Antarctic Circumpolar Current Transport Variability 189
We investigated the representation of the ACC in the currently available GRACE
solutions. For this purpose transport variability derived from GRACE OBP anoma-
lies has been compared to results obtained by Zlotnicki et al. (2006) as well as to
variations derived from simulated OBP anomalies and Drake Passage volume trans-
port. We utilized the Finite Element Sea Ice – Ocean Model (FESOM) described
in detail by Timmermann et al. (2009) which provides ocean bottom pressure
anomalies as well as the total transports.
In addition to the estimation of transport variability we analyzed the representa-
tion of SAM in the GRACE-derived OBP. We determined the connection between
the atmospheric variability (as represented by SAM) and transport variability that
can be derived from GRACE retrievals and model simulations. This analysis gives
an insight into how the atmospheric variability influences the mass distribution
measured by GRACE.
2 Data
We investigate the representation of ACC and SAM variability in GRACE using the
so-called de-striped solutions based on Chambers (2006). The de-striping algorithm
is based on the method developed by Swenson and Wahr (2006). They found the
errors, which manifest themselves as north-south stripes of opposite signs and dom-
inate spherical harmonic solutions, to be correlated – which allows for a systematic
correction of the data. However, they noted that due to their empirical method sig-
nals can be attenuated. Chambers (2006) optimized this algorithm for the purpose
of oceanic investigations with GRACE. The data (RL04) are provided by the three
JPL, CSR, and GFZ as monthly means of GRACE-derived ocean bottom pressure
for the period from January 2003 to April 2008. The de-striped data are available
ona1
◦
×1
◦
regular grid additionally smoothed with a 300, 500 or 750 km Gauss
filter (http://grace.jpl.nasa.gov/data/mass/). A time average over the complete data
set has been removed from the data. Since GRACE is not able to observe spher-
ical harmonics of degree and order 1 which are related to the geocenter motion,
those values have been derived from an estimation of geocenter motion. A PGR
model was used to correct the data for the effect of post-glacial rebound (see
Chambers, 2006).
We found that applying information about the oceanic circulation derived from
our ocean model FESOM for the filtering of GRACE solutions helps to obtain a
higher agreement of GRACE and in-situ data (Böning et al., 2008). As an update of
our work we used our algorithm of including properties of ocean bottom pressure
into the de-striped solutions of release 04. The 300 km Gauss filtered solutions were
first interpolated to our model domain. Instead of larger scale Gaussian filtering
we applied spatially varying patterns of coherent ocean bottom pressure anomalies.
Typical length scales for these patterns are 1,000 km in the Southern Ocean (Böning
et al., 2008). The results are time series that are more likely to represent fluctuations
in OBP than when applying an isotropic Gaussian filter. Our 300 km de-striped