Showing papers in "Journal of Geophysical Research in 2020"

Present‐Day Crustal Deformation of Continental China Derived From GPS and Its Tectonic Implications

Abstract: We process rigorously GPS data observed during the past 25 years from continental China to derive site secular velocities. Analysis of the velocity solution leads to the following results. (a) The deformation field inside the Tibetan plateau and Tien Shan is predominantly continuous, and large deformation gradients only exist perpendicular to the Indo‐Eurasian relative plate motion and are associated with a few large strike‐slip faults. (b) Lateral extrusions occur on both the east and west sides of the plateau. The westward extrusion peaks at ~6 mm/yr in the Pamir‐Hindu Kush region. A bell‐shaped eastward extrusion involves most of the plateau at a maximum rate of ~20 mm/yr between the Jiali and Ganzi‐Yushu faults, and the pattern is consistent with gravitational flow in southern and southeastern Tibet where the crust shows widespread dilatation at 10–20 nanostrain/yr. (c) The southeast borderland of Tibet rotates clockwise around the eastern Himalaya syntaxis, with sinistral and dextral shear motions along faults at the outer and inner flanks of the rotation terrane. The result suggests gravitational flow accomplished through rotation and translation of smaller subblocks in the upper crust. (d) Outside of the Tibetan plateau and Tien Shan, deformation field is block‐like. However, unnegligible internal deformation on the order of a couple of nanostrain/yr is found for all blocks. The North China block, under a unique tectonic loading environment, deforms and rotates at rates significantly higher than its northern and southern neighboring blocks, attesting its higher seismicity rate and earthquake hazard potential than its neighbors.

Development of an updated global land In situ‐based data set of temperature and precipitation extremes : HadEX3

Abstract: Robert Dunn was supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra (GA01101) and thanks Nick Rayner and Lizzie Good for helpful comments on the manuscript Lisa Alexander is supported by the Australian Research Council (ARC) Grants DP160103439 and CE170100023 Markus Donat acknowledges funding by the Spanish Ministry for the Economy, Industry and Competitiveness Ramon y Cajal 2017 Grant Reference RYC‐2017‐22964 Mohd Noor'Arifin Bin Hj Yussof and Muhammad Khairul Izzat Bin Ibrahim thank the Brunei Darussalam Meteorological Department (BDMD) Ying Sun was supported by China funding agencies 2018YFA0605604 and 2018YFC1507702 Fatemeh Rahimzadeh and Mahbobeh Khoshkam thank IR of Iranian Meteorological Organization (IRIMO) and the Atmospheric Science and Meteorological Organization Research Center (ASMERC) for Data and also sharing their experiences, especially Abbas Rangbar Jose Marengo was supported by the National Institute of Science and Technology for Climate Change Phase 2 under CNPq Grant 465501/2014‐1, FAPESP Grants 2014/50848‐9 and 2015/03804‐9, and the National Coordination for High Level Education and Training (CAPES) Grant 88887136402‐00INCT The team that worked on the data in West Africa received funding from the UK's National Environment Research Council (NERC)/Department for International Development DFID) Future Climate For Africa programme, under the AMMA‐2050 project (Grants NE/M020428/1 and NE/M019969/1) Data from Southeast Asia (excl Indonesia) was supported by work on using ClimPACT2 during the Second Workshop on ASEAN Regional Climate Data, Analysis and Projections (ARCDAP‐2), 25–29 March 2019, Singapore, jointly funded by Meteorological Service Singapore and WMO through the Canada‐Climate Risk and Early Warning Systems (CREWS) initiative This research was supported by Thai Meteorological Department (TMD) and Thailand Science Research and Innovation (TSRI) under Grant RDG6030003 Daily data for Mexico were provided by the Servicio Meteorologico Nacional (SMN) of Comision Nacional del Agua (CONAGUA) We acknowledge the data providers in the ECA&D project (https://wwwecadeu), the SACA&D project (https://saca-bmkgknminl), and the LACA&D project (https://ciifenknminl) We thank the three anonymous reviewers for their detailed comments which improved the manuscript

