against direct geological information is challenging. We present a new rock property catalogue containing density and susceptibility measurements on 320 rock samples from northern Victoria Land. This catalogue is used to assess the reliability of local and regional scale inverse results, including a new local high resolution magnetic inversion in the Mesa Range region and a previously published regional scale joint inversion of gravity and magnetic data in northern Victoria Land and the Wilkes Subglacial Basin. We compare our density and susceptibility measurements to global and local measurements from the literature to access the correlation to rock types and geological units. Furthermore, the measured values are compared against inverted values. The close correspondence between inverted and measured rock properties allows us to predict locations of rock types where currently such information is missing. The utility of measured susceptibility and density relationships for interpreting inversion output provides a strong incentive to incorporate local rock samples into geophysical studies of subglacial geology across Antarctica.

SummaryThis study focuses on improving the seafloor compliance noise removal method, which relies on estimates of the compliance transfer function frequency response (the deformation of the seafloor under long-period pressure waves). We first propose a new multi-scale deviation analysis of broadband ocean bottom seismometer data to evaluate stationarity properties that are key to the subsequent analysis. We then propose a new approach to removing the compliance noise from the vertical channel data, by stacking daily estimated transfer function frequency responses over a period of time. We also propose an automated transient event detection and data selection method based on a cross-correlation criterion. As an example, we apply the method to data from the Cascadia Initiative (network 7D2011). We find that the spectral extent of long-period forcing waves varies significantly over time so that standard daily transfer function calculation techniques poorly estimate the transfer function frequency response at the lowest frequencies, resulting in poor denoising performance. The proposed method more accurately removes noise at these lower frequencies, especially where coherence is low, reducing the mean deviation of the signal in our test case by 27 % or more. We also show that our calculated transfer functions can be transfered across time periods. The method should allow better estimates of seafloor compliance and help to remove compliance noise at stations with low pressure-acceleration coherence. Our results can be reproduced using the BRUIT-FM Python toolbox, available at https://gitlab.ifremer.fr/anr-bruitfm/bruit-fm-toolbox.

We analyzed electrostatic electron cyclotron harmonic waves observed by the interferometry observation mode of the Arase satellite. It is found that the magnitude of the phase difference varies with the satell...

Publication date: Available online 24 July 2024

Source: Advances in Space Research

Author(s): R. Mukesh, Sarat C. Dass, M. Vijay, S. Kiruthiga, M. Praveenkumar, M. Prashanth

Abstract

The formation of Lau Basin records an extreme event of plate tectonics, with the associated Tonga trench exhibiting the fastest retreat in the world (16 cm/yr). Yet paleogeographic reconstructions suggest that seafloor spreading in the Lau Basin only initiated around 6 Ma. This kinematics is difficult to reconcile with our present understanding of how subduction drives plate motions. Using numerical models, we propose that eastward migration of the Lau Ridge concurrent with trench retreat explains both the narrow width and thickened crust of the Lau Basin. To match the slab geometry and basin width along the Tonga-Kermadec trench, our models suggest that fast trench retreat rate of 16 cm/yr might start ~15 Ma. Tonga slab rollback induced vigorous mantle flow underneath the South Fiji Basin which is driving the extension and thinning of the basin and contributing to its observed deeper bathymetry compared to neighboring basins.

Abstract

The North Atlantic Oscillation (NAO) and North Atlantic tripole sea surface temperature (SST_tri) are important modes in the atmosphere and ocean over the North Atlantic, respectively. The link between the two is well-known. However, this link weakened during 1980–2001, which is particularly pronounced in late winter and was not detected in early winter. This phenomenon has not been well revealed. The role of NAO in the above correlation changes was discussed. In late winter, a significant eastward shift (up to 20° longitude) of NAO south center during 1980–2001 was observed in both observation and CMIP6, accompanied by the eastward expansion of NAO north center. Spatial shift of the NAO forced the region of strong air-sea interactions to shift and resulting in the collapse of NAO-related SST_tri. These findings deepen our understanding of the NAO on the subseasonal scale.

Abstract

The solar wind interaction with planetary magnetospheres dictates the mechanism through which energy is transported across planetary systems. The magnetohydrodynamic plasma description suggests that solar wind conditions in the outer solar system encourage the magnetopause boundaries at Uranus and Neptune to be more Kelvin-Helmholtz unstable, however, no quantitative assessment has been performed. To characterize the viscous solar wind interaction at Uranus and Neptune, we create an analytical model to determine where Kelvin-Helmholtz Instabilities (KHIs) may form along their magnetopauses by searching for regions where the minimum condition for KHI formation is satisfied. We run the model at solstice and equinox for a range of Interplanetary Magnetic Field (IMF) strengths, and rotation phases. We find minimal seasonal variation for low IMF strengths (B = 0.01 nT), with ∼70% of the magnetopause surface at Uranus and ∼80% at Neptune, enabling KHI formation. For periods of stronger IMF strength (B > 0.3 nT), KHIs were significantly suppressed. While KHIs depend on both the conditions inside the magnetopause boundary and the shocked solar wind IMF strength, we find that the IMF strength is the most significant criterion in determining whether or not KHIs are allowed to form at the magnetopause boundaries.

