• Issue

    Geophysical Research Letters: Volume 46, Issue 15

    8583-9299
    16 August 2019

Issue Information

Free Access

Issue Information

  • Pages: 8583-8587
  • First Published: 29 August 2019

Research Letters

Space Sciences

Free Access

Optical Signatures of the Outer Radiation Belt Boundary

  • Pages: 8588-8596
  • First Published: 15 July 2019
Key Points

  • Structured diffuse aurora marks the outer boundary of the outer radiation belt
  • Significant loss of 30- to 300-keV electrons precipitating from the radiation belts contributes up to 46% of the 4,278 Å emissions
  • The energetic electrons are nonadiabatically scattered from a thinning current sheet

Free Access

Dispersive Alfvén Wave Control of O+ Ion Outflow and Energy Densities in the Inner Magnetosphere

  • Pages: 8597-8606
  • First Published: 24 July 2019
Key Points

  • Dispersive Alfven wave energy density is enhanced during substorms and geomagnetic storms
  • Oxygen ion outflow energy and energy density are correlated with dispersive Alfven wave energy density
  • Observations support modeling that dispersive Alfven waves enhance oxygen ion energy density in the inner magnetosphere

Free Access

Lightning Contribution to Overall Whistler Mode Wave Intensities in the Plasmasphere

  • Pages: 8607-8616
  • First Published: 30 July 2019
Key Points

  • WWLLN data are used to classify spacecraft measurements according to concurrent lightning activity level
  • Lightning contributes substantially to the overall wave intensity, especially on the afternoon/evening/nightside
  • Frequencies of these waves start at 500 Hz with a bandwidth decreasing from 12 to 4 kHz for L between 1.5 and 5

Open Access

Variations in Solar Activity Across the Spörer Minimum Based on Radiocarbon in Danish Oak

  • Pages: 8617-8623
  • First Published: 22 July 2019
Key Points

  • This study presents the first European high-precision annual radiocarbon record across the Spörer Minimum
  • Band-pass filtering shows periods of enhanced and dampened variability of the Schwabe cycle
  • The new record confirms a flat, longer peak compared to the narrower and distinctly different peak associated with the Maunder Minimum

Free Access

Combined Effects of Equatorial Chorus Waves and High-Latitude Z-Mode Waves on Saturn's Radiation Belt Electrons

  • Pages: 8624-8632
  • First Published: 19 July 2019
Key Points

  • Chorus waves and Z-mode waves at different latitudes jointly control the fluxes of relativistic electrons in Saturn's radiation belt
  • Z-mode waves can efficiently inhibit the reversed energy spectrum of electrons driven by chorus waves
  • Chorus waves can further accelerate and scatter relativistic electrons accelerated by Z-mode waves toward larger and smaller pitch angles

Free Access

ULF Waves Modulating and Acting as Mass Spectrometer for Dayside Ionospheric Outflow Ions

  • Pages: 8633-8642
  • First Published: 01 August 2019
Key Points

  • Interaction between ULF waves and ionospheric outflow ions injected into the dayside magnetosphere was observed by MMS
  • ULF waves can serve as an auxiliary diagnostic for the detection of ionospheric outflow ions and inference of their mass species
  • Polarization drift plays a role in the modulation of oxygen ions

Free Access

Statistical Properties of Molecular Ions in the Ring Current Observed by the Arase (ERG) Satellite

  • Pages: 8643-8651
  • First Published: 22 July 2019
Key Points

  • Frequent TOF-mode ion composition measurements by Arase revealed statistical properties of molecular ions in the ring current
  • The molecular (O2+/NO+/N2+) ions are commonly observed during geomagnetically active periods even during small magnetic storms
  • The results suggest that the rapid ion outflow from the deep ionosphere occurs frequently during active periods

Free Access

The Effects of Solar Wind Dynamic Pressure on the Structure of the Topside Ionosphere of Mars

  • Pages: 8652-8662
  • First Published: 29 July 2019
Key Points

  • The topside ionosphere is globally depleted of plasma during times of high solar wind dynamic pressure
  • The topside ionosphere responds to changes in solar wind dynamic pressure in regions of both weak and strong crustal magnetic fields
  • Day-to-night plasma transport in the high-altitude nightside ionosphere is reduced during times of high solar wind dynamic pressure

Free Access

Jovian High-Latitude Ionospheric Ions: Juno In Situ Observations

  • Pages: 8663-8670
  • First Published: 01 August 2019
Key Points

  • The high-latitude ionosphere is observed between the magnetic latitudes bounded by the auroral oval and Io's magnetic flux shell
  • Two populations are observed at high latitudes: (1) magnetospheric ions consisting of H, S, and O ions and (2) cold ionospheric H+ ions
  • Observation of a loss cone suggests precipitating magnetospheric ions heat the upper ionosphere to heights ~0.5 RJ above the clouds