Martian Year 34 Column Dust Climatology from Mars Climate Sounder Observations: Reconstructed Maps and Model Simulations

Abstract: We have reconstructed longitude-latitude maps of column dust optical depth (CDOD) for Martian year (MY) 34 (May 5, 2017 --- March 23, 2019) using observations by the Mars Climate Sounder (MCS) aboard NASA's Mars Reconnaissance Orbiter spacecraft. Our methodology works by gridding standard and newly available estimates of CDOD from MCS limb observations, using the "iterative weighted binning" methodology. In this work, we reconstruct four gridded CDOD maps per sol, at different Mars Universal Times. Together with the seasonal and day-to-day variability, the use of several maps per sol allows to explore also the daily variability of CDOD in the MCS dataset, which is shown to be particularly strong during the MY 34 equinoctial Global Dust Event (GDE). Regular maps of CDOD are then produced by daily averaging and spatially interpolating the irregularly gridded maps using a standard "kriging" interpolator, and can be used as "dust scenario" for numerical model simulations. In order to understand whether the daily variability of CDOD has a physical explanation, we have carried out numerical simulations with the "Laboratoire de Meteorologie Dynamique" Mars Global Climate Model. Using a "free dust" run initiated at $L_s \sim 210^\circ$ with the corresponding kriged map, but subsequently free of further CDOD forcing, we show that the model is able to account for some of the observed daily variability in CDOD. The model serves also to confirm that the use of the MY 34 daily-averaged dust scenario in a GCM produces results consistent with those obtained for the MY 25 GDE.

The role of the stratosphere in subseasonal to seasonal prediction Part II: Predictability arising from stratosphere ‐ troposphere coupling

Abstract: The stratosphere can have a signi_cant impact on winter surface weather on subseasonal to seasonal (S2S) timescales. This study evaluates the ability of current operational S2S prediction systems to capture two important links between the stratosphere and tropo sphere: (1) changes in probabilistic prediction skill in the extratropical stratosphere by precursors in the tropics and the extratropical troposphere and (2) changes in surface predictability in the extratropics after stratospheric weak and strong vortex events. Prob abilistic skill exists for stratospheric events when including extratropical tropospheric precursors over the North Paci_c and Eurasia, though only a limited set of models captures the Eurasian precursors. Tropical teleconnections such as the Madden‐Julian Oscillation, the Quasi‐Biennial Oscillation, and El Nin~o Southern Oscillation increase the probabilistic skill of the polar vortex strength, though these are only captured by a limited set of models. At the surface, predictability is increased over the USA, Russia, and the Middle East for weak vortex events, but not for Europe, and the change in predictability is smaller for strong vortex events for all prediction systems. Prediction systems with poorly resolved stratospheric processes represent this skill to a lesser degree. Altogether, the analyses indicate that correctly simulating stratospheric variability and stratosphere‐troposphere dynamical coupling are critical elements for skillful S2S wintertime predictions.

Understanding Arctic Ocean Circulation: A Review of Ocean Dynamics in a Changing Climate

Abstract: 12 The Arctic Ocean is a focal point of climate change, with ocean warming, freshening, 13 sea-ice decline and circulation that link to the changing atmospheric and terrestrial en14 vironment. Major features of the Arctic and the interconnected nature of its windand 15 buoyancy-driven circulation are reviewed here by presenting a synthesis of observational 16 data interpreted from the perspective of geophysical fluid dynamics (GFD). The general 17 circulation is seen to be the superposition of Atlantic Water flowing into and around the 18 Arctic basin, and the two main wind-driven circulation features of the interior stratified 19 Arctic Ocean: the Transpolar Drift Stream and the Beaufort Gyre. The specific drivers 20 of these systems, including wind forcing, ice-ocean interactions and surface buoyancy fluxes, 21 and their associated GFD are explored. The essential understanding guides an assess22 ment of how Arctic Ocean structure and dynamics might fundamentally change as the 23 Arctic warms, sea-ice cover declines and the ice that remains becomes more mobile. 24

