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John Austin

Abstract

A coupled chemistry–climate model is integrated for the period March 1979–January 2000 with sea surface temperatures and sea ice amounts specified from observations. Greenhouse gas concentrations and halogen loading are also taken from observations. The model contains a detailed stratospheric chemistry package that responds to and in turn influences the model temperature fields. The simulated temperature and ozone trends over the period 1980 to 2000 are broadly in agreement with observations. However, the Arctic ozone depletion is smaller than observed, and the impact of the Antarctic ozone hole lasts too long into summer. The coupling between the model ozone and temperature trends is demonstrated to occur in a similar way to that inferred from observations and could be important in generating the observed interannual temperature variability. It is concluded that further improvement in climate models is necessary before future trends in stratospheric ozone and temperature can be predicted with confidence.

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Neal Butchart and John Austin

Abstract

A climatology of the middle atmosphere is determined from 11-yr integrations of the U.K. Meteorological Office Unified Model and compared with 18 years of satellite observations and 5 years of data assimilation fields. The model has an upper boundary at 0.1 mb, and above 20 mb uses Rayleigh friction as a substitute for gravity wave drag. Many of the results are, however, found to be relatively insensitive to enhancing the damping above 0.3 mb. As with most general circulation models, the polar night jet in both hemispheres is too strong and does not have the observed equatorward slope with height. The model suffers from the common “cold pole” problem and, apart from a local warm pool centered just below 100 mb in northern high latitudes in January, and another at about 30 mb at 70°S in July, has a cold bias throughout the stratosphere. At the level where polar stratospheric clouds occur, the temperature bias is about −4 K in the Northern Hemisphere and up to +6 K in the Southern Hemisphere. For the majority of the southern winters, local minimum temperatures in the lower stratosphere agree well with observations but in some years the behavior is more like the Northern Hemisphere with values rising rapidly in late winter. This feature of the simulation is also seen in the South Pole temperatures at 10 mb with midwinter warmings occurring in two of the years. At 10 mb, midwinter warming behavior at the North Pole is quite well reproduced, as is the annual cycle in extratropical circulation. In the Tropics, there is no quasi-biennial oscillation, and the semiannual oscillation in the upper stratosphere has a poorly simulated westerly phase, while the easterly phase lacks the observed seasonal asymmetry. Simulated stationary wave amplitudes in the upper stratosphere lack a strong hemispheric asymmetry and are overpredicted in both hemispheres despite having roughly the correct amplitudes at 100 mb. Interannual variability in the winter stratosphere is underestimated, and again there is evidence that the model does not produce the proper hemispheric asymmetries.

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James R. Holton and John Austin

Abstract

A global primitive equation model of the stratosphere and mesosphere is integrated for specified planetary wave forcing at the 100 mb level with mean zonal flow conditions corresponding to the westerly and easterly phases of the equatorial QBO, respectively. The responses in the two QBO phases were compared for integrations with wavenumber 1 forcing amplitude maxima at 100 mb and 60°N varying from 100 m to 400 m. The phase of the QBO had little effect on the results in the weak wave (100 m) cases, which did not produce warmings, and the strong wave (400 m) cases, which produced major sudden warmings. At intermediate forcing amplitudes (particularly around 250 m), large wave amplitudes and stratospheric warmings occurred in both westerly and easterly QBO simulations. The warmings occurred earlier, however, and wave variability was stronger for the easterly phase, in qualitative agreement with observations.

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Feng Li, John Austin, and John Wilson

Abstract

The strength of the Brewer–Dobson circulation (BDC) in a changing climate is studied using multidecadal simulations covering the 1960–2100 period with a coupled chemistry–climate model, to examine the seasonality of the change of the BDC. The model simulates an intensification of the BDC in both the past (1960–2004) and future (2005–2100) climate, but the seasonal cycle is different. In the past climate simulation, nearly half of the tropical upward mass flux increase occurs in December–February, whereas in the future climate simulation the enhancement of the BDC is uniformly distributed in each of the four seasons. A downward control analysis implies that this different seasonality is caused mainly by the behavior of the Southern Hemisphere planetary wave forcing, which exhibits a very different long-term trend during solstice seasons in the past and future. The Southern Hemisphere summer planetary wave activity is investigated in detail, and its evolution is found to be closely related to ozone depletion and recovery. In the model results for the past, about 60% of the lower-stratospheric mass flux increase is caused by ozone depletion, but because of model ozone trend biases, the atmospheric effect was likely smaller than this. The remaining fraction of the mass flux increase is attributed primarily to greenhouse gas increase. The downward control analysis also reveals that orographic gravity waves contribute significantly to the increase of downward mass flux in the Northern Hemisphere winter lower stratosphere.

