Storms and Storm Surges - Development of the storm climate

How did wind storm impact on storm surges and ocean waves develop in the past decades, and what may happen in the expected course of anthropogenic climate change?

Changes in storminess have a significant impact on a variety of socio-economic relevant activities and risks. An economic segment obviously sensitive to changes in the risk of wind-related damages is the insurance industry (Berz, 1993; Berz and Conrad, 1994). Other relevant aspects are related to ocean waves and storm surges, and their impact on off-shore activities, shipping, and coastal structures.

Using proxies (see 'How to determine decadal and longer variations in the storm climate?') indicates that a systematic roughening of storm-related risks has not happened in the past 200 years (see 'How has the storm climate developed in the last few decades and last few centuries?'), or so. On the other hand, a worsening has taken place in the past 50 years, and data during that period are good enough to examine the changes of storm surge and ocean wave statistics.

The availability of good weather analyses – on the global basis for instance the NCEP re-analyses (Kalnay et al., 1996) and, for the European region, the analyses provided by Feser et al. (2001) – allow a detailed analysis of changing ocean wave and storm surge conditions. To do so, 6-hourly (or even more frequent) wind- and air pressure analyses are used to run ocean wave (Günther et al., 1998; Sterl et al., 1998) and storm surge models (Flather et al., 1998b; Langenberg et al., 1999). In this way, homogeneous estimates of changes in the past 50, or so, years, can be constructed (Weisse and Plüß, 2006). Using the same models, also scenarios of expected climate change can be processed with respect to windstorms, ocean waves and storm surges (e.g., Flather et al., 1998a; Kauker, 1998; Debernard et al., 2003; Woth et al., 2005; Woth, 2005; Lowe and Gregory, 2001, 2005).

Along these lines, the analyses by Feser et al. (2001) have been used to examine changes in patterns of storminess (Weisse et al., 2005). In most parts of the Northeast Atlantic, storminess – given as annual frequency of gales per grid box – increased until the early 1990s, south of about 50°N there was a decrease. This pattern reversed almost completely in the early 1990s apart of the southern North Sea, where the trend towards more storms continued, albeit somewhat decelerated towards the end of the period, at least until 2004. Accordingly, simulations of high tide statistics reveal an increase of water levels of a few mm/year, both in the seasonal mean as well as in the high levels relative to the mean (Weisse and Plüß, 2005; Aspelien, 2006), in particular along the German Bight coast line.

Furthermore, in the HIPOCAS project (Soares et al., 2002) statistics of ocean (surface) waves have been derived. Extreme wave heights have increased in the Southeastern North Sea within the period 1958-2002 by rate of up to 1.8 cm/yr while for much of the UK coast a decrease is found. The increase in the Southeastern North Sea, however, is not constant in time. The frequency of high wave events has increased until about 1985-1990 and remained almost constant since that time (Weisse and Guenther 2006). This development closely follows that of storm activity (Weisse et al. 2005).

Scenarios of future wind conditions have been derived by several groups. The most useful is possibly the set of simulations with the model of the Swedish Rossby Center, which features not only an atmospheric component but also lakes and a dynamical description of the Baltic Sea (Räisänen et al., 2004). This model was run with boundary conditions taken from two global climate models; also the effect of two different emission scenarios has been simulated. In these simulations, strong westerly wind events are intensified by less than 10% at the end of the 21st century (Woth, 2005).

These changes of wind speed will have an effect on both North Sea storm surges and wave conditions. For the storm surges along the North Sea coast line, an intensification is expected, which may amount to an increase of 30 cm, or so, to the end of the century (Figure 5). To this wind-related change the mean level has to be added, so that for maximum values of 50 cm along the German Bight are plausible estimates for the increase of water levels during heavy storm surges. In the Elbe estuary, larger values up to 70 cm are derived. These numbers are associated with a wide range of uncertainty (± 50 cm) (Grossmann et al., 2006).

Figure 5: Expected changes in wind-related storm surge heights (left; maximum averaged across many years, RCAO model) and ocean wave heights (right; change of 99-percentile; averaged across a series of simulations using different models and scenarios. Shading indicates areas where signals from all models and scenarios have the same sign; red-positive, blue-negative.) in the North Sea at the end of the 21st century (emission scenario A2). Units: m. Courtesy Katja Woth and Iris Grabemann.

Scenarios of future wave conditions show large differences in the spatial patterns and the amplitude of the climate change signals. There is, however, agreement among models and scenarios that extreme wave heights may increase by up to 30 cm (7% of present values) in the Southeastern North Sea by 2085 (Weisse and Grabemann, in prep., Figure 5).

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References

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