A51F-01 08:04h
Climate change and North Sea storm surge extremes - an ensemble study (PRUDENCE)
Possible changes in North Sea storm surge climate are studied in a systematic manner. Following up on previous studies, we use a tide-surge model to derive storm surge climate and extremes from atmospheric conditions under present-day and enhanced greenhouse gas conditions. Results for modeled storm surges obtained by using regional model output from four RCMs, namely CLM (GKSS), RCAO (SMHI), REMO5 (MPI) and HIRHAM (DMI) are presented. The atmospheric regional simulations were prepared within the EU project PRUDENCE. The research strategy of PRUDENCE is to compare simulations of different regional models (RCMs) driven by the same global control and climate change simulations. These global conditions, representative for 1961-1990 and 2071-2100 were prepared by the Hadley Center based on the IPCC A2 SRES scenario. The effect on windiness of the enhanced greenhouse gas conditions, projected by these four regional climate models was in all cases similar, namely a moderate increase of high wind speeds in most parts of the North Sea during winter. These simulated surface wind and pressure data have been used to run a storm surge model. We show the expected storm-related changes in different storm surge parameters. For instance, the largest increase of high water levels, defined as the 99.5%ile during winter sampled every half hour would have to be expected along the southern and eastern North Sea coast, with maximum values of around 30 cm, which is beyond the range of normal year-to-year variations. Similar results can be found for all four experiments. Together with the expected rise of mean water levels of 40 cm by IPCC (2001), the total increase is 70 cm at the end of the 21st century under the assumptions of the rather severe A2 scenario. If an ECHAM-scenario A2 is used, quantitatively similar results are obtained, but in that case there are also significant increases along parts of the UK coast.
A51F-02 08:16h
NARCCAP North American Regional Climate Change Assessment Program A Multiple AOGCM and RCM Climate Scenario Project over North America
NARCCAP is a new international program that will serve the climate scenario needs of both the United States and Canada. We are systematically investigating the uncertainties in regional scale projections of future climate and producing high resolution climate change scenarios using multiple regional climate models (RCMs) and multiple global model responses to future emissions scenarios, by nesting the RCMs within multiple atmosphere ocean general circulation models (AOGCMs) forced with the A2 and A1B SRES scenarios, over a domain covering the conterminous US and most of Canada. The plan also includes a validation aspect through nesting the participating RCMs within reanalyses. The basic spatial resolution of the RCMs is 50 km. This program will include RCMs that participated in the European PRUDENCE program (HadRM3 and RegCM), the Canadian regional climate model (CRCM) as well as the NCEP regional spectral model (RSM) and MM5. Candidate AOGCMs include the Hadley Centre HadCM3, NCAR CCSM, the Canadian CGCM3 and the GFDL model. The resulting climate model runs will form the basis for multiple high resolution climate scenarios that can be used in climate change impacts assessments in the US and Canada. High-resolution global time slice experiments based on the GFDL atmospheric model and the NCAR atmospheric model (CAM3) will also be produced and will be compared with runs of the regional models. There also will be opportunities for double nesting over key regions through which additional modelers in the regional modeling community will be able to participate in NARCCAP. Additional key science issues will be investigated such as the importance of compatible physics in the nested and nesting models. Measures of uncertainty across the multiple runs will be developed by geophysical statisticians.
http://www.narccap.ucar.edu
A51F-03 08:28h
Are recent European summer climate trends and extremes consistent with future regional climate projections?
Summer climate over Europe in recent decades has been characterized by a drying trend and by the occurrence of especially devastating drought and flood events, such as in the summers of 2002 and 2003. We compare these trends with results from regional climate model simulations of future climate over Europe under increased greenhouse gas concentrations (GHG). We find that the projected changes in mean summer precipitation, surface temperature, and large-scale circulations are remarkably consistent with the observed changes in recent decades. Although we cannot directly attribute the observed changes to an anthropogenic GHG forcing, this result suggests that the observed drying trend over most of Europe might continue in the future. Our experiments additionally indicate substantial changes in the intensity and persistence of summer drought and flood. We identify the Central Mediterranean and Central/Western Europe to be especially vulnerable to increases in both summer drought and flood, suggesting that the frequency of devastating events such as those experienced in recent summers might also increase.
A51F-04 INVITED 08:40h
Using Regional Models to Assess the Potential for Extreme Climate Change
Scientists using regional mesoscale models have made exciting progress in the study of possible climate change and its impacts on human society. These models use the circulation pattern from General Circulation Models (GCMs) as background (lateral boundary) conditions to simulate climate change at the regional and local (or city) scale. Results from the mesoscale model can then be used in, for example, health impact studies, and/or for planning future water and energy use requirements. We present results from an observational and modeling study. The observational study is used to identify changes in the ocean-atmospheric system that affects summertime temperature anomalies over the eastern United States. These include changes in sea surface temperature, surface pressure over the southeastern U.S, as well as the frequency and timing of precipitation. Certain configurations of the "MM5" mesoscale model nested in the "GISS-GCM" reproduced facets of the observed climate-system better than others. We simulated future climate change in the 2050s and 2080s. In some years, the nested mesoscale model simulated temperature anomalies much larger than the GISS-GCM. The modeled characteristics of the ocean-atmospheric system have analogs in the observational data, suggesting that extreme climate change should be considered as one possible outcome of greenhouse forcing on the earth-climate system.
