bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinematics of the Pamir and Hindu Kush regions from GPS geodesy
Authors: Ischuk Anatoli , Bendick Rebecca , Rybin Anatoly , Molnar Peter , Khan Shah Faisal , Kuzikov Sergey , Mohadjer Solmaz , Saydullaev Umed , Ilyasova Zhyra , Schelochkov Gennady , Zubovich Alexander V.
Published: April 2013

GPS velocities measured in the Pamir and surrounding regions show a total of ~30 mm/yr of northward relative motion between stable Pakistan and Eurasia. The convergence budget is partitioned into 10–15 mm/yr of localized shortening across the Trans‐Alai Thrust, which bounds the Pamir on the north, consistent with southward subduction of intact lithosphere. Another 10–15 mm/yr of shortening is distributed across the Chitral Himalaya and Hindu Kush, suggesting that Hindu Kush seismicity might be related to northward subduction of Indian lithosphere. Modest shortening at <5 mm/yr occurs north of the Trans‐Alai Thrust, across the South Tien Shan and between the Ferghana Valley and Eurasia. Negligible north‐south shortening occurs within the high Pamir, but as much as 5 mm/yr, and perhaps 10 mm/yr, of east‐west extension occurs within this region. This extension is matched by a comparable amount of east‐west shortening in the Tajik Depression. The localization of shortening to the margins of the Pamir combined with observations of distributed internal extension implies that the east‐west vertically averaged, horizontal compressive normal stress is smaller than the north‐south compressive stress.
An integrated archeomagnetic and C14study on pre‐Columbian potsherds and associated charcoals intercalated between Holocene lacustrine sediments in Western Mexico: Geomagnetic implications
Authors: Aguilar Reyes B. , Goguitchaichvili A. , Morales J. , Garduño V. H. , Pineda M. , Carvallo C. , Moran Tomas González , Israde Isabel , Rathert Manuel Calvo
Published: April 2013

The firm relationship between archeomagnetic samples and their absolute ages, which is usually determined or estimated using a variety of methods (archaeological context, thermoluminiscense, C14, etc.), is crucial in any archaeomagnetic investigation. In this paper, we report 18 successful (from 65 analyzed) archaeointensity determinations of high technical quality performed on specimens obtained from three fragments of pre‐Columbian potsherds. These are unambiguously correlated with charcoal samples, as they were found together in three thin lacustrine sedimentary layers in western Mexico. Moreover, new radiometric results were obtained in this study from all charcoal samples (beta analytic) yielding ages of 1490 ± 40, 1510 ± 40, and 1580 ± 40 BP. In addition, detailed magnetic measurements were carried out on all studied ceramic fragments. These rock magnetic experiments, which included low field susceptibility versus temperature, hysteresis, and first‐order reversal curve measurements, indicate that the main magnetic carrier is Ti‐poor titanomagnetite. The average archeointensity values per ceramic fragments obtained in this study are very similar in all three cases: 35.2 ± 1.3 μT (N = 5), 36.8 ± 1.6 μT (N = 6), and 37.2 ± 3.4 μT (N = 7). This corresponds to an average intensity of 36.4 ± 1.1 μT and a virtual axial dipole moment value of 8.1 ± 0.2 × 1022Am2, which is slightly lower than the present field strength, corresponding at an age interval between 640 and 430 A.D. Although we detected some relationship between the Earth's magnetic field intensity and multidecadal climatic events, such observations can only be considered tentative at this stage.
Vortex generation in oscillatory canopy flow
Authors: Ghisalberti Marco , Schlosser Tamara
Published: April 2013

In this paper, we demonstrate for the first time the generation of coherent vortices at the top of a canopy in oscillatory (i.e., wave‐dominated) flow. Through a series of flow visualization experiments, vortex formation is shown to occur when two conditions described by the Keulegan–Carpenter (KC) and Reynolds (Re) numbers are met. First, the wave period must be sufficiently long to allow the generation of the shear‐driven instability at the top of the canopy; this occurs when KC ≳ 5. Second, the vortex instability must be able to overcome the stabilizing effects of viscosity; this occurs when Re ≳ 1000. The vortices greatly increase the rate of vertical mixing within the canopy, such that any prediction of residence time in a coastal canopy requires an understanding of whether vortex generation is occurring.
An experimental and numerical investigation on wave‐mud interactions
Authors: Hsu W. Y. , Hwung H. H. , Hsu T. J. , Torres‐Freyermuth A. , Yang R. Y.
Published: April 2013

