Sub-basin-scale sea level budgets from satellite altimetry, Argo floats and satellite gravimetry: a case study in the North Atlantic Ocean
In this study, for the first time, an attempt is made to close the sea level budget on a sub-basin scale in terms of trend and amplitude of the annual cycle. We also compare the residual time series after removing the trend, the semiannual and the annual signals. To obtain errors for altimetry and A...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-11-01
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| Series: | Ocean Science |
| Online Access: | http://www.ocean-sci.net/12/1179/2016/os-12-1179-2016.pdf |
| Summary: | In this study, for the first time, an attempt is made to close the
sea level budget on a sub-basin scale in terms of trend and amplitude of the
annual cycle. We also compare the residual time series after removing the
trend, the semiannual and the annual signals. To obtain errors for altimetry
and Argo, full variance–covariance matrices are computed using correlation
functions and their errors are fully propagated. For altimetry, we apply a
geographically dependent intermission bias [Ablain et al.(2015)], which leads to
differences in trends up to 0.8 mm yr<sup>−1</sup>. Since Argo float measurements
are non-homogeneously spaced, steric sea levels are first objectively
interpolated onto a grid before averaging. For the Gravity Recovery And
Climate Experiment (GRACE), gravity fields full variance–covariance matrices
are used to propagate errors and statistically filter the gravity fields. We
use four different filtered gravity field solutions and determine which
post-processing strategy is best for budget closure. As a reference, the
standard 96 degree Dense Decorrelation Kernel-5 (DDK5)-filtered Center
for Space Research (CSR) solution is used to compute the mass component (MC).
A comparison is made with two anisotropic Wiener-filtered CSR solutions up to
degree and order 60 and 96 and a Wiener-filtered 90 degree ITSG solution.
Budgets are computed for 10 polygons in the North Atlantic Ocean, defined in
a way that the error on the trend of the MC plus steric sea level remains
within 1 mm yr<sup>−1</sup>. Using the anisotropic Wiener filter on CSR gravity
fields expanded up to spherical harmonic degree 96, it is possible to close
the sea level budget in 9 of 10 sub-basins in terms of trend.
Wiener-filtered Institute of Theoretical geodesy and Satellite Geodesy (ITSG)
and the standard DDK5-filtered CSR solutions also close the trend budget if
a glacial isostatic adjustment (GIA) correction error of 10–20 % is
applied; however, the performance of the DDK5-filtered solution strongly
depends on the orientation of the polygon due to residual striping. In 7 of 10
sub-basins, the budget of the annual cycle is closed, using the
DDK5-filtered CSR or the Wiener-filtered ITSG solutions. The Wiener-filtered
60 and 96 degree CSR solutions, in combination with Argo, lack amplitude and
suffer from what appears to be hydrological leakage in the Amazon and Sahel
regions. After reducing the trend, the semiannual and the annual signals,
24–53 % of the residual variance in altimetry-derived sea level time
series is explained by the combination of Argo steric sea levels and the
Wiener-filtered ITSG MC. Based on this, we believe that the best overall
solution for the MC of the sub-basin-scale budgets is the Wiener-filtered
ITSG gravity fields. The interannual variability is primarily a steric signal
in the North Atlantic Ocean, so for this the choice of filter and gravity
field solution is not really significant. |
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| ISSN: | 1812-0784 1812-0792 |