The Remarkably Strong Arctic Stratospheric Polar Vortex of Winter 2020: Links to Record-Breaking Arctic Oscillation and Ozone Loss

Abstract: The Northern Hemisphere (NH) polar winter stratosphere of 2019/2020 featured an exceptionally strong and cold stratospheric polar vortex. Wave activity from the troposphere during December–February was unusually low, which allowed the polar vortex to remain relatively undisturbed. Several transient wave pulses nonetheless served to help create a reflective configuration of the stratospheric circulation by disturbing the vortex in the upper stratosphere. Subsequently, multiple downward wave coupling events took place, which aided in dynamically cooling and strengthening the polar vortex. The persistent strength of the stratospheric polar vortex was accompanied by an unprecedentedly positive phase of the Arctic Oscillation in the troposphere during January–March, which was consistent with large portions of observed surface temperature and precipitation anomalies during the season. Similarly, conditions within the strong polar vortex were ripe for allowing substantial ozone loss: The undisturbed vortex was a strong transport barrier, and temperatures were low enough to form polar stratospheric clouds for over 4 months into late March. Total column ozone amounts in the NH polar cap decreased and were the lowest ever observed in the February–April period. The unique confluence of conditions and multiple broken records makes the 2019/2020 winter and early spring a particularly extreme example of two‐way coupling between the troposphere and stratosphere.

Fifty Years of Research on the Madden-Julian Oscillation: Recent Progress, Challenges, and Perspectives

Abstract: Since its discovery in the early 1970s, the crucial role of the Madden-Julian Oscillation (MJO) in the global hydrological cycle and its tremendous influence of high-impact climate and weather extremes have been well recognised. The MJO also serves as a primary source of predictability for global Earth system variability on subseasonal time scales. The MJO remains poorly represeted in our state-of-the-art climate and weather forecasting models, however. Moreover, despite the advances made in recent decades, theories for the MJO still disagree at a fundamental level. The problems of understanding and modeling the MJO have attracted significant interest from the research community. As part of the AGU's Centennial collection, this article provides a review of recent progress, particularly over the last decade, in observational, modeling and theoretical study of the MJO. A brief outlook for near-future MJO research directions is also provided.

Evidence for a Diagenetic Origin of Vera Rubin Ridge, Gale Crater, Mars: Summary and Synthesis of Curiosity's Exploration Campaign

Abstract: This paper provides an overview of the Curiosity rover's exploration at Vera Rubin ridge and summarizes the science results. Vera Rubin ridge (VRR) is a distinct geomorphic feature on lower Aeolis Mons (informally known as Mt. Sharp) that was identified in orbital data based on its distinct texture, topographic expression, and association with a hematite spectral signature. Curiosity conducted extensive remote sensing observations, acquired data on dozens of contact science targets, and drilled three outcrop samples from the ridge, as well as one outcrop sample immediately below the ridge. Our observations indicate that strata composing VRR were deposited in a predominantly lacustrine setting and are part of the Murray formation. The rocks within the ridge are chemically in family with underlying Murray formation strata. Red hematite is dispersed throughout much of the VRR bedrock, and this is the source of the orbital spectral detection. Gray hematite is also present in isolated, gray‐colored patches concentrated towards the upper elevations of VRR, and these gray patches also contain small, dark Fe‐rich nodules. We propose that VRR formed when diagenetic event(s) preferentially hardened rocks, which were subsequently eroded into a ridge by wind. Diagenesis also led to enhanced crystallization and/or cementation that deepened the ferric‐related spectral absorptions on the ridge, which helped make them readily distinguishable from orbit. Results add to existing evidence of protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars’ rock record.