Wave attenuation over a mud (kaolinite) layer is investigated via laboratory experiments and numerical modeling. The rheological behavior of kaolinite exhibits hybrid properties of a Bingham and pseudoplastic fluid. Moreover, the measured time‐dependent velocity profiles in the mud layer reveal that the shear rate under wave loading is highly phase dependent. The measured shear rate and rheological data allow us to back‐calculate the time‐dependent viscosity of the mud layer under various wave loadings, which is also shown to fluctuate up to 1 order of magnitude during one wave period. However, the resulting time‐dependent bottom stress is shown to only fluctuate within 25% of its mean. The back‐calculated wave‐averaged bottom stress is well correlated with the wave damping rate in the intermediate‐wave energy condition. The commonly adopted constant viscosity assumption is then evaluated via linear and nonlinear wave‐mud interaction models. When driving the models with measured wave‐averaged mud viscosity (forward modeling), the wave damping rate is generally overpredicted under the low wave energy condition. On the other hand, when a constant viscosity is chosen to match the observed wave damping rate (inverse modeling), the predicted velocity profiles in the mud layer are not satisfactory and the corresponding viscosity is lower than the measured value. These discrepancies are less pronounced when waves become more energetic. Differences between the linear and nonlinear model results become significant under low‐energy conditions, suggesting an amplification of wave nonlinearity due to non‐Newtonian rheology. In general, the constant viscosity assumption for modeling wave‐mud interaction is only appropriate for more energetic wave conditions.
Optical properties of the Dead Sea
Authors: Boss Emmanuel , Gildor Hezi , Slade Wayne , Sokoletsky Leonid , Oren Aharon , Loftin James
Published: April 2013

The Dead Sea, located in the rift valley between Jordan and Israel, is a hypersaline lake, resulting in unique biogeochemistry and optical properties. In the spring of 2004 we conducted two days of physical and optical measurements in the lake. Because of the significant effect of dissolved salts on the optical properties of water, our analysis required a novel processing approach to obtain dissolved and total inherent optical properties from the measurements. In addition, we show that the lake's salinity can be estimated from measurements of hyper‐spectral absorption or attenuation spectra in the red and infrared parts of the spectrum, using published values of specific absorption of dissolved NaCl, despite the fact that the lake's salt chemistry is complex. In situ observations demonstrated that the lake has a two‐layer structure with a warm and more turbid layer at the top 20–30 m and a clearer colder layer below. Both the particulate and dissolved absorption are well approximated by exponentially decreasing functions with the spectral slope of the particulate absorption about half that of the dissolved fraction and consistent with other aquatic environments. Both have relatively low and similar magnitudes in the blue (O(0.15 m–1)). Mean particle size was observed to increase with depth, consistent with precipitating salt crystals (observed in past campaigns) shown here to play a major role in the lake's optical properties.
On the response of Southern Hemisphere subpolar gyres to climate change in coupled climate models
Authors: Wang Zhaomin
Published: April 2013

We investigate the responses of the Southern Hemisphere subpolar gyres to projected climate changes over the 21st century by Coupled Model Intercomparison Project Phase 3 and 5 models. Under increased greenhouse gas forcing, the Southern Hemisphere westerly winds consistently become intensified, resulting in increased cyclonic wind forcing in the subpolar region in these models. Under such wind forcing changes, it is a robust feature that there are consistent increases in the westward flow close to the coast of Antarctica, with strong implications to the mass balance of the Antarctic ice shelves and ice sheets. However, there are large discrepancies in the responses of the gyre axes and overall gyre strengths. Some models show equatorward expansions of the southern gyre limbs, resulting in consistent and large gyre strength increases, while some other models show poleward contractions of the gyres and generally small and less consistent gyre strength changes. These uncertainties are primarily a result of the uncertain simulations of eddy‐driven circulations in the Antarctic Circumpolar Current. The associated buoyancy forcing changes play a secondary role in driving these oceanic responses.

This study reveals that there are large uncertainties in the projections of the subpolar circulation in the current generation of coupled climate models, although CMIP5 models have considerably smaller inter‐model spreads in the present‐day and projected gyre strengths. To predict the subpolar circulation changes, future improved modelling studies need to particularly reduce the uncertainties in the projections of the westerly jet and to reduce the uncertainties in the eddy‐driven circulation responses to wind forcing changes.
A model based on stacked‐constraints approach for partitioning the light absorption coefficient of seawater into phytoplankton and non‐phytoplankton components
Authors: Zheng Guangming , Stramski Dariusz
Published: April 2013

Partitioning of the total non‐water absorption coefficient of seawater, anw(λ) (i.e., the light absorption coefficient after subtraction of pure water contribution), into phytoplankton, aph(λ), and non‐phytoplankton, adg(λ), components is important in the areas of ocean optics, biology, and biogeochemistry. We propose a partitioning model based on stacked‐constraints approach, which requires input of anw(λ) at a minimum of six specific light wavelengths. Compared with existing models, our approach requires much less restrictive assumptions about the spectral slope of adg(λ) and the spectral shape of aph(λ). Our model is based on several inequality constraints determined from an extensive, quality‐verified set of field data covering oceanic and coastal waters from low to high‐latitudes. With these constraints, the model first derives a wide range of speculative solutions for adg(λ) and aph(λ) and then identifies feasible solutions. Final model outputs include the optimal solution and a range of feasible solutions for adg(λ) and aph(λ). The optimal solutions agree well with measurements. For example, the median ratio of the model‐derived optimal solutions to measured adg(λ) and aph(λ) at 443 nm is very close to 1, i.e., 1.004 and 0.988, respectively. The median absolute percent difference between the optimal solutions and measured values of adg(443) and aph(443) is 6.5% and 12%, respectively. The range of feasible solutions encompasses the measured adg(λ) and aph(λ) with a probability >90% at most wavelengths. Our results support the prospect for the applications of the partitioning model using the input data of anw(λ) collected from various oceanographic and remote‐sensing platforms.
Predicting multiyear North Atlantic Ocean variability
Authors: Hazeleger W. , Wouters B. , van Oldenborgh G. J. , Corti S. , Palmer T. , Smith D. , Dunstone N. , Kröger J. , Pohlmann H. , von Storch J.‐S.
Published: April 2013

