[1] This study examines the temporal variability of ocean heat uptake in observations and in climate models. Previous work suggests that coupled Atmosphere-Ocean General Circulation Models (A-OGCMs) may have underestimated the observed natural variability of ocean heat content, particularly on decadal and longer timescales. To address this issue, we rely on observed estimates of heat content from the 2004 World Ocean Atlas (available at http://www.nodc.noaa.gov/OC5/indprod.html, hereinafter referred to as WOA-2004) compiled by Levitus et al., 2005. Given information about the distribution of observations in WOA-2004, we evaluate the effects of sparse observational coverage and the infilling that Levitus et al. use to produce the spatially complete temperature fields required to compute heat content variations. We first show that in ocean basins with limited observational coverage, there are important differences between ocean temperature variability estimated from observed and infilled portions of the basin. We then employ data from control simulations performed with eight different A-OGCMs as a test bed for studying the effects of sparse, space-varying and time-varying observational coverage. Subsampling model data with actual observational coverage has a large impact on the inferred temperature variability in the top 300 and 3000 m of the ocean. This arises from changes in both sampling depth and in the geographical areas sampled. Our results illustrate that subsampling model data at the locations of available observations increases the variability, reducing the discrepancy between models and observations.

[1] Stratospheric sulfate aerosol particles from strong volcanic eruptions produce significant transient cooling of the troposphere and warming of the lower stratosphere. The radiative impact of volcanic aerosols also produces a response that generally includes an anomalously positive phase of the Arctic Oscillation (AO) that is most pronounced in the boreal winter. The main atmospheric thermal and dynamical effects of eruptions typical of the past century persist for about two years after each eruption. In this paper we evaluate the volcanic responses in simulations produced by seven of the climate models included in the model intercomparison conducted as part of the preparation of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). We consider global effects as well as the regional circulation effects in the extratropical Northern Hemisphere focusing on the AO responses forced by volcanic eruptions. Specifically we analyze results from the IPCC historical runs that simulate the evolution of the circulation over the last part of the 19th century and the entire 20th century using a realistic time series of atmospheric composition (greenhouse gases and aerosols). In particular, composite anomalies over the two boreal winters following each of the nine largest low-latitude eruptions during the period 1860-1999 are computed for various tropospheric and stratospheric fields. These are compared when possible with observational data. The seven IPCC models we analyzed use similar assumptions about the amount of volcanic aerosols formed in the lower stratosphere following the volcanic eruptions that have occurred since 1860. All models produce tropospheric cooling and stratospheric warming as in observations. However, they display a considerable range of dynamic responses to volcanic aerosols. Nevertheless, some general conclusions can be drawn. The IPCC models tend to simulate a positive phase of the Arctic Oscillation in response to volcanic forcing similar to that typically observed. However, the associated dynamic perturbations and winter surface warming over Northern Europe and Asia in the post-volcano winters is much weaker in the models than in observations. The AR4 models also underestimate the variability and long-term trend of the AO. This deficiency affects high-latitude model predictions and may have a similar origin. This analysis allows us to better evaluate volcanic impacts in up-to-date climate models and to better quantify the model Arctic Oscillation sensitivity to external forcing. This potentially could lead to improving model climate predictions in the extratropical latitudes of the Northern Hemisphere.

Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, Aerosol Robotic Network, and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at middle to high latitudes and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be À0.19 ± 0.09 Wm À2 . This translates into an estimated global cooling of 0.05 to 0.12°C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.

