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Ann. Geophys., 22, 1465-1472, 2004
www.ann-geophys.net/22/1465/2004/
© European Geosciences Union 2004
Nonlinear sea level trends from European tide gauge records
S. M. Barbosa, M. J. Fernandes, and M. E. Silva
Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 687, 4169–007 Porto, Portugal
Abstract. Mean sea level is a variable of considerable interest in meteorological
and oceanographic studies, particularly long-term sea level variation
and its relation to climate changes.
This study concerns the analysis of monthly mean sea level data from
tide gauge stations in the Northeast Atlantic with long and continuous
records.
Much research effort on mean sea level studies has been focused on
identifying long-term linear trends, usually estimated through least-squares
fitting of a deterministic function. Here, we estimate nonparametric
and robust trends using lowess, a robust smoothing procedure based
on locally weighted regression. This approach is more flexible than
a linear trend to describe the deterministic part of the variation
in tide gauge records, which has a complex structure.
A common trend pattern of reduced sea levels around 1975 is found
in all the analysed records and interpreted as the result of hydrological
and atmospheric forcing associated with drought conditions at the tide
gauge sites. This feature is overlooked by a linear regression model.
Moreover, nonlinear deterministic behaviour in the time series, such
as the one identified, introduces a bias in linear trends determined
from short and noisy records.
Key words. Oceanography:
physical (sea level variations);
Hydrology (water balance)
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