We assess the skill of retrospective multiyear forecasts of North Atlantic ocean characteristics obtained with ocean‐atmosphere‐sea ice models that are initialized with estimates from the observed ocean state. We show that these multimodel forecasts can skilfully predict surface and subsurface ocean variability with lead times of 2 to 9 years. We focus on assessment of forecasts of major well‐observed oceanic phenomena that are thought to be related to the Atlantic meridional overturning circulation (AMOC). Variability in the North Atlantic subpolar gyre, in particular that associated with the Atlantic Multidecadal Oscillation, is skilfully predicted 2–9 years ahead. The fresh water content and heat content in major convection areas such as the Labrador Sea are predictable as well, although individual events are not captured. The skill of these predictions is higher than that of uninitialized coupled model simulations and damped persistence. However, except for heat content in the subpolar gyre, differences between damped persistence and the initialized predictions are not significant. Since atmospheric variability is not predictable on multiyear time scales, initialization of the ocean and oceanic processes likely provide skill. Assessment of relationships of patterns of variability and ocean heat content and fresh water content shows differences among models indicating that model improvement can lead to further improvements of the predictions. The results imply there is scope for skilful predictions of the AMOC.
Simulated heat flux and sea ice production at coastal polynyas in the southwestern Weddell Sea
Authors: Haid V. , Timmermann R.
Published: April 2013

Coastal polynyas are areas in an ice‐covered ocean where the ice cover is exported, mostly by off‐shore winds. The resulting reduction of sea ice enables an enhanced ocean‐atmosphere heat transfer. Once the water temperatures are at the freezing point, further heat loss induces sea ice production. The heat exchange and ice production in coastal polynyas in the southwestern Weddell Sea is addressed using the Finite‐Element Sea‐ice Ocean Model, a primitive‐equation, hydrostatic ocean circulation model coupled with a dynamic‐thermodynamic sea‐ice model, which allows to quantify the amount of heat associated with cooling of the water column. Three important polynya regions are identified: at Brunt Ice Shelf, at Ronne Ice Shelf and along the southern part of the Antarctic Peninsula. Multiyear winter means (May–September 1990–2009) give an upward heat flux to the atmosphere of 311 W/m2 in the Brunt polynyas, 511 W/m2 in Ronne Polynya and 364 W/m2 in the Antarctic Peninsula polynyas, whereof 57 W/m2, 49 W/m2 and 48 W/m2, respectively, are supplied as oceanic heat flux from deeper layers. The mean winter sea ice production is 7.2 cm/d in the Brunt polynyas corresponding to an ice volume of 1.3 ×1010 m3/winter, 13.2 cm/d at Ronne polynya (4.4 ×1010 m3/winter), and 9.2 cm/d in the Antarctic Peninsula polynyas (2.1 ×1010 m3/winter). The heat flux to the atmosphere inside polynyas is 7 to 9 times higher than the heat flux in the adjacent area; polynya ice production per unit area exceeds adjacent values by a factor of 9 to 14.
Observations of mixed layer deepening during an Antarctic gale
Authors: Lane‐Serff G. F. , Stansfield K. L.
Published: April 2013

Observations of mixed layer deepening made during a gale in February 2005 near an ice shelf, Fimbulisen, Antarctica, are reported. The observations were made from the RRS James Clark Ross in the lee of the ice shelf, using repeated downcasts (“yo‐yo”) of a conductivity‐temperature‐depth package, together with shipboard meteorological and other measurements. The mixed layer deepened from less than 40 m to over 120 m over the course of 27 h, with a very rapid deepening from 80 m to 120 m over a period of under 11 h. The mixed layer became both colder and fresher, with the change in salinity and heat content likely to be caused by melting ice. Oxygen isotope results suggest the source of the fresh water was melting sea ice rather than precipitation or ice shelf melt. The input of melt water at the surface stabilizes the mixed layer, so extra energy is required to deepen the mixed layer. The observations suggest that approximately 1.8% of the available “wind‐work” energy was used to mix the upper water column, while the stabilizing surface buoyancy flux inhibits the turbulence in the mixed layer, limiting the mixing length to 1.6 m. The eventual depth of the mixed layer is in line with estimates based on the planetary length scale u*/f. The rate of mixed layer deepening is given by Ue/u* = 0.035. The apparent peak ice melting rate was approximately 60 mm hr−1, although this is likely to be exaggerated by convergence and downwelling.
Particle dynamics of the Changjiang Estuary and adjacent coastal region determined by natural particle‐reactive radionuclides (7Be, 210Pb, and 234Th)
Authors: Huang Dekun , Du Jinzhou , Moore Willard S. , Zhang Jing
Published: April 2013