Mineralogy of Vera Rubin Ridge From the Mars Science Laboratory CheMin Instrument

Abstract: Vera Rubin ridge (VRR) is an erosion‐resistant feature on the northwestern slope of Mount Sharp in Gale crater, Mars, and orbital visible/short‐wave infrared measurements indicate it contains red‐colored hematite The Mars Science Laboratory Curiosity rover performed an extensive campaign on VRR to study its mineralogy, geochemistry, and sedimentology to determine the depositional and diagenetic history of the ridge and constrain the processes by which the hematite could have formed X‐ray diffraction (XRD) data from the CheMin instrument of four samples drilled on and below VRR demonstrate differences in iron, phyllosilicate, and sulfate mineralogy and hematite grain size Hematite is common across the ridge, and its detection in a gray‐colored outcrop suggested localized regions with coarse‐grained hematite, which commonly forms from warm fluids Broad XRD peaks for hematite in one sample below VRR and the abundance of FeO_T in the amorphous component suggest the presence of nano‐crystalline hematite and amorphous Fe oxides/oxyhydroxides Well‐crystalline akaganeite and jarosite are present in two samples drilled from VRR, indicating at least limited alteration by acid‐saline fluids Collapsed nontronite is present below VRR, but samples from VRR contain phyllosilicate with d(001) = 96 A, possibly from ferripyrophyllite or an acid‐altered smectite The most likely cementing agents creating the ridge are hematite and opaline silica We hypothesize late diagenesis can explain much of the mineralogical variation on the ridge, where multiple fluid episodes with variable pH, salinity, and temperature altered the rocks, causing the precipitation and crystallization of phases that are not otherwise in equilibrium

The Role of the Stratosphere in Subseasonal to Seasonal Prediction: 1. Predictability of the Stratosphere

Abstract: The stratosphere has been identified as an important source of predictability for a range of processes on subseasonal to seasonal (S2S) timescales. Knowledge about S2S predictability within the stratosphere is however still limited. This study evaluates to what extent predictability in the extratropical stratosphere exists in hindcasts of operational prediction systems in the S2S database. The stratosphere is found to exhibit extended predictability as compared to the troposphere. Prediction systems with higher stratospheric skill tend to also exhibit higher skill in the troposphere. The analysis also includes an assessment of the predictability for stratospheric events, including early and mid‐winter sudden stratospheric warming (SSW) events, strong vortex events, and extreme heat flux events for the Northern Hemisphere, and final warming events for both hemispheres. Strong vortex events and final warming events exhibit higher levels of predictability as compared to SSW events. In general, skill is limited to the deterministic range of one to two weeks. High‐top prediction systems overall exhibit higher stratospheric prediction skill as compared to their low‐top counterparts, pointing to the important role of stratospheric representation in S2S prediction models.

The Atlantic Meridional Overturning Circulation in High-Resolution Models

Abstract: The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.

Nine Outstanding Questions of Solar Wind Physics.

Abstract: In situ measurements of the solar wind have been available for almost 60 years, and in that time plasma-physics simulation capabilities have commenced, and ground-based solar observations have expa...

What martian meteorites reveal about the interior and surface of Mars

Abstract: Martian meteorites are our only samples from Mars, thus far. Currently, there are a total of 252 individual samples originating from ≥11 ejection sites with crystallization ages varying from 4.5...

