Geoscience Reference
In-Depth Information
For a more detailed study of the seasonal variability induced by the altimetric obser-
vations, the surface slopes and meridional velocities across 75N are presented in Fig.
9
together with the model-derived fields. The seasonal mean meridional velocities are esti-
mated by replacing MDT in Eq. (
2
) with absolute dynamic topography (ADT). Note that
ADT is determined as the sum of MDT and monthly mean sea-level anomaly (SLA) data.
The new high-resolution SLA data (obtained from the French CLS-led Sea Level Climate
Change Initiative project funded by ESA) are referenced to the time period 1993-2009 and
hence consistent with the DTU MSS data used in the calculation of GOCE MDT.
The main expected features of the flow toward and from the Fram Strait is revealed by
the mean velocities: the two branch northward flowing West Spitsbergen Current (WSC)
around 8 and 15E; the strong southbound EGC at 10W; and some minor, possibly
cyclonic, circulation features around 0E, likely related to circulation in the Boreas Basin.
Seasonal differences are most pronounced in the WSC. Both branches are strongest in
wintertime, with a near doubling of the easternmost branch, which is due to the general
(wind driven) intensification of the circulation in the region. This is consistent with
velocity retrievals and transport estimates reported by Mork and Skagseth (
2005
). The
western frontal branch stays relatively strong also during the rest of the year, likely due to
the summertime spread of buoyant surface water from the coast to the front (as seen further
south in the NwAC; Nilsen and Falck
2006
), maintaining a steep frontal surface slope.
In comparison, the model-based MDT slopes along 75N and the corresponding
meridional geostrophic velocities across the same latitude consistently reveal that the ATL
model has the steepest surface slopes and hence the strongest flow field for both the
northward flowing NwAC as well as the southward flowing EGC. Moreover, it is only the
ATL model that reproduces the double peak in the WSC current in agreement with the
mean and seasonal observation-based findings.
4 Volume Transport
By combining the GOCE-derived MDT and altimetric sea-level anomalies (SLA) with the
comprehensive hydrographic database, an estimate of the mean and variable transport of
Atlantic Water entering the Nordic seas is obtained for the period 1993-2011 at a spatial
resolution of 100 km. Using 44 CTD-sections in the Faroe north section normally taken to
represent the Iceland-Faroe Ridge (IFR) inflow (Hansen et al.
2010
), 84 CTD-sections for
the Faroe-Shetland Channel (FSC) and 76 CTD-sections taken along the Svinøy section
(see Fig.
8
for locations), the baroclinic velocity structures in the Atlantic Water defined by
salinity values S [35 were estimated across these sections. Combined with the barotropic
velocity values, the absolute velocities are then retrieved, and when these are multiplied by
the area covered by the Atlantic Water, we obtain estimates of the corresponding volume
transports of Atlantic Water across the 3 sections (see Table
2
).
From the combination of GOCE, altimetry and hydrography, the mean inflows of
Atlantic Water across the IFR and through the FSC are estimated to approximately 3.5 and
4.1 Sv, respectively (1 Sv = 10
6
m
3
s
-1
). The former is in very good agreement with
Hansen et al. (
2010
), but too low compared to Østerhus et al. (
2005
), while the latter is too
high compared to Østerhus et al. (
2005
) and too low compared to Sandø et al. (
2012
). In
comparison, the mean transport of the two branches of Atlantic Water crossing the Svinøy
section, e.g., the Norwegian Atlantic Slope Current (NwASC) and the Norwegian Atlantic
Front Current (NwAFC) is, respectively, 3.9 Sv and 3.0 Sv. The latter value is in
acceptable agreement with previous transport estimates for the NwASC reported by Mork