The timescale of transport processes in estuarine and coastal regions can be evaluated using natural radionuclides with different half‐lives. The distribution patterns of 7Be, 210Pb, and 234Th in the water column from April to July 2008 were used to calculate the removal and residence times in the Changjiang Estuary. The results showed that the maximum particulate activities of 7Be, 210Pb, and 234Th were observed approximately 150 km downstream (the turbidity maximum zone) of the freshwater end‐number. The mean distribution coefficients (Kd, cm3 g−1) of the high suspended particulate matter (SPM) group are higher than those of the lower‐SPM group for 7Be and 210Pb; for 234Th, the reverse is true. Based on a material balance in two‐dimensional models of 7Be, 210Pb, and 234Th in the Changjiang River mouth, the removal times of these nuclides were approximately 0.66–12, 1.6–21, and 1.2–5.4 days, respectively. The residence times increased toward the seaward side. In the coastal region, the removal times of 7Be, 210Pb, and 234Th calculated by material balance in one‐dimensional models were approximately 1.1–26, 1.2–27, and 0.70–23 days, respectively. Moreover, an enhanced resuspension process not only controlled the partitioning of 7Be, 210Pb, and 234Th between the particulate and dissolved phases, but also, this process may play a dominant role in controlling the dynamic behavior of SPM in the water column compared with the advection input/output fluxes of the SPM in the river mouth areas. The removal and the resuspension fluxes were comparable in the estuary. Meanwhile, old composition (whose deposition into the seabed for a long time is enough for 7Be to decay completely) occupied only a small part in the resuspended sediment during resuspension process.
The effects of geothermal heating on the East/Japan Sea circulation
Authors: Park Young‐Gyu , Park Jae‐Hun , Lee Ho Jin , Min Hong Sik , Kim Seon‐Dong
Published: April 2013

The magnitude of geothermal heating in the East/Japan Sea is about 100 mW/m2, twice that of a typical abyssal plain. In addition, bottom stratification in the East/Japan Sea is much weaker than that typical of the open ocean. Thus, geothermal heating could have more prominent effects in the East/Japan Sea than in the open ocean, and we tested this hypothesis via numerical modeling. With less than 100 mW/m2 bottom heat flux, we were able to reproduce bottom mixed layers that are thicker than ~1000 m as observed. Previously, no numerical model has been successful in reproducing such bottom mixed layers. Geothermal heating intensifies the bottom flows but the simulated flows are not as strong as the observed ones. Over the northern part of the East/Japan Sea, reduction in deep stratification strengthens deep water mass formation, intensifying cyclonic circulations located over this area, so the effects of the heating extend to the surface. As the cyclonic circulation becomes stronger, the water at the center of the gyre is trapped and more exposed to cold air, so it becomes cooler, and colder deep water is produced. When the geothermal heating is strong enough, the surface cooling effect dominates the bottom heating and the deep layer becomes cooler showing that the nonlinear effects of geothermal heating are far reaching. Thus, to account for the observed dynamics, the full three‐dimensional circulation at the basin scale is needed.
Evaluation of model nesting performance on the Texas‐Louisiana continental shelf
Authors: Marta-Almeida Martinho , Hetland Robert D. , Zhang Xiaoqian
Published: April 2013

A skill assessment of a model of the Texas‐Louisiana shelf, nested in a variety of different parent models, is performed using hydrographic salinity data. The nested models show improved salinity skill compared to the same model using climatological boundary conditions, as well as general skill score improvements over the parent models in the same region. Although a variety of parent models are used and these parent models have widely different skill scores when compared with regional hydrographic data sets, the skill scores for the nested models are generally indistinguishable. This leads to the conclusion that nesting is important for improving model skill, but it does not matter which parent model is used. The model is also used to create a series of ensembles, where the local forcing is varied with identical boundary conditions and where the boundary conditions are varied by nesting within the various parent models. The variance in the ensemble spread shows that there is a significant level of unpredictable, nonlinear noise associated with instabilities along the Mississippi/Atchafalaya plume front. The noise is seasonal and is greatest during summer upwelling conditions and weaker during nonsummer downwelling.
Spatial distribution of brominated very short‐lived substances in the eastern Pacific
Authors: Liu Yina , Yvon‐Lewis Shari A. , Thornton Daniel C. O. , Campbell Lisa , Bianchi Thomas S.
Published: April 2013

Seawater concentrations and distributions of brominated very short‐lived substances (BrVSLS), including bromoform (CHBr3), dibromomethane (CH2Br2), bromodichloromethane (CHBrCl2), chlorodibromomethane (CHClBr2), were measured in the upper water column (5–750 m) in the eastern Pacific. Inorganic nutrient, pigment concentrations, and picoplankton cell counts were measured to determine biogeochemical factors that affect the production and distribution of these BrVSLS. Elevated concentrations of BrVSLS were observed in coastal and tropical seawater. Concentration maxima for CHBr3, CH2Br2, and CHClBr2 were observed below the mixed layer, near the subsurface chlorophyll a maxima, which suggest BrVSLS production may be related to photosynthetic biomass production. Our results also suggest that heterotrophic bacteria may also contribute to CH2Br2 and CHBrCl2 production in the water column. The maximum CHBrCl2 concentration was observed at a depth much deeper than the euphotic zone, which suggests sources other than photosynthetic biomass. Elevated CHBrCl2 concentrations in deeper waters were coincident with elevated CHCl3 concentrations, which may be an evidence for successive chlorine substitution of CHBr3 in deeper and older water masses.
Dense water formation around islands
Authors: Spall Michael A.
Published: April 2013