A51F-05 09:00h
Climate Trends and Variability over North America Predicted by a Regional Climate Model for Increased CO2
The MM5 regional climate model is adapted for long-term runs and used to perform two 10-year simulations over North America for the present (1991-1999 AD) and for approximately doubled CO2 (2090-2099 AD), driven by an archived transient simulation of the NCAR CSM. Characteristic changes in surface temperature and precipitation are analyzed, emphasizing diurnal ranges, interdiurnal variability of temperature, and frequency and intensity of precipitation events. Relative to the present, strongest wintertime warming in 2091-2099 occurs poleward of 40oN, with increases of ~6oC in some regions. However, the southern and southeastern U.S. experience colder winters, with temperature decreases of 0.5 to 2oC. During the summer, most areas of North America become warmer and increases in daily maximum temperature are much higher than those of daily minimum. Southward of 40oN, winters become more variable with greater storm activity, interdiurnal variability, and increased cold events in winter and hot events in summer. In summer, zones of significant precipitation generally shift northward with increased CO2, leaving some central U.S. regions much drier. In winter, precipitation increases along most coastal regions of U.S. The frequency and intensity of precipitation change significantly over most of the domain with increased CO2 . Averaged over all land, the frequency of weak precipitation events is relatively constant, but the frequency of heavy precipitation events (>=64 mm/day) increases dramatically by 35%-40% in spring and summer. Precipitation intensity increases from March to August due to the greater number of heavy precipitation events, and the ratio of convective to total precipitation increases. These predicted shifts from weak to heavy precipitation events suggest that flooding may become more prevalent over much of North America due to increased CO2 towards the end of the next century.
A51F-06 09:12h
Response of eastern boundary current upwelling to regional-scale atmosphere-land cover feedbacks induced by elevated atmospheric carbon dioxide concentrations
The response of marine and terrestrial environments to global changes in atmospheric carbon dioxide concentrations will likely be governed both by responses to direct environmental forcing and by responses to Earth system feedbacks induced by that forcing. It has been proposed that anthropogenic greenhouse forcing will intensify coastal upwelling in eastern boundary current regions. To test the potential response of eastern boundary current upwelling to regional-scale atmosphere-land cover feedbacks, we have employed a regional climate model (RegCM2.5) asynchronously coupled to an equilibrium vegetation model (BIOME4), focusing on the California Current as a case study. Biophysical atmosphere-land cover feedbacks enhanced the radiative effects of carbon dioxide on land-sea thermal contrast, resulting in changes in total-seasonal upwelling and upwelling seasonality. Specifically, relative to greenhouse forcing, land cover-atmosphere feedbacks led to a stronger increase in peak- and late-season upwelling in the northern limb of the California Current and a stronger decrease in peak- and late-season upwelling in the southern limb. The response of coastal upwelling to atmosphere-land cover feedbacks was driven by changes in surface temperature over land. Of the total seasonal temperature response to elevated atmospheric carbon dioxide levels, up to 60 % was due to land cover change. In many areas, such as the Great Basin, albedo acted as the primary control on these changes in surface temperature. Along the central coast of California, soil moisture effects magnified the temperature response in JJA and SON, with negative surface soil moisture anomalies accompanied by negative evaporation anomalies, decreasing latent heating and further increasing surface temperature. Additionally, negative temperature anomalies were calculated at high elevation in California and Oregon in DJF, MAM and SON, indicating that future warming of these sensitive areas could be mitigated by changes in vegetation distribution and an associated muting of winter snow-temperature feedbacks. However, the mean regional temperature sensitivity to regional-scale land cover feedbacks did not exceed the large-scale sensitivity calculated elsewhere, indicating that spatial heterogeneity does not introduce non-linearities in the response of regional temperature to carbon dioxide-induced atmosphere-land cover feedbacks.
A51F-07 09:24h
Attribution of South West Australia observed rainfall trends
A statistical method, based on the idea of analogous synoptic situations, is used to study the observed rainfall trends in South-West of Western Australia (SWA). The method has been developed and optimized by relating patterns of atmospheric field (predictors) to station records of rainfall (predictands) in the SWA, for both rain occurrences and daily rain amounts. When tested on observational data, the analogue method is able to reproduce the observed trend and the inter-annual variability observed during the past 50 years, showing the sensitivity of the analogue approach to changed climatic conditions. Among the parameters considered, the set of predictors has one of the most important impacts on results. It is demonstrated that moisture fields explain an important part of the observed trend. The model is then applied to two ensembles of four simulations of the past century performed with the Parallel Climate Model (PCM) developed at NCAR. One ensemble is forced with natural forcings: volcanic eruptions and solar variability. The other ensemble is obtained adding human induced forcing to the natural forcings: greenhouse gases, ozone and aerosols. It is shown that the downscaling approach improve the reproduction of local rainfall compare with rainfall directly modeled to the point that realistic statistics of the local series are reproduced. A significant difference is observed between two sets of ensembles for the reproduction of the observed drying trend. With natural forcing only, the downscaling of the ensemble suggest an increase of rainfall in the SWA during the second half of the 20th century. When anthropogenic atmospheric forcings are added, the four-member ensemble gives an average reduction of the rainfall, which resemble what has been observed but with the magnitude is halved. One member of this ensemble was re-run with the effect of global land clearance taken into account. In this case, the drying trend is enhanced and resembles what was observed. These results suggest that the observed drying trend in the SWA is due to anthropogenic forcing and has been enhanced by global and local land clearance.