Planets

Free Access

Compositional Constraints on the North Polar Cap of Mars from Gravity and Topography

  • Pages: 8671-8679
  • First Published: 22 May 2019
Key Points

  • We constrain the density of the north polar ice cap of Mars using gravity and topography data
  • We estimate the mean density of the basal unit to be urn:x-wiley:00948276:media:grl59043:grl59043-math-0002 kg/m3, suggesting that it may contain more than 50% water ice
  • The basal unit may be the largest reservoir of water-ice on Mars after the two polar layered deposits

Open Access

The Young Age of the LAMP-observed Frost in Lunar Polar Cold Traps

  • Pages: 8680-8688
  • First Published: 01 July 2019
Key Points

  • The icy regolith top layer within shadowed lunar polar craters is eroded and lost in the presence of plasma sputtering and micrometeoric impacts
  • Recent impact rates suggest the top 500 nm of frost can only reside on the surface for <2,000 years
  • The finding suggests polar crater volatile surface and exosphere environments are dynamic

Free Access

Do Deltas Along the Crustal Dichotomy Boundary of Mars in the Gale Crater Region Record a Northern Ocean?

  • Pages: 8689-8699
  • First Published: 08 July 2019
Key Points

  • Twenty-two candidate deltas occur along the dichotomy boundary in the Gale crater region with delta front elevations spanning approximately 2,400 m
  • Front elevation variations do not correspond to modeled deformation from true polar wander or Tharsis emplacement
  • Based on modern topography, most deltas correspond to enclosed basins, which we infer to be paleo-lakes, not a northern ocean

Open Access

Ocean Dynamics of Outer Solar System Satellites

  • Pages: 8700-8710
  • First Published: 29 July 2019
Key Points

  • Ocean dynamics are important for the habitability of icy ocean worlds
  • Strong ocean currents likely exist in Enceladus, Titan, Europa, and Ganymede
  • Convective heat transfer in the ocean is predicted to vary with latitude, which would modify the thermophysical structure of the ice shell

Solid Earth

Free Access

Insights Into Intraplate Stresses and Geomorphology in the Southeastern United States

  • Pages: 8711-8720
  • First Published: 12 July 2019
Key Points

  • Stress indicators in the southeastern United States show substantial heterogeneity
  • Gravitational body forces from variable crustal thickness are important in explaining many features of observed stress patterns
  • Crustal weak zones beneath the Appalachian Mountains may help maintain a sharp passive margin escarpment (the Blue Ridge Escarpment)

Free Access

Source Model for the Tsunami Inside Palu Bay Following the 2018 Palu Earthquake, Indonesia

  • Pages: 8721-8730
  • First Published: 29 July 2019
Key Points

  • The series of tsunami waves that created the disaster in Palu was caused by a combination of seafloor uplift and multiple landslides
  • The seafloor vertical displacement was estimated using tsunami waveform and SAR data and is evaluated to be the source the largest tsunami
  • Ground subsidence of up to 1 m and a tide level of 1 m during the event enhanced the tsunami impact in Palu city

Open Access

Oxygen Vacancy Ordering in Aluminous Bridgmanite in the Earth's Lower Mantle

  • Pages: 8731-8740
  • First Published: 19 July 2019
Key Points

  • Al substitution in bridgmanite is dominated by the oxygen vacancy mechanism for peridotite mantle compositions
  • Short-range ordering of oxygen vacancies takes place forming oxygen vacancy clusters possibly due to migration into twin domain walls
  • Oxygen vacancy clusters are expected to have a major influence on transport properties of the lower mantle, such as the electrical conductivity

Free Access

Internal Drainage Has Sustained Low-Relief Tibetan Landscapes Since the Early Miocene

  • Pages: 8741-8752
  • First Published: 15 July 2019
Key Points

  • Robust age constraints of the youngest continental stratigraphic unit from the Lunpola Basin in central Tibet are reported
  • Aggradation and erosion of internal drainage dominated the formation of low-relief topography in central Tibet by the early Miocene times
  • Late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere

Free Access

Frictional Stabilities on Induced Earthquake Fault Planes at Fox Creek, Alberta: A Pore Fluid Pressure Dilemma