Basic constraints on the dense water formation rate and circulation resulting from cooling around an island are discussed. The domain under consideration consists of an island surrounded by a shelf, a continental slope, and a stratified ocean. Atmospheric cooling over the shelf forms a dense water that penetrates down the sloping bottom into the stratified basin. Strong azimuthal flows are generated over the sloping bottom as a result of thermal wind. Thermally direct and indirect mean overturning cells are also forced over the slope as a result of bands of convergent and divergent Reynolds stresses associated with the jets. The Coriolis force associated with the net mass flux into the downwelling region over the slope is balanced by these nonlinear terms, giving rise to a fundamentally different momentum budget than arises in semienclosed marginal seas subject to cooling. A similar momentum balance is found for cases with canyons and ridges around the island provided that the terms are considered in a coordinate system that follows the topography. Both eddy fluxes and the mean overturning cells are important for the radial heat flux, although the eddy fluxes typically dominate. The properties of the dense water formed over the shelf (temperature, diapycnal mass flux) are predicted well by application of baroclinic instability theory and simple heat and mass budgets. It is shown that each of these quantities depends only on a nondimensional number derived from environmental parameters such as the shelf depth, Coriolis parameter, offshore temperature field, and atmospheric forcing.
Role of asymmetric tidal mixing in the subtidal dynamics of narrow estuaries
Authors: Cheng Peng , de Swart Huib E. , Valle‐Levinson Arnoldo
Published: April 2013

The role of asymmetric tidal mixing (ATM) in subtidal estuarine dynamics is investigated using a series of generic numerical experiments that simulate narrow estuaries under different stratification and external forcing conditions. The focus is on quantifying the characteristics of ATM‐induced flow and its contributions to stratification and salt transport. The flow induced by ATM has a two‐layer vertical structure in periodically stratified estuaries, similar to that of the density‐driven flow. It has a three‐layer vertical structure in the central regime of weakly stratified estuaries, and a reverse two‐layer structure in highly stratified estuaries. The changes in vertical distribution of ATM‐induced flows result from the influence of stratification on the covariance of eddy viscosity and vertical shear. Such covariance represents the driving force of ATM‐induced flow in the tidally averaged momentum equation. Compared to density‐driven flow, ATM‐induced flow dominates in periodically stratified estuaries with strong tides, has the same order of magnitude in weakly stratified estuaries with moderate tides, and is less important in highly stratified estuaries with weak tides. In contrast to density‐driven flow that always increases estuarine stratification and transports salt landward, the ATM‐induced flow exhibits different behaviors because of its varying vertical structure. In estuaries with strong tides, ATM‐induced flow tends to enhance stratification and to transport salt landward, similar to density‐driven flow. In estuaries with weak tides, ATM‐induced flow tends to reduce stratification and to transport salt seaward.
Arctic sea ice circulation and drift speed: Decadal trends and ocean currents
Authors: Kwok R. , Spreen G. , Pang S.
Published: April 2013

We examine the basinwide trends in sea ice circulation and drift speed and highlight the changes between 1982 and 2009 in connection to regional winds, multiyear sea ice coverage, ice export, and the thinning of the ice cover. The polarity of the Arctic Oscillation (AO) is used as a backdrop for summarizing the variance and shifts in decadal drift patterns. The 28‐year circulation fields show a net strengthening of the Beaufort Gyre and the Transpolar Drift, especially during the last decade. The imprint of the arctic dipole anomaly on the mean summer circulation is evident (2001–2009) and enhances summer ice area export at the Fram Strait. Between 2001 and 2009, the large spatially averaged trends in drift speeds (winter: +23.6%/decade, summer: +17.7%/decade) are not explained by the much smaller trends in wind speeds (winter: 1.46%/decade, summer: −3.42%/decade). Notably, positive trends in drift speed are found in regions with reduced multiyear sea ice coverage. Over 90% of the Arctic Ocean has positive trends in drift speed and negative trends in multiyear sea ice coverage. The increased responsiveness of ice drift to geostrophic wind is consistent with a thinner and weaker seasonal ice cover and suggests large‐scale changes in the air‐ice‐ocean momentum balance. The retrieved mean ocean current field from decadal‐scale average ice motion captures a steady drift from Siberia to the Fram Strait, an inflow north of the Bering Strait, and a westward drift along coastal Alaska. This mean current is comparable to geostrophic currents from satellite‐derived dynamic topography.
Aerosol effect on the warm rain formation process: Satellite observations and modeling
Authors: Suzuki Kentaroh , Stephens Graeme L. , Lebsock Matthew D.
Published: April 2013