Abstract

Along a climate gradient in the Chilean coastal mountains, we investigated denudation rates using the meteoric cosmogenic nuclide 10Be and its ratio to stable 9Be, and chemical depletion fractions (CDFs) in bulk soil samples. We find that the fraction of 9Be released from bedrock is a sensitive indicator of weathering, similar to CDF. Meteoric 10Be decreases exponentially with depth, reflecting the reactive nature of this tracer. We also measured denudation rates by the well-understood in situ cosmogenic 10Be system on quartz. Assuming that both systems record the same denudation rate we calculated the depositional flux of meteoric 10Be for each study site. The flux agrees to that derived from atmospheric models in the mediterranean and humid areas. In contrast, in the arid and semi-arid areas, the calculated flux agrees with a precipitation-derived flux, indicating delivery of 10Be to be affected by small-scale climatic variations not reflected by current atmospheric models.

Abstract

In this paper, using a three-dimensional multifluid MHD model, we studied the effects of the interplanetary magnetic field (IMF) clock angle and the latitude position of the intense crustal magnetic field (ICMF) on the escape of ions O+, O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$, and CO2+ ${\mathrm{C}\mathrm{O}}_{\mathrm{2}}^{+}$ at Mars. The main results are as follows: (a) The IMF clock angle affects the ion escape at Mars. When the ICMF is on the dayside, the ion escape rate reaches a maximum at the IMF clock angles close to 60°–90° and a minimum at the IMF clock angles close to 120°–150°, because the ICMF can change the topology of the magnetic field and affect the interaction between the solar wind and Mars. The difference between the maximum and minimum ion escape rates due to the IMF clock angle can reach over 50%. (b) Compared with the −ESW hemisphere, the escape flux of O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$ and CO2+ ${\mathrm{C}\mathrm{O}}_{\mathrm{2}}^{+}$ in the +ESW hemisphere is more significant. However, O+ generally has a larger escape flux in the −ESW hemisphere. The different results in the ±ESW hemispheres might be due to the larger distribution of the hot oxygen corona, which changes the flow pattern of O+. (c) The latitude location of the ICMF can also affect the ion escape. When the ICMF is on the dayside, as the subsolar point varies from 25°S to 25°N, that is, the intense crustal magnetic field position keeps shifting southward, the ion escape rate shows a gradual increase.

Abstract

Sea surface temperature (SST) anomalies over the North Tropical Atlantic (NTA) during the early boreal spring can trigger El Niño-Southern Oscillation (ENSO) events in the following boreal winter. However, the future changes in the impact of the NTA on ENSO remain controversial. Here, we show distinct changes in the strength of the NTA−ENSO relationship due to global warming by comparing models from the Coupled Model Intercomparison Project (CMIP) 5 and CMIP6. The impact of the NTA on ENSO under greenhouse warming is notably enhanced in CMIP6 compared to CMIP5. A wetter mean state over the subtropical eastern North Pacific and increased oceanic sensitivity over the equatorial central Pacific are key factors that enhance the impact of the NTA SST on ENSO. Therefore, differences in the mean state under greenhouse warming between the CMIP5 and CMIP6 models can modulate the strength of the NTA−ENSO relationship.

Abstract

The Surface Water and Ocean Topography (SWOT) mission provides a good opportunity to study fine-scale processes in the global ocean but whether it can detect balanced submesoscale eddies is uncertain due to the “contamination” by unbalanced inertial gravity waves. Here, based on concurrent observations from SWOT and a mooring array in the northwestern Pacific, we successfully captured two submesoscale cyclonic eddies with negative sea level anomalies (SLAs) in spring 2023. We find that the SLA amplitude and equivalent radius of the first (second) eddy are 2.5 cm and 16.0 km (2.0 cm and 18.8 km), respectively. For both eddies, their vertical scales are around 150 m and their horizontal velocities and Rossby numbers exceed 15.0 cm/s and 0.4, respectively. Further analysis suggests that similar submesoscale eddies can commonly occur in the northwestern Pacific and that SWOT is capable to detect larger submesoscale eddies with scales greater than ∼10 km.

Abstract

In this study, we evaluated the performance of machine learning (ML) models (XGBoost) in predicting low-cloud fraction (LCF), compared to two generations of the community atmospheric model (CAM5 and CAM6) and ERA5 reanalysis data, each having a different cloud scheme. ML models show a substantial enhancement in predicting LCF regarding root mean squared errors and correlation coefficients. The good performance is consistent across the full spectrums of atmospheric stability and large-scale vertical velocity. Employing an explainable ML approach, we revealed the importance of including the amount of available moisture in ML models for representing spatiotemporal variations in LCF in the midlatitudes. Also, ML models demonstrated marked improvement in capturing the LCF variations during the stratocumulus-to-cumulus transition (SCT). This study suggests ML models' great potential to address the longstanding issues of “too few” low clouds and “too rapid” SCT in global climate models.

