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We present in Table 1 of this paper a series of free tropospheric radon profiles, obtained in the summer of 1994 on flights originating from Moffett Field, California (37.4°N, 122.0°W). These data were collected with the intent of acquiring a high-quality, statistically significant set of observations, in one season and at one location, suitable for use in the development and validation of three-dimensional global circulation and chemical transport models, as demonstrated in the companion paper [Stockwell, et al., this issue]. The profiles, which extend from the surface to 11.5 kilometers, were obtained on 11 flights flown in the period June 3 to August 16. A series of high-pressure whole-air samples (typically 11 or 12 per profile) were collected as the plane ascended to altitude, and analyzed immediately after each flight in a ground-based laboratory. A thorough discussion of sampling, analysis, calibration, and quality control procedures is provided.

Data assimilation is a vital technique in modern meteorology, allowing information from a wide range of observations to be combined to produce synoptic analyses of the atmospheric circulation. Data assimilation has a number of advantages over conventional analysis methods that stem from the use of a numerical model of the atmosphere as an integral part of the technique. The model provides a framework which allows information from observations to be combined with our understanding of the atmosphere, as embodied in the model, so that a consistent, evolving, three-dimensional picture of the atmospheric circulation may be constructed. The aim of this paper is to compare results from two different stratospheric data assimilation systems (from Goddard Space Flight Center and the U.K. Meteorological Office) to assess the reliability of analyses derived by using assimilation techniques. In particular, this paper concentrates on assessing the quality of the analyses for long-term transport studies. First, the zonal mean circulations are compared, with emphasis on the consistency of the residual circulation. Second, horizontal transport processes are compared, with particular emphasis on the representation of features in the neighborhood of the wintertime polar vortex. It is shown that the two analyses give a broadly consistent picture of the atmospheric circulation. Where differences are identified, they can often be attributed to particular shortcomings in one of the assimilation systems. The results presented here stress the usefulness of data assimilation for stratospheric studies.

[1] The aim of this paper is to define a method for determining reasonable estimates of rainfall modeled by global circulation models (GCMs) coupled with regional climate models (RCMs). The paper describes and uses two new procedures designed to give confidence in the interpretation of such rainfall estimates. The first of these procedures is the use of circulation patterns (CPs) to define quantile-quantile (Q-Q) transforms between observed and RCM-estimated rainfall (the CPs were derived from sea level pressure (SLP) fields obtained from reanalysis of historical daily weather in a previous study). The Q-Q transforms are derived using two downscaling techniques during a 20 year calibration period and were validated during a 10 year period of observations. The second novel procedure is the use of a double Q-Q transform to estimate the rainfall patterns and amounts from GCM-RCM predictions of SLP and rainfall fields during a future period. This procedure is essential because we find that the CP-dependent rainfall frequency distributions on each block are unexpectedly different from the corresponding historical distributions. The daily rainfall fields compared are recorded on a 25 km grid over the Rhine basin in Germany; the observed daily data are averaged over the grid blocks, and the RCM values have been estimated over the same grid. Annual extremes, recorded on each block during the validation period, of (1) maximum daily rainfall and (2) the lowest 5% of filtered rainfall were calculated to determine the ability of RCMs to capture rainfall characteristics which are important for hydrological applications. The conclusions are that (1) RCM outputs used here are good at capturing the patterns and rankings of CP-dependent rainfall; (2) CP-dependent downscaling, coupled with the double Q-Q transform, gives good estimates of the rainfall during the validation period; (3) because the RCMs offer future CP-dependent rainfall distributions that are different from the observed distributions, it is judged that these predictions, once modified by the double Q-Q transforms, are hydrologically reasonable; and (4) the climate in the Rhine basin in the future, as modeled by the RCMs, is likely to be wetter than in the past. The results suggest that such future projections may be used with cautious confidence.

As the most populated region of the world, Asia is a major source of aerosols with potential large impact over vast downstream areas, Papers published in this special section describe the variety of aerosols observed in China and their effects and interactions with the regional climate as part of the East Asian Study of Tropospheric Aerosols and their Impact on Regional Climate (EAST-AIRC), The majority of the papers are based on analyses of observations made under three field projects, namely, the Atmospheric Radiation Measurements (ARM) Mobile Facility mission in China (AMF-China), the East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE), and the Atmospheric Aerosols of China and their Climate Effects (AACCE), The former two are U,S,-China collaborative projects, and the latter is a part of the China's National Basic Research program (or often referred to as "973 project"), Routine meteorological data of China are also employed in some studies, The wealth of general and speCIalized measurements lead to extensive and close-up investigations of the optical, physical, and chemical properties of anthropogenic, natural, and mixed aerosols; their sources, formation, and transport mechanisms; horizontal, vertical, and temporal variations; direct and indirect effects; and interactions with the East Asian monsoon system, Particular efforts are made to advance our understanding of the mixing and interaction between dust and anthropogenic pollutants during transport. Several modeling studies were carried out to simulate aerosol impact on radiation budget, temperature, precipitation, wind and atmospheric circulation, fog, etc, In addition, impacts of the Asian monsoon system on aerosol loading are also simulated.

In the period following a geomagnetic storm the high-latitude, magnetospheric-driven convection pattern is normally weak. However, the neutral circulation, set up by ion-neutral momentum coupling during the main phase of the storm, may continue for several hours after the storm has ended. This persistent neutral circulation has the potential to drive Hall currents for some hours. In this paper we investi- gate these "flywheel" currents by simulating a storm which occurred on the 23 rd of November 1982 using the National Center for Atmospheric Research Thermosphere Ionosphere General Circulation Model (NCAR-TIGCM). The resulting high-latitude, height-integrated Hal/currents are dominated by the neutral-wind-driven component for several hours after the end of main phase of the storm. The direction of these currents is reversed from normal. Analysis of the neutral and ion components of this current system indicates that the neutral component may drive as much as 80% of the high-latitude current system immediately after the storm has ended, and may continue to dominate this system for 4 to 5 hours. stant in the E-region for the transfer of momentum from the plasma to the neutral gas implies that the neutral winds that were forced by the ions during the main phase of the geomag- netic storm can persist for a long time after the cessation of magnetospheric-driven forcing, and may drive significant ionospheric and magnetospheric field-aligned currents over the polar cap in a way that is similar to that of the dynamo in low and middle latitudes. Lyons et al. (1985) studied these winds and they showed that, in practice, the neutral circulation in the high-latitude E region can drive a significant Hall current system for up to six hours after the cessation of strong magne- tospheric convection - the so-called neutral "flywheel" effect. Some experimental support for this concept exists in the results published by several authors (e.g. Maezawa, 1976; Zanetti, 1984), who showed that ionospheric currents can reverse direction when B z is northward. In this paper we extend the Lyons et a1.(1985) study to investigate the link between the neutral circulation and these reversed currents during the period following a geomagnetic storm.

