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P. B. Duffy
,
C. Doutriaux
,
B. D. Santer
, and
I. K. Fodor

Abstract

It has been suggested that the apparent warming of the earth's surface during the twentieth century may be biased by large changes in the coverage of surface temperature measurements since 1900. This issue is investigated using climate model simulations. By imposing observed coverage changes on simulated surface temperatures, estimates are obtained of twentieth-century temperature change for both full global coverage and for actual historical coverage. In 10 out of 16 simulations including human climate perturbations, the temperature change from the globally complete model output is significantly larger than that derived from the historically masked model output. The remaining six simulations show no significant difference between complete and masked model output. Thus, these results do not support the hypothesis that the increase in the earth's surface temperature has been overestimated because of incomplete observational data. Rather, if the simulations analyzed are realistic, the true temperature increase over the last century is slightly larger than that estimated from available observations. Eight simulations of natural internal climate variability, which omit human climate perturbations, were analyzed. In none of these simulations does the temperature change during 100 yr—whether obtained from globally complete or masked model output—come close to the observed twentieth-century temperature increase.

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Amy C. Clement
,
Mark A. Cane
, and
Richard Seager

Abstract

Paleoclimatic data are increasingly showing that abrupt change is present in wide regions of the globe. Here a mechanism for abrupt climate change with global implications is presented. Results from a tropical coupled ocean–atmosphere model show that, under certain orbital configurations of the past, variability associated with El Niño–Southern Oscillation (ENSO) physics can abruptly lock to the seasonal cycle for several centuries, producing a mean sea surface temperature (SST) change in the tropical Pacific that resembles a La Niña. It is suggested that this change in SST would have a global impact and that abrupt events such as the Younger Dryas may be the outcome of orbitally driven changes in the tropical Pacific.

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C-P. Chang
,
Patrick Harr
, and
Jianhua Ju

Abstract

Since the 1970s, the inverse relationship between the Indian monsoon rainfall and the El Niño–Southern Oscillation (ENSO) has weakened considerably. The cause for this breakdown is shown to be most likely the strengthening and poleward shift of the jet stream over the North Atlantic. These changes have led to the recent development of a significant correlation between wintertime western European surface air temperatures and the ensuing monsoon rainfall. This western Europe winter signal extended eastward over most of northern Eurasia and remained evident in spring, such that the effect of the resulting meridional temperature contrast was able to disrupt the influence of ENSO on the monsoon.

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Kevin E. Trenberth
and
David P. Stepaniak

Abstract

To characterize the nature of El Niño–Southern Oscillation (ENSO), sea surface temperature (SST) anomalies in different regions of the Pacific have been used. An optimal characterization of both the distinct character and the evolution of each El Niño or La Niña event is suggested that requires at least two indices: (i) SST anomalies in the Niño-3.4 region (referred to as N3.4), and (ii) a new index termed here the Trans-Niño Index (TNI), which is given by the difference in normalized anomalies of SST between Niño-1+2 and Niño-4 regions. The first index can be thought of as the mean SST throughout the equatorial Pacific east of the date line and the second index is the gradient in SST across the same region. Consequently, they are approximately orthogonal. TNI leads N3.4 by 3 to 12 months prior to the climate shift in 1976/77 and also follows N3.4 but with opposite sign 3 to 12 months later. However, after 1976/77, the sign of the TNI leads and lags are reversed.

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Adrian M. Tompkins

Abstract

Tropical observations show convective activity increasing sharply above sea surface temperatures (SSTs) of around 26°C and then decreasing as the SST exceeds 30°C, with maximum observed SSTs of around 32°C.Although some aspects of this relationship are reasonably well understood, as of yet no theory has explained the decrease in convective activity above 30°C. Here it is suggested that this aspect of the relationship could result from a organizational positive feedback, sometimes termed “self aggregation,” whereby the occurrence of convection makes future convection more likely to occur in the same location. Using cloud-resolving simulations, it is shown that the feedback between convection and the water vapor field is a good candidate for this role.

