Annual sea level variability of the coastal ocean: The Baltic Sea-North Sea transition zone

The annual cycle is a major contribution to the nontidal variability in sea level. Its characteristics can vary substantially even at a regional scale, particularly in an area of high variability such as the coastal ocean. This study uses previously validated coastal altimetry solutions (from ALES data set) and the reference ESA Sea Level Climate Change Initiative data set to improve the understanding of the annual cycle during the Envisat years (2002-2010) in the North Sea-Baltic Sea transition area. This area of study is chosen because of the complex coastal morphology and the availability of in situ measurements. To our knowledge, this is the first time that the improvements brought by coastal satellite altimetry to the description of the annual variability of the sea level have been evaluated and discussed. The findings are interpreted with the help of a local climatology and wind stress from a reanalysis model. The coastal amplitude of the annual cycle estimated from ALES altimetry data is in better agreement with estimations derived from in situ data than the one from the reference data set. Wind stress is found to be the main driver of annual cycle variability throughout the domain, while different steric contributions are responsible for the differences within and among the subbasins. We conclude that the ALES coastal altimetry product is a reliable data set to study the annual cycle of the sea level at a regional scale, and the strategy described in this research can be applied to other areas of the coastal ocean where the coverage from the tide gauges is not sufficient.