[1] Climate research relies on realistic atmospheric data over long periods of time. Global reanalyses or observations are commonly used for this type of work. However, the many problems associated with both the reanalyses and observations cast doubts on the reliability of such data for climate applications, and users often need to know how large the errors and uncertainties associated with the different data sets are. This paper is a systematic assessment of the errors and uncertainties contained in the time mean (1979–1999) of many different climate quantities taken from a variety of global data sets, including four popular reanalyses, the output of the climate model developed at the Geophysical Fluid Dynamics Laboratory (GFDL), and a wide range of observations. We find that the ability of reanalyses to reproduce the observed climate mean state varies widely, with radiative quantities exhibiting the largest discrepancies. The different reanalysis products share many common errors, but overall the European Centre for Medium-Range Weather Forecasts 40-year reanalysis (ERA-40) matches best the observations. Interestingly, the climate model reproduces the observed climate mean state of certain quantities more faithfully than the reanalyses. This indicates that modern models have reached a high level of realism in their mean state and that care must be taken when reanalyses are used to validate models. A particular concern of this paper is the time mean uncertainty associated with specific observation-based atmospheric quantities. Observational uncertainties are estimated from the difference amongst alternative data sets for the same quantity. We show that for most quantities the observational uncertainty is smaller than the error of the reanalyses or the model. However, there are some notable exceptions. In particular, for the surface fluxes of heat, momentum, and radiation the observational uncertainties can be as large as the errors seen in the reanalyses or the model. The investigation of uncertainties in upper atmospheric quantities is restricted to reanalysis and model data, since no appropriate observations are available. In this case, the reanalyses uncertainties are generally smaller than the model errors, except for quantities which describe the meridional component of the atmospheric circulation.

[1] Vertical profiles of water vapor concentration at high latitudes (57-72 degreesN; 64-89 degreesS) were observed by the Improved Limb Atmospheric Spectrometer (ILAS) solar occultation sensor aboard the Advanced Earth Observing Satellite (ADEOS). These measurements were made continuously from November 1996 through June 1997 with some additional periods in September to October 1996. A validation study of the water vapor data processed with the version 5.20 ILAS retrieval algorithm is presented in this paper. Uncertainty and general characteristics of the ILAS water vapor measurements are briefly reviewed. Comparisons are made with data obtained by (1) the ILAS validation balloon campaigns at Kiruna, Sweden and at Fairbanks, Alaska; (2) the aircraft measurements under the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign; and (3) available satellite measurements of the version 19 Halogen Occultation Experiment (HALOE) and the version 6 Stratospheric Aerosol and Gas Experiment II (SAGE II). The agreement between ILAS water vapor and all independent reliable correlative measurements in the altitude region of 15-60 km is better than 10% for the majority of cases and better than 20% for all comparisons, with the exception of some isolated cases detailed in this paper. Climatological comparisons of ILAS data with Upper Atmosphere Research Satellite (UARS) climatology and HALOE data show the overall consistency of ILAS water vapor data considering the known features of atmospheric circulation. The characteristics of ILAS measurements, i.e., high sampling frequency in polar latitudes with high vertical resolution along with the good data quality, make the ILAS water vapor data set valuable for various polar stratospheric research applications.

Vb cyclones typically emerge in the Western Mediterranean and propagate to the Northeast into Central Europe. This paper explores the temporal characteristics of Vb cyclone occurrence based on cyclone tracks identified at the atmospheric levels of Z700 and sea level pressure, using JRA‐55 reanalysis data for the period 1959–2015. The risk of Vb occurrence was significantly high in the 1960s and has remained at a lower level since then. Vb cyclones do not occur fully randomly according to a Poisson point process. Eleven well‐separated and distinct clusters as well as 11 hiatus periods are identified, with average occurrence rates of 21.5 and 5.2 yrea−1, respectively. During the event of Vb, the large‐scale atmospheric circulation is changed into a state favoring the development of successive Vb cyclones. Clustering is very prominent in the case of Genoan Vb cyclones in summer as well as those Vb cyclones developing over the Iberian Peninsula or the North African Coast in winter. Superposition of the polar and the subtropical jet stream over the Western Mediterranean is identified as a main feature at the onset of Vb cyclones. Vb cyclone occurrence appears to be synchronized with the Northern Atlantic Oscillation (NAO; at Z500) and Arctic Oscillation (AO; at Z1000). Clusters have occurred when both NAO and AO were negative. This relation applies to Western Mediterranean cyclones not following a Vb track as well, however to a much weaker extent. In contrast, Vb cyclone frequency was particularly low from 1988 to 1997 during a sustained positive phase of both NAO and AO., Key Points In the past 50 years, the frequency of Vb cyclones was high in the 1960s and lower since thenVb cyclones were strongly synchronized with NAO and AO; high frequency clusters occurred when NAO and AO were negativeThe coupling of the polar and the subtropical jet stream over the Western Mediterranean is a driving mechanism of the onset of Vb cyclones

Californian hydroclimate is strongly seasonal and prone to severe water shortages. Recent changes in climate trends have induced shifts in seasonality, thus exacerbating droughts, wildfires, and adverse water shortage effects on the environment and economy. Previous studies have examined the timing of the seasonal cycle shifts mainly as temperature driven earlier onset of the spring season. In this paper, we address quantitative changes in the onset, amounts, and termination of the precipitation season over the past 6 decades, as well as the large-scale atmospheric circulation underpinning the seasonal cycle changes. We discover that the onset of the rainy season has been progressively delayed since the 1960s, and as a result the precipitation season has become shorter and sharper in California. The progressively later onset of the rainy season is shown to be related to the summer circulation pattern extending into autumn across the North Pacific, in particular, a delay in the strengthening of the Aleutian Low and later southward displacement of the North Pacific westerlies. Plain Language Summary The rainy season over California is projected to show a distinct sharpening of the mean seasonal cycle, with winter precipitation increasing, and both autumn and spring precipitation decreasing. Our analysis of the past 6 decades of data for California suggests autumn decrease is already underway. A delayed start of the rainy season of 27 days since 1960s can exacerbate seasonal droughts and prolong the wildfire season. This delay occurs due to a number of conditions that controls precipitation: the summer circulation pattern has been extending throughout November across the North Pacific, and the wintertime strengthening of the Aleutian Low is delayed. Accordingly, the southward migration of the North Pacific jet stream as well as extratropical storm tracks, which marks the start of the California rainy season, are delayed. More work, using climate models, will be needed to provide a better understanding of atmospheric conditions across the North America and the North Pacific. However, our findings provide observational evidence for the projected rainfall change over California and inform ongoing discussion about the drying/wetting tendencies of the rainy season in California.

Recent years have seen growing interest in improving seasonal predictions of Arctic sea ice conditions, including the timing of ice melt onset and retreat, especially on the regional scale. This paper investigates potential links between regional sea ice melt onset and retreat in the southern Laptev Sea and retreat of terrestrial snow cover. Past studies have shown that variability of snow extent over Eurasia can substantially impact regional atmospheric circulation patterns over the North Pacific and Arctic Oceans. It is shown here that for the Laptev Sea, earlier melt onset and retreat of sea ice are encouraged by earlier retreat of snow cover over the West Siberian Plain. Earlier snow retreat in spring encourages greater ridging (e.g., at 500 hPa) over the East Siberian Sea through the summer. This results in more frequently southerly flow of warm, moist air over the Laptev Sea. This relationship could provide modest improvements to predictive skill for sea ice melt onset and retreat in the southern Laptev Sea at lead times of approximately 50 and 90 days, respectively. The detrended time series of snow retreat in the West Siberian Plain explains 26 and 29% of the detrended variance of the timing of sea ice melt onset and retreat in southern Laptev Sea, respectively.