Future Evolution of Greenland's Marine-Terminating Outlet Glaciers

Abstract: Mass loss from the Greenland ice sheet (GrIS) has increased over the last two decades in response to changes in global climate, motivating the scientific community to question how the GrIS will contribute to sea-level rise on timescales that are relevant to coastal communities. Observations also indicate that the impact of a melting GrIS extends beyond sea-level rise, including changes to ocean properties and circulation, nutrient and sediment cycling, and ecosystem function. Unfortunately, despite the rapid growth of interest in GrIS mass loss and its impacts, we still lack the ability to confidently predict the rate of future mass loss and the full impacts of this mass loss on the globe. Uncertainty in GrIS mass loss projections in part stems from the nonlinear response of the ice sheet to climate forcing, with many processes at play that influence how mass is lost. This is particularly true for outlet glaciers in Greenland that terminate in the ocean because their flow is strongly controlled by multiple processes that alter their boundary conditions at the ice-atmosphere, ice-ocean, and ice-bed interfaces. Many of these processes change on a range of overlapping timescales and are challenging to observe, making them difficult to understand and thus missing in prognostic ice sheet/climate models. For example, recent (beginning in the late 1990s) mass loss via outlet glaciers has been attributed primarily to changing ice-ocean interactions, driven by both oceanic and atmospheric warming, but the exact mechanisms controlling the onset of glacier retreat and the processes that regulate the amount of retreat remain uncertain. Here we review the progress in understanding GrIS outlet glacier sensitivity to climate change, how mass loss has changed over time, and how our understanding has evolved as observational capacity expanded. Although many processes are far better understood than they were even a decade ago, fundamental gaps in our understanding of certain processes remain. These gaps impede our ability to understand past changes in dynamics and to make more accurate mass loss projections under future climate change. As such, there is a pressing need for (1) improved, long-term observations at the ice-ocean and ice-bed boundaries, (2) more observationally constrained numerical ice flow models that are coupled to atmosphere and ocean models, and (3) continued development of a collaborative and interdisciplinary scientific community. Plain Language Summary Increasing mass loss from the Greenland ice sheet (GrIS) in response to changes in global climate has motivated the scientific community to understand how much sea level rise will happen in the coming decades. Observations now indicate that the impact of a melting GrIS are more widespread than just sea-level rise and include changes to ocean properties and circulation, nutrient and sediment cycling, and ecosystem function. Major uncertainties still hamper accurate predictions of these impacts, particularly for outlet glaciers in Greenland that terminate in the ocean because their flow is strongly controlled by multiple processes that alter their boundary conditions at the ice-atmosphere, ice-ocean, and ice-bed interfaces. Many of these processes change on a range of overlapping timescales and are challenging to observe. Here we review the scientific progress in understanding how GrIS outlet glaciers respond to climate and how our understanding has changed over time as observations have increased. We conclude with recommendations for (1) improved, long-term observations at the ice-ocean and ice-bed boundaries, (2) more observationally-constrained ice flow models that are linked to atmosphere and ocean models, and (3) continued development of a collaborative and interdisciplinary scientific community. FEATURE ARTICLE 10.1029/2018JF004873 Key Points: • Outlet glacier changes are heterogeneous and result in large uncertainties in future sea-level rise contribution from Greenland • Uncertain understanding of outlet glacier changes is largely due to ice-ocean and ice sheet basal processes • Future research needs include expanded observations, improved modeling, and greater inclusion of new researchers Correspondence to: G. Catania, gcatania@ig.utexas.edu Citation: Catania G.A., L.A Stearns, T. Moon, E. Enderlin, & R.H. Jackson. (2020). Future evolution of greenland's marine-terminating outlet glaciers. Journal of Geophysical Research: Earth Surface, 125, e2018JF004873. https:// doi.org/10.1029/2018JF004873

A Lacustrine Paleoenvironment Recorded at Vera RubinRidge, Gale Crater: Overview of the Sedimentology and Stratigraphy Observed by the Mars ScienceLaboratory Curiosity Rover