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Jay A. Austin and John A. Barth

Abstract

Drifters released offshore of Oregon during predominantly downwelling favorable alongshore winds during three different deployments (October 1994, January 1998, and September 1998) display similar behavior: after being advected around in the offshore eddy field, they move onshore to a particular isobath and are advected poleward alongshore, without coming ashore. Numerical modeling results suggest that this may be due to downwelling circulation creating a marginally stable density gradient on the shelf inshore of the downwelling front, thereby increasing the vertical eddy diffusivity, which reduces the effective cross-shelf Ekman transport to nearly zero. The downwelling front itself is accompanied by a poleward jet, which carries drifters rapidly to the north. This behavior is consistent with previous modeling results.

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John Austin, John Wilson, Feng Li, and Holger Vömel

Abstract

Stratospheric water vapor concentrations and age of air are investigated in an ensemble of coupled chemistry-climate model simulations covering the period from 1960 to 2005. Observed greenhouse gas concentrations, halogen concentrations, aerosol amounts, and sea surface temperatures are all specified in the model as time-varying fields. The results are compared with two experiments (time-slice runs) with constant forcings for the years 1960 and 2000, in which the sea surface temperatures are set to the same climatological values, aerosol concentrations are fixed at background levels, while greenhouse gas and halogen concentrations are set to the values for the relevant years.

The time-slice runs indicate an increase in stratospheric water vapor from 1960 to 2000 due primarily to methane oxidation. The age of air is found to be significantly less in the year 2000 run than the 1960 run. The transient runs from 1960 to 2005 indicate broadly similar results: an increase in water vapor and a decrease in age of air. However, the results do not change gradually. The age of air decreases significantly only after about 1975, corresponding to the period of ozone reduction. The age of air is related to tropical upwelling, which determines the transport of methane into the stratosphere. Oxidation of increased methane from enhanced tropical upwelling results in higher water vapor amounts. In the model simulations, the rate of increase of stratospheric water vapor during the period of enhanced upwelling is up to twice the long-term mean. The concentration of stratospheric water vapor also increases following volcanic eruptions during the simulations.

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John Austin, Larry W. Horowitz, M. Daniel Schwarzkopf, R. John Wilson, and Hiram Levy II

Abstract

Results from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the effect of eruptions is shown to increase the amount of HNO3, reducing ozone destruction by the NOx catalytic cycle. In the presence of anthropogenic chlorine, after the eruption of El Chichón and Mt. Pinatubo, chlorine radicals increased and the chlorine reservoirs decreased. The net volcanic effect on nitrogen and chlorine chemistry depends on altitude and, for these two volcanoes, leads to an ozone increase in the middle stratosphere and a decrease in the lower stratosphere. Model lower-stratospheric temperatures are also shown to increase during the last three major volcanic eruptions, by about 0.6 K in the global and annual average, consistent with observations.

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Neal Butchart, John Austin, Jeffrey R. Knight, Adam A. Scaife, and Mark L. Gallani

Abstract

Results are presented from two 60-yr integrations of the troposphere–stratosphere configuration of the U.K. Met. Office’s Unified Model. The integrations were set up identically, apart from different initial conditions, which, nonetheless, were both representative of the early 1990s. Radiative heating rates were calculated using the IS92A projected concentrations of the well-mixed greenhouse gases (GHGs) given by the Intergovernmental Panel on Climate Change, but changes in stratospheric ozone and water vapor were not included. Sea surface conditions were taken from a separate coupled ocean–atmosphere experiment. Both integrations reproduced the familiar pattern of tropospheric warming and a stratospheric cooling increasing with height to about −1.4 K per decade at 1 mb. There was good agreement in the trends apart from in the polar upper stratosphere and, to a greater extent, the polar lower-to-middle stratosphere, where there is significant interannual variability during the winter months. Even after decadal smoothing, the trends in the northern winter were still overshadowed by the variability resulting from the planetary wave forcing from the troposphere. In general, the decadal variability of the Northern Hemisphere stratosphere was not a manifestation of a uniform change throughout each winter but, as with other models, there was a change in the frequency of occurrence of sudden stratospheric warmings. Unlike previous studies, the different results from the two simulations confirm the change in frequency of warmings was due to internal atmospheric variability and not the prescribed changes in GHG concentrations or sea surface conditions. In the southern winter stratosphere the flux of wave activity from the troposphere increased, but any additional dynamical heating was more than offset by the extra radiative cooling from the growing total GHG concentration. Consequently the polar vortex became more stable, with the spring breakdown delayed by 1–2 weeks by the 2050s. Polar stratospheric cloud (PSC) amounts inferred from the predicted temperatures increased in both hemispheres, especially in the early winter. In the Southern Hemisphere, the region of PSC formation expanded both upward and equatorward in response to the temperature trend.