Data from the satellite-based Special Sensor Microwave Imager (SSM/I) show that the total atmospheric moisture content over oceans has increased by 0.41 kg/m 2 per decade since 1988. Results from current climate models indicate that water vapor increases of this magnitude cannot be explained by climate noise alone. In a formal detection and attribution analysis using the pooled results from 22 different climate models, the simulated ''fingerprint'' pattern of anthropogenically caused changes in water vapor is identifiable with high statistical confidence in the SSM/I data. Experiments in which forcing factors are varied individually suggest that this fingerprint ''match'' is primarily due to humancaused increases in greenhouse gases and not to solar forcing or recovery from the eruption of Mount Pinatubo. Our findings provide preliminary evidence of an emerging anthropogenic signal in the moisture content of earth's atmosphere.climate change ͉ climate modeling ͉ detection and attribution ͉ water vapor '' F ingerprint'' studies, which seek to identify the causes of recent climate change, involve rigorous statistical comparisons of modeled and observed climate change patterns (1). Such work has been influential in shaping the ''discernible human influence'' conclusions of national and international scientific assessments (2-4). Most fingerprint studies have focused on temperature changes at the earth's surface (5, 6), in the free atmosphere (7,8), or in the oceans (9), or have considered variables whose behavior is directly related to changes in atmospheric temperature (10).Despite a growing body of empirical evidence documenting increases in moisture-related variables (11,12), and climate model evidence of a number of robust hydrological responses to global warming (13,14), there have been no formal fingerprint studies involving changes in the total amount of atmospheric water vapor, W. Other aspects of moisture changes have received attention in recent fingerprint work, with identification of an anthropogenic signal in observed records of continental river runoff (15), zonal mean rainfall (16), and surface specific humidity (17).Warming induced by human-caused changes in well mixed greenhouse gases (GHGs) should increase W (11,12). Under the assumption that relative humidity remains approximately constant, for which there is considerable empirical support (13,18,19), the increase in W is estimated to be Ϸ6.0-7.5% per degree Celsius warming of the lower troposphere (13, 18). The observed increase in W over the global ocean, as inferred since late 1987 from microwave radiometry measurements made with the satellite-borne Special Sensor Microwave Imager (SSM/I), is broadly consistent with theory (12,18,20). Observational and Model DataThe SSM/I atmospheric moisture retrievals are based on measurements of microwave emissions from the 22-GHz water vapor absorption line. The distinctive shape of this line provides robust retrievals that are less problematic than other types of satellite measurement. For example, the si...

Changes in the vertical structure of atmospheric temperature have been proposed as a possible "fingerprint" of greenhouse-gas-induced climate change4-'. Until recently, most of our information about the structure of such a fingerprint has been derived from equilibrium COa doubling experiments performed with atmospheric General Circulation Models ( AGCMs) coupled to mixed-layer oceansgJo. These experiments yielded a fingerprint pattern characterized by stratospheric cooling, tropospheric warming, a warming maximum in the tropical upper troposphere, and (for annual mean zonally-averaged changes) an approximate hemispheric symmetry of the temperature response (see Fig. la).One recent study '' has compared such model-predicted patterns of temperature change with observed latitude-height temperature-change profiles. The latter were 2 obtained from the radiosonde analyses of Oort, and span the period 1963-8712*13. The conclusion reached by this work was that the observed data showed an increasing expression of the equilibrium temperature-change signal predicted by two different AGCMs in response to C02 doubling. This time-increasing similarity was judged to be significant, and it was further concluded that the individual pattern signatures of El Nifio-Southern Oscillation (ENSO) events and stratospheric ozone reduction were spatially dissimilar to the searched-for COa fingerprint.Although suggestive of a causal relationship between increasing levels of atmospheric C 0 2 and the vertical structure of atmospheric temperature changes, this investigation did not claim attribution of all or even part of the observed changes to the specific cause of changes in CO2. The principal uncertainties were related to the quality and short record length of the radiosonde data, the lack of a dynamic ocean in the model experiments, the neglect of other anthropogenic forcings (such as changes in sulphate aerosol loadings) and concerns regarding the estimation of significance by resampling of the observed data1'J4. One further concern was whether natural climatic variability could mimic the model-predicted greenhouse fingerprint, as preliminary analyses of observations and model control runs had s u g g e~t e d '~-~~. All of these factors hampered more confident statements regarding detection of a significant change, and attribution of (some fraction of) that change to increasing C02.Our investigation differs from this earlier work in three ways. First, we examine the relative detectability of vertical temperature-change signals from recent experiments with individual and combined changes in atmospheric C02 and anthropogenic sulphate aerosolslg. Second, we consider how a combined C02+S04 vertical temperature-change signal might be modified by observed changes in stratospheric ozone. It is highly likely that recent reductions in stratospheric ozone are in part attributable to industrial production of halocarbons2'. These changes may have a complex signature in the thermal structure of the atmosphere, varying as a function of latitude, alt...