This paper examines the characteristics of the heatwave that affected western and central Europe in June 2017. Using a novel algorithm, we show that its extension, intensity, and persistence were comparable to those of other European mega‐heatwaves, but it occurred earlier in the summer. The most affected area was Iberia, which experienced devastating forest fires with human casualties and the warmest temperatures of the reanalysis period from daily to seasonal scales. The peak of the mega‐heatwave displayed an unprecedented warm air intrusion due to a record‐breaking subtropical ridge with signatures closer to those of July and August. The atmospheric circulation was the main triggering factor of the event. However, thermodynamical changes of the last decades made a substantial contribution to the event, by increasing the likelihood of surpassing high‐temperature thresholds. This episode could be a good example of a coming future, with high‐summer mega‐heatwaves occurring earlier., This work was supported by the Ministerio de Economía y Competitividad through the PALEOSTRAT (CGL2015‐69699‐R) and ISIPEDIA (ERA4CS) (PCIN‐2017‐046) projects. Ricardo M. Trigo and Pedro M. Sousa are supported by project IMDROFLOOD: Improving Drought and Flood Early Warning, Forecast and Mitigation using real‐time hydroclimatic indicators, JPND‐WaterJPI/0004/2014, financed by FCT/MCTES (PIDDAC). A. Sánchez‐Benítez was funded by grant FPU15/03958 from the Ministerio de Educación, Cultura y Deporte (MECD).

The east coast of Australia has a relatively high frequency of midlatitude cyclones, locally known as East Coast Lows (ECLs), which can cause severe weather including widespread flooding and coastal erosion. The elevated topography close to the east coast has been hypothesized to play a role in both the genesis and impacts of cyclones in this region, but existing studies have been limited to case studies of individual events. In this paper we present the results from two 20-year WRF simulations over the Australian region, and assess the results from removing all topography in the region on both mean atmospheric circulation and ECL frequency. Removing topography results in an increase in sea level pressure to the south of Australia and an increase in moisture flux convergence and rainfall near the east coast, as well as a decrease in potential vorticity to the north of the ECL region. This results in a change in the spatial distribution of cyclones, with a 37% decrease in the frequency of cyclones that develop to the south of the ECL region but a 20% increase in cyclones near the east coast. This results in little overall change in the frequency of ECLs, and suggests coarse topography is unlikely to be responsible for the difficulties in simulating coastal cyclones in global climate models.

This paper was intended as an introductory and superficial survey of the earth's neutral upper atmosphere, mainly at heights between 100 and 1000 km, to prepare for the more specialized papers which followed. The density, temperature, and winds and their variations are briefly discussed.

[1] Atmospheric Rivers (ARs) are being increasingly identified as associated with some extreme floods. More generally, such floods may be associated with tropical moisture exports (TMEs) that exhibit relatively robust teleconnections between moisture source regions and flood regions. A large-scale flood event that persisted over Western Europe in January 1995 is studied here. During the last 10 days of the month, two rare flooding events, associated with heaviest rainfall in 150 years, occurred in two places, one over Brittany (West of France), and the second in the France-Germany border region and parts of neighboring countries. In this paper, we explore the month-long evolution of TMEs and their connection to the precipitation events that led to the Brittany event. The persistent large-scale atmospheric circulation patterns that led to the birth, death, and evolution of these TMEs as ARs with landfalls in Western Europe are identified, and the relationship of daily extreme precipitation to these patterns is examined. Singular value decomposition analysis and a generalized linear model are used to assess whether knowledge of the atmospheric circulation patterns from the prior record is useful for explaining the occurrence of their rare events. The analysis establishes the importance of both global and regional atmospheric circulation modes for the occurrence of such persistent events and the hydrologic importance of diagnosing global atmospheric moisture pathways.

[1] The meteorological excitation of Mars' rotation refers to the change of Mars' orientation parameters caused by meteorological processes, e.g., that portion due to variations of atmospheric circulation and surface pressure, and a cryospheric part due to the variable Martian polar ice caps associated with the CO2 sublimation/condensation effects. Global dust storms (GDSs) are unique meteorological events on Mars that cause changes in surface pressure, polar caps extent, and circulation patterns on a global scale, which in turn alter Mars' rotation under the conservation of Mars' total angular momentum. In this paper, the GDS signal in the meteorological excitation of Mars' rotation is investigated on seasonal, diurnal, and semidiurnal time scales, based on the Martian Climate Database. On seasonal scales, the cryospheric excitation plays a less important role in general, due to its location at high latitudes, than its atmospheric counterpart. The GDS brings (−3%, +15%) changes in the amplitude of annual polar motion and length-of-day (LOD) variation and (+22%, −34%) changes in the amplitude of semiannual polar motion and LOD variation. On diurnal and semidiurnal scales, the cryospheric excitation can be fully neglected in comparison to its atmospheric counterpart. A significant GDS signal is found with respect to the diurnal polar motion and semidiurnal LOD excitation variation, because of strong atmospheric tides arising from GDS effects. The amplitude increases in diurnal polar motion and semidiurnal LOD variation during some periods of the Martian year may reach 3–5 times as large as those during the same stage in a typical Martian year.

[1] This paper investigates the effect of Last Glacial Maximum (LGM) versus high CO2 world boundary condition on the Southern Hemisphere atmospheric circulation, in particular on the strength and latitudinal position of the near surface Southern Westerly Winds (SWW). PMIP2 and PMIP3 experiments, as well as the “abrupt 4 × CO2” simulations from CMIP5, were analyzed. Robust findings include poleward expansion of the Mean Meridional Circulation (MMC) and intensified and poleward-shifted SWW in the 4 × CO2 simulations (consistent with recent observations and 21st century climate change projections); and for the LGM, stronger and southward shifted northern hemisphere MMC, and weakened southern Hadley cell. However, six of the eight LGM simulations show a decrease in the SWW, the other two models simulate the opposite. A critical difference between the models is strong coupling between sea-ice extent, surface temperature gradients, SWW, and Ferrel cell in the two models with stronger and poleward-shifted SWW.

To determine the complex dependencies of currents and electric fields within the Global Electric Circuit (GEC) on the underlying physics of the atmosphere, a new modeling framework of the GEC has been developed for use within global circulation models. Specifically, the Community Earth System Modeling framework has been utilized. A formulation of atmospheric conductivity based on ion production and loss mechanisms (including galactic cosmic rays, radon, clouds, and aerosols), conduction current sources, and ionospheric potential changes due to the influence of external current systems are included. This paper presents a full description of the calculation of the electric fields and currents within the model, which now includes several advancements to GEC modeling as it incorporates many processes calculated individually in previous articles into a consistent modeling framework. This framework uniquely incorporates effects from the troposphere up to the ionosphere within a single GEC model. The incorporation of a magnetospheric potential, which is generated by a separate magnetospheric current system, acts to modulate or enhance the surface level electric fields at high-latitude locations. This produces a distinct phasing signature with the GEC potential that is shown to depend on the observation location around the globe. Lastly, the model output for Vostok and Concordia, two high-latitude locations, is shown to agree with the observational data obtained at these sites over the same time period.

