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Syndicate content Wiley: Geophysical Research Letters: Table of Contents
Table of Contents for Geophysical Research Letters. List of articles from both the latest and EarlyView issues.
Updated: 1 day 19 hours ago

Depth Dependent Dynamics Explain the Equatorial Jet Difference Between Jupiter and Saturn

Tue, 03/12/2024 - 13:24
Abstract

Jupiter's equatorial eastward zonal flows reach wind velocities of ∼100 m s−1, while on Saturn they are three times as strong and extend about twice as wide in latitude, despite the two planets being overall dynamically similar. Recent gravity measurements obtained by the Juno and Cassini spacecraft uncovered that the depth of zonal flows on Saturn is about three times greater than on Jupiter. Here we show, using 3D deep convection simulations, that the atmospheric depth is the determining factor controlling both the strength and latitudinal extent of the equatorial zonal flows, consistent with the measurements for both planets. We show that the atmospheric depth is proportional to the convectively driven eddy momentum flux, which controls the strength of the zonal flows. These results provide a mechanistic explanation for the observed differences in the equatorial regions of Jupiter and Saturn, and offer new understandings about the dynamics of gas giants beyond the Solar System.

Vertical Velocity Diagnosed From Surface Data With Machine Learning

Tue, 03/12/2024 - 04:14
Abstract

Submesoscale vertical velocities, w, are important for the oceanic transport of heat and biogeochemical properties, but observing w is challenging. New remote sensing technologies of horizontal surface velocity at O(1) km resolution can resolve surface submesoscale dynamics and offer promise for diagnosing w subsurface. Using machine learning models, we examine relationships between the three-dimensional w field and remotely observable surface variables such as horizontal velocity, density, and their horizontal gradients. We evaluate the machine learning models' sensitivities to different inputs, spatial resolution of surface fields, the addition of noise, and information about the subsurface density. We find that surface data is sufficient for reconstructing w, and having high resolution horizontal velocities with minimal errors is crucial for accurate w predictions. This highlights the importance of finer scale surface velocity measurements and suggests that data-driven methods may be effective tools for linking surface observations with vertical velocity and transport subsurface.

How Does the Southern Annular Mode Control Surface Melt in East Antarctica?

Tue, 03/12/2024 - 04:09
Abstract

Surface melt in East Antarctica is strongly correlated with the Southern Annular Mode (SAM) index, but the spatiotemporal variability of the relationship, and the physical processes responsible for it, have not been examined. Here, using melt flux estimates and climate variables from the RACMO2.3p3 regional climate model, we show that a decreasing SAM index is associated with increased melt in Dronning Maud Land primarily owing to reduced precipitation and greater absorption of solar radiation. Conversely, in Wilkes Land, a decreasing SAM index corresponds to increased melt because of greater incoming longwave radiation from a warmer atmosphere. We also demonstrate that SAM-melt correlations are strongest in December as the melt season develops, and that the SAM’s influence on peak melt intensities in January occurs indirectly through the snowmelt-albedo feedback. Future work must account for such variability in the physical processes underlying the SAM-melt relationship to reduce uncertainty in surface melt projections.

Wildfire Smoke Directly Changes Biogenic Volatile Organic Emissions and Photosynthesis of Ponderosa Pines

Mon, 03/11/2024 - 18:14
Abstract

Wildfires are increasing across the USA. While smoke events affect human exposure and air quality, wildfire smoke effects on ecosystem-atmosphere interactions are poorly understood. We investigate smoke effects on biogenic volatile organic compound (VOC) emissions and photosynthesis for ponderosa pines. During several wildfire smoke events, we observed photosynthetic reduction with evidence for stomatal plugging and changes in leaf-level uptake and emission of both biogenic and wildfire VOCs. During intense smoke events, photosynthesis and VOC emissions were almost entirely suppressed, but increased dramatically upon stomatal opening. We propose four types of VOC responses to this burst in stomatal opening: post-burst emissions, pulsed emissions, surge emissions, and post-burst uptake. Our observations suggest that wildfire smoke can affect plant physiology and leaf-atmosphere gas exchange.