  • Pages: 8753-8762
  • First Published: 19 July 2019
Key Points

  • The slip tendency along the fault planes is determined using a quantitative model for the stress tensors at the epicentral locations
  • The faults cannot be stable at the ambient high pore fluid pressures within the Duvernay Formation
  • Triggered planes of weakness need not be perfectly optimally oriented to the stress field

Free Access

Migration of Very Long Period Seismicity at Aso Volcano, Japan, Associated With the 2016 Kumamoto Earthquake

  • Pages: 8763-8771
  • First Published: 29 July 2019
Key Points

  • Backprojection technique based on differential-time and phase weighted stacking is used to map clusters of very long period seismicity
  • Clusters of very long period seismicity indicate clear relationship between distant earthquake and volcanic activities
  • The 2016 Kumamoto earthquake suddenly induced very long period seismicity at west of Aso Caldera lasting for five months

Free Access

Pervasive Foreshock Activity Across Southern California

  • Pages: 8772-8781
  • First Published: 30 July 2019
Key Points

  • We analyze foreshock activity in a catalog of more than 1.8 million earthquakes in southern California
  • Foreshock occurrence significantly exceeds the background seismicity rate for 72% of candidate mainshocks
  • Durations of elevated foreshock activity range from days to weeks for these sequences

Free Access

Dynamic Wedge Failure and Along-Arc Variations of Tsunamigenesis in the Japan Trench Margin

  • Pages: 8782-8790
  • First Published: 30 July 2019
Key Points

  • Inelastic wedge deformation can induce large seafloor uplift efficiently landward from trench and diminish near-trench slip on the fault
  • Along-arc variation of sediment thickness in the Japan Trench margin can lead to important along-arc variations of tsunamigenesis
  • The large tsunami heights along the Sanriku coast in the 2011 Tohoku earthquake can be due to inelastic wedge deformation

Free Access

Volcanoes Erupt Stressed Quartz Crystals

  • Pages: 8791-8800
  • First Published: 22 July 2019
Key Points

  • Explosive and effusive eruptions from volcanoes contain deformed quartz crystals with the same residual stresses
  • Residual stresses are proposed to originate from the stresses required to fragment magma
  • Residual stress may also originate from crystal-crystal contacts in crystal-rich magmatic mushes

Free Access

Hindcasting Magma Reservoir Stability Preceding the 2008 Eruption of Okmok, Alaska

  • Pages: 8801-8808
  • First Published: 25 July 2019
Key Points

  • Statistical data assimilation is used to retroactively forecast the 2008 eruption of Okmok volcano, Alaska, which lacked clear precursors
  • Numerical models track the evolving stress state of the Okmok magma system and forecast eruption likelihood at each time step
  • Model forecasts indicate that the system was trending toward tensile failure in the weeks leading up to the 2008 eruption

Hydrology and Land Surface Studies

Open Access

Climate Variability Alters Flood Timing Across Africa

  • Pages: 8809-8819
  • First Published: 01 July 2019
Key Points

  • Modes of climate variability in the Pacific and Indian Oceans drive significant changes in flood timing across sub-Saharan Africa
  • The hydrological system enhances and alters climate variability signals seen in rainy season onset, duration, and total rainfall
  • Understanding of how climate variability changes the timing of floods can support flood-based farming systems

Open Access

Quality Control for Crowdsourced Personal Weather Stations to Enable Operational Rainfall Monitoring

  • Pages: 8820-8829
  • First Published: 22 July 2019
Key Points

  • A real-time applicable quality control methodology for crowdsourced personal weather stations is suggested
  • The quality control successfully identifies typical errors for this data source without requiring auxiliary data
  • High-resolution nationwide rainfall maps can be produced from the quality-controlled crowdsourced personal weather stations

Open Access

Developing and Testing Physically Based Triggering Thresholds for Runoff-Generated Debris Flows

  • Pages: 8830-8839
  • First Published: 17 July 2019
Key Points

  • Slope-dependent thresholds of dimensionless discharge and Shields stress can predict postfire debris flow initiation
  • We propose a process-based method for calculating rainfall intensity-duration thresholds
  • Rainfall intensity-duration thresholds derived from process-based hydrologic modeling are comparable to empirical thresholds

Open Access

Cross-Scale Interactions Dictate Regional Lake Carbon Flux and Productivity Response to Future Climate

  • Pages: 8840-8851
  • First Published: 01 August 2019
Key Points

  • Future regional water balance dictated variation in regional lake CO2 emissions, carbon burial, and primary productivity
  • Increased air temperature reduced regional lake productivity due to reduced lake volume but had no effect on CO2 emissions or carbon burial
  • Cross-scale interactions dictated the magnitude and direction of lake-specific responses to future climate projections

