ABSTRACT
The overall objective of STOWASUS-2100 is to study severe storms, surges and waves in the present climate and in a scenario with increased CO2-concentration. More specifically the project is a joint atmospheric/oceanographic numerical modelling effort aiming at constructing and analysing storm, wave and surge climatologies for the North Atlantic/European region in a climate forced by increasing amounts of greenhouse gases and to compare with present day conditions. It is investigated whether any systematic anomalies regarding frequency, intensity or area of occurrence are found for these extreme events. Also physical mechanisms responsible for possible scenario anomalies are investigated.
INTRODUCTION
Off-shore industries, fisheries, shipping companies, and the insurance business are highly sensitive to extra-tropical strong wind events and the associated ocean waves and surges. It is likely that impacts of possible future changes in the occurrence of extreme type events like these and others will be more severe than modulations of the long term mean climate. This is the rationale behind the STOWASUS-2100 project which aims at setting up climate change scenarios for storms, waves and surges on a variety of spatial scales. On the larger scales, studies on storminess in the North Atlantic region will be performed, while detailed studies on storminess, surges and waves will be carried out in the Adriatic, The North Sea and the Norwegian Sea. On the local scales, storms and surges will be studied in estuaries, low lying coastal areas along the North Mediterranean and North-western European coasts.
The project builds to considerable degree on the results obtained in another project called WASA, which has been described by “The WASA Group” (1998). In WASA it was found that the storm and wave climate has roughened in recent decades, but that the present intensity of the storm and wave climate seems comparable with that at the beginning of the 20th century. The WASA project furthermore analysed and used the output from a high-resolution (T106 spectral truncation) climate change scenario experiment, mimicking global warming due to increase greenhouse gas concentrations. It was found that storm and extreme wave activity was slightly increased in the Bay of Biscay and in the North Sea in a warmer climate, while this activity was slightly weakened at several other places. The experimental set-up of the climate model simulations on which these results were based has been described by Beersma et al. (1997) who pointed out that the projected anthropogenic changes in storm activity fall well within the limits of variability observed in the past considering the length of the (control and scenario) simulations which was only 5-years.
Recently, two so called time slice simulations with the ECHAM4 model also at T106 horizontal resolution have been performed at the Danish Meteorological Institute (DMI) in a collaboration between the Max Planck Institute for Meteorology in Hamburg and DMI. These simulations each covered a period of 30 years, i.e. 6 times longer than the simulations used in WASA. Thus they should be much more suited for studies of storminess and associated impacts since the sampling problem is considerably reduced. The STOWASUS-project therefore uses these new simulations as a backbone to drive very high resolution regional atmospheric climate models and wave and surge models of different resolution. It is these secondary simulations which will be used to set up climate change scenarios of storms, waves and surges along European shelf and in European Estuaries and to compare these with present day conditions. The project is logically divided into 12 working tasks some of which will be described briefly in the following three sections together with preliminary results – to the extend they are available at this early state of the project.
INTENSIVE STORMS
As mentioned above the backbone in STOWASUS-2100 consists of two 30 year time slice simulations with the ECHAM4 atmospheric climate model at T106 horizontal resolution. The experimental design of these simulations which are not part of the project is described in May and Roeckner (1998). The project includes an analysis of the storm and extreme wind climate in these simulations. Fig. 1 (left column) shows the long term mean sea level pressure (MSLP) in winter (DJF) as obtained from the European Re-Analysis data (ERA), from the control simulation and from the scenario simulation. It is seen, that the ECHAM4 model simulates the atmospheric mass field well except for a too high pressure over and immediately to the west of the Iberian peninsula which leads to a too zonal flow over NE Atlantic region. The figure also shows the difference in the MSLP between the scenario and control simulations in the bottom panel, and it is seen that there is a significant increase in the zonality over the northern part of the area associated with a decrease in the high latitude MSLP in the scenario. The right column in Fig. 1 similarly shows the standard deviation of the band pass filtered (2.5-6 day filter) 500 hPa winter height fields which is commonly used as an estimate of storm activity. Also here the model behaves well and a significant increase in storm activity is seen over Northern Europe in the scenario simulation relative to the control simulation together with a corresponding decrease in storm activity around the east coast of US. Fig.2 shows the 1% percentile wind speed for the winters (DJF). Comparing the ERA data and the control run shows that the ECHAM4 model has more severe storms along the south and east coast of Greenland than the ERA data. The difference between the scenario run compared to the control run (fig. 2d) shows more severe storms in the Atlantic north of 60N, and less severe storms south of this latitude. These changes are in accordance with the changes in the 500 hPa variability (fig. 1d). The changes in near surface wind (Fig. 2) are important to the wave and surge simulations (see below) as one can expect that the enhanced wind speeds will also lead to more severe wave and surge activity. This has, however, not yet been shown in the project.
The atmospheric investigations will also cover atmospheric modelling with very high resolution regional climate models (HIRHAM and BOLAM) to perform of intensive systems that are not well resolved at T106 resolution: intensive extra-tropical baroclinic developments, polar lows and highly convective systems (with some apparent similarities to polar lows) in the Mediterranean. Fig. 3 shows the orography in the T106 model and in the BOLAM model over the Mediterranean region and it is seen that one may expect much larger impact from orographic effects at very high resolution than at T106. All the simulations with HIRHAM and BOLAM will take boundary conditions from the T106 time slice simulations and will to considerable degree focus on analysing and understanding the processes associated with possible changes in scenario cases relative to control cases.