Geoscience Reference
In-Depth Information
Fig. 2 Schematic illustration of
the relationship between the
absolute and mean dynamic
topography (ADT and MDT), the
mean sea surface and the geoid
referenced to the same ellipsoid.
Note the difference between the
instantaneous sea surface and the
MDT
insufficient to calculate a reliable MDT. Also, the presence of sea ice may hamper the
computation of the MSS and hence the MDT. Though care is taken to avoid erroneous data
some of the data that have been used to calculate the MSS may represent the top of the sea
ice floes rather than the sea surface. In particular, off the coasts of the Canadian Archi-
pelago and northern Greenland the high values of the GOCE MDT may be caused by the
influence of the permanent and thick sea ice cover.
The Arctic Ocean displays an elevation change reaching up to about 0.45 m associated
with the high in the Beaufort Gyre, and with the corresponding dominant orientation of the
slope mostly aligned from Siberia to the northern shores of Greenland. According to Steele
and Ermold (
2007
), the dynamic height in the Arctic Ocean is predominantly influenced by
salinity. In the Nordic Seas, the general shape of the MDT favors the cyclonic circulation
pattern displaying steepest MDT slopes of 0.4 m/100 km between the Faroe and Shetland
Islands, along the northwest coast of Norway and in the northern part of the EGC. In
comparison, the slope across the Gulf Stream reaches 1 m/100 km. This spatial pattern in
the MDT agrees well with the spatial pattern in the mean steric height derived from
hydrographic data (Nilsen et al.
2008
) for the period 1950-2010, respectively, referenced
to 500, 1,000 and 1,500 m as shown in Figs.
4
and
5
b.
The steric height calculation is done according to Siegismund et al. (
2007
), where the
steric height is referenced to a constant density q
0
from salinity of 35 and temperature of
0 C. More information on the concept and application of the steric height is given by
Tomczak and Godfrey (
2003
). The difference in these height fields primarily reveals the
effect of the vertical distribution of temperature and salinity in the upper 1500 m, pre-
dominantly influenced by the advection and spreading of the Atlantic Water. Apart from
the changes occurring in the Lofoten Basin, the overall structure remains largely
unchanged when the density structures from 1,000 to 1,500 m are included. This suggests
that the baroclinic circulation in the Nordic Seas is driven by the temperature and salinity
structures of the Atlantic Water in the upper 1,000 m.
In the Nordic Seas, the total range in the MDT derived from the combined GOCE and
altimetry data is around 0.50-0.55 m as seen in Fig.
5
a. In comparison, the range of the
mean steric height of 0.30 m (Fig.
5
b) suggests that there might be a significant contri-
bution to the MDT pattern from the large-scale atmospheric pressure field and the deep
barotropic currents in some of the sub-basins. Siegismund et al. (
2007
) moreover con-
cluded that the seasonal cycle of the steric height (for the period 1950-1999) is predom-
inantly associated with the temperature variations in agreement with previous studies on
global scale (e.g., Gill and Niiler
1973
; Stammer
1997
; Mork and Skagseth
2005
). By
subtracting the hydrographic-based steric height associated with the baroclinic structure in