A51F-08 09:36h
Modes of Variability in the West African Monsoon and their Relevance to Climate Change
Two modes of regional climate variability on interannual time scales over West Africa are investigated, and their relevance for past and future climate is examined. One mode involves a low-level westerly jet, recently identified in high-resolution satellite observations and well simulated in a regional climate model (RCM). In today's climate, relatively wet conditions can be associated with an intensification of this jet, which is accompanied by increased moisture transport into Sahelian Africa from the Atlantic. RCM simulations of the African Humid Period, 6-8 ka, show that the westerly jet was also stronger during that time of enhanced summertime insolation. The "greening" of the Sahara and Sahel during that time was related to enhanced moisture advection and convergence by the westerly jet and not with, for example, enhanced southerly monsoon flow across the Guinean coast. Another prominent mode of interannual variability over West Africa is drying across Sahelian Africa during years of relatively warm sea surface temperatures in the Gulf of Guinea. When the Gulf of Guinea is 1-2 K warmer than the climatology, rainfall along the Guinean coast to the north is enhanced due to stronger evaporation over the Gulf. The resulting strong condensation heating induces northerly flow over the Sahel, which sinks to conserve potential vorticity. Convection is suppressed, and anomalously low precipitation rates are induced. This mode of variability has implications for future climate, and the relationship between greenhouse-gas induced warming of the Gulf of Guinea and the occurrence of Sahelian drought is explored.
A51F-09 09:48h
The Impact of Water Table Dynamics on Regional Climate
The water cycles in the land and atmosphere make up a fundamentally coupled system, with complex interactions among all reservoirs (atmosphere, soil-vegetation, groundwater, and rivers) over a wide range of space and time scales. Our focus is on the role of the groundwater reservoir in this land-atmosphere coupling: the water table's dynamical interaction with infiltration, stream flow, and evapotranspiration, its control of the spatio-temporal organization of soil moisture, and hence its impact on atmospheric processes such as precipitation. Our hypothesis is that the water table dynamics plays an important role in land-atmosphere interactions and feedbacks, and thus the regional climate system. To date, however, no study has investigated the three-dimensional, dynamic coupling between all of the reservoirs, atmospheric, surface, and subsurface, of the terrestrial water cycle. Our goal is to address this fundamental gap in our knowledge. The key to understanding the role of the water table in the two-way coupling between land and atmosphere lies in its influence on the soil water content near the surface and in the root zone. In particular, the water table depth controls the equilibrium soil water profile in the unsaturated zone. Therefore, the characteristic spatial and temporal organization of water table depth (that reflects its governing dynamics) is communicated to the soil moisture field, and in turn, via surface-atmosphere fluxes, to the atmosphere. At the same time, the surface and subsurface hydrology responds strongly to atmospheric processes such as evaporative demand and the distribution, frequency, and intensity of precipitation. We wish to understand the implications of this two-way linkage for our current understanding of land-atmosphere feedbacks, e.g., between soil moisture and convective rainfall. Answering these questions requires a tool capable of studying the integrated terrestrial water cycle. Until now, no such tool was available. To address this need, we have built the fundamental groundwater and river flow processes into the Regional Atmospheric Modeling System (RAMS), including a prognostic, process-based boundary condition (i.e., the water table) for the unsaturated soil column, a mass balance for the saturated storage which governs the water table dynamics, lateral groundwater flow from cell to cell, two-way groundwater-stream interaction within a cell, surface runoff to local rivers, and continental river routing to the ocean. With our new tool, it is now possible to study the co-evolution of the water table, soil moisture, atmospheric circulation, clouds, and precipitation, along with the feedbacks that link them, in the context of one internally consistent model system. Here we report on results from preliminary numerical experiments with our new modeling system over the continental U.S. during the warm season. These experiments show that the inclusion of full water table dynamics in a regional climate simulation has a strong impact on the spatial and temporal organization of soil moisture at continental and large river basin scales, and that this impact is felt by the surface fluxes of sensible and latent heat, thereby driving changes in atmospheric dynamics and precipitation. Implications for land-atmosphere feedbacks will be discussed. In addition, the potential of this new tool for confronting a wide range of future integrated water cycle problems, e.g., in the areas of seasonal-to-interannual prediction, predicting and managing water resources, planning for weather-related emergencies, and projecting future coupled climatic-hydrologic changes due to natural and anthropogenic forcings, will also be discussed.