Abstract: For ~500 Martian solar days (sols), the Mars Science Laboratory team explored Vera Rubin ridge (VRR), a topographic feature on the northwest slope of Aeolis Mons. Here we review the sedimentary facies and stratigraphy observed during sols 1,800–2,300, covering more than 100 m of stratigraphic thickness. Curiosity's traverse includes two transects across the ridge, which enables investigation of lateral variability over a distance of ~300 m. Three informally named stratigraphic members of the Murray formation are described: Blunts Point, Pettegrove Point, and Jura, with the latter two exposed on VRR. The Blunts Point member, exposed just below the ridge, is characterized by a recessive, fine‐grained facies that exhibits extensive planar lamination and is crosscut by abundant curvi‐planar veins. The Pettegrove Point member is more resistant, fine‐grained, thinly planar laminated, and contains a higher abundance of diagenetic concretions. Conformable above the Pettegrove Point member is the Jura member, which is also fine‐grained and parallel stratified, but is marked by a distinct step in topography, which coincides with localized meter‐scale inclined strata, a thinly and thickly laminated facies, and occasional crystal molds. All members record low‐energy lacustrine deposition, consistent with prior observations of the Murray formation. Uncommon outcrops of low‐angle stratification suggest possible subaqueous currents, and steeply inclined beds may be the result of slumping. Collectively, the rocks exposed at VRR provide additional evidence for a long‐lived lacustrine environment (in excess of 106 years via comparison to terrestrial records of sedimentation), which extends our understanding of the duration of habitable conditions in Gale crater.

Atmospheric Escape Processes and Planetary Atmospheric Evolution

Abstract: The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical, and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our solar system: It is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H‐dominated thermosphere for astrophysicists or the Sun‐like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so‐called Xenon paradox.

