Alfred Wegener Institute for Polar and Marine Research
INFLUENCE OF DEEP OCEAN WARMING
ON THE INTERPRETATION OF SEA LEVEL RISE
M. Wenzel, J. Schr¨oter, H. Hellmer and M. Schodlok
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Introduction
Two global data assimilation experiments, B0ntp and B2ntp, are performed with the goal of a better understanding of sea level rise. In both cases satellite altimetry referenced to a GRACE geoid is assimilated together with a set of oceanographic data. The experiments differ in the treatment of the Weddell Sea. In the first case, B0ntp, climatological hydrography is used for assimilation while in the second experiment, B2ntp, hydrographic lines (see Fig.1) derived from a detailed shelf-ice/sea-ice/ocean model are used in addition.
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DOVETAIL section WOCE section SR4
Fig. 1: Weddell Sea sector of the ocean models bottom topography showing the additional data sections across the Wedddel Sea (WOCE section SR4) and along the South Scotia Ridge (DOVETAIL section).
assimilation
experiment Weddell Sea data
B0ntp NO
B2ntp YES
The OGCM that is used to study the impact of the different treatment of the Weddell Sea on the ocean state is based on the Hamburg Large Scale Geostrophic model LSG. The main improvement of the model is the ability to estimate the single contributions to sea level change, the steric (thermosteric, halosteric) and the non-steric effects (local freshwater balance, mass redistribution) seperately.
The model has a 2◦ × 2◦ horizontal resolution, 23 vertical layers and a ten day timestep. Nine years (1993-2001) of TOPEX/Poseidon sea surface height anomalies, provided by GfZ Potsdam, are assimilated into the model. In addition the SHOM98.2 mean sea surface relative to the GRACE geoid (GfZ) as well as sea surface temperatures and ice cover information from Reynolds (2002) are assimilated into the model. Furthermore background information from the Levitus WOA98 is used.
To adjust the model to the data the adjoint method is employed. The control parameters of this optimization are the models initial temperature and salinity state as well as the forcing fields (windstress, air temperature and surface freshwater flux). The forcing is optimized via an empiri- cal orthogonal function (EOF) decomposition, with the first guess taken from the NCEP reanaly- sis.
Both assimilation experiments, B0ntp and B2ntp, start from the same first guess. They differ only in the additional section data used!
Ocean Model Sea Surface Height vs. Data
Figure 2 shows that in both experiments, B0ntp as well as B2ntp, the model follows the altimetric anomalies and trend well. This is true especially for the interannual variability, while the amplitu- de of the annual cycle is underestimated by the models. The spatial distribution of the temporal RMS difference (Fig.3) is very simular for both experiments. Also their global mean RMS values, the measure of success in the assimilation, appear to be comparable (2.86cm and 2.81cm respective- ly). The same good correspondence between the two experiments one finds for the differences bet- ween the modeled temporal mean sea level and the SHOM98.2 sea level (Fig.4). These also have a comparable spatial RMS (12.24cm and 10.84cm respectively).
1993 1994 1995 1996 1997 1998 1999 2000 2001
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cm
T/P(GfZ) B0ntp B2ntp
global mean sea level
Fig. 2: Global mean sea level anomaly from the two assimila- tion experiments, B0ntp and B2ntp, as compared to the TO- PEX/Poseidon data
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B0ntp vs. T/P(GfZ)
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1993-2001
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sea surface height anomaly
temporal RMS difference
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2.806 area RMS:
B2ntp vs. T/P(GfZ)
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1993-2001
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sea surface height anomaly
temporal RMS difference
Fig. 3: Local temporal RMS of the modeled SSHA diffe- rence between model and TOPEX/Poseidon data, for experiment B0ntp (top) and B2ntp (bottom).