Issue Information

Mon, 03/11/2024 - 18:04

No abstract is available for this article.

A Mid‐Lithospheric Discontinuity Detected Beneath 155 Ma Western Pacific Seafloor Using Sp Receiver Functions

Mon, 03/11/2024 - 05:28
Abstract

This study probes the lithosphere-asthenosphere system beneath 155 Ma Pacific seafloor using teleseismic S-to-p receiver functions at the Pacific Lithosphere Anisotropy and Thickness Experiment project ocean-bottom-seismometers. Within the lithosphere, a significant velocity decrease at 33–50 km depth is observed. This mid-lithospheric discontinuity is consistent with the velocity contrast between the background mantle and thin, trapped layers of crystallized partial melt, in the form of either dolomite or garnet granulite. These melts possibly originated from deeper asthenospheric melting beneath the flanks of spreading centers, and were transported within the cooling lithosphere. A positive velocity increase of 3%–6% is observed at 130–155 km depth and is consistent with the base of a layer with partial melt in the asthenosphere. A shear velocity decrease associated with the lithosphere-asthenosphere boundary at 95–115 km depth is permitted by the data, but is not required.

A Pre‐Monsoon Signal of False Alarms of Indian Monsoon Droughts

Mon, 03/11/2024 - 05:24
Abstract

Current knowledge suggests a drought Indian monsoon (perhaps a severe one) when the El Nino Southern Oscillation and Pacific Decadal Oscillation each exhibit positive phases (a joint positive phase). For the monsoons, which are exceptions in this regard, we found northeast India often gets excess pre-monsoon rainfall. Further investigation reveals that this excess pre-monsoon rainfall is produced by the interaction of the large-scale circulation associated with the joint phase with the mountains in northeast India. We posit that a warmer troposphere, a consequence of excess rainfall over northeast India, drives a stronger monsoon circulation and enhances monsoon rainfall over central India. Hence, we argue that pre-monsoon rainfall over northeast India can be used for seasonal monsoon rainfall prediction over central India. Most importantly, its predictive value is at its peak when the Pacific Ocean exhibits a joint positive phase and the threat of extreme drought monsoon looms over India.

On the Role of Indian Ocean SST in Influencing the Differences in Atmospheric Variability Between 2020–2021 and 2021–2022 La Niña Boreal Winters

Mon, 03/11/2024 - 04:50
Abstract

The difference in observed atmospheric anomalies over the Northern Hemisphere winter between 2021–22 and 2020–21 La Niña years indicated a tripole pattern consisting of a Japan cyclone, a Bering Sea anticyclone, and a cyclone over the North American continent. This feature, however, was not replicated in the North American Multi-Model Ensemble (NMME) forecasts. A set of model sensitivity experiments was performed to better understand the cause of this discrepancy. The results revealed the possible role of the influence of sea surface temperature (SST) anomalies, particularly over the Indian Ocean, on the observed circulation differences that was further modulated by internal atmospheric variability. The failure in predicting circulation changes in NMME was next attributed to the errors in SST predictions over the Indian Ocean and highlights the need for improvements in SST forecasts over this region.

Deciphering Controls of Pore‐Pressure Evolution on Sediment Bed Erosion by Debris Flows

Mon, 03/11/2024 - 04:34
Abstract

Pore-fluid pressure (PP) plays an important role in bed erosion, but the mechanisms that control PP evolution and the resulting feedbacks on flow dynamics are unclear. Here, we develop a general formulation, allowing quantification of the propensity for PP evolution of saturated and unsaturated bed sediments. We conduct erosion experiments by systematically varying grain composition and water content of beds, for investigating effects of PP evolution on flow erosion. With increasing water content, PP shows a slight rise in deforming beds with drained behavior but significant larger rise in undrained beds. Regardless of bed composition, the erosion rate of beds presents a synchronous change tendency with PP evolution due to the loss in basal friction. PP instigates positive feedback that induces a remarkable gain of flow velocity and momentum on wet beds with undrained behavior. Our results help explain observations of volume growth and long run out of debris flows.

