Showing papers in "Geophysical Research Letters in 2000"

Evolution of the Sun's Spectral Irradiance Since the Maunder Minimum

Abstract: Because of the dependence of the Sun's irradi- ance on solar activity, reductions from contemporary levels are expected during the seventeenth century Maunder Min- imum. New reconstructions of spectral irradiance are de- veloped since 1600 with absolute scales traceable to space- based observations. The long-term variations track the en- velope of group sunspot numbers and have amplitudes con- sistent with the range of Ca II brightness in Sun-like stars. Estimated increases since 1675 are 0.7%, 0.2% and0.07% in broadultraviolet, visible/nearinfraredandinfraredspectral bands, with a total irradiance increase of 0.2%.

Global patterns of ENSO‐induced precipitation

Abstract: Although there have been many analyses of El Nino/Southern Oscillation (ENSO) induced precipitation anomalies, global patterns from these analyses remain incomplete. Here we combine recent satellite estimates of oceanic precipitation and historical rain-gauge records to derive a global climatology of ENSO-induced precipitation anomalies using empirical orthogonal function (EOF) analyses. The patterns suggest that the re-arrangement of convection centers of the Walker circulation during ENSO events induces large precipitation anomalies in the tropics, while associated changes in the monsoon systems (through the Hadley cell) over the Pacific, Indian and Atlantic Oceans, and their interactions with midlatitude westerlies generate coherent anomaly patterns over the extratropics. Our results can be used to evaluate climate models and forecast ENSO-induced precipitation anomalies.

Interplanetary acceleration of coronal mass ejections

Abstract: Using an observed relation between speeds of CMEs near the Sun and in the solar wind, we determine an “effective” acceleration acting on the CMEs. We found a linear relation between this effective acceleration and the initial speed of the CMEs. The acceleration is similar to that of the slow solar wind in magnitude. The average solar wind speed naturally divides CMEs into fast and slow ones. Based on the relation between the acceleration and initial speed, we derive an empirical model to predict the arrival of CMEs at 1 AU.

A physically‐based treatment of elemental carbon optics: Implications for global direct forcing of aerosols

Abstract: To date, global models of direct radiative forcing have treated elemental carbon (EC) as completely externally mixed or well-mixed internally No global study has treated EC as a core in an internal mixture It is hypothesized that the well-mixed treatment is unphysical and reality lies between the externally-mixed and core treatments It is also suggested, but not proven, that most EC particles are coated to some degree; hence, the core treatment may be more representative than the external-mixture treatment Global simulations with the core treatment resulted in EC forcing 50% higher and 40% lower than forcings obtained with the externally-mixed and well-internally-mixed treatments, respectively In the core case, EC's positive forcing more than offset negative forcing due to all other anthropogenic aerosol components combined Further studies are needed to understand the mixing state of EC and determine the accuracy of the core treatment

On the Teleconnectivity of the "Arctic Oscillation"

Abstract: The term “Arctic Oscillation” (AO) has recently been introduced to describe the leading structure of SLP variability over the Northern Hemisphere. A key feature of the AO is its zonally symmetric appearance, with a primary center of action over the Arctic and opposing anomalies in midlatitudes. Does the AO's annular appearance result from significant temporal correlations between SLP anomalies at distant longitudes? The results presented indicate that the temporal coherence between the Arctic and midlatitudes is strongest over the Atlantic sector, with weak correlations between the Atlantic and Pacific midlatitudes, both on intraseasonal and interannual time scales during the past 50 yrs. Hence, the “annular” character of the AO is more a reflection of the dominance of its Arctic center of action than any coordinated behavior of the Atlantic and Pacific centers of action in the SLP field. The AO is nearly indistinguishable from the leading structure of variability in the Atlantic sector (e.g., the North Atlantic Oscillation): their temporal correlation is 0.95 for monthly data.