One frequently observed effect of thermospheric storms is the reduction of atomic oxygen relative to molecular nitrogen at high and middle latitudes. These composition changes lead to a decrease in thermospheric O I 130.4-nm emissions in the sunlit hemisphere. Such decreases have been observed by various satellite-based instruments including the Dynamics Explorer 1 (DE 1) Spin-Scan Auroral Imager. In contrast, this paper focuses on enhancements of the terrestrial 130.4-nm dayglow emission observed with DE 1. Following the onset of a geomagnetic storm at 1610 UT on February 5, 1983, an increase (>20%) in the O I 130.4-nm emission was observed at middle latitudes in the morning sector of the Southern Hemisphere. The increased O I 130.4-nm emission indicates an increase of atomic oxygen relative to molecular nitrogen. The brightness enhancement coincided with the passage of a large-scale gravity wave that was observed in measurements from a global network of ionosondes. The global FUV images and ionosonde observations are complemented by a TIMEGCM simulation of the February 1–6 period, which provides a framework for combining the ionosonde and DE 1 observations. The primary mechanisms for the FUV variations considered here are increases in the relative abundance of atomic oxygen in the morning sector caused by (1) the large-scale gravity wave launched by the onset of magnetic activity and (2) corotation of previously affected parcels onto the dayside. The investigation leads to a physical interpretation of the observed FUV features in terms of the gravity wave effects and a transient Hadley circulation cell. This work represents the first detection of a large-scale gravity wave from an orbiting FUV imager.

Snow water equivalent anomalies (SWEA) measured around April 1 by stations in the Columbia basin area in British Columbia, Canada, were studied for their interannual variability during the period 1950–1999, particularly in relation to El Niño/La Niña events and to high and low Pacific–North American (PNA) atmospheric circulation patterns. Composites of the SWEA showed that SWEA were negative during El Niño years, positive during La Niña years, negative during high PNA years, and positive during low PNA years. High PNA appeared to have the most impact on the SWEA, followed by La Niña, El Niño, and low PNA. In the Columbia basin area, La Niña effects (relative to El Niño effects) on SWEA decrease northward and eastward but strengthen with elevation. Composites of the Pacific sea surface temperature anomalies (SSTA) during the 10 lowest SWEA years revealed weak signals, with El Niño warm SSTA present only during spring and early summer in the preceding year and the SSTA pattern consistent with a high PNA present by fall and winter. In contrast, composites of the SSTA during the 10 highest SWEA years showed strong La Niña cool SSTA starting around May in the preceding year and lasting onto winter. An edited version of this paper was published by AGU. Copyright 2001 American Geophysical Union.

An advanced algorithm to retrieve the radiative properties of terrestrial surfaces sampled by the Meteosat visible instrument was derived in a companion paper [Pinty et al., this issue]. Preliminary applications of this algorithm against a limited set of Meteosat data is performed and the required procedures to screen “clear-sky” conditions only and to retrieve the “likely” solution of the inverse problem are presented and evaluated. The accumulation of results over two periods of 20 days each during the Northern Hemisphere summer and winter permits establishing sample geophysical maps of the algorithm products, including the surface albedo (i.e., directional hemispherical reflectance factors) over the entire African continent. The seasonal albedo changes occurring at a continental scale are interpreted on the basis of the most prominent environmental factors, namely the atmospheric circulation controlling the seasonal monsoon events and the biomass burning activities. The results of this study, supported by additional radiation transfer simulations, suggest that anthropogenic fire activities induce significant perturbations of the surface albedo values in the intertropical zones at the continental scale.

Signatures of the North Atlantic Oscillation (NAO) are widely detected in climatic variability, particularly in the extratropical latitudes surrounding the North Atlantic Ocean. However, it is still controversial whether the NAO is the most important pattern of nonseasonal variability of atmospheric circulation related to precipitation over Europe. This paper is an attempt to contribute to this issue. The spatial modes of nonseasonal variability of monthly fields of mean sea level pressure (MSLP) over the northeastern Atlantic and western Europe and precipitation over Europe are investigated mainly by using principal component analysis. The relationships between the two fields are studied via canonical correlation analysis (CCA). The data sets used refer to the period from 1911 to 1990. The most important spatial mode of MSLP is the NAO pattern, its corresponding principal component being closely related to the NAO index. Interestingly however, the NAO pattern seems to be responsible only for the second empirical orthogonal function in precipitation, while the most important spatial mode of precipitation corresponds to the third EOF of MSLP (North Sea pattern). Furthermore, the second EOF of MSLP (Scandinavian pattern) is highly associated with the third EOF of precipitation. Significant pairs of canonical patterns between the MSLP and precipitation fields obtained from canonical correlation analysis are coherent with the conclusions above. These results can potentially be used to assess possible changes of precipitation over Europe due to increasing greenhouse gases based on the variability of MSLP simulated by general circulation models.

During the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) intensive observing period (IOP) (November 1992 to February 1993), two pronounced Madden-Julian oscillation (MJO) events were observed and have been intensively studied. To provide a necessary climatological basis for the case study as well as to evaluate how typical the COARE MJO cases are and to what degree conclusions made from these cases can be generalized, we compare the MJOs that occurred during the COARE IOP with a 15-year climatology. In this paper, along with a description of the characteristics of large-scale background including flow patterns, OLR and SST, the emphasis is placed on the intensity and propagation of the oscillations exhibited in organized convection and atmospheric circulation. Moreover, the maintenance of perturbation kinetic energy (PKE) associated with 30- to 60-day variability containing the MJOs is compared as well. The COARE IOP occurred in a lingering period between the persistence and the rejuvenation of the warm ENSO episode of 1992–1993. The intraseasonal oscillations in the convective activity and large-scale circulation are intensified during the IOP, and the maximum intensification is located in the region of central Pacific. Two MJO events in the COARE IOP are typical in terms of phase propagation. The major features of the energy transformation processes, such as the predominant role of the generation of the perturbation available potential energy (PAPE) by convective heating and its conversion into the PKE in the maintenance of the MJOs, found in the COARE IOP, are also displayed in the climatology indicating the generality of the results obtained from the COARE case study.

In this study we determined characteristic temporal modes of atmospheric variability at the decadal and interdecadal timescales. This was done on the basis of 1000 year long integrations of a global coupled atmosphere-ocean climate model of moderate complexity including the troposphere, stratosphere, and mesosphere. The applied model resolves explicitely the basic features of the large-scale long-term atmospheric and oceanic variables. The synoptic-scale processes are described in terms of autocorrelation and crosscorrelation functions. The paper includes an extended description and validation of the model as well as the results of analyses of two 1000 year long model integrations. One model run has been performed with the fully coupled model of the atmosphere-ocean system. The performed time-frequency analyses of atmospheric fields reveal strong decadal and interdecadal modes with periods of about 9, 18, and 30 years. To quantify the influence of the ocean on atmospheric variations an additional run with seasonally varying prescribed sea surface temperatures has been carried out, which is characterized by strong decadal modes with periods of about 9 years. The comparison of both runs suggests that decadal variability can be understood as an inherent atmospheric mode due to the nonlinear dynamics of the large-scale atmospheric circulation patterns whereas interdecadal climate variability has to be regarded as coupled atmosphere-ocean modes.

