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R. S. Lindzen and B. Farrell

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Hal B. Gordon and Siobhan P. O’Farrell

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The CSIRO coupled model has been used in a “transient” greenhouse experiment. This model contains atmospheric, oceanic, comprehensive sea-ice (dynamic/thermodynamic plus leads), and biospheric submodels. The model control run (over 100 years long) employed flux corrections and displayed only a small amount of cooling, mainly at high latitudes. The transient experiment (1% increase in CO2 compounding per annum) gave a 2°C warming at time of CO2 doubling. The model displayed a “cold start” effect with a (maximum) value estimated at 0.3°C. The warming in the transient run had an asymmetrical response as seen in other coupled models, with the Northern Hemisphere (NH) warming more than the Southern Hemisphere (SH). However, the land surface response in this model is different from some other transient experiments in that there is not a pronounced drying of the midlatitudes in the NH in summer.

In the control run the ice model gave realistic ice distributions at both poles, with the NH ice in particular displaying considerable interdecadal variability. In the transient run the ice amount decreased more in the NH than the SH (corresponding with a greater NH warming). The NH ice extent and volume in summer was considerably reduced in depth and extent compared to the control. However, the model ice dynamics and thermodynamics allowed for a successful regrowth of the ice during the winter season to give a coverage comparable to that of the control run, although thinner.

During the transient run there is a freshening of the surface salinity in the oceans at high latitudes. In the SH this is caused mainly by increases in precipitation over evaporation. The same is true for the NH, but it is found that there is a similar magnitude contribution to the polar freshening from ice melt and land runoff changes. The freshening in the North Atlantic reduces the strength of the meridional overturning by 35% at time of doubling of CO2. Other changes in the global climate at the end of the transient run relative to the control run are also investigated.

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R. S. Lindzen, A. J. Rosenthal, and B. Farrell

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It is suggested that barotropically unstable easterly jets may be approximated by broken line profiles. It is then possible to use solutions to the Charney problem for baroclinic instability in order to solve the barotropic problem. Results are presented for easterly jets with an arbitrary degree of asymmetry.

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Anthony C. Hirst, Siobhan P. O’Farrell, and Hal B. Gordon

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The Gent and McWilliams (GM) parameterization for large-scale water transport caused by mesoscale oceanic eddies is introduced into the oceanic component of the Commonwealth Scientific and Industrial Research Organisation global coupled ocean–atmosphere model. Parallel simulations with and without the GM scheme are performed to examine the effect of this parameterization on the model behavior for integrations lasting several centuries under conditions of constant atmospheric CO2. The solution of the version with GM shows several significant improvements over that of the earlier version. First, the generally beneficial effects of the GM scheme found previously in studies of stand-alone ocean models, including more realistic deep water properties, increased stratification, reduced high-latitude convection, elimination of fictitious horizontal diffusive heat transport, and more realistic surface fluxes in some regions, are all maintained during the coupled integration. These improvements are especially pronounced in the high-latitude Southern Ocean. Second, the magnitude of flux adjustment is reduced in the GM version, mainly because of smaller surface fluxes at high southern latitudes in the GM ocean spinup. Third, the GM version displays markedly reduced climate drift in comparison to the earlier version. Analysis in the present study verifies previous indications that changes in the pattern of convective heat flux are central to the drift in the earlier version, supporting the view that reduced convective behavior in the GM version contributes to the reduction in drift. Based on the satisfactory behavior of the GM model version, the GM coupled integration is continued for a full 1000 yr. Key aspects of the model behavior during this longer period are also presented. Interannual variability of surface air temperature in the two model versions is briefly examined using some simple measures of magnitude. The variability differs between the two versions regionally, but is little changed on the global scale. In particular, the magnitude of variability in the tropical Pacific is little changed between the versions.

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R. S. Lindzen, A. Y. Hou, and B. F. Farrell

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The role of the parameterization of vertical convection in calculating the climate impact of doubling CO2 is assessed using both one-dimensional radiative-convective vertical models and in the latitude-dependent Hadley-baroclinic model of Lindzen and Farrell (1980). Both the conventional 6.5 K km−1 and the moist-adiabat adjustments are compared with a physically-based, cumulus-type parameterization. The model with parameterized cumulus convection has much less sensitivity than the 6.5 K km−1 adjustment model at low latitudes, a result that can be to some extent imitated by the moist-adiabat adjustment model. However, when averaged over the globe, the use of the cumulus-type parameterization in a climate model reduces sensitivity only ∼34% relative to models using 6.5 K km−1 convective adjustment. Interestingly, the use of the cumulus-type parameterization appears to eliminate the possibility of a runaway greenhouse.

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R. S. Lindzen, B. Farrell, and A. J. Rosenthal

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We have approached the barotropic instability of mean zonal flows over the Bay of Bengal for the months June, July and August from the perspective of pulse asymptotics rather than most rapidly growing normal modes. The results are in good agreement with observations of monsoon depressions.

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Wenju Cai, Jozef Syktus, Hal B. Gordon, and Siobhan O’Farrell

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The response of a coupled oceanic–atmospheric–sea ice climate model to an imposed North Atlantic high-latitude freshening is examined. The imposed freshening lasts for 5 yr with a total salt deficit equivalent to about eight times the observed Great Salinity Anomaly during the late 1960s and early 1970s.