African dust and the demise of Caribbean Coral Reefs

Abstract: The vitality of Caribbean coral reefs has undergone a continual state of decline since the late 1970s, a period of time coincidental with large increases in transatlantic dust transport. It is proposed that the hundreds of millions of tons/year of soil dust that have been crossing the Atlantic during the last 25 years could be a significant contributor to coral reef decline and may be affecting other ecosystems. Benchmark events, such as near synchronous Caribbean-wide mortalities of acroporid corals and the urchin Diadema in 1983, and coral bleaching beginning in 1987, correlate with the years of maximum dust flux into the Caribbean. Besides crustal elements, in particular Fe, Si, and aluminosilicate clays, the dust can serve as a substrate for numerous species of viable spores, especially the soil fungus Aspergillus. Aspergillus sydowii, the cause of an ongoing Caribbean-wide seafan disease, has been cultured from Caribbean air samples and used to inoculate sea fans.

Ultrafine aerosol formation via ion‐mediated nucleation

Abstract: The role of background ionization in the generation and evolution of ultrafine atmospheric particles is developed through modeling and data analysis. It is found that charged molecular clusters condensing around natural air ions can grow significantly faster than corresponding neutral clusters, and thus preferentially achieve stable, observable sizes. Detailed microphysical simulations of this process seem to explain recent measurements of ultrafine particle behavior, as well as the diurnal variation seen in tropospheric mobility spectra. The proposed ion-mediated nucleation mechanism leads to the production of new particles under conditions that are unfavorable for binary homogeneous nucleation, and provides a consistent explanation for a variety of tropospheric observations.

The effect of mineral bond strength and adsorbed water on fault gouge frictional strength

Abstract: Recent studies suggest that the tendency of many fault gouge minerals to take on adsorbed or interlayer water may strongly influence their frictional strength. To test this hypothesis, triaxial sliding experiments were conducted on 1 5 different single-mineral gouges with various water-adsorbing affinities. Vacuum dried samples were sheared at 100 MPa, then saturated with water and sheared farther to compare dry and wet strengths. The coefficients of friction, µ, for the dry sheet- structure minerals (0.2-0.8), were related to mineral bond strength, and dropped 20-60% with the addition of water. For non-adsorbing minerals (µ =0.6-0.8), the strength remained unchanged after saturation. These results confirm that the ability of minerals to adsorb various amounts of water is related to their relative frictional strengths, and may explain the anomalously low strength of certain natural fault gouges.

Increased carbon sequestration by a boreal deciduous forest in years with a warm spring

Abstract: A boreal deciduous forest in Saskatchewan, Canada, sequestered 144±65, 80±60, 116±35 and 290±50 g C m−2 y−1 in 1994, 1996, 1997 and 1998, respectively. The increased carbon sequestration was the result of a warmer spring and earlier leaf emergence, which significantly increased ecosystem photosynthesis, but had little effect on respiration. The high carbon sequestration in 1998 was coincident with one of the strongest El Nino events of this century, and is considered a significant and unexpected benefit.

GPS geodetic constraints on Caribbean-North America plate motion

Abstract: We describe a model for Caribbean plate motion based on GPS velocities of four sites in the plate interior and two azimuths of the Swan Islands transform fault. The data are well fit by a single angular velocity, with average misfits approximately equal to the 1.5–3.0 mm yr−1 velocity uncertainties. The new model predicts Caribbean-North America motion ∼65% faster than predicted by NUVEL-1A, averaging 18–20±3 mm yr−1 (2σ) at various locations along the plate boundary. The data are best fit by a rotation pole that predicts obliquely convergent motion along the plate boundary east of Cuba, but are fit poorly by a suite of previously published models that predict strike-slip motion in this region. The data suggest an approximate upper bound of 4–6 mm yr−1 for internal deformation of the Caribbean plate, although rigorous estimates await more precise and additional velocities from sites in the plate interior.