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B0ntp vs. SHOM98
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1993-2001
c.i. 10 cm
mean sea level difference
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10.836 area RMS:
B2ntp vs. SHOM98
cm
1993-2001
c.i. 10 cm
mean sea level difference
Fig. 4: Temporal mean sea level for the assimilation experiments B0ntp (top) and B2ntp (bottom) compared to the SHOM98.2 mean sea surface height referenced to the GRACE geoid
Ocean Model Heat Content
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Fig. 5: Mean potential temperature on the N-S section through the Atlantic Ocean at 30◦W, for experiment B0ntp (top) and B2ntp (bottom).
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linear trend
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linear trend
Fig. 6: Corresponding linear temperature trends on the N-S section through the Atlantic Ocean at 30◦W, for experiment B0ntp (top) and B2ntp (bottom).
1993 1994 1995 1996 1997 1998 1999 2000 2001
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total top middle bottom
dashed: B0ntp straight: B2ntp
global ocean heat content anomaly
Fig. 7: Global ocean heat content anomaly for the depth ranges: total=[ζ-bottom], top=[ζ- 512m], middle=[512m-2250m] and bottom=[2250m-bottom] , for experiment B0ntp (das- hed lines) and B2ntp (straight lines).
Using better information for the Weddell Sea leads to an impro- vement of the circulation in the South Atlantic. But the assimi- lation experiments, B0ntp and B2ntp respectively, do not only end up with a different mean state (e.g. for temperature, Fig.5) but exhibit also different trends (Fig.6) which are most notable in the convective as well as in the subduction regions. Here the downward transport of relatively warm and saline water is signi- ficantly reduced in B2ntp. Associated with this is a reduction in the global warming of the ocean (Fig.7) especially in the deep layers. This consequently leads to a reduced sea level rise due to steric effects in B2ntp.
Comparing Model Sea Level Trends
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total steric non-steric
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global mean sea level
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total steric non-steric
B2ntp
global mean sea level
Fig. 8: Temporal evolution of the global mean sea level decomposed into its steric and non-steric part for the model solutions B0ntp (left) and B2ntp (right).
Figure 8 shows that in both experiments, B0ntp and B2ntp respectively, nearly all the ’short term’ tempo- ral variability of the global mean sea level is resampled by the non-steric part, while the steric contribution ap- pears more or less as a straight line. The global sea level rise due to thermal expansion is about twice as large for the model solution without the additional hy- drographic section data (B0ntp) compared to the case utilizing this data (B2ntp). Consequently a strong eva- poration surplus is needed in B0ntp to fit the measured global mean sea level curve (Fig.2).
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GfZ
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1993-2001
c.i. 0.5 cm/year
sea surface height anomaly local linear trend
Fig. 9: Local linear trend of the TOPEX/Poseidon (GfZ) sea sur- face height
Both model solutions, B0ntp and B2ntp respectively, retrieve the measured local sea level trends to a good quality although the extrema are partly shifted in space (Figs. 9 and 10a). The main part of the spatial variab- lity is given by the steric contribution (Figs.10b), whi- le the non-steric part (Figs.10c) exhibits a much wea- ker signal. Nevertheless both components show strong differences among the experiments. But while these differences are more or less restricted to the southern hemisphere for the steric one, they are more global for the non-steric part giving a negative trend nearly ever- ywhere in experiment B0ntp.
A more detailed comparison of the local steric con- tribution to sea level rise from the two experiments is given in Figs. 11 and 12. In both experiments the main contribution results from the top 500m for the thermo- steric as well as for the halosteric part, but the diffe- rences appear mainly in the thermosteric in the deeper layers of the circumpolar belt, the South Atlantic and the Southwest Pacific.
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0.225 area mean:
B0ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly local linear trend
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B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly local linear trend
Fig. 10a: Local sea level trend of the model solutions B0ntp (up- per row) and B2ntp (lower row).
