Executive Summary
The European project WASA has been set up for verifying, or falsifying hypotheses of a worsening storm and wave climate in the Northeast Atlantic and its adjacent seas in the present century. Its main conclusion is that the storm- and wave climate in most of the Northeast Atlantic and in the North Sea has undergone significant variations on time scales of tens of years; it has indeed roughened in the past decades, but the present intensity of the storm- and wave-climate seems to compare with the intensity at the beginning of this century. Part of the variability is found to be related to the North Atlantic Oscillation. There is a slight increase of probabilities of high waves derived from conventional extreme value statistics in the North West approaches of the North Sea.
An analysis of a high-resolution climate change experiment, mimicking global warming due to increased greenhouse gas concentrations, results in weak increase of storm activity and (extreme) wave heights in the Bay of Biscay and in the North Sea, while storm action and waves slightly decrease along the Norwegian coast and in most of the remaining North Atlantic area in this scenario. A weak to moderate increase of storm surges in the southern and eastern part of the North Sea is expected. These projected anthropogenic changes at the time of CO2 doubling fall well within the limits of variability observed in the past.
A major methodical obstacle for the assessment of changes
in the intensity of storm and wave events are the inhomogeneities of the
observational record, both in terms of local observations and of analyzed
products (such as weather maps), which usually produce an artificial increase
of extreme winds. This is so because older analyses were based on fewer
observations and with limited conceptual and numerical models of the dynamical
processes. Therefore the assessment about changes in storminess is based
on local observations of air pressure and high-frequency variance at tide
gauges. Data of this sort is available for 100 years and sometimes more.
The assessment about changes in the wave climate is achieved by a two-step
procedure; first a state-of-the-art wave model is integrated with 40 years
of wind analysis; the results are assumed to be reasonably homogeneous
in the area south of 70N and east of 20W; then a regression is built which
related monthly mean air-pressure distributions to intra-monthly percentiles
of wave heights at selected locations with the help of the 40 year simulated
data; finally observed monthly mean air pressure fields from the beginning
of this century are fed into the regression model derive best guesses of
wave statistics throughout the century.
Evidence
In the following, we present some of the evidence upon which our executive summary is based. A complete summary paper is presently prepared.
Standardized annual 95% (diamonds and full line) and
99% (crosses and dotted line) quantile time series (1880-1995) from pressure
triangles in the Scandinavian, Finish and Baltic Sea region (top) and in
the British-Isle, North Sea and Norwegian Sea region. The lines are obtained
from the yearly data by applying a Gaussian filter with standard deviation
of 3 years.
From Alexandersson et al., 1999.
Variations in geostrophic wind statistics are considered a homogeneous proxy for variations in near surface wind variations. In our two diagrams, the percentiles of the annual geostrophic wind speed distributions are plotted as a function of years. There is obviously a clear increase in the percentiles in the last 30 or so years, but the present levels of wind speeds are comparable to wind speeds early this century.
Obviously, an analysis of a few decades of data is
insufficient for the assessment of the natural variability of the storm
climate.
Trend in the intra-annual 90% quantiles of significant
wave height as derived from the WAM hindcast 1955-94 forced with DNMI wind
analyses. Units: cm/year.
From Günther et al., 1998.
Note that the stronger trends along the margin of the simulation area may be due to inhomogeneities in the wind analyses, but the results in the interior of the model domain is considered fairly "clean". The trend in the inner North Sea amounts to 1 cm/year, i.e., 40 cm in the considered time interval 1955-94. The maximum of the Scottish coast is 1.5 - 2 cm/year.
When comparing the trends derived from the 1955-94
hindcast with the geostrophic wind quantile time series shown above, it
appears likely that the present wave heights are comparable to the heights
in the early part of this century. Indeed, a statistical model confirms
this hypothesis.
Change in the intra-annual 90% quantiles of wind speed
(top) and significant wave height (bottom) as derived from a paired atmospheric
circulation model run with present and doubled carbon dioxide conditions.
The simulated wind data have been used in a simulation in the wave model
WAM. Units: m/s and m.
From Beersma et al., 1997 and Rider et al., 1997.
The atmospheric circulation model used is a T106 version
of ECHAM 3 which has been forced with present SST and sea ice conditions
as well as superimposed SST and sea ice anomalies simulated in a fully
coupled atmosphere ocean GCM. Also the carbon dioxide concentration has
been doubled. Because of the computational costs, both the control, and
the "2CO2" runs have been simulated only for about 5 years.
In the Bay of Biscay the 90% quantiles of wind speed
are simulated to be increased by up to 1.5 m/s and in the Central North
Sea up to 0.5 m/s. In area colored in green, the wind speed in the climate
model is decreased by up to 1 m/s.
The 90% wave heigh quantiles are found to increase in
the Bay of Biscay and in the North Sea by up to 0.5 m, whereas in most
of the North Atlantic, the wave heights are decreasing.
The changes of wind speed and wave height statistics are derived from one relatively short climate simulation; therefore, the found changes in wind speed and wave heights may reflect mere sample (year-to-year) variations unrelated to a deterministic signals related to changes in the composition of the atmosphere.
The new EU sponsored project STOWASUS, coordinated by
Eigil Kaas, DMI, will create a new scenario of this sort, with a more modern
atmospheric model integrated over a long time. This project will begin
in 1998.
Organizational
WASA is an abbreviations of Waves and Storms in the North Atlantic. The project was funded by the European Union's Environment program from 1994-1996. Coordinator is Hans von Storch, GKSS, PO Box, 21502 Geesthacht, Germany, e-mail: hvonstorch (at)web.de.
Project participants are:
Hans von Storch, 30/1/2000
