JERICHO: UK wave climate analyses

INTRODUCTION:
As part of JERICHO, a study of the large scale wave climate around the British Isles was carried out, based on an analysis of satellite altimeter data. Altimeter data from the ERS-1, ERS-2 and TOPEX/Poseidon satellites were processed to generate a monthly mean significant wave height data set on a 1° latitude by 2° longitude grid. These data covered the period October 1992 to December 1997. Mean, variance, sum of squares, and other derived values relevant for compiling statistics are retained in monthly files. The full grid of data generated for JERICHO covers the region 80°W to 10°E, and 20°-70°N. We believe that this is the first investigation of the large scale wave climate in the seas around the UK that is based purely on measured satellite data.

UK WAVE CLIMATE CHARACTERISTICS:
The wave climate can be considered as consisting of three parts: the long term mean climate, the annual/seasonal cycle and non-seasonal variability on both the short term (within year) and long term (year to year, or inter-annual).

Mean Climate
Of the seas around Britain, the west coast of Ireland and the Outer Hebrides experience the highest wave heights (long term mean significant wave height, Hs, of 3.0 m). Considering only the English and Welsh coastline, south-west Wales and western Cornwall see the highest waves (mean Hs of 2.0-2.5 m), whilst the English Channel and Eastern English coastline are the most sheltered (long term mean Hs of 1m or less).

Annual Cycle
The annual range in Hs (i.e. the difference between winter and summer) has a similar spatial pattern to the long term mean. The winter to summer range is greatest in the north and west and lowest in the south and east (see Figure 1).
In JERICHO we investigated the strength of the contributions of the seasonal cycle to the overall variability in the monthly data. Whilst the annual cycle was found to describe most of the variability in the north east Atlantic (~70%) it described less than half (30-50%) in the North Sea and English Channel.

Inter-Annual Variability
The seasonal cycle explains a large part of the variability in the data but inter-annual variability is also important, some winters can be much stormier than others. It is important to identify any pattern to this behaviour, and to understand what the causes may be. The mean winter wave height in the north-east Atlantic increased significantly (by 50% or more) between the 1960's and 1990's. Although this trend may have extended as far east as the northern North Sea, there is no evidence to suggest any similar increases in the central and southern North Sea. Recent work has linked this increase to the North Atlantic Oscillation (NAO) -the anomaly in the pressure gradient between the Azores and Iceland.

Figure 1. Mean winter (top) and summer (bottom) significant wave height from altimeter data.

WAVE CLIMATE TELE-CONNECTIONS:
Statistical techniques can be used to identify patterns of variability in data, and to measure the significance of the connection to known external signals. In JERICHO such techniques were used to investigate the possible connection between the British wave climate and the NAO. We found that there was a strong link between the NAO and the wave climate to the north and west of Britain, but not to the east. Figure 2 highlights (in red) the areas that are most influenced by the NAO, and Figure 3 shows the changes in the NAO over the last 130 years. An increase between 1960 and 1990 is clearly evident. Table 1 shows the extent that the wave climate of UK coastline is influenced by the NAO. It is clear that the east coast wave climate is subject to other influences, and further studies are needed to investigate what these may be.

Table 1. Characteristics of significant wave height climate at selected UK sites.

Figure 2. The influence of the NAO on the mean Hs (as % of inter-annual variance explained) from a canonical correlation analysis.

Figure 3. Time series of the winter North Atlantic Oscillation (taken between Lisbon and Iceland)

EXTREME VALUES:
If the wave height data can be assumed to fit a certain form of distribution, defined by 2 parameters, then the values of mean and variance of the gridded data set can be used to generate estimates of multi-year return values (the value of Hs that could be expected to be exceeded once in these years), and exceedance probabilities (the probability that a given Hs will be exceeded). Figure 4 gives estimates of 100 year return values for Hs.
Note that true maps of extremes would require a more thorough analysis of the data. Nonetheless, spot checks against reliable references suggests that the values calculated from the altimeter data provide a good indication of expected values away from the coast.

Figure 4. 100 year return significant wave height, estimated from altimeter data.

THE PARTNERSHIP (see Contact) : The Environment Agency was the customer for JERICHO, it wished to develop its long term strategy for the protection of the English and Welsh coastline. The Centre for Coastal and Marine Science — Proudman Laboratory, and Halcrow Maritime provided expertise in shallow water wave modelling. Southampton Oceanography Centre undertook analyses of large scale wave climate variability and provided computing support, and Satellite Observing Systems were project managers and carried out analyses of satellite and in situ data. JERICHO was supported by the British National Space Centre under the Earth Observation LINK programme.