Alfred Wegener Institute for Polar and Marine Research
GLOBAL OCEAN HEAT CONTENT VARIATIONS DERIVED FROM
SATELLITE ALTIMETRY AND HYDROGRAPHIC DATA
M. Wenzel and J. Schr¨oter
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Introduction
Sea surface elevations as measured by the satellite altimetry together with a limited set of hy- drographic measurements are assimilated into a global OGCM that has a free surface and that conserves mass rather than volume. The combination of both types of measurements appeared to be necessary to get a reasonable estimate of the oceanic circulation. Further improvement in esti- mating sea level change was achieved by including the steric effects (thermosteric and halosteric) into the modelled sea surface elevation, because local sea level trends vary substantial in space and time. They are closely associated to heat and salt anomalies in the ocean. The resulting heat content variations of the ocean will be analysed and compared to independent estimates like e.g.
to the analysis of Willis et al (2004).
The OGCM that is used in this study is based on the Hamburg Large Scale Geostrophic model LSG. The model has a 2◦ × 2◦ horizontal resolution, 23 vertical layers and a ten day timestep.
Furthermore the model is able to estimate the single contributions to sea level change, the steric (thermosteric, halosteric) and the non-steric effects (local freshwater balance, mass redistribution) seperately.
Eleven years (1993-2003) 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 EIGEN-GRACE01S geoid (GfZ) as well as sea surface temperatures and ice cover infor- mation from Reynolds (2002) are assimilated into the model. The WGHC climatology combined with the monthly anomalies from WOA01 is used as background information for temperature and salinity. Furthermore data from high resolution regional model runs are supplied in the Ross Sea and in the Weddell Sea.
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.
Ocean Model Sea Surface Height vs. Data
The temporal RMS differences between the modeled SSHA and the data is shown in Fig.1. The global RMS value, which is the measure of success in the assimi- lation, is 2.9cm although locally we find higher RMS values (up to 7cm) especially in the tropical Pa- cific and in the western bounda- ry currents. For the temporal mean SSH the deviations between the model and the data are well below 5cm in most part of the ocean gi- ving a global RMS value of 14cm (Fig.2). As for the anomalies the largest deviations (up to ∼ 30cm) are found in the regions with strong currents, i.e. the western bounda- ry currents as well as the Antarctic Circumpolar Current (ACC). Espe- cially the signature in the ACC region implies that these currents are represented too broadly by the model. For the surface tempera- ture the corresponding RMS diffe- rences between the model and the data are 0.30K for the temporal mean and 0.51K for the anomalies (not shown).
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area RMS: cm cm
roWE vs. TOPEX
1993
2.74
- 2003
area mean:
local RMS deviation
sea surface height anomaly
Fig. 1: Local temporal RMS difference between the modeled SSHA and the TO- PEX/Poseidon data.
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mean difference 1993 - 2003
area RMS: 14.00 area mean: 0.00
MSSH
Fig. 2: Modeled temporal mean sea level compared to the SHOM98.2 mean sea surface height referenced to the GRACE geoid
Ocean Model Heat Content
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 -0.6
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1023 J
Levitus ’05 Willis ’04 roWE (model)
top 700m
global ocean
linear trend [W/m2] Levitus :
Willis : model :
0.6548 0.7431 0.9736
heat content anomaly
Fig. 3: Global ocean heat content anomaly for the depth range [ζ-700m] compa- red to the WOA01 annual anomaly data (Levitus, red line) and to the Willis data (green line).
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linear trend
1993 - 2003
area RMS: 2.99 area mean: 0.97
[ - 700m ] heat content anomaly
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linear trend
1993 - 2003
area RMS: 2.17 area mean: 0.66
[ 700 - 3000m ] heat content anomaly
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Undef -5 -4 -3 -2 -1 0 1 2 3 4 5 W/m2 roWE
linear trend
1993 - 2003
area RMS: 0.95 area mean: 0.17
[ 3000m - bottom ] heat content anomaly
Fig. 4: Modeled local linear trend of the oceans heat content for the depth ranges:
[ζ-700m] = top, [700-3000m] = middle and [3000m-bottom] = bottom panel.
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W/m2 using Levitus annual HC anomalies 1993 to 2003
heat content / top 700m / local trend
Fig. 5: Local linear trend of the upper ocean heat content [0-700m] as derived from the WOA01 annual anomalies (top) and from the Willis analysis (bottom).
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W/m2 roWE
linear trend
1993 - 2003
area RMS: 4.41 area mean: 1.74
[ - bottom ] heat content anomaly
Fig. 6: Modeled local linear trend of the oceans heat content (total water column).
Using additional information in the Weddell Sea and the Ross Sea areas leads to a better circulation in these regions and it improves the evolution of the global upper ocean heat content. Its trend now fits well to the estimates derived analysing the WOA01 and the Willis data respectively (Fig. 3). Furthermore the regional distribution of these trends (top panel of Fig.4) compares well to measurements (Fig.5).
Fig.4 also shows that there is net ocean warming in the deeper layers too (middle and bottom panel). Especially in the Atlantic, the South Pacific and the circumpolar belt one finds essential contributions to the overall trend of the total water columns heat content (Fig.6) thus
contributing to the thermosteric sea level change. References:
Conkright M. E. et al.: World Ocean Atlas 2001: Objective Analysis, Data Statistics and Figures, CD-ROM Documentation, National Oceanographic Data Center, Silver Springs, MD, 17pp., 2002
Gouretski V. V. and K. P. Koltermann: WOCE Global Hydrographic Climatology, A Technical Report, Berichte des Bundesamtes f¨ur Seeschifffahrt und Hydrographie, No. 35, 50pp. + 2 CD-ROM, 2004
Levitus S. et al.: Warming of the world ocean 1955–2003, Geophysical Research Letters, Vol. 32, L02604, doi: 10.1029/2004GL021592, 2005
Reynolds, R. W. et al: An improved in situ and satellite SST analysis for climate, J. Cli- mate, 15, 1609–1625, 2002
Willis J. K. et al: Interannual variability in the upper ocean heat content, temperature and thermosteric expansion on global scales, Journal of Geo- physical Research, Vol. 109, C12036, doi: 10.1029/2003JC002260, 2004
Corresponding e-mail adresses:
mwenzel@awi-bremerhaven.de jschroeter@awi-bremerhaven.de
