Polar amplification: is atmospheric heat transport important?

Surface albedo feedback is widely believed to be the principle contributor to polar amplification. However, a number of studies have shown that coupled ocean-atmosphere models without ice albedo feedbacks still produce significant polar amplification in 2 × CO₂ runs due to atmospheric heat transports and their interaction with surface conditions. In this article, the relative importance of atmospheric heat transport and surface albedo is assessed using a conceptual 2-box energy balance model in a variety of different model climates. While both processes are shown to independently contribute to the polar amplified response of the model, formal feedback analysis indicates that a strong surface albedo response will tend to reduce the effect of atmospheric heat transport in the full model. We identify several scenarios near the present day climate in which, according to this formal feedback analysis, atmospheric heat transport plays no role in shaping the equilibrium warming response to uniform forcing. However, a closer analysis shows that even in these scenarios the presence of atmospheric heat transport feedback does play a significant role in shaping the trajectory by which the climate adjusts to its new equilibrium.     

Baseline atmospheric condensation nuclei at Cape Grim 1977–1987

The meaning of baseline as applied to measurements of CN, at atmospheric monitoring sites such as Cape Grim, is discussed and the idea that a baseline exists only in a statistical sense advocated. Use of the concentration frequency-distribution geometric-mean is proposed as the best measure of CN concentrations. Variations in CN concentrations at Cape Grim for periods greater than 12 months are derived by Kalman filtering the monthly estimates of geometric mean concentration obtained from 1977 to 1987. Significant variations are shown to be present on this timescale with quasi-periodic oscillations in concentration. Some association between major fluctuations and El-Nino Southern Oscillation perturbations in the regional circulation is suggested.     

Coal’s medium-run future under atmospheric greenhouse gas stabilization

We assess the future of coal under alternative climate stabilization regimes, investigating how the quantity and location of future coal production, trade and use depends upon five factors: the supply-side constraint of resource depletion, diversification and deepening of international trade, economic growth, trends in energy intensity, and the availability of coal-fired carbon-free electric generation technology (IGCC-CCS). Using the Phoenix computable general equilibrium model of the world economy, we find that coal is sensitive to demand-side assumptions about economic growth and energy-saving structural or technological change. In a 550 ppm stabilization emission tax scenario, the gobal coal industry initially declines sharply and then rebounds, in 2050 reaching roughly the same size as it is today—but only if IGCC-CCS is available by 2020. Under alternative stabilization regimes, IGCC-CCS penetration is a key influence on production and imports in major coal regions, where it interacts with extraction costs driven by the rate of depletion relative to trade partners.     

Application of theE-ε turbulence closure model to separated atmospheric surface-layer flows

Neutrally buoyant atmospheric surface-layer flow over a thin vertical wall has been studied using a turbulence closure scheme designed specifically to address flow problems containing high shears. The turbulent flow model consists of a general solution of the time averaged, steady state, twodimensional Navier-Stokes equations, where theE- turbulence model has been used to close the system of equations. Model output compares favorably with measurements made in both a full-scale field study and in an atmospheric wind tunnel. In the simulation of flow over a solid wall, two recirculation eddies are produced. The smallest eddy is located windward of the wall with a separation point located atx/h=–0.8, and the largest is located in the lee of the wall atx/h=5.8. Immediately downwind of the wall top, the turbulent kinetic energy, the energy dissipation rate, and the momentum flux all reach a local maximum. These peak values generally maintain their height positionz/h=1.0, but decrease progressively downwind. The turbulent viscosity is strongly modified under the influence of the wall, with a local maximum forming in the lee of the wall top, and a local minimum forming at a heightz/h=2.0 above the lee recirculation eddy. The surface momentum flux reduction due to the presence of the wall begins atx/h=–10.0. Minimum zero fluxes occur at the surface separation points, and a local peak in momentum flux is produced at the centers of each recirculation eddy. Downwind of the wall, the modeled surface flux reaches an equilibrium at roughlyx/h=30.0.     

