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David J. Karoly
and
Qigang Wu

Abstract

Trends in surface temperature over the last 100, 50, and 30 yr at individual grid boxes in a 5° latitude–longitude grid are compared with model estimates of the natural internal variability of these trends and with the model response to increasing greenhouse gases and sulfate aerosols. Three different climate models are used to provide estimates of the internal variability of trends, one of which appears to overestimate the observed variability of surface temperature at interannual and 5-yr time scales. Significant warming trends are found at a large fraction of the individual grid boxes over the globe, a much larger fraction than can be explained by internal climate variations. The observed warming trends over the last 50 and 30 yr are consistent with the modeled response to increasing greenhouse gases and sulfate aerosols in most of the models. However, in some regions, the observed century-scale trends are significantly larger than the modeled response to increasing greenhouse gases and sulfate aerosols in the atmosphere. Warming trends consistent with the response to anthropogenic forcing are detected at scales on the order of 500 km in many regions of the globe.

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Michael E. Mann
,
Scott Rutherford
,
Eugene Wahl
, and
Caspar Ammann

Abstract

Two widely used statistical approaches to reconstructing past climate histories from climate “proxy” data such as tree rings, corals, and ice cores are investigated using synthetic “pseudoproxy” data derived from a simulation of forced climate changes over the past 1200 yr. These experiments suggest that both statistical approaches should yield reliable reconstructions of the true climate history within estimated uncertainties, given estimates of the signal and noise attributes of actual proxy data networks.

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Jeffrey T. Kiehl
,
Julie M. Caron
, and
James J. Hack

Abstract

Climate model simulations of the latter part of the twentieth century indicate a warming of the troposphere that is equal to or larger than the warming at the surface, while satellite observations from the Microwave Sounding Unit (MSU) indicate little warming of the troposphere relative to surface observations. Recently, Fu et al. proposed a new approach to retrieving free tropospheric temperature trends from MSU data that better accounts for stratospheric cooling, which contaminates the tropospheric signal and leads to a smaller trend in tropospheric warming. In this study, climate model simulations are used as a self-consistent dataset to test these retrieval algorithms. The two methods of retrieving tropospheric temperature trends are applied to three climate model simulations of the twentieth century. The Fu et al. algorithm is found to be in very good agreement with the model-simulated tropospheric warming, indicating that it accurately accounts for cooling from the lower stratosphere.

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Rong Zhang
and
Thomas L. Delworth

Abstract

In this study, a mechanism is demonstrated whereby a large reduction in the Atlantic thermohaline circulation (THC) can induce global-scale changes in the Tropics that are consistent with paleoevidence of the global synchronization of millennial-scale abrupt climate change. Using GFDL’s newly developed global coupled ocean–atmosphere model (CM2.0), the global response to a sustained addition of freshwater to the model’s North Atlantic is simulated. This freshwater forcing substantially weakens the Atlantic THC, resulting in a southward shift of the intertropical convergence zone over the Atlantic and Pacific, an El Niño–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions.

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Shaleen Jain
,
Martin Hoerling
, and
Jon Eischeid

Abstract

Assessing climate-related societal vulnerability and mitigating impacts requires timely diagnosis of the nature of regional hydrologic change. A late-twentieth-century emergent trend is discovered toward increasing year-to-year variance (decreasing reliability) of streamflow across the major river basins in western North America—–Fraser, Columbia, Sacramento–San Joaquin, and Upper Colorado. Simultaneously, a disproportionate increase in the incidence of synchronous flows (simultaneous high or low flows across all four river basins) has resulted in expansive water resources stress. The observed trends have analogs in wintertime atmospheric circulation regimes and ocean temperatures, raising new questions on the detection, attribution, and projection of regional hydrologic change induced by climate.

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Brian J. Soden
,
Anthony J. Broccoli
, and
Richard S. Hemler

Abstract

Uncertainty in cloud feedback is the leading cause of discrepancy in model predictions of climate change. The use of observed or model-simulated radiative fluxes to diagnose the effect of clouds on climate sensitivity requires an accurate understanding of the distinction between a change in cloud radiative forcing and a cloud feedback. This study compares simulations from different versions of the GFDL Atmospheric Model 2 (AM2) that have widely varying strengths of cloud feedback to illustrate the differences between the two and highlight the potential for changes in cloud radiative forcing to be misinterpreted.

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Oleg A. Saenko
,
Andreas Schmittner
, and
Andrew J. Weaver

Abstract

A global, oceanic teleconnection of salinity, meridional overturning circulation (MOC), and climate of the North Atlantic and North Pacific is proposed. Simulations with a global climate model show that an extraction of freshwater from the Pacific results not only in an increase of salinity there, but also in a decrease of salinity in the Atlantic. As a result, a Pacific MOC develops while the Atlantic MOC collapses without freshwater perturbation in the Atlantic. Similarly, an input of freshwater to the Atlantic leads not only to a decrease of salinity there, but also to an increase of salinity in the Pacific. The Atlantic MOC collapses, whereas the Pacific MOC develops without freshwater perturbation in the Pacific. The mechanism behind this antiphase Atlantic– Pacific relationship is the positive feedback between ocean circulation and salinity contrasts, originally proposed by Stommel to operate between low and high latitudes. Here the authors show that the same mechanism operates on the Atlantic–Pacific interbasin scale, with the Southern Ocean acting as a pivot point for the interbasin seesaw. The proposed Atlantic–Pacific seesaw effect helps to explain some major out-of-phase oscillations of the climate states between the North Atlantic and North Pacific during the last deglaciation.

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Laura M. Ciasto
and
David W. J. Thompson

Abstract

The authors examine wintertime atmosphere–ocean interaction on weekly time scales over the North Atlantic sector. Consistent with previous results, it is found that the strongest interactions between the ocean and atmosphere occur when the atmosphere leads. However, the authors also find a spatially coherent and statistically significant pattern of sea surface temperature anomalies over the Gulf Stream extension region that precedes changes in the leading mode of Northern Hemisphere atmospheric variablilty by ∼2 weeks.

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Xavier Rodó
and
Miquel-Angel Rodriguez-Arias

Abstract

High-resolution sedimentary proxies from low latitudes are rare but nevertheless important to understanding the role of tropical regions in the global climate. The reanalysis of a sedimentary record from Lake Pallcacocha (Andes) shows that ENSO was present throughout the Holocene. Even from 10 000 to 7000 calendar years before present, when the Tropics underwent a period of low variance, there is still evidence of a weak ENSO. This weakening, however, has been strongly overestimated. A frequency decomposition shows that all frequency components, except the millennial band (which has a different origin), covary synchronically for more than 6000 yr. A need to reconcile methodologies and results from climate studies at different time scales thus arises.

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Agatha M. de Boer
and
Doron Nof

Abstract

During glacial periods, climate records are marked by large-amplitude oscillations believed to be a result of North Atlantic (NA) freshwater anomalies, which weakened the thermohaline circulation (THC) and introduced instabilities. Such oscillations are absent from the present interglacial period. With the aid of a semiglobal analytical model, it is proposed that the Bering Strait (BS) acts like an exhaust valve for the above NA freshwater anomalies. Specifically, it is suggested that large instabilities in the THC are only possible during glacial periods because, during these periods, the BS is closed. During interglacial periods (when the BS, the exhaust valve, is open), low-salinity anomalies are quickly flushed out of the North Atlantic by the strong Southern Ocean winds.

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