Free Access

Ecohydrology Controls the Geomorphic Response to Climate Change

  • Pages: 8852-8861
  • First Published: 30 July 2019
Key Points

  • Vegetation exerts a major control on equilibrium slopes, erosion rates, and response times to climate shifts
  • Vegetated and unvegetated landscapes could show contrasting response patterns to climate change
  • The interpretations of erosional records of past climates need to account for the effect of coevolving vegetation

Free Access

Disentangling the Simultaneous Effects of Inertial Losses and Fracture Dilation on Permeability of Pressurized Fractured Rocks

  • Pages: 8862-8871
  • First Published: 29 July 2019
Key Points

  • The competition between inertial losses and fracture dilation in controlling fractured rock permeability was analyzed
  • An analytical expression for pressure-dependent permeability considering inertial effect and dilation was derived and tested with experiment
  • Transition from inertial effect-dominated to fracture dilation-dominated permeability evolution phases happens at Forchheimer number = 1/3

Open Access

Alpine Glacier Shrinkage Drives Shift in Dissolved Organic Carbon Export From Quasi-Chemostasis to Transport Limitation

  • Pages: 8872-8881
  • First Published: 01 August 2019
Key Point

  • Drivers of DOC yield shift from chemostasis to transport limitation as mountain glaciers shrink

The Cryosphere

Open Access

Projected Changes in Interannual Variability of Peak Snowpack Amount and Timing in the Western United States

  • Pages: 8882-8892
  • First Published: 08 August 2019
Key Points

  • Interannual variability of peak snowpack amount in the western United States will decrease in regions transitioning from snow to rain
  • Peak snowpack timing will become more interannually variable and occur across a broader range of months
  • Consecutive snow droughts will become much more common across mountains of the western United States

Free Access

Ross Ice Shelf Icequakes Associated With Ocean Gravity Wave Activity

  • Pages: 8893-8902
  • First Published: 01 August 2019
Key Points

  • Rift and icefront icequakes were identified
  • Icefront icequakes are most frequent during the austral summer when swell forcing is greatest in the absence of sea ice
  • An empirical ocean-to-ice shelf displacement transfer function is obtained

Free Access

Seasonal Variations in Iceberg Freshwater Flux in Sermilik Fjord, Southeast Greenland From Sentinel-2 Imagery

  • Pages: 8903-8912
  • First Published: 01 August 2019
Key Points

  • Freshwater fluxes from iceberg melt in Sermilik Fjord have a seasonal signal, peaking across August and September in 2017 and 2018
  • Fluxes decrease with distance down-fjord from Helheim Glacier, with ~8691% of iceberg volume lost before reaching the fjord mouth
  • We present a simple and effective tool for monitoring iceberg freshwater fluxes across a range of Greenlandic fjords

Oceans

Free Access

Energetics of Radiatively Heated Ice-Covered Lakes

  • Pages: 8913-8925
  • First Published: 22 July 2019
Key Points

  • The mechanical energy budget for shallow, ice-covered lakes energized by penetrative solar radiation is derived
  • Energy pathways and water-ice heat transfer are sensitive to dynamics of available potential energy
  • Efficiencies and energy transfer rates can be estimated using under-ice turbulence measurements

Free Access

Heavy Noble Gas Isotopes as New Constraints on the Ventilation of the Deep Ocean

  • Pages: 8926-8932
  • First Published: 01 August 2019
Key Points

  • Ar, Kr, and Xe isotope ratios were measured in the deep North Pacific to investigate mechanisms of solubility disequilibrium
  • Seawater solubility fractionation factors of heavy noble gas isotope ratios were determined, indicating a weak dependence on salinity
  • Observed isotopic disequilibria point to incomplete cooling-driven diffusive gas uptake prior to deep-water formation

Free Access

Diagnosing Ocean-Wave-Turbulence Interactions From Space

  • Pages: 8933-8942
  • First Published: 30 July 2019
Key Points

  • We exploit spectral characteristics of sea surface height (SSH) to partition ocean motions into balanced motions and internal gravity waves
  • We use a simple shallow-water model to diagnose internal gravity wave motions from SSH
  • We test a dynamical framework to recover the interactions between internal gravity waves and balanced motions from SSH

Free Access

The Distinction Between the Gulf Stream and Its North Wall

  • Pages: 8943-8951
  • First Published: 01 August 2019
Key Points

  • The Gulf Stream and its North Wall are close to each other west of 71°W but depart sharply in their properties farther downstream
  • Differences in the behaviors of the Gulf Stream and the North Wall are related to mesoscale eddies between them
  • Recognition of distinct properties of Gulf Stream and the North Wall is necessary for understanding their dynamics and impact on climate