Modeling climate shift of El Nino variability in the Holocene

Abstract: A coupled ocean-atmosphere general circulation model is used to investigate climatic shift of El Nino in the Holocene. The model simulates a reduced ENSO intensity in the early and mid- Holocene, in agreement with paleoclimate record. The ENSO reduction is proposed to be caused by both an intensified Asian summer monsoon and a warm water subduction from the South Pacific into the equatorial thermocline.

The solar wind interaction with Mars: Locations and shapes of the bow shock and the magnetic pile-up boundary from the observations of the MAG/ER Experiment onboard Mars Global Surveyor

Abstract: The Mars Global Surveyor spacecraft was inserted into an elliptical orbit around Mars on September 12, 1997. It includes the MAG/ER instrument with two magnetometers providing in-situ sensing of the ambient magnetic field and an electron reflectometer measuring the local distribution function of the electrons in the energy range of 10 eV to 20 keV. This statistical study deals with the identification and the position of the Bow Shock (BS) and of another plasma boundary, the Magnetic Pile-up Boundary (MPB), proved as permanent by MAG/ER. During the first year of the MGS mission, a total of 290 orbits have been considered to fit the geometric characteristics of these boundaries. The position and shape of these boundaries are compared with previous studies. Good agreement is found with the Phobos 2 observations, suggesting than the mean bow shock and MPB locations are independent of solar cycle phase. The great number of crossings shows that the Bow Shock position and nightside MPB position are highly variable.

Hydrological responses to dynamically and statistically downscaled climate model output

Abstract: Daily rainfall and surface temperature series were simulated for the Animas River basin, Colorado using dynamically and statistically downscaled output from the National Center for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) re-analysis. A distributed hydrological model was then applied to the downscaled data. Relative to raw NCEP output, downscaled climate variables provided more realistic simulations of basin scale hydrology. However, the results highlight the sensitivity of modeled processes to the choice of downscaling technique, and point to the need for caution when interpreting future hydrological scenarios.

Abrupt shift of the ITCZ over West Africa and intra‐seasonal variability

Abstract: The onset of the monsoon system over West Africa is linked to the northward migration of the Inter-Tropical Convergence Zone (ITCZ) during the northern spring and summer. By using daily gridded rainfall data and NCEP/NCAR wind reanalyses over the period 1968–1990, we show that this migration is characterised by an abrupt latitudinal shift of the ITCZ in late June from a quasi-stationary location at 5N in May–June to another quasi-stationary location at 10N in July–August. A composite analysis based on the shift dates shows that this northward shift is associated with the occurrence of a westward-travelling monsoon depression pattern over the Sahel with characteristic periodicities of 20–40 days.

Evaluating uncertainties in the prediction of regional climate change

Abstract: Uncertainties in regional climate change simulations for the 21st century by five coupled atmosphere-ocean General Circulation Models (AOGCMs) (two of them including ensembles of simulations), for different anthropogenic forcing scenarios and 23 regions in the World, are examined. Seasonally and regionally averaged precipitation and surface air temperature for the future period of [2070–2099] as compared to the period of [1961–1990] are considered. The dominant source of uncertainty in the simulation of average regional climate change is due to inter-model variability with inter-scenario and internal model variability playing secondary roles. The range of predicted climate changes by different realizations of the same ensemble is small, and simulated changes exhibit a high level of coherency among different forcing scenarios. Uncertainties in regional changes are 3 K or greater for temperature and 25% of present day values or greater for precipitation. The model biases in reproducing present day climate are ≤1 K to over 5 K for temperature and ≤10% to over 100% for precipitation.

