Contact: oaspacecenter'dk ESACNES 15 Years progress in Satellite Altimetry, Venice, Italy, 2006,

1
Altimetric Mean Sea Surfaces and Inter-annual
sea level variability (DNSC05-MSS)
Ole B. Andersen, Anne L. Vest and Per Knudsen,
Danish National Space Center.
ABSTRACT The DNSC-MSS05 mean Sea Surface (MSS)
is an updated version of the older KMS04 MSS. It
is the physically observed time-averaged height
of the oceans surface derived from a combination
of 12 years of altimetry from a total of 6
different satellites covering the period
1993-2004. In this presentation we will show
several applications of an accurate MSS and
especially applications related to the way that
the DNSC-MSS05 has been derived. One application
is the ability to adjust the MSS to map the
inter-annual sea level variability within a
sub-period of the 12 years. This is achieved by
simultaneously solving for the MSS, the annual
sea level signal and the sea level trend. We
will also demonstrate how this MSS can be used to
derive a synthetic geoid model based on a mean
sea surface and a mean dynamic topography (MDT)
with consistent modeling of the inter-annual
ocean variability.
Inter-annual sea level variability
By subtracting the mean as well as the linear sea
level change and the annual sea level change from
the altimetric observations the inter-annual sea
level anomalies can be obtained by averaging the
residual observations within each year. These
values are shown above for the period 1993-2004,
computed from T/P and JASON-1 satellite
altimetry. The El Nino La Nina related sea
level signals can be seen for almost all years,
but the El Nino in 1997-1998 is particularly
dominating. During this period sea level where
displaced by more than 15 cm in the central
Pacific Ocean averaged over the entire year.
A Synthetic Geoid Model from the DNSC-05MSS The
connection between MSS, MDT, and geoid is MDT
MSS geoid  Consequently, changes in the MSS
are directly related to changes in the MDT as the
geoid is approximately stationary over two given
periods. G (period1) G (period2) This
means that MDT(period1) MDT(period2)
?MSS(period1) - ?MSS(period2) The OCCAM MDT
model covers the period 1993-1995, and the
DNSC-MSS05 covers the period 1993-2004. The OCCAM
MDT adjusted for the inter-annual sea level
anomalies for the 1993-2004 period is
consequently OCCAM(93-04) OCCAM(93-95)
?DNSC05(93-04) - ?DNSC05(93-95) The final
synthetic geoid can then computed as the
difference between the MSS and the corrected MDT.
The DNSC-MSS05 mean sea surface.
MSS and inter-annual ocean variability In the
computation of the long wavelength part of the
MSS, a 4 parameter fit to the altimetric time
series of TOPEX/JASON-1 is made in which the
largest contributions to sea level variations are
modeled. This is the mean value, a linear sea
level change (over the 12 years) and the annual
cycle in sea level like hobs h0 h1
t h2 cos(? ann t) h3 sin (? ann t)
e where ? ann is the annual cycle.
The error field associated with the DNSC-MSS05
Mean sea surface. The colorscale ranges between 0
(purple) and 10 centimeters (red).
The linear sea level change (trend) or the h1
parameter above. Increase in sea level is shown
with yellow and red colors and decrease with blue
and green colors. Range /- 2 cm/year.
The OCCAM MDT for the period 1993-1995 (range is
/1 2 meters) courtesy of R. Bingham.
The Arctic Ocean The connection between MSS, the
Mean Dynamic Topography and geoidMDT MSS
geoid  With the increased accuracy in both geoid
and MSS determination, geostrophic currents can
be mapped in most parts of the Arctic sea. The
MDT derived from the difference between the
DNSC-MSS05 and the EGM96 geoid model (Lemoine et
al. 1998)and the difference between the
DNSC-MSS05 and the more accurate GRACE GGM01
(Tapley and Chambers, 2003) geoid model is shown
in the figure below. Notice the un-realistic
geostrophic circulation structures imposed by
errors in the EGM96 and how these have vanished
with the use of GGM01. Also notice the very
realistic extension of the North Atlantic Current
along the west coast of Norway splitting into a
recirculation structure in the Norwegian-Greenland
-Iceland sea and a branch into the Barents Sea.
The sea level anomaly for the period 1993-1995
used to adjust the OCCAM MDT to the period
1993-2004 similar to the averaging period for the
altimetric MSS. The color ranges from 5 cm
(blue) to 5 cm (red).
The MDT in the Arctic Ocean derived from the
difference between the DNSC_MSS05 MSS and the
EGM96 geoid model to the left and the GRACE GGM01
geoid model to the right.
The Syntheric geoid model somputed as the
difference between the DNSC05-MSS and the MDT
corrected for ocean variatility over the 12-year
period. The figure will be visually identical to
the MSS .