CS2 Cycling in Seawater: Dark Production and UV Light Driven Consumption

Sat, 03/09/2024 - 07:00
Abstract

Carbon disulfide (CS2) has recently gained attention as an important precursor for the atmospheric trace gas carbonyl sulfide (OCS), which delivers sulfur to the stratospheric sulfur layer and impacts the radiative budget of the Earth. CS2 is naturally produced in the ocean and emitted to the atmosphere. However, the magnitude of its marine emissions is only poorly constrained due to lacking understanding of its production and consumption processes. Here, we present incubation experiments with and without UV light treatment and provide evidence for a previously not considered UV-light-driven degradation process of CS2 in seawater, following first-order kinetics. In addition to its already known photochemical production process, CS2 production is found in the dark, depending on the amount of dissolved organic sulfur present in seawater. We provide novel production and consumption rates of CS2 in seawater that pave the way toward mechanistically quantifying marine emissions of this important trace gas.

Reconstruction of 3D DPR Observations Using GMI Radiances

Sat, 03/09/2024 - 06:49
Abstract

Three-dimensional global precipitation observation is crucial for understanding climate and weather dynamics. While spaceborne precipitation radars provide precise but limited observations, passive microwave imagers are available much more frequently. In this study, we propose a deep learning approach to reconstruct active radar observations using passive microwave radiances. We introduce the Hybrid Deep Neural Network (HDNN) model, which utilizes reflectivity profiles from the Dual-frequency Precipitation Radar (DPR) onboard the Global Precipitation Measurement (GPM) Core Observatory Satellite as the “target” and combines radiances from the GPM Microwave Imager (GMI) with supplementary reanalysis data to serve as the “features.” Results underscore the HDNN's exemplary performance, with a root mean square error below 4 dBZ across all altitude levels, and a consistent accuracy across different precipitation types. Its efficacy is further illustrated when applied to typhoon cases of Haishen and Khanun, emerging as a superior tool for capturing 3D structures of expansive precipitation systems.

Byrd Ice Core Debris Constrains the Sediment Provenance Signature of Central West Antarctica

Sat, 03/09/2024 - 06:40
Abstract

Provenance records from sediments deposited offshore of the West Antarctic Ice Sheet (WAIS) can help identify past major ice retreat, thus constraining ice-sheet models projecting future sea-level rise. Interpretations from such records are, however, hampered by the ice obscuring Antarctica's geology. Here, we explore central West Antarctica's subglacial geology using basal debris from within the Byrd ice core, drilled to the bed in 1968. Sand grain microtextures and a high kaolinite content (∼38–42%) reveal the debris consists predominantly of eroded sedimentary detritus, likely deposited initially in a warm, pre-Oligocene, subaerial environment. Detrital hornblende 40Ar/39Ar ages suggest proximal late Cenozoic subglacial volcanism. The debris has a distinct provenance signature, with: common Permian-Early Jurassic mineral grains; absent early Ross Orogeny grains; a high kaolinite content; and high 143Nd/144Nd and low 87Sr/86Sr ratios. Detecting this “fingerprint” in Antarctic sedimentary records could imply major WAIS retreat, revealing the WAIS's sensitivity to future warming.