Thermoelastic properties and crystal structure of MgSiO3 perovskite at lower mantle pressure and temperature conditions

Abstract: A new synchrotron X-ray diffraction study of MgSiO3 perovskite at high-pressure and high-temperature has been carried out in a laser-heated diamond-anvil cell to 94 GPa and temperatures above 2500 K. MgSiO3 perovskite is shown to be stable in this P–T range and adopts an orthorhombic structure (space group Pbnm), thus ruling out any phase transition or decomposition to an assemblage of denser oxides. Structural refinements show an increase of the orthorhombic distortion with increasing pressure, counterbalanced by a decrease of this distortion at high temperature, as evidenced by the Si-O-Si angle evolution. In addition, thermoelastic parameters identified from this new pressure-volume-temperature data set indicate that a significant amount of magnesiowustite is required to match PREM bulk modulus profile, thus making a pure perovskite lower mantle unlikely.

Mean annual temperature trends and their vertical structure in the tropical Andes

Abstract: Mean annual temperature trends in the tropical Andes were determined over the last six decades (1939–1998), to investigate the apparent inconsistency between the observed glacier retreat and the reported slight cooling trend in the lower tropical troposphere after 1979. Our results indicate that temperature in the tropical Andes has increased by 0.10°–0.11°C/decade since 1939. The rate of warming has more than tripled over the last 25 years (0.32°–0.34°C/decade) and the last two years of the series, associated with the 1997/98 El Nino, were the warmest of the last six decades. Temperature trends vary with altitude and show a generally reduced warming with increasing elevation. However, despite the lower rate of warming, the trend toward increased temperatures is still significant at the 95% confidence level, even at the highest elevations. Clearly high elevation surface stations in the Andes do not reflect the slight cooling trend observed in the tropical lower-troposphere.

Evidence for a recent change in the link between the North Atlantic Oscillation and Arctic Sea ice export

Abstract: Evidence for a recent change in the link between the North Atlantic Oscillation (NAO) and Arctic sea ice export through Fram Strait during wintertime (DJFM) is presented from the analysis of simulated Arctic sea ice and observations for the period 1958–1997. Whereas no correlation between the two time series is found from 1958 to 1977 (r=0.1), the correlation increased significantly thereafter (r=0.7, 1978–1997). The increased coherency between the NAO and Arctic sea ice export through Fram Strait during the last two decades resulted from anomalous meridional wind components near Fram Strait which were associated with the more easterly position of the NAO's centers of interannual variability compared to 1958–1977.

Seismo‐ionospheric signatures prior to M≥6.0 Taiwan earthquakes

Abstract: This paper examines variations of the greatest plasma frequency in the ionosphere, foF2, recorded by the Chung-Li ionosonde (25.0° N, 121.1° E) before M≥6.0 earthquakes during 1994–1999. The 15-day running median and the associated inter-quartile range are utilized as the reference and the upper or lower bounds to monitor the ionospheric foF2 variations for finding seismo-ionospheric signatures (precursors) of the earthquakes. It is found that precursors, in the form of the recorded foF2 falling below its associated lower bound around 1200–1700 LT, appear 1–6 days prior to these earthquakes. On September 20, 1999 UT (September 21, Taiwan local time) a large Mw=7.7 earthquake struck central Taiwan near the small town of Chi-Chi. We analyzed the foF2 and found three clear precursors 1, 3, and 4 days prior to the Chi-Chi earthquake.

The thermal state of the upper mantle; No role for mantle plumes

Abstract: A variety of geophysical data indicates that long wavelength temperature variations of the asthenosphere depart from the mean by ±200°C, not the ±20°C adopted by plume theoreticians. The ‘normal’ variation, caused by plate tectonic processes (subduction cooling, continental insulation, small‐scale convection) encompasses the temperature excesses that have been attributed to hot jets and thermal plumes. Geophysical estimates of the average potential temperature of the upper mantle are about 1400°C. Asthenospheric convection at ridges, rifts and fracture zones and at the onset of continental breakup is intrinsically 3D, giving rise to shallow pseudoplume‐like structures without deep thermal instabilities. Deep narrow thermal plumes are unnecessary and are precluded by uplift and subsidence data. The locations and volumes of ‘midplate’ volcanism appear to be controlled by lithospheric architecture, stress and cracks.