This paper documents significant long-term changes in the tropospheric circulation and progression of the seasonal cycle over the South Pole and relates aspects of the changes to the enhanced springtime stratospheric ozone depletion of recent decades. During the winter-to-summer transition over Antarctica, the formation of the thermal tropopause above 150 hPa is now delayed by a month compared to the early 1960s. A similar delay occurs in the breakdown of the winter polar stratospheric vortex, a prerequisite to the formation of the strong summertime tropopause near 300 hPa. A delay in tropopause formation implies delays in those aspects of the transition from winter to summer in the dynamics of the troposphere that are sensitive to its upper boundary condition, including the contraction of the Rossby radius of deformation during the shallow troposphere phase of summer. Consistent with this expectation, the normal springtime decline of baroclinic-eddy activity in the lower troposphere now occurs later by a month. Similar delays occur in the time of an early summer minimum in total cloud fraction that coincides with the time of the maximum stratification between 150 and 100 hPa. In addition, the analyses reveal (1) an increase of 15 to 20% in spring and summer cloud fraction over the last four decades; (2) a separation of spring and summer cloudiness into two transport regimes with a distinct intervening minimum in cloud fraction; (3) a decadal modulation of springtime cloudiness that is coincident with decadal shifts in the directional mode of 300-hPa winds following solar minima; (4) a long-term increase in total inversion strength, inversion depth, and 650- to 300-hPa thickness; and (5) a long-term decrease in wintertime surface wind speed, surface temperature, and near-surface lapse rate. The study also introduces a covariance technique that exploits the control by continental-scale orography of synoptic and surface flow regimes, increasing the value of lower tropospheric observations for the study of climate change over Antarctica. Finally, a cautionary note arises from the apparent effect of the Weddell Polynyas and the reduction of the maximum extent of sea ice of the middle to late 1970s, with increased tropospheric and lower stratospheric temperatures and an earlier time of formation of the tropopause. This observation suggests that the occurrence of decadal and subdecadal variability in the ocean and the atmosphere of the Southern Hemisphere may obscure the detection of long-term trends in the circulation over Antarctica.

The Smoke, Clouds, and Radiation-Brazil (SCAR-B) field experiment was conducted in central Brazil and southern Amazonia during the period August 15 to September 20, 1995. This paper presents an overview of atmospheric conditions during the SCAR-B period. A meteorological office was established in Brasilia to support the mission flights and field activities. All the meteorological data collected during the SCAR-B have been archived and are available to the scientific community. The meteorological conditions throughout the SCAR-B period of August and September 1995 were near climatological conditions. Few synoptic-scale waves traveling from southeastern Pacific reached Brazil, therefore the SCAR-B period was characterized by long periods of low humidity, dryness, little cloudiness, or rain, and haze events. The stable situation was interrupted due to the penetration of a frontal system on September 20, 1995, and a second and more rain-producing one on September 28, 1995. Those two frontal systems in a period of 10 days brought an end to the dry season. Air particle trajectories show that most of the smoke below 4 km from Amazonia was transported to the south and then to the east exiting the continent at 25°S–30°S. Radiative transfer calculations carried out for the clear sky gaseous and aerosol atmosphere show the decrease of the solar radiation absorption in the atmosphere-surface system due to smoke aerosol loading at the value from 5 to 50 W m−2 depending on the aerosol optical thickness, single-scattering albedo, and solar zenith angle.

This paper analyzes the impact of the 1997–1998 El Nino on tropospheric column ozone and tropospheric water vapor derived respectively from the Total Ozone Mapping Spectrometer (TOMS) on Earth Probe and the Microwave Limb Scanning instrument on the Upper Atmosphere Research Satellite. The 1997-1998 El Nino, characterized by an anomalous increase in sea-surface temperature (SST) across the eastern and central tropical Pacific Ocean, is one of the strongest El Nino Southern Oscillation (ENSO) events of the century, comparable in magnitude to the 1982–1983 episode. The major impact of the SST change has been the shift in the convection pattern from the western to the eastern Pacific affecting the response of rain-producing cumulonimbus. As a result, there has been a significant increase in rainfall over the eastern Pacific and a decrease over the western Pacific and Indonesia. The dryness in the Indonesian region has contributed to large-scale burning by uncontrolled wildfires in the tropical rainforests of Sumatra and Borneo. Our study shows that tropospheric column ozone decreased by 4–8 Dobson units (DU) in the eastern Pacific and increased by about 10–20 DU in the western Pacific largely as a result of the eastward shift of the tropical convective activity as inferred from National Oceanic and Atmospheric Administration (NOAA) outgoing longwave radiation (OLR) data. The effect of this shift is also evident in the upper tropospheric water vapor mixing ratio which varies inversely as ozone (O3). These conclusions are qualitatively consistent with the changes in atmospheric circulation derived from zonal and vertical wind data obtained from the Goddard Earth Observing System data assimilation analyses. The changes in tropospheric column O3 during the course of the 1997–1998 El Nino appear to be caused by a combination of large-scale circulation processes associated with the shift in the tropical convection pattern and surface/boundary layer processes associated with forest fires in the Indonesian region.

During the Tropical Ocean-Global Atmosphere (TOGA) program substantial progress was made in the development of coupled general circulation models with regard to representation of the tropical mean state and climate variability. This paper provides a review of the main developments, focusing on the tropical Pacific region. Early coupled general circulation models were relatively crude; with coarse resolution and limited physical parameterizations and poor surface fluxes, the model drift from the observed mean state was often substantial, and their use for investigating climate variability such as El Nino-Southern Oscillation (ENSO) was limited. Improvements in resolution and parameterizations led to rapid progress. Through the TOGA program it has been possible to assess coupled model mean states and variability against high-quality observations, particularly for the tropical Pacific. Both components of the coupled system (ocean and atmosphere) have benefited from an improved understanding of the physics. Coupled experiments have revealed deficiencies in each model component that were concealed in separate forced runs. Several general systematic errors have yet to be eliminated, especially in the east Pacific. The variety of behavior obtained with coupled models provides evidence that more than one mechanism is active in the generation and evolution of ENSO events. There are also indications that interannual variability is linked to the mean structure of the equatorial Pacific.

The clear-sky greenhouse effect (GE) is determined primarily by the amount and vertical distribution of water vapor in the atmospheric column. GE hampers surface radiative cooling and is maintained through surface evaporative cooling. This paper examines the intimate space-time relationships between the patterns of radiative heating of the atmosphere and surface evaporative cooling. We use data derived from satellite and in situ observations to show that tropical maritime GE is decoupled in space and time from latent heat flux (LHF), its source of water vapor. Large scale transport of atmospheric water vapor responsible for the observed relationships between GE and LHF is discussed. The spatial patterns of average GE and LHF are imbedded in the Walker and Hadley circulations and reinforce these circulations with strong evaporative cooling in the subtropical highs and greenhouse warming in the equatorial trough zones. Throughout tropical areas characterized by strong seasonality, the seasonal cycles of GE and LHF are out of phase. Much of the moisture that feeds GE in these off-equatorial regions is advected by the Hadley circulation from tropical moisture source regions of the opposite hemisphere. An out-of-phase relationship between GE and LHF also turns up on El Nino - Southern Oscillation timescales, most notably in the central tropical Pacific. The “super” greenhouse effect (SGE), a situation when GE absorption increases more than colocated surface longwave emission, is a seasonal feature of extensive tropical off-equatorial areas where it is maintained by moisture convergence and convection. On interannual timescales, the same dynamical processes appear to assert the SGE in the central equatorial Pacific. GE and LHF regimes are also described for the equatorial cold tongue and warm pool regions.