Free Access

Extreme Conditions in the Bering Sea (2017–2018): Record-Breaking Low Sea-Ice Extent

  • Pages: 8952-8959
  • First Published: 14 August 2019
Key Points

  • Wind patterns have changed in the Bering Sea, with a marked increase in warm southerly winds since 2016
  • These changes in winds have resulted in decrease in sea ice, with the lowest extent ever observed in the Bering Sea occurring in 2017/2018
  • The low ice extent restructured the horizontal temperature patterns, resulting in an almost nonexistent cold pool in August 2018

Open Access

An Index to Better Estimate Tropical Cyclone Intensity Change in the Western North Pacific

  • Pages: 8960-8968
  • First Published: 16 July 2019
Key Points

  • The depth-averaged temperature more realistically characterizes ocean response than pre-TC sea surface temperature
  • The parameterization of the air-sea exchange process is important in computing the thermodynamic energy budget for TCs
  • A revised predictor including two key factors shows significant improvement in the TC intensity change prediction

Climate

Free Access

Fast-Forward to Perturbed Equilibrium Climate

  • Pages: 8969-8975
  • First Published: 21 May 2019
Key Points

  • A simple method for estimating the equilibrium climate sensitivity is proposed
  • The method allows to simulate the stationary climate corresponding to any given radiative perturbation with a limited computational cost
  • The method can be applied to any atmosphere-ocean coupled climate model

Free Access

Climate Response to Pulse Versus Sustained Stratospheric Aerosol Forcing

  • Pages: 8976-8984
  • First Published: 05 August 2019
Key Points

  • We simulate the climate impact of a volcanic-like pulse and a geoengineering-like sustained stratospheric sulfate aerosol forcing
  • For the same global mean cooling, decrease in land temperature, precipitation, and runoff is much larger under the pulse forcing
  • Different timescales of aerosol forcing related to volcanic eruptions and geoengineering would produce quite different climate outcomes

Free Access

Allometric Models to Estimate Leaf Area for Tropical African Broadleaved Forests

  • Pages: 8985-8994
  • First Published: 11 July 2019
Key Points

  • A destructive data set of leaf area was collected for 13 tree species representative of African semideciduous forests
  • Operational allometric models are provided to estimate leaf area and leaf area index, which could apply to forests in this ecosystem
  • Model exponents agree with theoretical expectations, and leaf area index in dense tropical forests seems higher than previously assessed

Free Access

Land Cover Regulates the Spatial Variability of Temperature Response to the Direct Radiative Effect of Aerosols

  • Pages: 8995-9003
  • First Published: 18 July 2019
Key Points

  • The local climate response to aerosols is strongly modulated by the biophysical attributes of the underlying surface
  • Asymmetries between aerosol shortwave and longwave radiative effect reduce the diurnal temperature range, particularly in arid regions
  • Long-term enhancement in local climate sensitivity to aerosols is seen in equatorial climate, possibly driven by deforestation

Free Access

Wintertime Modulation of the Local Cloud and Diagnostic Fields by the Hadley Cell Subsiding Boundary Over the Western North Pacific

  • Pages: 9004-9012
  • First Published: 18 July 2019
Key Points

  • The poleward shift of the local Hadley cell subsiding edge (LHCSE) is marked by increases of the midlevel cloud fraction over the climatological sinking zone
  • LHCSE-correlated radiation anomalies are mainly contributed by the response of cumulus congestus
  • Modulated cumulus congestus and moist instability lead to increased precipitation over the northeast Asian coastal area

Free Access

Spatiotemporal Variations of Riverine Discharge Within the Amazon Basin During the Late Holocene Coincide With Extratropical Temperature Anomalies

  • Pages: 9013-9022
  • First Published: 18 July 2019
Key Points

  • Sediments in fluvial rias reveal hydrological changes in Amazonia during the last millennia
  • Compound-specific plant wax isotope records suggest a decrease in rainfall in Amazon lowlands over the last 4,000 yr
  • A shift in the extratropical temperature gradient from 2,600 to 1,400 cal yr BP potentially led to lower Amazon River water discharge

Free Access

Explaining Differences Between Recent Model and Satellite Tropospheric Warming Rates With Tropical SSTs

  • Pages: 9023-9030
  • First Published: 01 August 2019
Key Points

  • CMIP5 models simulate more tropical tropospheric warming than observations
  • Observations and models agree on the interannual relationship between tropical SST and tropospheric temperatures
  • Recent SST trends in regions of deep convection explain divergence in tropospheric trends

