[1] We demonstrate first measurements of the aerosol indirect effect using ground-based remote sensors at a continental US site. The response of nonprecipitating, icefree clouds to changes in aerosol loading is quantified in terms of a relative change in cloud-drop effective radius for a relative change in aerosol extinction under conditions of equivalent cloud liquid water path. This is done in a single column of air at a temporal resolution of 20 s (spatial resolution of $100 m). Cloud-drop effective radius is derived from a cloud radar and microwave radiometer. Aerosol extinction is measured below cloud base by a Raman lidar. Results suggest that aerosols associated with maritime or northerly air trajectories tend to have a stronger effect on clouds than aerosols associated with northwesterly trajectories that also have local influence. There is good correlation (0.67) between the cloud response and a measure of cloud turbulence.

[1] The contribution of coastal oceans to the global air-sea CO 2 flux is poorly quantified due to insufficient availability of observations and inherent variability of physical, biological and chemical processes. We present simulated air-sea CO 2 fluxes from a high-resolution biogeochemical model for the North American east coast continental shelves, a region characterized by significant sediment denitrification. Decreased availability of fixed nitrogen due to denitrification reduces primary production and incorporation of inorganic carbon into organic matter, which leads to an increase in seawater pCO 2 , but also increases alkalinity, which leads to an opposing decrease in seawater pCO 2 . Comparison of simulations with different numerical treatments of denitrification and alkalinity allow us to separate and quantify the contributions of sediment denitrification to air-sea CO 2 flux. The effective alkalinity flux resulting from denitrification is large compared to estimates of anthropogenically driven coastal acidification.

Energetic constraints on precipitation are useful for understanding the response of the hydrological cycle to ongoing climate change, its response to possible geoengineering schemes, and the limits on precipitation in very warm climates of the past. Much recent progress has been made in quantifying the different forcings and feedbacks on precipitation and in understanding how the transient responses of precipitation and temperature might differ qualitatively. Here, we introduce the basic ideas and review recent progress. We also examine the extent to which energetic constraints on precipitation may be viewed as radiative constraints and the extent to which they are confirmed by available observations. Challenges remain, including the need to better demonstrate the link between energetics and precipitation in observations and to better understand energetic constraints on precipitation at sub-global length scales.

Arctic amplification (AA)—referring to the enhancement of near-surface air temperature change over the Arctic relative to lower latitudes—is a prominent feature of climate change with important impacts on human and natural systems. In this review, we synthesize current understanding of the underlying physical mechanisms that can give rise to AA. These mechanisms include both local feedbacks and changes in poleward energy transport. Temperature and sea ice-related feedbacks are especially important for AA, since they are significantly more positive over the Arctic than at lower latitudes. Changes in energy transport by the atmosphere and ocean can also contribute to AA. These energy transport changes are tightly coupled with local feedbacks, and thus their respective contributions to AA should not be considered in isolation. It is here emphasized that the feedbacks and energy transport changes that give rise to AA are sensitively dependent on the state of the climate system itself. This implies that changes in the climate state will lead to changes in the strength of AA, with implications for past and future climate change.

[1] The role of stratospheric ozone recovery in the Southern Hemisphere climate system, in the coming decades, is examined by contrasting two 10-member ensembles of Community Atmospheric Model (CAM3) integrations, over the period . Model integrations in the first ensemble are conducted with a complete set of forcings: greenhouse gas concentrations from the A1B scenario, SSTs from corresponding ocean-atmosphere coupled model integrations, and ozone starting with severe depletion over the South Pole and recovering by mid-century. The integrations in the second ensemble are very similar to the first, except that only the transient ozone forcing is specified, and all other forcings are kept at year 2000 levels. Specifying ozone recovery in isolation allows us to determine unambiguously how it impacts the atmospheric circulation. We find that, in DJF, most key indices of atmospheric circulation show significant trends in the second ensemble, due to the closing of the ozone hole. In the first ensemble, however, trends are found to be statistically insignificant for nearly all key circulation indices. This suggests that ozone recovery will result in a nearly complete cancellation (and possible reversal) of the atmospheric circulation effects associated with increasing greenhouse gases, in Southern Hemisphere summer, over the coming half century. Citation: Polvani, L. M., M. Previdi, and C. Deser (2011), Large cancellation, due to ozone recovery, of future Southern Hemisphere atmospheric circulation trends, Geophys. Res. Lett., 38, L04707,

We review what is presently known about the climate system response to stratospheric ozone depletion and its projected recovery, focusing on the responses of the atmosphere, ocean and cryosphere. Compared with well‐mixed greenhouse gases (GHGs), the radiative forcing of climate due to observed stratospheric ozone loss is very small: in spite of this, recent trends in stratospheric ozone have caused profound changes in the Southern Hemisphere (SH) climate system, primarily by altering the tropospheric midlatitude jet, which is commonly described as a change in the Southern Annular Mode. Ozone depletion in the late twentieth century was the primary driver of the observed poleward shift of the jet during summer, which has been linked to changes in tropospheric and surface temperatures, clouds and cloud radiative effects, and precipitation at both middle and low latitudes. It is emphasized, however, that not all aspects of the SH climate response to stratospheric ozone forcing can be understood in terms of changes in the midlatitude jet. The response of the Southern Ocean and sea ice to ozone depletion is currently a matter of debate. For the former, the debate is centred on the role of ocean eddies in possibly opposing wind‐driven changes in the mean circulation. For the latter, the issue is reconciling the observed expansion of Antarctic sea‐ice extent during the satellite era with robust modelling evidence that the ice should melt as a result of stratospheric ozone depletion (and increases in GHGs). Despite lingering uncertainties, it has become clear that ozone depletion has been instrumental in driving SH climate change in recent decades. Similarly, ozone recovery will figure prominently in future climate change, with its impacts expected to largely cancel the impacts of increasing GHGs during the next half‐century.

The radiative kernel technique is employed to quantify twenty-first century changes to the tropospheric energy budget in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models in order to better understand changes in global-mean precipitation. The strongest feedbacks on the tropospheric radiative cooling are found to be associated with increases in temperature and water vapor, with the water vapor feedback offsetting a significant portion (∼39%) of the increase in radiative cooling due to higher temperatures. Cloud and surface sensible heat flux feedbacks, though not as large in magnitude as the temperature and water vapor feedbacks, are important contributors to the intermodel difference in the global precipitation response to warming, or hydrological sensitivity. The direct effects of radiative forcing agents on the tropospheric energy budget are also important. Rising CO 2 levels reduce tropospheric radiative cooling and hence limit the increase in global rainfall. Additionally, in some of the models, further reductions in radiative cooling occur due to increases in absorbing aerosol, suggesting that differences in aerosol forcing can explain part of the difference in hydrological sensitivity between models.

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