Well-documented present-day distributions of stable water isotopes (HDO and H218O) show the existence, in middle and high latitudes, of a linear relationship between the mean annual isotope content of precipitation (δD and δ18O) and the mean annual temperature at the precipitation site. Paleoclimatologists have used this relationship, which is particularly well obeyed over Greenland and Antarctica, to infer paleotemperatures from ice core data. There is, however, growing evidence that spatial and temporal isotope/surface temperature slopes differ, thus complicating the use of stable water isotopes as paleothermometers. In this paper we review empirical estimates of temporal slopes in polar regions and relevant information that can be inferred from isotope models: simple, Rayleigh-type distillation models and (particularly over Greenland) general circulation models (GCMs) fitted with isotope tracer diagnostics. Empirical estimates of temporal slopes appear consistently lower than present-day spatial slopes and are dependent on the timescale considered. This difference is most probably due to changes in the evaporative origins of moisture, changes in the seasonality of the precipitation, changes in the strength of the inversion layer, or some combination of these changes. Isotope models have not yet been used to evaluate the relative influences of these different factors. The apparent disagreement in the temporal and spatial slopes clearly makes calibrating the isotope paleothermometer difficult. Nevertheless, the use of a (calibrated) isotope paleothermometer appears justified; empirical estimates and most (though not all) GCM results support the practice of interpreting ice core isotope records in terms of local temperature changes.

An earlier study by Dickey et al. [1992] established the existence of globally coherent interannual fluctuations in atmospheric angular momentum (AAM), associated with the El Nino-Southern Oscillation (ENSO) cycle. In this paper, we pursue the origin and the structure of these fluctuations using an ensemble of experiments generated by the National Centers for Environmental Prediction, medium range forecast model version 9. In the control experiments, where the observed sea surface temperatures (SSTs) were used as the lower boundary conditions, the model captures the characteristic V-like structure in time-latitude plots of zonally averaged AAM found by Dickey et al., while experiments with climatological SSTs and those with either perpetual warm or cold ENSO conditions superimposed on the climatological SSTs failed to reproduce this structure. The numerical results indicate that these AAM structures are related to SST variations associated with transitions between different phases of the ENSO cycle and have both propagating and standing components. The largest zonal wind contribution from the levels studied (850, 500, and 200 hPa) is at 200 hPa, where the tropical convective outflow is the strongest. Composites of zonal wind and geopotential height show a clear relationship between the stages of the global AAM oscillation and the ENSO cycle. The strong similarity between the simulated and observed AAM series attests to the model's ability to realistically simulate the interannual response of the atmosphere to ENSO SST anomalies.

This paper reports on a successful general circulation model simulation of a rapid warming phenomenon observed in the Martian winter polar atmosphere during global dust storm conditions. The model includes a self-consistent simulation of the dust distribution which is forced with a prescribed surface source. The warming is shown to be largely a response to the development of a pole-to-pole solstitial Hadley circulation resulting from the greatly increased dust loading. A crucial aspect of the simulation is a sufficiently deep computational domain that allows for the full development of this circulation. These simulations indicate that the thermal tides play a contributing role by providing zonal momentum flux divergence at the winter pole and by the advection of aerosol.

The focus of this paper is the role of rain forests in large-scale atmospheric circulations. The significance of this role is investigated by studying the response of the tropical atmosphere to a perturbation in the state of vegetation (deforestation) over three regions: the Amazon, Congo, and Indonesia. A theory is developed to relate tropical deforestation and the resulting changes in the large-scale atmospheric circulation. Field observations and numerical simulations support the argument that tropical deforestation reduces the total net surface radiation, including terrestrial and solar forms. However, the energy balance at the land-atmosphere boundary dictates that for equilibrium conditions, any reduction in net surface radiation has to be balanced by a similar reduction in the total flux of heat, including sensible and latent forms. Since these fluxes supply heat as well as entropy from the forest into the atmospheric boundary layer, a reduction in the total flux of heat reduces the boundary layer entropy. In a moist atmosphere, that satisfies a quasi equilibrium between moist convection and radiative forcing, the equilibrium temperature profile is uniquely related to the boundary layer entropy. Under such conditions, large-scale deforestation reduces boundary layer entropy relative to the surroundings, cools the upper troposphere, and results in subsidence, divergent flow in the boundary layer, and weakening of the large-scale circulation. These changes are simulated using a simple linear model of atmospheric flow. The comparison of the model predictions with observations of atmospheric circulations over the Amazon, Congo, and Indonesia suggests a significant role for vegetation in maintaining large-scale atmospheric circulations in the tropics.

[1] Radiative flux perturbation (RFP) is defined as the top-of-the-atmosphere (TOA) radiative imbalance after the atmosphere-land system adjusts fully to an external perturbation, and serves as a useful metric for quantifying climate forcing. This paper presents an effort to address the issue of whether a forcing imposed persistently over a specific region may alter the radiative balance elsewhere through atmospheric circulation, thus giving rise to a nonlocal component of RFP. We perturb the climate simulated with an atmospheric general circulation model (AGCM) solely by increasing the cloud droplet number concentration over the land, and observe widespread positive (warming) RFP over the ocean, along with the expected negative (cooling) RFP over the land. A detailed analysis suggests that the oceanic (or nonlocal) RFP is closely associated with a reduction in low cloud amount, which can be attributed primarily to the horizontal advection of drier land boundary layer air and to the oceanic boundary layer top entrainment of drier free troposphere air. By examining how the land surface and atmospheric energy balances are re-established, we are able to identify the physical mechanisms behind the strong hydrological impact of a tropical land shortwave (SW) forcing (the multiplier effect). In contrast, the hydrological cycle is relatively insensitive to an extratropical forcing.

This paper presents an investigation of a lake-land breeze atmospheric circulation responsible for strong horizontal and vertical inhomogeneities of moisture distribution which affected the refractivity in the neighborhood of a microwave line-of-sight link, as observed during a multi-instrument radiometeorological experiment carried out in Sao Paulo, Brazil. A boundary layer and mesoscale atmospheric model was used to simulate the local circulation within the link region and to compute the time evolution of the vertical structure of the atmospheric refractive index. A ray-tracing model was also used to calculate the intensity of the received signals from the measured refractive index vertical profiles. The integrated simulation results were then compared with the measurements. The good agreement obtained between the measured and the simulated characteristics of the temperature and moisture advection, as well as between the measured and the calculated received signal levels, indicates that the lake-land breeze circulation mechanism is a likely source of the propagation phenomena observed in the microwave link.

In this study, the seasonal variation of Earth's rotational velocity is studied from a dynamical point of view. Many studies have been conducted in recent years by methods of correlation analysis, dynamical calculation, and numerical simulation. In this paper, we use an analytical method that has not been attempted in this study area to establish a model in which the angular momentum conservation, short-wave and long-wave radiation, the radiation transmitting process, and the anomaly of atmospheric circulation are considered. On the basis of the angular momentum conservation law of the Earth-atmosphere system, we obtain an analytical solution for variation of Earth's angular velocity. Our model shows that the semiannual variation is mainly caused by solar short-wave radiation and that the annual change is generated by short-wave radiation from the Sun and long-wave radiation from the atmospheric underlying surface. Our model also shows that long-wave radiation from the atmosphere affects the nonuniformity of Earth's rotation by reducing amplitude and changing phase. The results suggest that the action of ocean for seasonal variation is unimportant, that the nonlinear action of zonal averaged field can be neglected, and that only a secondary part of atmospheric circulation anomaly is decisive for the seasonal variation. Our analytical solution depends on general astronomical parameters and therefore can be applied to other planets.