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Tapio Schneider
and
Isaac M. Held

Abstract

An approach to identifying climate changes is presented that does not hinge on simulations of natural climate variations or anthropogenic changes. Observed interdecadal climate variations are decomposed into several discriminants, mutually uncorrelated spatiotemporal components with a maximal ratio of interdecadal-to-intradecadal variance. The dominant discriminants of twentieth-century variations in surface temperature exhibit large-scale warming in which, particularly in the Northern Hemisphere summer months, localized cooling is embedded. The structure of the large-scale warming is consistent with expected effects of increases in greenhouse gas concentrations. The localized cooling, with maxima on scales of 1000–2000 km over East Asia, eastern Europe, and North America, is suggestive of radiative effects of anthropogenic sulfate aerosols.

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Franco Biondi
,
Alexander Gershunov
, and
Daniel R. Cayan

Abstract

Climate in the North Pacific and North American sectors has experienced interdecadal shifts during the twentieth century. A network of recently developed tree-ring chronologies for Southern and Baja California extends the instrumental record and reveals decadal-scale variability back to 1661. The Pacific decadal oscillation (PDO) is closely matched by the dominant mode of tree-ring variability that provides a preliminary view of multiannual climate fluctuations spanning the past four centuries. The reconstructed PDO index features a prominent bidecadal oscillation, whose amplitude weakened in the late l700s to mid-1800s. A comparison with proxy records of ENSO suggests that the greatest decadal-scale oscillations in Pacific climate between 1706 and 1977 occurred around 1750, 1905, and 1947.

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Thomas L. Delworth
and
Keith W. Dixon

Abstract

Most projections of greenhouse gas–induced climate change indicate a weakening of the thermohaline circulation (THC) in the North Atlantic in response to increased freshening and warming in the subpolar region. These changes reduce high-latitude upper-ocean density and therefore weaken the THC. Using ensembles of numerical experiments with a coupled ocean–atmosphere model, it is found that this weakening could be delayed by several decades in response to a sustained upward trend in the Arctic/North Atlantic oscillation during winter, such as has been observed over the last 30 years. The stronger winds over the North Atlantic associated with this trend extract more heat from the ocean, thereby cooling and increasing the density of the upper ocean and thus opposing the previously described weakening of the THC. This result is of particular importance if the positive trend in the Arctic/North Atlantic oscillation is a response to increasing greenhouse gases, as has been recently suggested.

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Brian J. Soden
and
Steven R. Schroeder

Abstract

Multiple satellite records of tropical-mean water vapor are compared with a general circulation model (GCM) simulation to assess the ability to monitor and to predict low-frequency changes in total precipitable water. Particular attention is focused on the drying between 1979 and 1995 recorded by a TOVS statistical retrieval that is calibrated to radiosondes. Both a GCM integrated with observed SSTs and microwave and TOVS physical retrievals that overlap the drying period show no sustained drying. This discrepancy is consistent with the suggestion by Ross and Gaffen that the TOVS statistical algorithm is vulnerable to radiosonde instrumentation changes over this period that introduce an artificial drying trend into the retrieval.

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Ning Zeng
and
J. David Neelin

Abstract

Using a coupled atmosphere–land–vegetation model of intermediate complexity, the authors explore how vegetation–climate interaction and internal climate variability might influence the vegetation distribution in Africa. When the model is forced by observed climatological sea surface temperature (SST), positive feedbacks from vegetation changes tend to increase the spatial gradient between desert regions and forest regions at the expense of savanna regions. When interannual variation of SST is included, the climate variability tends to reduce rainfall and vegetation in the wetter regions and to increase them in the drier regions along this gradient, resulting in a smoother desert–forest transition. This effect is most dramatically demonstrated in a model parameter regime for which multiple equilibria (either a desertlike or a forestlike Sahel) can exist when strong vegetation–climate feedbacks are allowed. However, the presence of a variable SST drives the desertlike state and the forestlike state toward an intermediate grasslike state, because of nonlinearities in the coupled system. Both vegetation and interannual variability thus play active roles in shaping the subtropical savanna ecosystem.

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