Shelf break upwelling driven by Brazil Current Cyclonic Meanders

Abstract: Analysis of hydrographic data and results of numerical modeling show the occurrence of upwelling associated with the cyclonic meanders of the Brazil Current, in the region known as the Southeast Brazil Bight or Santos Bight. The temperature and salinity distributions collected during summer and winter confirm the idea that meander-induced shelf break upwelling plays an important role in the pumping of South Atlantic Central Water from the slope region onto the continental shelf.

Pore scale study of flow in porous media: Scale dependency, REV, and statistical REV

Abstract: Flow in porous media is studied at the pore-scale with lattice Boltzmann simulations on pore geometries reconstructed from computed microtomographic images. Pore scale results are analyzed to give quantities such as permeability, porosity and specific surface area at various scales and at various locations. With this, some fundamental issues such as scale dependency and medium variability can be assessed quantitatively. More specifically, the existence and size of the well known concept, representative elementary volume (REV), can be quantified. It is found that the size of an REV varies spatially and depends on the quantity being represented. For heterogeneous media, a better measure may be the so called “statistical REV”, which has weaker requirements than does the deterministic REV.

Geoengineering Earth's radiation balance to mitigate CO2-induced climate change

Abstract: To counteract anthropogenic climate change, several schemes have been proposed to diminish solar radiation incident on Earth's surface. These geoengineering schemes could reverse global annual mean warming; however, it is unclear to what extent they would mitigate regional and seasonal climate change, because radiative forcing from greenhouse gases such as CO2 differs from that of sunlight. No previous study has directly addressed this issue. In the NCAR CCM3 atmospheric general circulation model, we reduced the solar luminosity to balance the increased radiative forcing from doubling atmospheric CO2. Our results indicate that geoengineering schemes could markedly diminish regional and seasonal climate change from increased atmospheric CO2, despite differences in radiative forcing patterns. Nevertheless, geoengineering schemes could prove environmentally risky.

The time-dependence of climate sensitivity

Abstract: A doubled CO2 coupled ocean-atmosphere experiment has been run for over 800 years. The ‘effective’ equilibrium climate sensitivity to a doubling of CO2 (the equilibrium response of the model assuming the feedbacks remained constant at the value found at any given point of the transient response) is calculated throughout the run and found to increase by around 40%. The time-dependence is associated with differences in cloud feedback arising from inter-hemispheric temperature differences due to the slower warming rate of the Southern Ocean. The time-dependence of the climate response has implications for the use of simpler models in scaling GCM results to different scenarios of forcing.

Sensing global Birkeland currents with iridium® engineering magnetometer data

Abstract: The Iridium system consists of >70 satellites in low altitude, 780km, polar orbits in six equally spaced orbit planes with at least eleven satellites in each plane. Each satellite carries an engineering magnetometer with 48nT resolution. Techniques have been developed at JHU/APL to process Iridium magnetic field data and obtain global maps of magnetic perturbations due to field aligned currents (FACs). The noise level in the processed data is typically 70 to 100 nT and readings above the noise level occur at high magnetic latitudes consistent with an auroral signature. Time series also display well known features characteristic of FACs. Synoptic maps derived using three hours of data from 17 February, 2000, AE ∼100–300nT, show patterns consistent with the Region 1/2 currents previously determined statistically. The Indium data set provides new global measurements of the Birkeland currents in both hemispheres on time scales of a few hours or less.