This paper focuses on the comparison of cloud amounts derived from an atmospheric general circulation model (AGCM), Satellite-observed clouds, and Ground-based cloud observations. This is distinctly different from Earth Radiation Budget Experiment (ERBE)-type comparisons because it does not mix potential errors in the cloud amount with those in the radiation code embedded in the model. Long term cloud climatologies were used to compare global cloud amounts and regional seasonal cycles. The results obtained were surprising in many respects. The AGCM successfully reproduced the signatures of the warm pool and North Pacific seasonal cycle cloudiness but failed in the low stratus region off the coast of South America, a known problem for AGCMs. The data sets also reproduced the anomaly signature associated with El Nino in the warm pool region, but the model amounts were lower. Global results had a similar success rate, with the model generally producing lower total cloud amounts compared to the satellite and in situ measurements. Also, an attempt was made to compare cloud vertical distributions between the data sets. Because of the inherent differences in the measuring processes among the three data sets, the cloud height may need to be validated using the corresponding radiation fields. Unfortunately there were also some large discrepancies between the two observed cloud data sets. We conclude that the character of the observed cloud data sets, while tremendously improved over the last decade, must be substantially enhanced before they will be useful in validating AGCMs by any but the crudest levels of comparison.

[1] We use a three-dimensional atmospheric model to study the airglow emissions from molecular oxygen in the Martian atmosphere. We estimate the O2 Herzberg I & II, the Chamberlain, and the Infrared Atmospheric band emissions from different sets of kinetic parameters available in the literature. As expected, the enhanced production of atomic oxygen during daytime leads to stronger emissions at 12 hour local time than at 00 hour local time. Nevertheless, at night, the strongest emissions are found in the subtropics and around the terminator where the photochemistry of atomic oxygen is more active. Among the simulated emissions, we find that the Infrared Atmospheric emission is the most intense, as expected, and has maximum intensity reaching a few megarayleighs over the poles during the equinoctial seasons, and an average intensity over the equatorial latitudes of 50 kilorayleighs. We investigate the impact of different levels of water and dust content on airglow and we observe that the airglow structure is modulated by variations in the background atmospheric conditions. Moreover, comparisons of the emission with observations from instruments on board Mars orbiters and with ground-based measurements from Earth allow us to validate the consistency of our airglow model. Finally, we observe that the emission profiles from all band systems show structures; a double-layer profile is very frequent, and is representative of the vertical distribution of the current measurements of O2 nightglow. This paper emphasizes the advantage of using three-dimensional global circulation models for the diagnostic of O2 photochemistry in CO2-dominated atmospheres.

[1] Polynyas, regions of reduced sea ice concentration relative to their surroundings, are important features of the polar climate system in which enhanced fluxes of heat, moisture, and momentum can occur between the atmosphere and ocean. As such, they play a significant role in many atmospheric, oceanographic and biological processes. There are concerns that in a warming climate, in which there is a trend towards a reduction in sea ice cover, that the location, size and duration of many polynyas may change resulting in climatological and ecological impacts. In this paper, we identify an early summer manifestation of the Wrangel Island polynya that forms in the western Chuckchi Sea. We show that over the past 30 years there has been an increased frequency of occurrence as well as a doubling in the size of the polynya. The polynya is shown to form when there is an enhanced easterly flow over the Chukchi Sea that is associated with an anomalously intense Beaufort Sea High (BSH), a closed anti-cyclonic atmospheric circulation that forms over the Beaufort Sea. We also show that there has been a concomitant trend towards a more intense BSH over the same time period and we propose that this trend is responsible for the observed changes in the Wrangel Island polynya. Given its large and increasing size, the early summer polynya may also play an important and unaccounted role in the physical and biological oceanography of the western Chukchi Sea.

In this paper, we investigate the relative importance of local vs remote control on cloud radiative forcing using a cumulus ensemble model. It is found that cloud and surface radiation forcings are much more sensitive to the mean vertical motion associated with large scale tropical circulation than to the local SST. When the local SST is increased with the mean vertical motion held constant, increased surface latent and sensible heat flux associated with enhanced moisture recycling is found to be the primary mechanism for cooling the ocean surface. Large changes in surface shortwave fluxes are related to changes in cloudiness induced by changes in the large scale circulation. These results are consistent with a number of earlier empirical studies, which raised concerns regarding the validity of the cirrus-thermostat hypothesis (Ramanathan and Collins, 1991). It is argued that for a better understanding of cloud feedback, both local and remote controls need to be considered and that a cumulus ensemble model is a powerful tool that should be explored for such purpose.

Diurnal asymmetry of the recent surface warming, reported for several continental areas, is believed to be related to changes in cloudiness, humidity, atmospheric circulation patterns, winds and soil moisture. This paper presents linear trend analyses of maximum and minimum temperature data at 121 stations in India during the period 1901–87. While the mean temperature trends over India are similar to the global and hemispheric trends, there are marked differences in their diurnal manifestation from those reported for other areas. The increase in the mean temperatures over India is almost solely contributed by the maximum temperatures, with the minimum temperatures remaining practically trendless, leading to an increase in the diurnal range of temperatures. These trends do not show any significant urban or altitude bias. There are some differences in the trends on smaller spatial and temporal scales, but the increase of maximum temperature is predominant over a major part of India, particularly in winter and post-monsoon seasons.

[1] Inference of surface CO2 fluxes from atmospheric CO2 observations requires information about large-scale transport and turbulent mixing in the atmosphere, so transport errors and the statistics of the transport errors have significant impact on surface CO2 flux estimation. In this paper, we assimilate raw meteorological observations every 6 hours into a general circulation model with a prognostic carbon cycle (CAM3.5) using the Local Ensemble Transform Kalman Filter (LETKF) to produce an ensemble of meteorological analyses that represent the best approximation to the atmospheric circulation and its uncertainty. We quantify CO2 transport uncertainties resulting from the uncertainties in meteorological fields by running CO2 ensemble forecasts within the LETKF-CAM3.5 system forced by prescribed surface fluxes. We show that CO2 transport uncertainties are largest over the tropical land and the areas with large fossil fuel emissions, and are between 1.2 and 3.5 ppm at the surface and between 0.8 and 1.8 ppm in the column-integrated CO2 (with OCO-2-like averaging kernel) over these regions. We further show that the current practice of using a single meteorological field to transport CO2 has weaker vertical mixing and stronger CO2 vertical gradient when compared to the mean of the ensemble CO2 forecasts initialized by the ensemble meteorological fields, especially over land areas. The magnitude of the difference at the surface can be up to 1.5 ppm.

The ice-core record of the carbon-13 content of atmospheric methane (δ13CH4) has largely been used to constrain past changes in methane sources. The aim of this paper is to explore, for the first time, the contribution that changes in the strength of a minor methane sink―oxidation by atomic chlorine in the marine boundary layer (ClMBL)―could make to changes in δ13CH4 on glacial-interglacial timescales. Combining wind and temperature data from a variety of general circulation models with a simple formulation for the concentration of ClMBL, we find that changes in the strength of this sink, driven solely by changes in the atmospheric circulation, could have been responsible for changes in δ13CH4 of the order of 10% of the glacial-interglacial difference observed. We thus highlight the need to quantify past changes in the strength of this sink, including those relating to changes in the sea-ice source of sea salt aerosol.