Free Access

The Signal-to-Noise Paradox for Interannual Surface Atmospheric Temperature Predictions

  • Pages: 9031-9041
  • First Published: 22 July 2019
Key Points

  • Signal-to-noise paradox is a signature of an underconfident, unreliable prediction system
  • CMIP5 models are unable to reproduce observed persistence of surface atmospheric temperature
  • In CMIP5 models 70% of the globe exhibits the signal-to-noise paradox in SAT

Open Access

Western Mediterranean Climate Response to Dansgaard/Oeschger Events: New Insights From Speleothem Records

  • Pages: 9042-9053
  • First Published: 29 July 2019
Key Points

  • Western Mediterranean precipitation responded to Greenland interstadials and stadials in a coherent manner
  • Dansgaard/Oeschger interstadials were accompanied by humid climate conditions over the western Mediterranean
  • Atmospheric circulation patterns did not induce spatial differences in precipitation during the last glacial as they did during the Holocene

Open Access

The Decadal Variability of the Global Monsoon Links to the North Atlantic Climate Since 1851

  • Pages: 9054-9063
  • First Published: 29 July 2019
Key Points

  • The coral δ18O record from the northern South China Sea captures the migration of the regional ITCZ on decadal timescales
  • The Global Monsoon varied in an anti-phase inter-hemispheric mode on decadal timescales associated with the ITCZ migration since 1851 A.D
  • The decadal variability of the Global Monsoon was found to be linked to the Atlantic Multidecadal Oscillation

Free Access

Contrasting Influence of Gobi and Taklimakan Deserts on the Dust Aerosols in Western North America

  • Pages: 9064-9071
  • First Published: 01 August 2019
Key Points

  • There is a distinct relationship of dust aerosol in Gobi and Taklimakan deserts with the dust aerosols in western North America
  • The difference of terrain and background wind account for the different remote influences of dust aerosol in Gobi and Taklimakan deserts
  • The influences of dust aerosol in Gobi desert on the western North America experienced an obvious regime shift around 1999

Free Access

Variability of Acoustically Evidenced Methane Bubble Emissions Offshore Western Svalbard

  • Pages: 9072-9081
  • First Published: 22 July 2019
Key Points

  • Hydroacoustically derived methane flow rates from three areas offshore Svalbard range from 725 to 1,125 t CH4/year
  • Seasonal migration of the landward limit of the methane hydrate stability zone influences locations of bubble seepage
  • Alternating CH4-bubble seepage between two areas suggests geological interconnectivity

Free Access

The 2005 Amazon Drought Legacy Effect Delayed the 2006 Wet Season Onset

  • Pages: 9082-9090
  • First Published: 15 July 2019
Key Points

  • Precipitation observations from TRMM show a postponed wet season onset in southern Amazonia in 2006
  • Both deuterium retrievals from TES and a new evapotranspiration (ET) product suggest an ET reduction during the late transition of 2006
  • ET reduction due to 2005 Amazonian drought legacy effect on forest delays the wet season onset in 2006

Open Access

Initial 40Ar-39Ar Ages of the Paleocene-Eocene Boundary Impact Spherules

  • Pages: 9091-9102
  • First Published: 23 July 2019
Key Points

  • Radiometric age of P-E boundary impact spherules is 54.2 ± 2.5 Ma, consistent with their depositional age
  • Their origin as reworked K-Pg ejecta is not supported by these data

Free Access

Varying Sensitivity of East Asia Summer Monsoon Circulation to Temperature Change Since Last Glacial Maximum

  • Pages: 9103-9109
  • First Published: 29 July 2019
Key Points

  • Sensitivity of East Asia summer monsoon circulation to temperature was larger over Holocene than that over the last deglaciation
  • Diverse sensitivity under orbital and CO2 forcing and their varying confounding effect determined this varying sensitivity
  • Seasonal synchrony of temperature variation over ocean is a key to determine diverse sensitivity under each forcing

Free Access

Ongoing Poleward Migration of Tropical Cyclone Occurrence Over the Western North Pacific Ocean

  • Pages: 9110-9117
  • First Published: 29 July 2019
Key Points

  • Tropical cyclone occurrence has been shifting poleward to the coast of East Asia from areas south of 20°N from 1982 to 2018
  • The preferential tropical cyclone passage has switched from westward moving to northward recurving since 1998
  • The poleward migration may be primarily attributed to the cyclonic anomaly of the steering flow over East Asia

Free Access

Different Sources of 10- to 30-day Intraseasonal Variations of Autumn Snow over Western and Eastern Tibetan Plateau