On the existence of Lloró (the rainiest locality on earth): enhanced ocean-land-atmosphere interaction by a low-level jet

Abstract: The department of Choco, on the Colombian Pacific coast experiences 8,000 to 13,000 mm of average annual precipitation. Lloro (5°30′N, 76°32′W, 120m) has received above 12,700 mm (1952–1960). Using the NCEP/NCAR Reanalysis data, we show that the ocean-land-atmosphere interaction over the easternmost fringe of the tropical Pacific, enhanced by the dynamics of a low-level westerly jet (“CHOCO”), contributes to explain the existence of such record-breaking hydrological region. Deep convection develops from low-level moisture convergence by the CHOCO jet, combined with high-level easterly trade winds, orographic lifting on the western Andes, low surface pressures and warm air. Precipitation is organized in mesoscale convective complexes, in turn dynamically linked to the jet. The strength of the CHOCO jet (centered at 5°N) is associated with the gradient of surface air temperatures between western Colombia and the Nino 1+2 region, thereby exhibiting strong annual and interannual variability, which contributes to explaining Colombia's hydro-climatology and its anomalies during ENSO.

The increase in stratospheric water vapor from balloonborne, frostpoint hygrometer measurements at Washington, D.C., and Boulder, Colorado

Abstract: Stratospheric water vapor concentrations measured at two midlatltude locations in the northern hemisphere show water vapor amounts have increased at a rate of 1-1.5% yr'1 (0.05-0.07 ppmv yr"1) for the past 35 years. At Washington, D.C., meas- urements were made from 1964-1976, and at Boulder, Colorado, observations began in 1980 and continue to the present. While these two data sets do not comprise a single time series, they individually show increases over their respective measurement periods. At Boulder the trends do not show strong seasonal differences; significant increases are found throughout the year in the altitude range 16-28 km. In winter these trends are significant down to about 13 km.

On the control of stratospheric humidity

Abstract: We present a hypothesis on the dehydration and transfer of air from the tropical troposphere into the stratosphere. The hypothesis is based on the existence of a thick “tropopause layer,” in which vertical and horizontal mixing are both significant. Air is rapidly dehydrated upon entering this layer in vigorous convective overshoots, then slowly ascends through the layer before fully entering the stratosphere. Dehydration and genuine entry into the stratosphere are separate processes that happen on much different time scales.

A study of the 1999 monsoon rainfall in a mountainous region in central Nepal using TRMM products and rain gauge observations

Abstract: Raingauge data from the 1999 monsoon were compared with precipitation derived from the precipitation radar (PR) and the microwave imager instruments on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The raingauges are part of a new hydrometeorological network installed in the Marsyandi river basin, which extends from the edge of the Tibetan Plateau to the Gangetic basin. TRMM-derived precipitation showed better detection of rain at low altitude stations as compared with high elevation stations, with good scores for the PR product for rain rates >0.5 mm/hr. The 3D PR rain rates suggest strong interaction between mesoscale convective systems and steep terrain at elevations of 1–2 km, which is consistent with the very high rainfall measured at those locations. Analysis of the raingauge data shows that even at altitudes as high as 4,000 m the cumulative monsoon rainfall is comparable to the highest amount recorded in the Indian subcontinent.

New particle formation in the continental boundary layer: Meteorological and gas phase parameter influence

Abstract: New particle formation in the polluted continental boundary layer was studied, based on 1.5-year observations of the particle size distribution, meteorological and gas phase parameters. Events of new particle formation involving significant ultrafine particle number concentrations (>104 cm−3 in the size range 3–11 nm) were observed on 20% of all days, pointing out that a frequent particle production from gaseous precursors can occur despite the relatively high pre-existing particle surface area in the area of investigation. The maximum in the observed particle size distributions was mostly above 3 nm, suggesting the actual particle nucleation to take place upwind of the measurement site. A particle growth analysis yielded 2.3±1.4 h as an upper limit of the time for the particles to grow from the critical cluster size till the observation of the peak in ultrafine number concentration. On 80% of the significant events of new particle formation (though not on all), SO2 concentrations increased considerably (by an average factor of 7), most likely by entrainment from aloft. Particle surface area was, on average, higher on event days compared to non-event days, indicating only a weak competition between condensation onto the pre-existing particle surface area and the new particle formation process. The highest statistical correlation was found between the events of new particle formation and solar radiation, indicating a high degree of meteorological control.