Magnetic Reconnection in the Space Sciences: Past, Present, and Future

Abstract: Magnetic reconnection converts, often explosively, storedmagnetic energy to particle energy in space and in the laboratory. Through processes operating on length scales that are tiny, it facilitates energy conversion over dimensions of, in some cases, hundreds of Earth radii. In addition, it is the mechanism behind large current disruptions in fusion machines, and it can explain eruptive behavior in astrophysics. We have known about the importance of magnetic reconnection for quite some time based on space observations. Theory and modeling employed magnetized fluids, a very simplistic description. While successful at modeling the large‐scale consequences of reconnection, it is ill suited to describe the engine itself. This is because, at its heart, magnetic reconnection in space is kinetic, that is, governed by the intricate interaction of charged particles with the electromagnetic fields they create. This complex interaction occurs in very localized regions and involves very short temporal variations. Researching reconnection requires the ability to measure these processes as well as to express them in models vastly more complex than fluid approaches. Until very recently, neither of these capabilities existed. With the advent of NASA's Magnetospheric Multiscale mission and modern modeling advances, this has now changed, and we have now determined its small‐scale structure in exquisite detail. In this paper, we review recent research results to predict what will be achieved in the future. We discuss how reconnection contributes to the evolution of larger‐scale systems, and its societal impacts in the context of threatening space hazards, customarily referred to as “space weather.” Plain Language Summary In space, huge amounts of energy are released explosively by a mysterious mechanism: magnetic reconnection. Reconnection can abruptly convert energy stored in magnetic fields to energy in charged particles, and power such diverse phenomena as solar and stellar flares, magnetic storms and aurorae in near‐Earth space, and major disruptions in magnetically confined fusion devices. It is behind many of the dangerous effects associated with space weather, including damage to satellites, endangering astronauts, and impacting the power grid and pipelines. Understanding reconnection enables us to quantitatively describe and predict these magnetic explosions. Therefore, magnetic reconnection has been at the forefront of scientific interest for many years, and will be for many more. Measuring reconnection is incredibly difficult. However, recently scientists have been able to peek into its machinery. Combining measurements from NASA's Magnetospheric Multiscale mission with supercomputer modeling, scientists have now been able to analyze the inner workings of this elusive mechanism. Even though open questions remain, this new understanding has broad implications. Here, we describe magnetic reconnection, where it plays a role, its impacts on society, and what we now know about it. We point to future research challenges, including implications and the utility of our recently developed knowledge. There is an imprecise—but useful—analogy to rubber bands that helps us picture magnetic reconnection. A loose rubber band cannot hold a pile of pencils in place, but a stretched rubber band can. This is because it takes energy to stretch the rubber band, and that energy can be thought of as stored in the rubber band. The energy in the stretched rubber band holds the pencils in place. The more you stretch a rubber band, themore energy it stores. Eventually, if you stretch a rubber band toomuch, it breaks, providing a painful lesson of how much energy it can hold! There are ways that magnetic fields are analogous to rubber bands. Magnetic fields are invisible and intangible but permeate all space. Earth itself acts like a large magnet, and its magnetic field is felt © 2019. The Authors. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. COMMENTARY 10.1029/2018JA025935 Key Points: • Magnetic reconnection is a key energy conversion and transport process in plasmas • There has been recent, considerable, research progress understanding how reconnection works • Many exciting research challenges await, while we can reap the benefits of our new understanding Correspondence to: M. Hesse, michael.hesse@uib.no Citation: Hesse, M., & Cassak, P. A. (2020). Magnetic reconnection in the space sciences: Past, present, and future. Journal of Geophysical Research: Space Physics, 125, e2018JA025935. https:// doi.org/10.1029/2018JA025935 Received 23 SEP 2019 Accepted 3 DEC 2019 Accepted article online 15 DEC 2019 HESSE AND CASSAK 1 of 24 hundreds of thousands of miles from Earth. The Sun also has a magnetic field, shaped vastly different than Earth's, which is felt 10 billion miles away! Like a rubber band, when magnetic fields get stretched, they store energy. It was historically thought that lines of magnetic field could not break, no matter how stretched they get. Unlike a rubber band, a single magnetic field line cannot break because magnetic fields cannot have free ends. However, if there is another stretched magnetic field line nearby pointing in the opposite direction, they can simultaneously break and cross connect so that there is never a free end. This is sketched in Figure 1. The light blue lines at the top and bottom are magnetic field lines pointing to the left and right, respectively, against a backdrop of the electric current. Magnetic field lines enter the shaded box in the center of Figure 1, called the diffusion region, where they effectively break and the broken ends from each immediately cross connect with each other. The field lines are said to have reconnected. The resulting two strongly bent magnetic field lines, threading the diffusion region, are stretched like rubber bands. They straighten out to the left and right and release their energy. The matter in space is typically a super‐heated gas called a plasma. Plasmas, the “fourth state of matter” joining solids, liquids, and gases, are so hot that some or all the atoms making them up cannot stay together—they break up into negatively charged electrons and positively charged ions that move fast enough to not recombine into neutral matter. To be a plasma, its temperature must be high enough and its density low enough. It is said that 99% of the knownmaterial in the universe is in the plasma state. The fact that the plasma is present is very important for magnetic reconnection. The magnetic field lines in Figure 1 thread the ambient plasma, shown as light blue circles. The charged particles move with the magnetic field as they move toward the diffusion region. As the bent reconnected magnetic field lines sling out like rubber bands, the plasma mostly moves with the magnetic field. This produces two jets of plasma moving away from the diffusion region, shown by the ovals to its left and right. As the magnetic field line slings out, the plasma residing at the boundaries between magnetic fields that have reconnected already and those that have not is also accelerated. Depending on where magnetic reconnection in space is happening, the jets can be faster than a million miles per hour! In addition to producing jets, magnetic reconnection heats the plasma, which is depicted by the shading of the plasma in the jets. The heating can happenwithin the diffusion region or at the boundaries between reconnected and unreconnected magnetic fields. This heating can be significant—depending on the setting, up to ~1/2 of the energy released by magnetic fields can go into heating the plasma. Figure 1. Simplified two‐dimensional schematic diagram of magnetic reconnection. Oppositely directed magnetic fields (light blue lines) and ambient plasma (light blue circles) move into the diffusion region (shaded box in the center), where magnetic reconnection occurs. The plasma is heated and accelerated into jets to the left and right (shaded blue ovals). 10.1029/2018JA025935 Journal of Geophysical Research: Space Physics HESSE AND CASSAK 2 of 24 The reason we care about magnetic reconnection is that it efficiently converts energy, both in the form of directed motion (the jets) and random motion (the increase in temperature). It is quite common for a magnetic field to change directions, so magnetic reconnection occurs in many settings. It is the mechanism behind solar flares, enormous bursts of light in the atmosphere of the Sun, which release up to 10,000,000,000,000,000,000,000,000 (10) joules of energy. For perspective, if we could harness all the energy released in a single large flare for human use, it would supply enough energy for the whole world for about 20,000 years! Magnetic reconnection also happens in the space surrounding Earth. Earth's magnetic field forms a bubble around Earth protecting it from the solar wind, a stream of plasma and magnetic fields emanating from the Sun. When the magnetic field in the solar wind points in the opposite direction as Earth's, magnetic reconnection at the edge of the bubble occurs. This sets the magnetic field and plasma inside Earth's magnetic bubble into motion, accumulating its magnetic field on the side of Earth away from the Sun. There, magnetic fields are again oppositely directed, and magnetic reconnection drives hot plasma toward Earth during what are called geomagnetic storms or substorms. Some of the ambient plasma penetrates all the way down to Earth's atmosphere, where it excites molecules in the atmosphere, and they give off light as they de‐excite. This is the origin of the auroral lights seen near Earth's poles. Consequently, researching magnetic reconnection is a major aspect of understanding the Sun and the Sun's impa