Abstract

The impact of land variables on temperature forecasts in atmospheric cycling is often underestimated or overlooked. This oversight primarily occurs due to the abundance of meteorological measurements available for assimilation and partly because soil states are assumed to be quickly reset by atmospheric forcing, such as precipitation, justifying no spin-ups or no updates of soil states during cycling. In this study, by updating soil moisture every 6 hr using different analysis data sets for May 2019, considerable discrepancies were found, highlighting large uncertainties in soil moisture analysis. Different soil moisture analyses produced systematically different temperature forecasts, with errors growing over cycles to be comparable to a typical error magnitude of 2-m temperature observations (∼2°K). This study demonstrates that temperature forecasts are significantly influenced by whether and how soil moisture is updated, not only near the surface but also up to the low-mid troposphere and throughout the cycles.

Abstract

Variations in southern African precipitation have a major impact on local communities, increasing climate-related risks and affecting water and food security, as well as natural ecosystems. However, future changes in southern African precipitation are uncertain, with climate models showing a wide range of responses from near-term projections (2020–2040) to the end of the 21st century (2080–2100). Here, we assess the uncertainty in southern African precipitation change using five Ocean-Atmosphere General Circulation single model initial-condition large ensembles (30–50 ensemble members) and four emissions scenarios. We show that the main source of uncertainty in 21st Century projections of southern African precipitation is the internal climate variability. In addition, we find that differences between ensemble members in simulating future changes in the location of the Angola Low explain a large proportion (∼60%) of the uncertainty in precipitation change. Together, the internal variations in the large-scale circulation over the Pacific Ocean and the Angola Low explain ∼64% of the uncertainty in southern African precipitation change. We suggest that a better understanding of the future evolutions of the southern African precipitation may be achieved by understanding better the model's ability to simulate the Angola Low and its effects on precipitation.

Earth systems could be "tipped" into unstable states if warming overshoots the 1.5°C target, but impacts could be minimized if warming is swiftly reversed.

Abstract

Interplanetary (IP) shocks are perturbations observed in the solar wind. IP shocks correlate well with solar activity, being more numerous during times of high sunspot numbers. Earth-bound IP shocks cause many space weather effects that are promptly observed in geospace and on the ground. Such effects can pose considerable threats to human assets in space and on the ground, including satellites in the upper atmosphere and power infrastructure. Thus, it is of great interest to the space weather community to (a) keep an accurate catalog of shocks observed near Earth, and (b) be able to forecast shock occurrence as a function of the solar cycle (SC). In this work, we use a supervised machine learning regression model to predict the number of shocks expected in SC25 using three previously published sunspot predictions for the same cycle. We predict shock counts to be around 275 ± 10, which is ∼47% higher than the shock occurrence in SC24 (187 ± 8), but still smaller than the shock occurrence in SC23 (343 ± 12). With the perspective of having more IP shocks on the horizon for SC25, we briefly discuss many opportunities in space weather research for the remainder years of SC25. The next decade or so will bring unprecedented opportunities for research and forecasting effects in the solar wind, magnetosphere, ionosphere, and on the ground. As a result, we predict SC25 will offer excellent opportunities for shock occurrences and data availability for conducting space weather research and forecasting.

No abstract is available for this article.

Abstract

This study investigates the effects of sea spray on extreme precipitation forecast in Beijing of China between 28 July and 2 August 2023 as a case test. In this case, fully coupled model increased upward moisture in the Bohai and Yellow Seas and increased accumulated rainfall by 21% in North China. For the extreme precipitation events with the 5-day accumulated precipitation exceeding 500 mm, the atmosphere-only model did not forecast the events; the coupled model without sea spray performed well with the 0.29 threat score (TS) and 88 mm root mean square error (RMSE); in the fully coupled model, the effects of sea spray increased atmospheric instability, which increased the precipitation around Beijing and yielded a more accurate forecast with the 0.37 TS and 65 mm RMSE. This paper suggests a scientific clue to improve numerical simulation for extreme rainfall events, however, more cases are still needed for statistical evaluation.

Aerosol particles are tiny. Swirling suspended in the air around us, most are smaller than the smallest bug, thinner than the thinnest hair on your head, gossamer specks practically invisible to the naked eye. Newly formed ones are nano-sized. Yet their influence is gargantuan.

Abstract

The Earth exhibits an equatorial flattening specified by the ellipticity and the east longitude (or orientation) of the equatorial major axis, which is uniquely determined by the degree 2 and order 2 gravitational coefficients, C22 and S22. The 31-year SLR (satellite laser ranging) and 22-year GRACE/GRACE-FO (gravity recovery and climate experiment) data are analyzed to study the climate-related secular and 5.7 years to decadal variations in C22 and S22, in turn, the drift and decadal variation in the Earth’s equatorial ellipticity and orientation of the principal axis of the least moment of inertia. The effects of the surface floating mass changes (including atmosphere, ocean and surface water redistribution and the melting of the mountain and polar glaciers) and the interior fluid convective (Earth’s core flows) were evaluated. Results reveal that the equatorial ellipticity of the Earth is linearly increasing along with a remarkable decadal variation and the Earth’s equator is flattening by ~ 0.16 mm/yr.