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0.523 area mean:
B0ntp
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1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
total steric component local linear trend
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0.219 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
total steric component local linear trend
Fig. 10b: same as Fig.10a but for the total steric component
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B0ntp
cm/year
1993-2001
c.i. 0.1 cm/year
sea surface height anomaly
non-steric component local linear trend
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0.022 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.1 cm/year
sea surface height anomaly
non-steric component local linear trend
Fig. 10c: same as Fig.10a but for the non-steric component
Thermosteric Sea Level Trends
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0.303 area mean:
B0ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
thermosteric component
[ - 512m]
local linear trend
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0.163 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
thermosteric component
[ - 512m]
local linear trend
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0.130 area mean:
B0ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
thermosteric component
[512 - 2250m]
local linear trend
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0.040 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
thermosteric component
[512 - 2250m]
local linear trend
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0.098 area mean:
B0ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
thermosteric component
[2250m - bottom]
local linear trend
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0.017 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
thermosteric component
[2250m - bottom]
local linear trend
Fig. 11: Thermosteric sea level trends from ocean model solutions B0ntp (left column) and B2ntp (right column), giving the contribu- tion from the depth ranges (topmost to undermost): [ζ-512m], [512m-2250m] and [2250m-bottom].
Halosteric Sea Level Trends
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0.043 area mean:
B0ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
halosteric component
[ - 512m]
local linear trend
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-0.007 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
halosteric component
[ - 512m]
local linear trend
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-0.014 area mean:
B0ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
halosteric component
[512 - 2250m]
local linear trend
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0.0
-0.5
30 60 90 120 150 180 210 240 270 300 330 360 -90
-60 -30 0 30 60 90
-90 -60 -30 0 30 60 90
0.001 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
halosteric component
[512 - 2250m]
local linear trend
0.0 0.0 0.0
0.0
0.0
0.0 0.0
0.0 0.0
0.0
0.0
0.0 -0.5 0.0
0.0
0.0 0.0
0.0
0.0
0.0 0.0
0.0 -0.5
0.0 0.5
0.0
-0.5 0.0
0.0
30 60 90 120 150 180 210 240 270 300 330 360 -90
-60 -30 0 30 60 90
-90 -60 -30 0 30 60 90
-0.037 area mean:
B0ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
halosteric component
[2250m - bottom]
local linear trend
0.5 1.0
0.0 0.0
0.0
0.0
0.0 0.0
0.0 0.0
0.0 0.0
0.0
0.0
0.5
0.0 0.0
0.5
-0.5
0.0 0.0
0.5 0.0
0.0 0.0
0.0 0.5
30 60 90 120 150 180 210 240 270 300 330 360 -90
-60 -30 0 30 60 90
-90 -60 -30 0 30 60 90
0.005 area mean:
B2ntp
cm/year
1993-2001
c.i. 0.5 cm/year
sea surface height anomaly
halosteric component
[2250m - bottom]
local linear trend
Fig. 12: Halosteric sea level trends from ocean model solutions B0ntp (left column) and B2ntp (right column), giving the contribution from the depth ranges (topmost to undermost): [ζ-512m], [512m-2250m] and [2250m-bottom].
Summary
• The ocean model fits the altimetric data with equal quality no matter if the additional hydrographic section data are used or not.
• The use of the section data in the Weddell Sea results in a much less warming of the global ocean, approximately half the value, than without these data.
• The resulting difference in the steric global sea level rise of the ocean model solutions is balanced by the non-steric contribution (net global surface freshwater flux).
• On regional scale the differences in the steric part are mainly restricted to the southern hemispere, while the non-steric differences show a distinct global extent.
• Not only the thermosteric sea level change is effected by the additional use of Weddell Sea data, but the halosteric part as well.
• The steric differences are most evident in the deeper ocean layers.
Corresponding e-mail adresses:
mwenzel@awi-bremerhaven.de jschroeter@awi-bremerhaven.de hhellmer@awi-bremerhaven.de mschodlok@awi-bremerhaven.de