Particulate Nitrate Photolysis as a Possible Driver of Rising Tropospheric Ozone

Sat, 03/09/2024 - 06:33
Abstract

Tropospheric ozone is an air pollutant and a greenhouse gas whose anthropogenic production is limited principally by the supply of nitrogen oxides (NOx) from combustion. Tropospheric ozone in the northern hemisphere has been rising despite the flattening of NOx emissions in recent decades. Here we propose that this sustained increase could result from the photolysis of nitrate particles (pNO3 −) to regenerate NOx. Including pNO3 − photolysis in the GEOS-Chem atmospheric chemistry model improves the consistency with ozone observations. Our simulations show that pNO3 − concentrations have increased since the 1960s because of rising ammonia and falling SO2 emissions, augmenting the increase in ozone in the northern extratropics by about 50% to better match the observed ozone trend. pNO3 − will likely continue to increase through 2050, which would drive a continued increase in ozone even as NOx emissions decrease. More work is needed to better understand the mechanism and rates of pNO3 − photolysis.

Unraveling the Role of Vegetation CO2 Physiological Forcing on Climate Zone Shifts in China

Fri, 03/08/2024 - 09:09
Abstract

Increasing atmospheric CO2 causes substantial spatial and seasonal changes in air temperature and precipitation through its radiative (RAD) and vegetation physiological (PHY) effects. However, it remains poorly understood on how these two effects impact the integrated climate zone shifts over China. Here, we disentangle the RAD and PHY effects on the shifts of Köppen-Geiger climate zones from pre-industrial to 4 × CO2 in China using nine Earth system models. We find that climate zone changes over approximately 56.1% of China, and PHY contributes 15.2% of such changes at 4 × CO2. PHY shifts regional climate to warmer and wetter classifications, shrinking (−42.8%) the arid zone distributions and promoting (26.8%) the tropical zone northward extensions. Our findings highlight the critical role of vegetation in reshaping the overall climate zone distributions, yet introduce potential risk to climate mitigation and adaptation.

The Climatology of Mars Thermospheric Polar Warming at Aphelion

Fri, 03/08/2024 - 08:59
Abstract

Thermospheric polar warming (TPW) is observed conclusively for the first time at Mars during the aphelion/Northern Summer season using solar occultation (SO) measurements made by the Extreme Ultraviolet Monitor (EUVM) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter. Aphelion data from Mars Year (MY) 33–36 are analyzed revealing TPW to be present at dawn but not dusk. This is consistent with an earlier analysis of accelerometer data from the Mars Global Surveyor that showed aphelion TPW is also not evident at 15 hr local time. Separating the data into individual MYs reveals TPW is observed during each year except MY 35. TPW is markedly intensified during MY 34, which is attributed to enhanced circulation caused by a northern-hemisphere dust storm coinciding with the observations. Simulations from the Mars Climate Database predict the large TPW enhancement in MY 34 relative to MY 33 observed by EUVM SO, but predicts approximately 20K less overall TPW for both years than that observed by EUVM SO.

Mesoscale Meridional Heat Transport Inferred From Sea Surface Observations

Fri, 03/08/2024 - 06:20
Abstract

The ocean regulates the Earth's climate by transporting heat from the equator to the poles. Here, we use satellite-based sea surface observations of air-sea heat fluxes and eddy detection to investigate the mesoscale heat transport. “Mesoscale” refers to both the Eulerian perspective as the spatio-temporal scales of ∼100 km and ∼1 month, as well as the Lagrangian aspect as isolated vortices identified from the dynamic topography. Paradoxically, there are a considerable number of mesoscale eddies inconsistent between their surface thermal and dynamic signals, that is, cold-core anticyclones and warm-core cyclones are globally prevalent. On account of such inconsistency, we show that the mesoscale meridional heat transport carried by geostrophic components is 10 times larger than (and opposite in direction to) that of the wind-driven Ekman components. An offset between SSH-SST coherent and incoherent eddies in the Ekman heat transport is apparent, whereas the geostrophic heat transport is contained within coherent eddies.