  • Pages: 9118-9125
  • First Published: 05 August 2019
Key Points

  • The dominant intraseasonal snow cover variation over the Tibetan Plateau (TP) is 10-30 days during autumn and early winter
  • The intraseasonal snow cover variations over the western and eastern TP have different moisture sources
  • The intraseasonal snow cover variations over the western and eastern TP are related to the AO and NAO, respectively

Free Access

Choice of Irrigation Water Management Practice Affects Indian Summer Monsoon Rainfall and Its Extremes

  • Pages: 9126-9135
  • First Published: 05 August 2019
Key Points

  • Representation of irrigation and paddy fields is added to a regional climate model to study that impacts on Indian Summer Monsoon
  • Excess irrigation over northern India causes northwestward shift in September month monsoon rainfall over the land mass
  • Irrigation intensifies widespread extreme events over Central India

Open Access

Pliocene Warmth Consistent With Greenhouse Gas Forcing

  • Pages: 9136-9144
  • First Published: 01 August 2019
Key Points

  • Pliocene SSTs calculated from the alkenone proxy do not support a “permanent El Niño”
  • Pliocene model simulations can reproduce proxy-inferred SST patterns and gradients
  • The pattern of Pliocene warmth supports a weakening of Walker circulation under higher CO2

Free Access

Enhanced Tropical Cyclone Intensity in the Western North Pacific During Warm Periods Over the Last Two Millennia

  • Pages: 9145-9153
  • First Published: 02 August 2019
Key Points

  • The coarse fraction of Chinese coastal sediments is used as a proxy for TC intensity
  • A 2,000-year record of TC intensity in the western North Pacific is reconstructed
  • TC intensity increased during relatively warm periods on a multicentennial scale

Atmospheric Science

Free Access

Aerosol-Mediated Glaciation of Mixed-Phase Clouds: Steady-State Laboratory Measurements

  • Pages: 9154-9162
  • First Published: 18 July 2019
Key Points

  • Holographic measurements of steady-state mixed-phase clouds are made in a laboratory cloud-convection chamber
  • The ice fraction in the cloud is determined by varying the ice-nucleating particle concentration while holding CCN concentration constant
  • Measurements are in agreement with an extension of Korolev-Mazin theory for critical forcing needed for mixed-phase conditions

Open Access

Seasonal Prediction Potential for Springtime Dustiness in the United States

  • Pages: 9163-9173
  • First Published: 19 July 2019
Key Points

  • A regression model and ensembles from a seasonal prediction model initialized on 1 December are used to predict springtime dustiness
  • About 71% of the variances of dustiness over the Great Plains and 63% over the southwestern United States from 2004 to 2016 are captured
  • Springtime climatic factors play a more important role in variations in spring dustiness than wintertime factors

Open Access

Diurnal Surface Flux Variability Over Western Boundary Currents

  • Pages: 9174-9182
  • First Published: 17 July 2019
Key Points

  • Diurnal turbulent flux variability in wintertime is evident over both the Kuroshio Extension and the Gulf Stream
  • The diurnal flux is tightly tied to the location of the maximum surface fluxes along the western boundary currents
  • Higher winds are associated with moister surface layer air, indicating surface fluxes, not entrainment mixing, are key to this process

Open Access

Investigating the Fast Response of Precipitation Intensity and Boundary Layer Temperature to Atmospheric Heating Using a Cloud-Resolving Model

  • Pages: 9183-9192
  • First Published: 22 July 2019
Key Points

  • Atmospheric heating preferentially reduces weak precipitation
  • A cancellation between thermodynamic and dynamic effects maintains the intensity of strong precipitation
  • Free-tropospheric heating warms the boundary layer mainly through reducing rain reevaporation

Free Access

The Seasonality and Regionality of MJO Impacts on North American Temperature

  • Pages: 9193-9202
  • First Published: 29 July 2019
Key Points

  • The MJO is linked to significant temperature variability over North America outside of winter
  • Peaks in MJO covariability with temperature occur outside of winter for much of North America
  • The MJO contributes to predictive skill of North American temperatures in spring, summer, and fall

Free Access

The Relationship of Cloud Number and Size With Their Large-Scale Environment in Deep Tropical Convection

  • Pages: 9203-9212
  • First Published: 01 August 2019
Key Points

  • The relationship between a convective cloud ensemble and its environment is studied to improve the representation of tropical convection in models
  • The strongest rainfall intensities are found with few large cells those exist in a dry and subsiding environment with both high CAPE and CIN
  • The convective cell number and size are quantified and their relationship to atmospheric stability, vertical motion, and humidity is studied