International audience; Many studies with atmospheric general circulation models (AGCMs) have demonstrated their usefulness in reconstructing past climates. In a new approach, we have used an AGCM to investigate the link between tracer cycles and climate. We consider in this paper the atmospheric cycle of windblown dust material from desertic areas and in part 2 the water isotope cycles. Studies from ice cores have shown a strong increase of the dust deposits during glacial periods, both over East Antarctica and Greenland. We do not know, however, whether this past increase is global or just a local feature, where the dust came from, and what are the mechanisms yielding this increase. We try to answer to these questions by using an AGCM including a model for the desert dust cycle to simulate the present-day and Last Glacial Maximum climates for February and August. The model simulates only a weak increase of the global atmospheric dust content. Stronger variations are obtained at a regional scale and are in good agreement with observations from deep-sea sediments. However, the model does not reproduce the great increase of the dust concentration in snow that has been observed in ice cores. Several model deficiencies can induce this model-data discrepancy as, for example, inaccuracies of the circulation patterns or of the dust model. However, most likely, the model fails to simulate the actual sources of dust.

[1] This paper presents a high-resolution dual-Doppler analysis of lake breeze mesoscale circulation induced by an elongated reservoir with a mean width of ∼2 km and an area of 125 km2. Compared to previous meso-γ-scale lake breeze studies based on point measurements or aircraft observations, the present study provides both a unique quantification and a high-resolution spatial and temporal 3-D visualization (several hundred meters and 5 min, respectively) of the kinematic structure of the lake breeze initiation and evolution. Visible satellite images show a cloud-free zone over the lake and adjacent land areas that was promoted by subsidence associated with the lake breeze circulation. The background synoptic-scale wind flow was almost parallel to the major axis of the lake, and distinct lake breeze frontal zones formed along both shorelines spanning the length of the lake. Dual-Doppler analyses showed updrafts in the frontal zones, perturbation horizontal velocities of 1.5 m s−1 on both sides of the lake, and maximum downdrafts of 2 m s−1 centered over the lake. Vertical vorticity in the boundary layer was produced by differential friction between the smooth lake and adjacent land. The circulations are robust and are maintained during the day against a minor change in background wind direction. The study shows that the circulations produced by a small lake can generate significant mesoscale circulations that influence local climatology and identifies the importance of including such small mesoscale processes in global forecast models.

[1] Using a series of relaxation hindcast experiments, this paper shows that the northern extratropics have a significant impact on the skill of the ECMWF forecast system in predicting the Madden Julian Oscillation (MJO) during boreal winter. The westerly wind burst associated with the March 1997 MJO event is a high-profile example of an event that was influenced by extratropical circulation anomalies. The propagation speed of MJO events is overall improved when the northern extratropics are relaxed towards analysis fields. Additional experiments in which the relaxation is confined to different areas of the Northern Hemisphere extratropics suggest that it is atmospheric circulation anomalies in the North Pacific region which most strongly affect the skill in predicting the MJO.

[1] In this paper, changes in isentropic circulations associated with global warming in the AlB model outputs for the 20th and 21st centuries are analyzed. The changes in the circulations on dry and moist isentropes are quantified through the use of three bulk measures of the circulations: mass transport, entropy transport and effective stratification. The circulation on dry isentropes is expected to weaken due to a reduction of the meridional heat transport and to an increase in stratification. In contrast, the moist branch of the circulation, measured in terms of the difference between the circulations on moist and dry isentropes, strengthens during the winter months. This intensification is characterized not only by an increase in the eddy latent heat transport but also by an increase in the mass transport. This indicate a larger poleward mass flow of warm moist subtropical air into the stormtracks leading to enhanced moist ascent within baroclinic eddies.

In this paper we discuss a regional climate change scenario over Europe induced by doubling of carbon dioxide concentration (what we call the 2XCO2 experiment) as simulated with a limited area model (LAM) nested in a general circulation model (GCM). The models used are a 4.5×7.5 latitude×longitude resolution (R15) version of the comrnunity climate model (CCM) of the National Center for Atmospheric Research (NCAR) and a 70-km resolution version of the NCAR/Pennsylvania State University mesoscale model (MM4). Overall, the model produces a 2XCO2 warming over Europe in all seasons, which is in the range of 1.5°–7°. The response of the large scale circulation patterns, and therefore precipitation, over various European subregions varies from season to season. For example, the mid-latitude jet stream over Europe is weaker in the perturbed climate run than in the control run in January and October, while it is stronger in July and April. As a consequence, simulated 2XCO2 changes in regional precipitation amounts also vary for different seasons, e.g., from −20% of present-day values over the Alpine region in October to +177% over the western Mediterranean in July. Because of its finer representation of European topography and coastlines the nested MM4 produces a temperature and precipitation change scenario which shows finer spatial detail than that of the driving CCM. The 2XCO2-induced average temperature modifications predicted by the two models differ locally by up to a few degrees, and predicted precipitation changes differ in some instances not only in magnitude but also in sign. Similarly, the nested MM4 predicts changes in snow depth and other components of the surface hydrology, such as surface runoff and soil water drainage, which show a great deal of sub-GCM grid-scale detail. These results emphasize the inadequacy of simply interpolating coarse resolution GCM output to obtain estimates of local changes in surface climatic variables for impact assessments.

The gross moist stability relates the net lateral outflow of moist entropy or moist static energy from an atmospheric convective region to some measure of the strength of the convection in that region. If the gross moist stability can be predicted as a function of the local environmental conditions, then it becomes the key element in understanding how convection is controlled by the large-scale flow. This paper provides a guide to the various ways in which the gross moist stability is defined and the subtleties of its calculation from observations and models. Various theories for the determination of the gross moist stability are presented and its roles in current conceptual models for the tropical atmospheric circulation are analyzed. The possible effect of negative gross moist stability on the development and dynamics of tropical disturbances is currently of great interest.

[1] The relationships among the boreal spring Southern Hemisphere Annular Mode (SAM), the Indian Ocean (IO) sea surface temperature (SST), and East Asian summer monsoon (EASM) are examined statistically in this paper. The variability of boreal spring SAM is closely related to the IO SST. When the SAM is in its strong positive phase in boreal spring, with low-pressure anomalies over the south pole and high-pressure anomalies over middle latitudes, SSTover the subtropics and middle latitudes of the South Indian Ocean (SIO) increases, which persists into the summer. Following the positive SST anomalies over the subtropics and midlatitudes of the SIO, SST in the equatorial Indian Ocean and Bay of Bengal increases in summer. Moreover, the variability of SST in the equatorial Indian Ocean and Bay of Bengal is closely related to EASM. When SST in the equatorial Indian Ocean and Bay of Bengal increases, EASM tends to be weak. Therefore the IO SST may play an important role bridging boreal spring SAM and EASM. The atmospheric circulations and surface heat exchanges contribute to the SST anomalies in the SIO. When the spring SAM is in its strong positive phases, the regional Ferrel Cell weakens, and the anomalous upward motions at 20S–30S cause an increase of low cloud cover and downward longwave radiation flux. The surface atmospheric circulations also transport more (less) warmer (cooler) air from middle latitudes north of 50S (high latitudes south of 60S) into 50S–60S and warm the air, which reduces the temperature difference between the ocean and atmosphere and consequently reduces sensible heat flux from the ocean to atmosphere. The increased downward longwave radiation and decreased sensible heat are responsible for the SST increase in the SIO. The atmospheric circulation and surface heat flux anomalies are of opposite signs following the strong negative phases of SAM.

Measurements of the tropospheric horizontal wind field at Christmas Island during the ALOHA-90 campaign are presented in this paper. A diurnal cycle is observed on several days but is not consistent from day-to-day. Significant variance associated with the high frequency wave motions occurred between March 22 and 26, 1990. This large wave activity in the troposphere is consistent with mesospheric wave activity during the same time interval.