Changes in External Forcings Drive Divergent AMOC Responses Across CESM Generations

Fri, 03/08/2024 - 06:13
Abstract

It has been suggested that the Atlantic meridional overturning circulation (AMOC) in many CMIP6 models is overly sensitive to anthropogenic aerosol forcing, and it has been proposed that this is due to the inclusion of aerosol indirect effects for the first time in many CMIP6 models. We analyze the AMOC response in a newly released ensemble of simulations performed with CESM2 forced by the CMIP5 input data sets (CESM2-CMIP5). This AMOC response is then compared to the CMIP5-generation CESM1 large ensemble (CESM1-LE) and the CMIP6-generation CESM2 large ensemble (CESM2-LE). A key conclusion, only made possible by this experimental setup, is that changes in aerosol-indirect effects cannot explain differences in AMOC response between CESM1-LE and CESM2-LE. Instead, we hypothesize that the difference is due to increased interannual variability of anthropogenic emissions. This forcing variability may act through a nonlinear relationship between the surface heat budget of the North Atlantic and the AMOC.

The Energetic Oxygen Ion Beams in the Martian Magnetotail Current Sheets: Hints From the Comparisons Between Two Types of Current Sheets

Thu, 03/07/2024 - 11:22
Abstract

Using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we explore the plasma properties of Martian magnetotail current sheets (CS), to further understand the solar wind interaction with Mars and ion escape. There are some CS exhibit energetic oxygen ions that show narrow beam structures in the energy spectrum, which primarily occurs in the hemisphere where the solar wind electric field (Esw) is directed away from the planet. On average, these CS have a higher escaping flux than that of the CS without energetic oxygen ion beams, suggesting different roles in ion escape. The CS with energetic oxygen ion beams exhibits different proton and electron properties to the CS without energetic oxygen ion beams, indicating their different origins. Our analysis suggests that the CS with energetic oxygen ion beams may result from the interaction between the penetrated solar wind and localized oxygen ion plumes.

New Magnetotelluric Data Reveal Deep Fault Boundaries and Contrasting Late Cenozoic Fault Kinematics Between the Qilian Shan Thrust Wedge and Beishan‐Alxa Block, Western China

Thu, 03/07/2024 - 11:21
Abstract

The structural connectivity and kinematic relationship between the Altyn Tagh sinistral strike-slip fault (ATF) and Qilian Shan fold-and-thrust belt along the north Tibetan margin east of 96°E is an important question for tectonicists interested in the evolving active deformation field of Central Asia and associated earthquake hazards of China's Hexi Corridor region. New results from a detailed 130-km-long N-S magnetotelluric (MT) survey from the Qilian Shan to Beishan elucidates the locations and down-dip orientations of major faults. Importantly, the results indicate that the Heishan-Jinta’Nanshan fault system roots steeply into the lower crust, is unconnected to the Qilian Shan thrust wedge, and has reactivated the margin of the North China Craton and an older, regional ductile shear belt. The structurally linked ATF-Heishan-Jinta’Nanshan system defines a fundamental kinematic boundary in central Asia between the NE directed Qilian Shan thrust belt to the south and the eastwardly extruding Beishan-Alxa Block to the north.

Annual 10Be Record for 1510–1701 CE Obtained From Endogenic Travertine at Baishuitai, China: A New Proxy Record of Annual Solar Activity

Thu, 03/07/2024 - 11:21
Abstract

The 10Be record in laminated travertines is a potential proxy for reconstructing past solar activity down to the annual scale; however, correcting for the potential influence of climatic or environmental variations remains challenging. Here, we present an annually resolved 10Be record using travertines from Baishuitai, China, covering the period from 1510 to 1701 CE, along with environmental proxies, to evaluate climatic influences and implement corrections to accurately reconstruct solar activity. We demonstrate that the 10Be deposition in travertines exhibits two environmental impacts: the transport efficiency of atmospheric 10Be into travertine and the additional 10Be inflow from overland flow associated with rainfall. We show that these impacts can be corrected based on iron and potassium contents. The resulting corrected record agrees with ice-core and tree-ring records, demonstrating the feasibility of using such carbonate sediment 10Be records to reconstruct past solar activity.

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