This study demonstrates how aerosols influence the liquid precipitation formation process. This demonstration is provided by the combined use of satellite observations and global high‐resolution model simulations. Methodologies developed to examine the warm cloud microphysical processes are applied to both multi‐sensor satellite observations and aerosol‐coupled global cloud‐resolving model (GCRM) results to illustrate how the warm rain formation process is modulated under different aerosol conditions. The observational analysis exhibits process‐scale signatures of rain suppression due to increased aerosols, providing observational evidence of the aerosol influence on precipitation. By contrast, the corresponding statistics obtained from the model show a much faster rain formation even for polluted aerosol conditions and much weaker reduction of precipitation in response to aerosol increase. It is then shown that this reduced sensitivity points to a fundamental model bias in the warm rain formation process that in turn biases the influence of aerosol on precipitation. A method of improving the model bias is introduced in the context of a simplified single‐column model (SCM) that represents the cloud‐to‐rain water conversion process in a manner similar to the original GCRM. Sensitivity experiments performed by modifying the model assumptions in the SCM and their comparisons to satellite statistics both suggest that the auto‐conversion scheme has a critical role in determining the precipitation response to aerosol perturbations and also provide a novel way of constraining key parameters in the auto‐conversion schemes of global models.
Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability
Authors: Rajagopalan Balaji , Molnar Peter
Published: April 2013

Despite recent challenges, conventional wisdom has held that heating over the Tibetan Plateau leads to increased Indian summer monsoon rainfall via enhancement of cross‐equatorial circulation aloft, and a concurrent strengthening of both the Somali Jet and westerly winds that bring moisture to southern India. We show that such heating, quantified by monthly estimates of moist static energy in the atmosphere just above the surface, correlates with summer monsoon rainfall, but only in the early (20 May to 15 June) and late (September 1 to 15 October) monsoon season. Correlations during the main monsoon season (15 June to 31 August) are small and insignificant. The positive correlations with early and late monsoon season, however, allow for heating over Tibet to modulate as much as ~30% of the total rainfall. Furthermore, we demonstrate that heating over Tibet is independent of the El Niño Southern Oscillation, so that together they explain a substantial portion of variability in the early and late season rainfall, providing potential predictability. These links may also explain the wet conditions over India during early Holocene time and provide a quantitative link between a rise of Tibet and stronger Somali Jet.
Intraseasonal oscillation associated with the Indian winter monsoon
Authors: Dimri A. P.
Published: April 2013

The Indian winter (December–February) monsoon (IWM) contributes almost one third of the annual precipitation over the western Himalayas (WH). In winter, eastward moving synoptic weather systems, or “western disturbances” (WDs), yield precipitation in either liquid or solid form. Although previous studies have examined the interannual variation (IAV) of the IWM, little is known of the intraseasonal oscillation (ISO) associated with the IWM. The present study examines the ISO and its plausible effects on the IWM using 28 years (1980–2007) of precipitation, height, wind, and outgoing longwave radiation (OLR) fields. The dominant ISO mode is found during the active IWM phase with well‐defined cyclonic circulation in the midtroposphere over the WH. The relationship between OLR and circulation indicates that this ISO mode is driven by moisture convergence. During the peak phase, a strong moisture influx from the Arabian Sea dominates. This moisture incursion adds to the precipitation over the WH. Successive growth and decay of anomalous cyclonic and anticyclonic circulation takes place within ISO periodicity. Strong convection always seems to precede anomalous cyclonic circulation. In addition, in‐phase wind and convection (over the WH region) are associated with the ISO phase.
Relationship between mesospheric Na and Fe layers from simultaneous and common‐volume lidar observations at Arecibo
Authors: Yue Xianchang , Zhou Qihou , Raizada Shikha , Tepley Craig , Friedman Jonathan
Published: April 2013

We compare the mesospheric Na and Fe layers by using simultaneous and common‐volume lidar measurements made at the Arecibo Observatory (18.3°N, 66.75°W), Puerto Rico, in 2003. The temporal variations of the two species are highly correlated at practically all heights, although not always positively. Positive correlations occur in the bottom and top sides while negative correlation is observed in a relatively narrow region in the middle part. Chemical and dynamical effects are discussed to interpret this particular relationship between Na and Fe layers. It is shown that gas phase chemistry determines the structure of the Na and Fe layers after the metals are ablated from meteoroids entering the atmosphere. The observed region of negative correlation appears to be slightly lower than that of the expected region of negative correlation based on inert response to dynamics. It appears that such a difference may be due to temperature‐dependent chemistry. Overall, the observed correlations between Na and Fe layers can be well explained by their responses to wave dynamics.
Global analysis of night marine air temperature and its uncertainty since 1880: The HadNMAT2 data set
Authors: Kent Elizabeth C. , Rayner Nick A. , Berry David I. , Saunby Michael , Moat Bengamin I. , Kennedy John J. , Parker David E.
Published: April 2013