Observational climatology and characteristics of wintertime atmospheric blocking over Ural–Siberia

This study investigates the climatological aspects and temporal characteristics of wintertime Ural-Siberian blocking (USB, centered over 30°–100°E), for the period 1980/1981–2009/2010. Sixty-eight events are identified and their physical structure is diagnosed using thermodynamic and geostrophic vorticity tendency equations. In climatology, horizontal advections play a fundamental role in constructing a USB event, in which the anticyclonic center is a warm core in the troposphere and a cold core in the lower stratosphere. The decay of the thermal structure is related to diabatic cooling along the vertical structure and warm advection in the lower stratosphere. Meanwhile, the collapse of the height structure is caused primarily by cyclonic vorticity advection. A strong interrelationship exists between the intensity and extension of USB events. The temporal characteristics of USB events are analyzed by examining strong and weak events, which are of high and low intensity. The strong events are probably preceded by an open ridge over Europe and a cyclogenesis over the Mediterranean Sea, and their formation is followed by the stronger amplification of a Rossby wave packet across Eurasia. On the other hand, the weak events are likely to be triggered by surface cold anomalies over Siberia. Overall, the evolution of a USB event forms a dynamic linkage with the Siberian high, in which the decay stage of the USB event is accompanied by a southeastward migration of the Siberian high and a subsequent cold air outbreak in East Asia. These results advance our understanding of USB and its relationship with East Asian winter monsoon activities.     

Variations of atmospheric 14C concentrations over the Alleroød-Younger Dryas transition

Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolution AMS ¹⁴C-dated records, Lake Go?ci??, and a floating Late Weichselian glacial varve chronology at the Allerød-Younger Dryas transition allowed us to assess in detail atmospheric Δ¹⁴C changes between late Allerød and early Preboreal. The combined data set shows a drastic rise in Δ¹⁴C during the first 200 years or so of Younger Dryas and the two following about 500 year-long ¹⁴C plateaux. Model experiments which included variations in the geomagnetic field, atmospheric CO₂ variations and a drastic reduction in North Atlantic Deep Water flux at the onset of Younger Dryas allowed to reproduce the distinct rise in Δ¹⁴C during the first 200 years of Younger Dryas fairly well. Also the drop in Δ¹⁴C at the Younger Dryas/Holocene boundary seems reasonably explained by changes in North Atlantic Deep Water circulation. However, the reason behind the anomalous behaviour of the Δ¹⁴C signal in the middle of Younger Dryas remains an open question.     

Is atmospheric carbon dioxide removal a game changer for climate change mitigation?

The ability to directly remove carbon dioxide from the atmosphere allows the decoupling of emissions and emissions control in space and time. We ask the question whether this unique feature of carbon dioxide removal technologies fundamentally alters the dynamics of climate mitigation pathways. The analysis is performed in the coupled energy-economy-climate model ReMIND using the bioenergy with CCS route as an application of CDR technology. BECCS is arguably the least cost CDR option if biomass availability is not a strongly limiting factor. We compare mitigation pathways with and without BECCS to explore the impact of CDR technologies on the mitigation portfolio. Effects are most pronounced for stringent climate policies where BECCS is a key technology for the effectiveness of carbon pricing policies. The decoupling of emissions and emissions control allows prolonging the use of fossil fuels in sectors that are difficult to decarbonize, particularly in the transport sector. It also balances the distribution of mitigation costs across future generations. CDR is not a silver bullet technology. The largest part of emissions reductions continues to be provided by direct mitigation measures at the emissions source. The value of CDR lies in its flexibility to alleviate the most costly constraints on mitigating emissions.     

Spatial response of two European atmospheric circulation classifications (data 1901–2010)

Air pressure field and circulation pattern frequencies were investigated to (1) locate and compare positions of the underlying pressure fields, (2) analyse the spatial dimension of affected areas, (3) create schematic maps of important circulation types and (4) compare the classification types in their response to the data. Two manual classifications were used, selected for the length of their time series and their applicability to a larger region: the Grosswetterlagen classification (GWLc) and the Vangengeim–Girs classification (VGc). Their time series were correlated with a global set of gridded monthly sea-level pressure data. Results show the different conceptual orientation of VGc (hemispheric) and GWLc (continental). The highest correlation values and the largest affected areas are visible in winter, where patterns frequently extended into northern Africa and western Asia. Schematic maps, illustrating the average location of main pressure centres, are provided for basic classes of both classifications. Re-arranging GWLc subtypes increases the classifications comparability with the VGc. Analysis of moving correlation coefficients reveals high fluctuations in the relation of both classifications over time.     

Sea‐level response at Nain,Labrador, to atmospheric pressure and wind

Abstract

Two years of subtidal sea‐level data from Nain, Labrador, are analysed in terms of local atmospheric pressure and the two components of geostrophic wind or stress. Frequency‐dependent response coefficients are determined by multiple regression analysis involving inversion of the cross‐spectral matrix of the inputs. At very low frequencies the response to pressure is isostatic and the wind stress coefficients are consistent with those determined by Thompson et al. (1985) from analysis of a longer series of monthly means. There is very little change in the response between icy and ice‐free seasons. The wind, or stress, coefficients correspond to geostrophic set‐up by a narrow longshore current but do not show as much of an increase of phase lag with increasing frequency as expected. The pressure response is less than isostatic and lags as the frequency increases from zero to about 0.02 cph. Possible reasons for this are discussed. Removal of wind as well as pressure effects ffom the sea‐level data makes only minor changes to the monthly mean residual sea‐level.     

