Paolo Cipollini (National Oceanography Centre, UK); Ellis Ash (Satellite Oceanographic Consultants, UK); Francisco Calafat (National Oceanography Centre, UK)

UK Sea Level SpaceWatch is a service designed to support the UK agencies responsible for the management and planning of national flood defences and for the preservation of coastal habitats threatened by sea level change.

Using data from satellite altimeters together with tide gauge data, Sea Level SpaceWatch provides, through an easy to use web-interface, the latest figures on observed sea level around the UK, supported by careful analyses of these data in terms of long-term trends, regional variability and confidence intervals showing the lower and upper limit for the current mean sea levels. The service complements and supplements the sea level change scenario information available from UK Climate Projections, offering planners the opportunity to verify the regional variability of sea level around the UK at multiple time scales and observe the presence of any significant inter-annual changes.

In the past it has been difficult to retrieve useful data from satellite altimeters close to the coast, because of land contamination of the return waveform. To address this problem, NOC has developed the “ALES” altimeter re-tracker for coastal regions. With this re-tracker, altimeter data from Jason-1, Jason-2, Envisat and Altika has been reprocessed to generate a 14 year times series of sea level data for the UK coastal zone (2002-2015).

This multi-year sea level data set has been validated against tide gauge data and then further analysis carried out to provide a characterisation of regional variability in sea level, in terms of the annual cycle, and inter-annual variability.

Key findings are:
- There is preliminary evidence of a geographical structure in the long-term trend, larger on the South and East than in the North-West.
- There was clear consistency between the annual cycle parameters (amplitude and phase) from tide gauge and altimeter data, with some localised differences. The annual cycle peaks between early October in the south-east and early November in the west coast and has an amplitude ranging from 5 to 9 cm.
- There is good agreement between the de-trended de-seasoned sea level from altimetry and from the tide gauges.
- There is significant spatial coherence in sea level on inter-annual timescales, with the leading EOF capturing over 50% of the variability.

The Development of Sea Level Space Watch has been funded by the UK Space Agency under the Space for Smarter Government Programme.