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Rolf H. Reichle, Gabrielle J. M. De Lannoy, Qing Liu, Randal D. Koster, John S. Kimball, Wade T. Crow, Joseph V. Ardizzone, Purnendu Chakraborty, Douglas W. Collins, Austin L. Conaty, Manuela Girotto, Lucas A. Jones, Jana Kolassa, Hans Lievens, Robert A. Lucchesi, and Edmond B. Smith

Abstract

The Soil Moisture Active Passive (SMAP) mission Level-4 Soil Moisture (L4_SM) product provides 3-hourly, 9-km resolution, global estimates of surface (0–5 cm) and root-zone (0–100 cm) soil moisture and related land surface variables from 31 March 2015 to present with ~2.5-day latency. The ensemble-based L4_SM algorithm assimilates SMAP brightness temperature (Tb) observations into the Catchment land surface model. This study describes the spatially distributed L4_SM analysis and assesses the observation-minus-forecast (OF) Tb residuals and the soil moisture and temperature analysis increments. Owing to the climatological rescaling of the Tb observations prior to assimilation, the analysis is essentially unbiased, with global mean values of ~0.37 K for the OF Tb residuals and practically zero for the soil moisture and temperature increments. There are, however, modest regional (absolute) biases in the OF residuals (under ~3 K), the soil moisture increments (under ~0.01 m3 m−3), and the surface soil temperature increments (under ~1 K). Typical instantaneous values are ~6 K for OF residuals, ~0.01 (~0.003) m3 m−3 for surface (root zone) soil moisture increments, and ~0.6 K for surface soil temperature increments. The OF diagnostics indicate that the actual errors in the system are overestimated in deserts and densely vegetated regions and underestimated in agricultural regions and transition zones between dry and wet climates. The OF autocorrelations suggest that the SMAP observations are used efficiently in western North America, the Sahel, and Australia, but not in many forested regions and the high northern latitudes. A case study in Australia demonstrates that assimilating SMAP observations successfully corrects short-term errors in the L4_SM rainfall forcing.

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Leo J. Donner, Bruce L. Wyman, Richard S. Hemler, Larry W. Horowitz, Yi Ming, Ming Zhao, Jean-Christophe Golaz, Paul Ginoux, S.-J. Lin, M. Daniel Schwarzkopf, John Austin, Ghassan Alaka, William F. Cooke, Thomas L. Delworth, Stuart M. Freidenreich, C. T. Gordon, Stephen M. Griffies, Isaac M. Held, William J. Hurlin, Stephen A. Klein, Thomas R. Knutson, Amy R. Langenhorst, Hyun-Chul Lee, Yanluan Lin, Brian I. Magi, Sergey L. Malyshev, P. C. D. Milly, Vaishali Naik, Mary J. Nath, Robert Pincus, Jeffrey J. Ploshay, V. Ramaswamy, Charles J. Seman, Elena Shevliakova, Joseph J. Sirutis, William F. Stern, Ronald J. Stouffer, R. John Wilson, Michael Winton, Andrew T. Wittenberg, and Fanrong Zeng

Abstract

The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for the atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol–cloud interactions, chemistry–climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical system component of earth system models and models for decadal prediction in the near-term future—for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud droplet activation by aerosols, subgrid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with ecosystem dynamics and hydrology. Its horizontal resolution is approximately 200 km, and its vertical resolution ranges approximately from 70 m near the earth’s surface to 1 to 1.5 km near the tropopause and 3 to 4 km in much of the stratosphere. Most basic circulation features in AM3 are simulated as realistically, or more so, as in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks remains problematic, as in AM2. The most intense 0.2% of precipitation rates occur less frequently in AM3 than observed. The last two decades of the twentieth century warm in CM3 by 0.32°C relative to 1881–1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of 0.56° and 0.52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol–cloud interactions, and its warming by the late twentieth century is somewhat less realistic than in CM2.1, which warmed 0.66°C but did not include aerosol–cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud–aerosol interactions to limit greenhouse gas warming.

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