Linking Arctic sea‐ice and atmospheric circulation anomalies on interannual and decadal timescales

Abstract

The relationship between Arctic sea‐ice concentration anomalies, particularly those associated with the “Great Salinity Anomaly” of 1968–1982, and atmospheric circulation anomalies north of 45°N is investigated. Empirical orthogonal function (EOF) analyses are performed on winter Arctic ice concentration from 1954 to 1990, sea level pressure and 500‐hPa heights from 1947 to 1994, and 850‐hPa temperatures from 1963 to 1994. Variability on both interannual and decadal timescales is apparent in the time series of the leading winter EOFs of all variables. The first EOF of winter sea‐ice concentration was found to characterize the patterns of ice variability associated with the Great Salinity Anomaly in the northern North Atlantic from 1968–82. Spatial maps of temporal correlation coefficients between the time series of the first EOF of winter sea‐ice concentration and the winter atmospheric anomaly fields are calculated at lags of 0 and ±7 year. Maximum correlations were found to exist when the time‐series of this ice EOF 1 leads the atmospheric anomaly fields by one year. A particularly interesting result is the connection between the presence of ice anomalies in the Greenland and Barents Seas and subsequent pressure anomalies of the same sign over the Irminger Basin and the Canadian Arctic. The main emphasis of the paper is to identify connections between Arctic sea‐ice and atmospheric circulation anomalies at interannual time‐scales.     

Phase‐locked and asymmetric correlations of the wintertime atmospheric patterns with the ENSO

Abstract

Teleconnections between sea surface temperature (SST) anomalies over the Pacific and the dominant patterns of wintertime Northern Hemisphere 500‐hPa height are examined by applying statistical techniques such as rotated principal component analysis and composite analysis. It is shown that the Pacific/North American (PNA) patterns in December through March are correlated most significantly with the ENSO‐related SST anomalies in the previous October, while the western Pacific (WP) patterns in December through February are most closely linked to the ENSO‐related SST anomalies in the same season. In addition, the PNA response to the ENSO signal during La Niña events is more significant than that during El Niño events, while the WP response is stronger during El Niño events than during La Niña events. A composite analysis shows that in the El Niño winters the North Pacific centre of the PNA pattern is located about 10 degrees east of its normal position, leading to a less significant correlation between the ENSO signal and the PNA pattern in these winters.

The ENSO‐related SST anomalies include a large centre of action over the tropical Pacific and an oppositely signed anomaly centre over the North Pacific. The North Pacific centre appears to the west of the dateline in September and October. This ENSO‐related seed of SST anomalies slowly moves eastward in the following months, gradually cutting off its connection with SST anomalies over the tropical Pacific and being coupled with the PNA pattern. It is pointed out that, although the wintertime SST anomaly over the North Pacific may appear as a mode linearly independent of the ENSO signal in the same season, it is partially related to the ENSO signal in the preceding autumn.

Possible dynamical explanations of the above results are discussed. It is suggested that the WP pattern can be linked to the tropical Pacific heat source via advection of vorticity by the upper‐tropospheric divergent/convergent flow, and the intensification of vorticity gradients associated with a stronger east Asian jet is likely to be responsible for a more significant WP pattern response to the ENSO signal in the El Niño winters. On the other hand, the ENSO‐related PNA pattern could be considered a manifestation of the eastward extension (El Niño) or westward withdrawal (La Niña) of the east Asian jet stream due to the local Hadley cell over the Pacific. In addition, the ENSO‐related seed of extratropical SST anomaly over the western Pacific in autumn may also play an important role in the development of the PNA pattern in the following winter.     

The use of σ T as a temperature scale in the atmospheric surface layer

The standard deviation of temperature _T is proposed as a temperature scale and as a velocity scale to describe the behaviour of turbulent flows in the Atmospheric Surface Layer (ASL), instead of _* andu _* of the Monin—Obukhov similarity theory, and ofT _f andU _f used for free convection stability conditions. On the basis of experimental evidence reported in the literature, it is shown that _T T _f andv _* U _f in the free convection region, and _{T *} andv _* U _* in nearneutral and stable conditions. This implies that the proposed scales can be applied for all stabilities. Furthermore, a new length scale is proposed and its relation with Obukhov length is given. Also, a simple semi-empirical expression is presented with which _T andv _* can be evaluated in a rather simple way. Some examples of practical applications are given, e.g., a stability classification for unstable conditions.     