The thermohaline circulation associated with North Atlantic Deep Water Formation (NADWF) initially weakens, but it recovers within 20 yr of the imposed freshening being removed. The effect of the weakened NADWF is gradually transmitted from high latitudes to the entire Atlantic Ocean. The response at the equator lags that at 62°N by about 10 yr. In the midlatitude (from 30° to 58°N) region, the lag causes a warming during the initial weakening and a cooling during the recovery. Changes in the thermohaline circulation significantly modify the large-scale North Atlantic circulation. In particular, the barotropic Gulf Stream weakens by about 18%.

An interesting feature is the dipole structure of the initial response in sea surface temperature, with cooling in the sinking region and warming south of it. This dipole structure plays an important role for the recovery of the NADWF once the imposed freshening is removed. It increases the surface density in the sinking region and increases the north–south pressure gradient. Thus, the conditions set up during the initial weakening contribute to the recovery process.

Modifications of the thermal structure of the ocean surface lead to changes in the atmospheric circulation, in particular, a weakening of the westerlies over the midlatitude North Atlantic and a southward shift over Western Europe. The North Atlantic oscillation (NAO) index under the imposed freshening is negative, consistent with findings from observational studies. The associated climate changes are similar to those observed with negative NAO values.

Effects of various oceanic and atmospheric feedbacks are discussed. The results are also compared with those from ocean-only models, where the atmosphere–ocean interactions and some of the oceanic feedbacks are excluded.

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Bernadett Weinzierl, A. Ansmann, J. M. Prospero, D. Althausen, N. Benker, F. Chouza, M. Dollner, D. Farrell, W. K. Fomba, V. Freudenthaler, J. Gasteiger, S. Groß, M. Haarig, B. Heinold, K. Kandler, T. B. Kristensen, O. L. Mayol-Bracero, T. Müller, O. Reitebuch, D. Sauer, A. Schäfler, K. Schepanski, A. Spanu, I. Tegen, C. Toledano, and A. Walser

Abstract

North Africa is the world’s largest source of dust, a large part of which is transported across the Atlantic to the Caribbean and beyond where it can impact radiation and clouds. Many aspects of this transport and its climate effects remain speculative. The Saharan Aerosol Long-Range Transport and Aerosol–Cloud-Interaction Experiment (SALTRACE; www.pa.op.dlr.de/saltrace) linked ground-based and airborne measurements with remote sensing and modeling techniques to address these issues in a program that took place in 2013/14. Specific objectives were to 1) characterize the chemical, microphysical, and optical properties of dust in the Caribbean, 2) quantify the impact of physical and chemical changes (“aging”) on the radiation budget and cloud microphysical processes, 3) investigate the meteorological context of transatlantic dust transport, and 4) assess the roles of removal processes during transport.

SALTRACE was a German-led initiative involving scientists from Europe, Cabo Verde, the Caribbean, and the United States. The Falcon research aircraft of the Deutsches Zentrum für Luft- und Raumfahrt (DLR), equipped with a comprehensive aerosol and wind lidar payload, played a central role. Several major dust outbreaks were studied with 86 h of flight time under different conditions, making it by far the most extensive investigation on long-range transported dust ever made.

This article presents an overview of SALTRACE and highlights selected results including data from transatlantic flights in coherent air masses separated by more than 4,000-km distance that enabled measurements of transport effects on dust properties. SALTRACE will improve our knowledge on the role of mineral dust in the climate system and provide data for studies on dust interactions with clouds, radiation, and health.

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J. K. Andersen, Liss M. Andreassen, Emily H. Baker, Thomas J. Ballinger, Logan T. Berner, Germar H. Bernhard, Uma S. Bhatt, Jarle W. Bjerke, Jason E. Box, L. Britt, R. Brown, David Burgess, John Cappelen, Hanne H. Christiansen, B. Decharme, C. Derksen, D. S. Drozdov, Howard E. Epstein, L. M. Farquharson, Sinead L. Farrell, Robert S. Fausto, Xavier Fettweis, Vitali E. Fioletov, Bruce C. Forbes, Gerald V. Frost, Sebastian Gerland, Scott J. Goetz, Jens-Uwe Grooß, Edward Hanna, Inger Hanssen-Bauer, Stefan Hendricks, Iolanda Ialongo, K. Isaksen, Bjørn Johnsen, L. Kaleschke, A. L. Kholodov, Seong-Joong Kim, Jack Kohler, Zachary Labe, Carol Ladd, Kaisa Lakkala, Mark J. Lara, Bryant Loomis, Bartłomiej Luks, K. Luojus, Matthew J. Macander, G. V. Malkova, Kenneth D. Mankoff, Gloria L. Manney, J. M. Marsh, Walt Meier, Twila A. Moon, Thomas Mote, L. Mudryk, F. J. Mueter, Rolf Müller, K. E. Nyland, Shad O’Neel, James E. Overland, Don Perovich, Gareth K. Phoenix, Martha K. Raynolds, C. H. Reijmer, Robert Ricker, Vladimir E. Romanovsky, E. A. G. Schuur, Martin Sharp, Nikolai I. Shiklomanov, C. J. P. P. Smeets, Sharon L. Smith, Dimitri A. Streletskiy, Marco Tedesco, Richard L. Thoman, J. T. Thorson, X. Tian-Kunze, Mary-Louise Timmermans, Hans Tømmervik, Mark Tschudi, Dirk van As, R. S. W. van de Wal, Donald A. Walker, John E. Walsh, Muyin Wang, Melinda Webster, Øyvind Winton, Gabriel J. Wolken, K. Wood, Bert Wouters, and S. Zador
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