Predicting the Downward and Surface Influence of the February 2018 and January 2019 Sudden Stratospheric Warming Events in Subseasonal to Seasonal (S2S) Models.

Abstract: Using the real-time predictions from 11 models, this study analyzes the prediction of the downward propagation and surface impact of the 2018 and 2019 sudden stratospheric warmings (SSWs). These two SSWs differed both in their morphology types (2018: split; 2019: displacement followed by split) and magnitudes (the former being stronger). With a large sample size (>2,200) of multimodel ensemble forecasts, it is revealed that the strength of the SSW is more important than the vortex morphology in determining the magnitude of its downward impact, with strong SSWs more likely to propagate downward than weak SSWs. Therefore, based on the probabilistic forecasts, the observed strong SSW in February 2018 was more likely to have a downward and surface impact than the January 2019 SSW. The relationship between the 10-hPa dominant wave number and the 100-hPa polar cap height (or the Northern Annular Mode) is weak, implying that the dominant wave number might not be the primary factor determining the downward propagation of SSWs in the two analyzed cases. Hence, the high polar cap height (or negative Northern Annular Mode) response in the lower stratosphere and troposphere following the February 2018 SSW is mainly attributed to its strong intensity rather than the split morphology. Further, the 2-m temperature anomaly pattern following the January 2019 SSW is not forecasted due to its weak downward propagation, whereas the 2-m temperature in North Eurasia, Middle East, south China, and eastern United States could be forecasted for the downward propagating February 2018 SSW. However, regional rainfall anomalies are poorly forecasted (both in a deterministic and probabilistic sense) for both SSWs.

A Jovian Magnetodisc Model for the Juno Era.

Abstract: The Jovian magnetosphere assumes a disc-like geometrical configuration ("magnetodisc") owing to the persistent presence of a system of azimuthal currents circulating in a washer-shaped volume aligned with, or near, the magnetic equatorial plane. A Voyager era empirical model of the magnetodisc is fitted to vector magnetic field measurements obtained during the Juno spacecraft's first 24 orbits. The best fitting (within 30 Jovian radii) magnetodisc model is characterized by an inner and outer radius of 7.8 and 51.4 Jovian radii, a half-thickness of 3.6 Jovian radii, with a surface normal at 9.3° from the Jovigraphic pole and 204.2° System 3 west longitude. We supplement the magnetodisc model with a second current system, also confined to the magnetic equatorial plane, consisting of outward radial currents that presumably effect the transfer of angular momentum to outward flowing plasma. Allowing for variation of the magnetodisc's azimuthal and radial current systems from one 53-day orbit to the next, we develop an index of magnetospheric activity that may be useful in interpretation of variations in auroral observations.

The Transpolar Drift as a Source of Riverine and Shelf-Derived Trace Elements to the Central Arctic Ocean

Abstract: A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river-influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high-resolution pan-Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25�50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle-reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv (106 m3 s�1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.