The skill of atmospheric linear inverse models in hindcasting the Madden–Julian Oscillation

A suite of statistical atmosphere-only linear inverse models of varying complexity are used to hindcast recent MJO events from the Year of Tropical Convection and the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation mission periods, as well as over the 2000–2009 time period. Skill exists for over two weeks, competitive with the skill of some numerical models in both bivariate correlation and root-mean-squared-error scores during both observational mission periods. Skill is higher during mature Madden–Julian Oscillation conditions, as opposed to during growth phases, suggesting that growth dynamics may be more complex or non-linear since they are not as well captured by a linear model. There is little prediction skill gained by including non-leading modes of variability.     

The 1958�C2009 Greenland ice sheet surface melt and the mid-tropospheric atmospheric circulation

In order to assess the impact of the mid-tropospheric circulation over the Greenland ice sheet (GrIS) on surface melt, as simulated by the regional climate model MAR, an automatic Circulation type classification (CTC) based on 500?hPa geopotential height from reanalyses is developed. General circulation correlates significantly with the surface melt anomalies for the summers in the period 1958?C2009. The record surface melt events observed during the summers of 2007?C2009 are linked to the exceptional persistence of atmospheric circulations favouring warm air advection. The CTC emphasizes that summer 500?hPa circulation patterns have changed since the beginning of the 2000s; this process is partly responsible for the recent warming observed over the GrIS.     

Time changes of atmospheric precipitation in Russia from the corrected data during 1936–2000

Homogeneous time series of atmospheric precipitation with corrected systematic errors of measurements at 100 stations in Russia for the period of 1936–2000 are obtained. Combined effects are considered of all kinds of systematic errors of standard network precipitation-measuring instruments (the raingauge with the Nifer shield and the Tret’yakov raingauge) on the measured precipitation totals. Comparative analysis is carried out of the measured and corrected long-term mean characteristics of precipitation amounts (annual totals, warm and cold season totals, and different types of precipitation). On the basis of the obtained archives of precipitation homogeneous time series, linear trends are estimated for the period under consideration with estimation of their statistical significance. Schematic charts are plotted and analyzed of time changes in the annual precipitation amounts and in the amounts of different types of precipitation.     

An intimate coupling of ocean–atmospheric interaction over the extratropical North Atlantic and Pacific

The inter-basin teleconnection between the North Atlantic and the North Pacific ocean–atmosphere interaction is studied using a coupled ocean–atmosphere general circulation model. In the model, an idealized oceanic temperature anomaly is initiated over the Kuroshio and the Gulf Stream extension region to track the coupled evolution of ocean and atmosphere interaction, respectively. The experiments explicitly demonstrate that both the North Pacific and the North Atlantic ocean–atmosphere interactions are intimately coupled through an inter-basin atmospheric teleconnection. This fast inter-basin communication can transmit oceanic variability between the North Atlantic and the North Pacific through local ocean-to-atmosphere feedbacks. The leading mode of the extratropical atmospheric internal variability plays a dominant role in shaping the hemispheric-scale response forced by oceanic variability over the North Atlantic and Pacific. Modeling results also suggest that a century (two centuries) long observations are necessary for the detection of Pacific response to Atlantic forcings (Atlantic response to Pacific forcing).     

Characterization of atmospheric aerosols across Canada from a ground‐based sunphotometer network: AEROCAN

Abstract

Ground‐based sunphotometry measurements acquired under clear sky conditions can be used to investigate atmospheric aerosol optical properties. Such measurements are not only important in their own right as a technique for monitoring generic aerosol dynamics, but also represent a direct means of evaluating the contribution of aerosol induced radiative forcing in the modelling of climate change. In this paper we analyze derived aerosol optical properties using datasets from the Canadian AEROCAN (AERosol CANada) sunphotometer network.

The AEROCAN network currently includes eight sunphotometers distributed across Canada at sites chosen in order to obtain a diverse sampling of continental, maritime and arctic aerosols. Some of these sites have been operational since 1993 as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). These instruments permit standard and automatic multi‐wavelength measurements of solar extinction radiance centred on the solar disk as well as sky radiance scans off the solar disk. These data yields aerosol optical depth, the Ångström exponent, aerosol particle volume size distribution, refractive index, column‐averaged single scattering albedo, and precipitable water vapour content.

Spatial and temporal trends of these parameters as well as observed inter‐correlations are discussed. The results demonstrate the utility and significance of these types of measurements and illustrate the potential applications of networked sunphotometry data.