Free Access

Influence of the QBO on the MJO During Coupled Model Multiweek Forecasts

  • Pages: 9213-9221
  • First Published: 18 June 2019
Key Points

  • MJO amplitude is strengthened during coupled model forecasts that are initialized during austral summer when QBO is in its easterly phase
  • MJO is promoted by enhanced tropopause destabilization during QBO easterly phase, and this effect is captured in the initialized forecasts
  • Reduction of tropopause static stability is more in-phase with MJO convection during QBO easterly phase, promoting stronger MJO events

Free Access

Skillful Prediction of Monthly Major Hurricane Activity in the North Atlantic with Two-way Nesting

  • Pages: 9222-9230
  • First Published: 30 July 2019
Key Points

  • For the first time prediction skill on anomalous monthly hurricane and major hurricane activity is demonstrated in a dynamical model
  • The 8-km nest better resolves intense hurricanes and hence better predicts monthly major hurricane occurrence and accumulated cyclone energy

Free Access

Hemispheric Asymmetry of Tropical Expansion Under CO2 Forcing

  • Pages: 9231-9240
  • First Published: 25 July 2019
Key Points

  • There is twice as much Hadley cell expansion in Southern Hemisphere relative to Northern Hemisphere in models under CO2 forcing
  • Degree of asymmetry depends on season but is highly robust across models and Hadley cell definitions
  • Due to smaller Northern Hemisphere sensitivity to static stability increase, pattern of sea surface warming and direct CO2 radiative effects

Open Access

A Thermodynamic Pathway Leading to Rapid Intensification of Tropical Cyclones in Shear

  • Pages: 9241-9251
  • First Published: 30 July 2019
Key Points

  • The nearly saturated lower-middle troposphere in the inner core region precedes the tropical cyclone (TC) rapid intensification in shear
  • The moistening of the inner core region is achieved by a competition between surface heat fluxes, radiation, and ventilation effects
  • Vortex alignment benefits the moistening of the TC inner core by reducing ventilation effect

Free Access

On the Role of Reduced Air Density Along the Lightning Leader Path to Ground in Increasing X-Ray Production Relative to Normal Atmospheric Conditions

  • Pages: 9252-9260
  • First Published: 05 August 2019
Key Points

  • Lightning leaders in previously created but decayed (cooled down) channels can produce more detectable X-rays than the first leader
  • The friction curve for air at 3000 K allows all electrons to run away in fields exceeding 3 to 4 MV/m versus 30 to 40 MV/m in cold air
  • X-ray emission was observed at the time of collision of opposite-polarity streamers but not at the time of stepping

Free Access

The Polar Stratosphere as an Arbiter of the Projected Tropical Versus Polar Tug of War

  • Pages: 9261-9270
  • First Published: 01 August 2019
Key Points

  • The late 21st century temperature anomalies in the upper-level tropics and the Arctic are imposed in a high-top atmospheric model using nudging
  • In winter, warmer tropics induce a weakening of the polar vortex that reinforces the response to Arctic amplification (no tug of war)
  • The polar stratosphere has the potential to shape the tug of war over the North Atlantic in future climate scenarios

Free Access

In Search of Winter Blocking in the Western North Pacific Ocean

  • Pages: 9271-9280
  • First Published: 07 June 2019
Key Points

  • The western North Pacific blocking events identified by the anomaly type are mostly originated and moved from the eastern North Pacific
  • In situ development of the western North Pacific blocking is reasonably captured well by the reversal type
  • The two types of detection methods capture unique blocking events that have very different regional surface impacts

Open Access

Southern Hemisphere Atmospheric Blocking in CMIP5 and Future Changes in the Australia-New Zealand Sector

  • Pages: 9281-9290
  • First Published: 05 August 2019
Key Points

  • We provide a comprehensive survey of Southern Hemisphere blocking using a large number of coupled models
  • Future projections generally suggest an overall decrease in Southern Hemisphere blocking frequency
  • There is strong model agreement on blocking frequency decreases to the south of Australia during austral winter

Open Access

Interactions Between Nonlinear Internal Ocean Waves and the Atmosphere

  • Pages: 9291-9299
  • First Published: 25 July 2019
Key Points

  • The first quantitative analysis detailing interactions between a nonlinear internal ocean wave packet and the atmosphere is presented
  • The mean wind velocity and Reynolds stress components responded directly to internal wave-induced roughness variability
  • Evidence for responses in the humidity variance and gradient were found, but temperature was not directly impacted by the internal waves