An updated version of the Met Office Hadley Centre's monthly night marine air temperature data set is presented. It is available on a 5° latitude‐longitude grid from 1880 as anomalies relative to 1961–1990 calendar‐monthly climatological average night marine air temperature (NMAT). Adjustments are made for changes in observation height; these depend on estimates of the stability of the near surface atmospheric boundary layer. In previous versions of the data set, ad hoc adjustments were also made for three periods and regions where poor observational practice was prevalent. These adjustments are re‐examined. Estimates of uncertainty are calculated for every grid box and result from measurement errors, uncertainty in adjustments applied to the observations, uncertainty in the measurement height, and under‐sampling. The new data set is a clear improvement over previous versions in terms of coverage because of the recent digitization of historical observations from ships' logbooks. However, the periods prior to about 1890 and around World War II remain particularly uncertain, and sampling is still sparse in some regions in other periods. A further improvement is the availability of uncertainty estimates for every grid box and every month. Previous versions required adjustments that were dependent on contemporary measurements of sea surface temperature (SST); to avoid these, the new data set starts in 1880 rather than 1856. Overall agreement with variations of SST is better for the updated data set than for previous versions, supporting existing estimates of global warming and increasing confidence in the global record of temperature variability and change.
Impacts of boundary conditions on the precipitation simulation of RegCM4 in the CORDEX East Asia domain
Authors: Park Ju‐Hee , Oh Seok‐Geun , Suh Myoung‐Seok
Published: April 2013

The impact of boundary conditions (BCs) on simulations of precipitation characteristics using the Regional Climate Model version 4 (RegCM4) driven by two BCs (ERA‐Interim: ERA, NCEP/DOE 2: R2) was investigated using 18 years (1989–2006) of simulations from the Coordinated Regional Climate Downscaling Experiment (CORDEX) East Asia region. The RegCM4 adequately simulated spatial distribution of precipitation over the East Asia region and its temporal variations, irrespective of the BC used. It overestimated precipitation over the central region of the model domain except for India and South Korea and underestimated precipitation over equatorial ocean regions. The RegCM4's simulation skills were better in winter than in summer regardless of the BC, which can be related to the low spatial resolution (50 km) of the model, but detailed simulations of precipitation are significantly impacted by the BCs, although the impact varies with geographical location and season. Driven by ERA, the RegCM4 reproduces more precipitation in the central inland region of the model's domain and less precipitation in the southern coastal areas than when driven by R2. The differences occur because RegCM4 driven by ERA simulated the lower troposphere as being warmer and more humid and the upper troposphere as being cooler than that by R2. The transport of moisture into the central region of the model domain is also enhanced when the RegCM4 is driven by ERA. Although the RegCM4 accurately simulated the seasonal variations in precipitation, it failed to capture diurnal variations over South Korea, particularly when driven by R2, and overestimated and underestimated light precipitation (<25 mm/day) and heavy precipitation (> 50 mm/day), respectively.
A large‐eddy simulation of the phase transition of ammonium nitrate in a convective boundary layer
Authors: Aan de Brugh J. M. J. , Ouwersloot H. G. , Vilà‐Guerau de Arellano J. , Krol M. C.
Published: April 2013

Under warm and dry conditions, ammonium nitrate aerosol outgasses to form ammonia and nitric acid in the lower atmospheric boundary layer. In the upper boundary layer, where the temperature is lower and the relative humidity is higher, nitric acid and ammonia condense back to the aerosol phase. Measurements show that aerosol nitrate mixing ratios increase with altitude, confirming this phase transition. Since phase equilibrium is not reached instantaneously, updrafts transport aerosol‐poor air from the surface to high altitudes and aerosol‐rich air subsides from high altitudes to the surface under turbulent conditions. As a result, the partitioning deviates from equilibrium, so the horizontal and temporal variabilities of the aerosol nitrate mixing ratio are enhanced and a continuous downward aerosol nitrate flux emerges. We postulate that observations of this variability and flux should not be interpreted as instrument noise and deposition of nitrate. In an idealized large‐eddy simulation (LES) experiment of a convective boundary layer, we find that the larger variability and flux occurred at about one third of the boundary layer height. Both are largest when the gas‐aerosol partitioning time scale is assumed to be about half the time scale of turbulence. Our LES result shows negatively skewed nitrate mixing ratios. Under colder conditions, a smaller fraction of ammonium nitrate aerosol outgasses at the surface, so the absolute effect of nitrate repartitioning becomes smaller. However, dimensionless statistical properties do not change as long as the turbulent properties of the boundary layer remain similar. This indicates that the identified processes are also present under colder conditions.
Influence of climate variability on seasonal extremes over Australia
Authors: Min Seung‐Ki , Cai Wenju , Whetton Penny
Published: April 2013

It is well understood that Australian climate is affected by natural climate variability such as El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Southern Annular Mode (SAM), depending on seasons and regions. However, studies on Australian climate extremes associated with natural climate variability remain limited. This study examines the observed impact of natural climate variability on inter‐annual changes in seasonal extremes of rainfall and temperature over Australia during 1957–2010. We use non‐stationary Generalized Extreme Value (GEV) analysis, where GEV parameters are specified as a linear function of modes of climate variability, and compare results with the case when climate variability is not considered. Results from two station‐based observational data sets consistently suggest that extreme responses overall resemble mean responses to climate variability. Anomalously drier and hotter conditions occur over northeastern Australia and the southern coast during El Niño and a positive phase of the IOD in the cold seasons, while wetter and cooler conditions are observed during La Niña and a negative phase of IOD. A positive (negative) phase of the SAM brings wetter and cooler (drier and warmer) conditions over much of the eastern continent in summer. Covariation and relative importance of ENSO and the IOD as well as an inverse relationship between rainfall and daily maximum temperature are also found to hold for extremes. This suggests that teleconnection mechanisms responsible for seasonal mean variations may be at work for inter‐annual changes in extremes, providing important implications for climate model evaluations and regional climate change projections.
Evaluating adjusted forcing and model spread for historical and future scenarios in the CMIP5 generation of climate models
Authors: Forster Piers M. , Andrews Timothy , Good Peter , Gregory Jonathan M. , Jackson Lawrence S. , Zelinka Mark
Published: April 2013

