Attribution of Climate Variability During the Last 100 Years II
Presiding: M P Hoerling, NOAA Climate Diagnostics Center; A Kumar, NOAA Climate Prediction Center
A23A-01 13:30h
At What Horizontal Scale can Significant Climate Change be Detected in Surface Temperatures?
The assessment of the possible causes of observed climate change in the IPCC Third Assessment Report concluded that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations" (Mitchell et al. 2001). This conclusion was based on many studies of global and very large scale climate variations. That assessment also concluded that "surface temperature changes are detectable only on scales greater than 5,000 km". Since then, it has been shown that an anthropogenic climate change signal is detectable in continental-scale regions using surface temperature changes over the 20th century. For example, Stott (2003) used simulations with the HadCM3 model to show that most of the observed warming over the last 50 years in six separate regions of the globe, including North America, Eurasia and Australia, was likely to be due to the increase in greenhouse gases in the atmosphere. Almost no studies have considered the detection of surface temperature trends at a regional scale such as at the scale of an individual model grid box. This is because detection of anthropogenic climate change is a signal-to-noise problem and the noise associated with natural variations of surface temperatures at a regional scale is greater than at larger continental or global scales. Here, we report the results of exactly such a study. We show that observed trends in surface air temperature at individual grid boxes of size 500km are significant compared with estimates of internal variability at the majority of grid boxes over the globe and over a range of periods from 100 years down to the most recent 30 years. These observed trends are consistent with the response to increasing greenhouse gases in the atmosphere, but we cannot eliminate the possibility that other forcing factors, such as land surface change may have contributed to the observed trends in some regions. The IPCC TAR conclusion on the scale of detectable surface temperature changes is no longer correct.
A23A-02 14:00h
On the Nature of the Changes in Predictability of Drought and Pluvial Conditions in the U.S. Great Plains over the Last 100 Years
In this study we examine the predictability of extended periods of wet and dry conditions over the central United States using century-long simulations with the NASA Seasonal-to-Interannual Prediction Project (NSIPP-1) atmospheric General circulation model forced with observed SSTs. Moderately large ensembles of runs allow us to diagnose the changes over time in predictability assuming the SSTs are known. Extended (multi-year) periods of enhanced and reduced predictability in both drought and pluvial conditions are diagnosed in terms of changes in the "signal" (forced by the SST) and "noise" (determined by the unpredictable components of atmospheric and land variability). The changes in predictability are interpreted in terms of their dependence on the strength of soil moisture feedbacks, long-term (decadal) changes in the SST, as well as ENSO. The realism of the changes in predictability is assessed by comparing the simulated and observed properties of the seasonal mean precipitation statistics.
A23A-03 14:15h
The NAO, the AO, and Global Warming: How Closely Related?
The North Atlantic Oscillation (NAO) and closely related Arctic Oscillation (AO) strongly affect Northern Hemisphere (NH) surface temperatures with patterns reported similar to the global warming trend. The NAO and AO have been in a positive trend for much of the 1970s and 1980s with historic highs in the early 1990s, and it has been suggested that they contribute significantly to the global warming signal. We examine the trends in standard indices of the AO, NAO, and NH average surface temperature for Dec-Feb, 1950-2004, and the associated patterns in surface temperature anomalies. This analysis shows that the AO and NAO indices have been recently decreasing and the overall AO and NAO trends for the past thirty years are weak to non-existent, and strongly dependent on the choice of start and end date. In clear distinction, the wintertime hemispheric warming trend has been vigorous and consistent throughout the entire period. Also analyzed are factors previously identified as relating to the NAO, AO, and their positive trend: North Atlantic sea surface temperatures (SSTs), Indo-Pacific warm pool SSTs, stratospheric circulation, and Eurasian snow cover. While the NAO and AO may contribute to hemispheric and regional warming for multi-year periods, our analysis suggests that the large-scale features of the global warming trend over the last 30 years are unrelated to the AO and NAO. The other factors may also be clearly distinguished, with warm pool SSTs linked to the warming trend, while the others are linked to the NAO and AO. The results are insensitive to choice of index or dataset.
