Spatial variability of the annual cycle in coastal sea level: a regional study

Marcello Passaro (GSNOCS University of Southampton / European Space Agency - ESRIN, United Kingdom)

CoAuthors

Paolo Cipollini (Marine Physics and Ocean Climate Group, National Oceanography Centre, Southampton, UK); Jérôme Benveniste (European Space Agency - ESRIN, Italy)

Event: 2014 Ocean Surface Topography Science Team Meeting

Session: Science Results from Satellite Altimetry: Finer scale ocean processes (mesoscale and coastal)

Presentation type: Type Oral

The annual cycle is a major contribution to the non-tidal variability in sea level. Its amplitude and phase are influenced by both atmospheric (pressure, winds) and oceanographic (steric expansion, circulation) processes. These dependences suggest that its characteristics can differ substantially even at a regional scale, particularly in an area of high variability and strong gradients such as the coastal ocean.

Tide gauges accurately capture the annual cycle variability, but their validity is limited to the coast. Satellite altimetry, particularly missions with a dense spatial coverage such as Envisat, provides a useful mean to estimate the annual component, but its accuracy in the coastal ocean needs to be demonstrated, since sea level retrieval along the coast has long been considered sub-optimal, due to land and coastal waters interference, degradation of atmospheric corrections and tidal model accuracy.

This study uses previously validated, up-to-date coastal altimetry solutions to derive a spatial map of the annual cycle during the Envisat years (2002-2010) in an area that includes the Eastern North Sea, the Danish Straits and the Arkona Basin in the Baltic Sea. This area was chosen because of the challenging coastal morphology and the availability of in-situ coastal data in order to verify the consistency of altimetry data.
The area has been divided into sub-basins and our results show that the annual cycle amplitude estimated from altimetry data within 15 Km from the coast is in agreement with estimations derived from in-situ data within 1.5 cm. Moreover in the Eastern North Sea (Skagerrak Sea) significant spatial variations of the amplitude, which could not be detected by tide gauges, are found in good agreement with the local bathymetry and possibly with wind and circulation patterns. Compared to Sea Level CCI multi-mission product, our data are able to better represent the slope between coastal and open sea annual cycle amplitude in the Skagerrak Sea and improve by 2 cm the estimation of the amplitude in the West Arkona sub-basin w.r.t. in-situ data.
To our knowledge, this is the first time that the improvements that coastal satellite altimetry can bring to the description of the sea level seasonal variability are evaluated, discussed and interpreted with the help of other observed and modelled variables that affects sea level, such as sea surface temperature and wind intensity

The study is part of a wider project that aims at:

1) deploying the same methodology in areas where the lack of in-situ data prevents an accurate description of the annual variability, such as the seas of Indonesia, a hotspot for sea level rise and water exchange between Pacific and Indian Ocean.

2) extending the time series to current missions with a high spatial coverage such as CryoSat-2 and AltiKa
 

Oral presentation show times:

RoomStart DateEnd Date
Red salon Wed, Oct 29 2014,15:00 Wed, Oct 29 2014,15:15
Marcello Passaro
GSNOCS University of Southampton / European Space Agency - ESRIN
United Kingdom
marcello.passaro@noc.soton.ac.uk