We utilize energy budget diagnostics from the Coupled Model Intercomparison Project phase 5 (CMIP5) to evaluate the models' climate forcing since preindustrial times employing an established regression technique. The climate forcing evaluated this way, termed the adjusted forcing (AF), includes a rapid adjustment term associated with cloud changes and other tropospheric and land‐surface changes. We estimate a 2010 total anthropogenic and natural AF from CMIP5 models of 1.9 ± 0.9 W m−2 (5–95% range). The projected AF of the Representative Concentration Pathway simulations are lower than their expected radiative forcing (RF) in 2095 but agree well with efficacy weighted forcings from integrated assessment models. The smaller AF, compared to RF, is likely due to cloud adjustment. Multimodel time series of temperature change and AF from 1850 to 2100 have large intermodel spreads throughout the period. The intermodel spread of temperature change is principally driven by forcing differences in the present day and climate feedback differences in 2095, although forcing differences are still important for model spread at 2095. We find no significant relationship between the equilibrium climate sensitivity (ECS) of a model and its 2003 AF, in contrast to that found in older models where higher ECS models generally had less forcing. Given the large present‐day model spread, there is no indication of any tendency by modelling groups to adjust their aerosol forcing in order to produce observed trends. Instead, some CMIP5 models have a relatively large positive forcing and overestimate the observed temperature change.
Contribution of isotopologue self‐shielding to sulfur mass‐independent fractionation during sulfur dioxide photolysis
Authors: Ono S. , Whitehill A. R. , Lyons J. R.
Published: April 2013

Signatures of sulfur mass‐independent fractionation (S‐MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO2 is thought to be the most likely source for these S‐MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S‐MIF production. Laboratory SO2 photolysis experiments are carried out with a flow‐through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO2 in 0.25 to 1 bar N2 bath gas, in order to test the effect of SO2 pressure on the production of S‐MIF. Elemental sulfur products yield high δ34S values up to 140 ‰, with δ33S/δ34S of 0.59 ± 0.04 and Δ36S/Δ33S ratios of −4.6 ± 1.3 with respect to initial SO2. The magnitude of the isotope effect strongly depends on SO2 partial pressure, with larger fractionations at higher SO2 pressures, but saturates at an SO2 column density of 1018 molecules cm−2. The observed pressure dependence and δ33S/δ34S and Δ36S/Δ33S ratios are consistent with model calculations based on synthesized SO2 isotopologue cross sections, suggesting a significant contribution of isotopologue self‐shielding to S‐MIF for high SO2 pressure (>0.1 mbar) experiments. Results of dual‐cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ36S/Δ33S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.
Does laboratory‐scale physics obstruct the development of a theory for climate?
Authors: Essex Christopher
Published: April 2013

Despite best efforts, there still is no physical theory for climate based on the physics of the laboratory regime (i.e., fluid mechanics, radiative transfer, classical thermodynamics, etc.). This paper builds from previous discussions on how laboratory‐regime assumptions may lock our current theoretical efforts into the laboratory regime and how we might get around this problem. Using ultralong time photographic exposures (known as solargraphs) for inspiration, it draws into question classical thinking about intensive thermodynamic variables for theoretical climate purposes while using the fact that physical flows remain well defined even for climate regimes. These flows are characterized here as “generalized wind.” A simple example based on radiative energy and entropy transfer illustrates how these generalized wind fields can partially replace what is lost in moving away from laboratory‐regime physics. These winds are shown to carry the dynamics in a modified form of radiation‐like fluid dynamics that, together with radiation, might be possible to close in climate regimes.
Global seasonal variations of midday planetary boundary layer depth from CALIPSO space‐borne LIDAR
Authors: McGrath‐Spangler Erica L. , Denning A. Scott
Published: April 2013

We present a new global analysis of the depth of the planetary boundary layer (PBL) and consider regional variations throughout the year. PBL depth is estimated from the vertical variance of CALIPSO space‐borne LIDAR backscatter associated with aerosol and shallow clouds during midday satellite overpasses and is only retrieved in the absence of optically thick clouds. The resulting analysis of over 100 million retrievals per year is therefore only a sample with higher frequency over deserts and other regions of strong subsidence, and lower frequency over regions of deep convection such as the ITCZ, tropical rainforests, and the Asian Monsoon. The mean of sampled PBL depths ranges from 500 m over cold oceans to more than 3000 m over hot deserts. The seasonal cycle of analyzed PBL depth is stronger over land than over water, and seasonality over land and midlatitude oceans is of opposite sign. Wintertime storm tracks and stratocumulus regions over subtropical oceans are prominent features of the analysis. Although evaluation of the new analysis is difficult due to previous sparse sampling by other methods, comparison of LIDAR‐retrieved PBL depth with data collected by commercial aircraft generally shows good agreement.