A23A-04 14:30h
Climatic Feedbacks during the 2003 European Heatwave
During the summer of 2003, a record heat wave over Europe occurred, to which the deaths of over 12,000 people in 18 countries were attributed, 10,000 of those in France alone. There were widespread wildfires, and the wildlife in the region was also dramatically affected, with high tolls being taken in the bird and fish populations. The Danube River reached its lowest level in more than a century, and Mediterranean Sea surface temperatures off the coast of Spain reached the highest levels observed in 45 years (32° C). Temperatures across Europe were above normal for most of the summer, but reached their peak during the first 2 weeks of August, when most of the deaths occurred. Ensemble simulations done with a recent version of the Center for Ocean-Land-Atmosphere Studies (COLA) atmospheric general circulation model (AGCM) have been analyzed over the European region. The simulations were forced by weekly mean observed sea-surface temperature (SST) obtained from the U.S. National Centers for Environmental Prediction (NCEP) and were initialized in late 1981. Relative to the 1982-2001 period, the COLA AGCM simulated anomalous warmth over the European region during June, July and August 2003, in response to the observed SST, however the simulated magnitude was smaller than observed. Additional simulations done with climatological SST verified that the simulated warmth was indeed related to SST. A series of simulations in which the observed SST was used only south of 25N or only north of 25N suggest that it was the influence of the local SST rather than remote SST that was an important influence on the heatwave. Analysis of the near real-time calculated global soil wetness provided by NCEP reveals that by early June the soil over much of the European region was anomalously dry, which is consistent with the below normal precipitation observed over the region in the preceding months. The soil wetness anomalies obtained from NCEP were adapted for use in the COLA AGCM and the model was restarted in early June with the resultant anomalies imposed on the soil wetness from the control simulations. Although the model soil wetness was then allowed to evolve as usual, the simulations with the imposed initial soil wetness anomaly enhanced the simulated surface temperature anomaly during June, July and August by 1-2° C. The experiments suggest that both the warm local SST and the dry local soil were important in intensifying the 2003 European heat wave.
A23A-05 14:45h
The Uncertainty in the Simulated Climate in Forcing and Prediction Experiments With the NASA GISS GCM II' due to the Uncertainty in the Forcing by Soil Dust Aerosols
The magnitude and even the sign of direct radiative forcing by soil dust aerosols is one of the most uncertain factors in climate change studies, and this contributes to an uncertainty in the response of the simulated climate due to this forcing as well as to other forcings like greenhouse gases. Our objective is to estimate the uncertainty in the simulated climate attributed to imprecisely known quantities such as the dust single scatter albedo and vertical distribution in transient climate forcing and prediction experiments carried out using the National Aeronautics and Space Administration Goddard Institute for Space Studies (NASA GISS) general circulation model II prime (GCM II'). Although various forcings like greenhouse gases, stratospheric aerosols, tropospheric sulfate, black and organic carbon are time dependent in these experiments, a three-dimensional distribution of soil dust aerosols that consists of an unvarying seasonal cycle is prescribed for the whole simulated period from 1951 to 2050. The forcing and prediction experiments with the dust single scattering albedo prescribed based upon measurement of Saharan dust are repeated using values that assume more reflection and absorption, respectively. We also compute the climate effect of concentrating the dust distribution closer to the surface. For each assumed vertical distribution and single scatter albedo of the dust particles, a 5-member ensemble is calculated with different initial conditions to account for the internal variability of the model climate. We analyze the uncertainty of main climate variables such as surface temperature, precipitation, cloud cover, and sea level pressure to the varied parameters and compare the response to the internal climate variability. We present results for the global average as well as for the geographic distribution of these variables.