1. Modelling Steric Sea Level Rise
Felix Landerer
Jochem Marotzke Max Planck Institute for Meteorology (MPI-M)
Centre for Marine and Atmospheric Sciences (ZMAW)
Bundesstrasse 53, Hamburg, Germany

2. Overview: Steric Sea Level Change
Simulated vs. Observed
Regional Patterns and Mechanisms
Dynamical Aspects: A Wider Perspective

3. How to evaluate steric sea level rise in AOGCMs?
Hydrography (T and S), giving thermo-/halosteric changes; problem: incomplete coverage
Altimetry (1992-today): nearly global SL, high accuracy; problem: it includes eustatic effects, which is not properly represented in the models (no changing land ice)
Gravity (since 2002): can constrain the eustatic effect; problem: record too short for detecting long-term trend
Moreover
AOGCMS cannot be expected to reproduce interdecadal variability with the correct phase
Models are Boussinesq (Mass = Volume)
Attribution of overall freshening: Melting land ice or sea ice (Wadhams & Munk 2005)?
HOW WELL then do models simulate the ocean warming or thermal expansion in the last 50 years or last decade?

4. Simulated vs. observed thermosteric sea level rise
Similar trends (observed: 0.4±0.05mm/yr, AOGCMs 0.54±0.26 mm/yr), but considerable spread
PCMDI, modelling groups, J. Gregory

5. Simulated vs. observed thermosteric sea level rise
J. Church (pers. comm. 2006)

6. Regional Patterns and Mechanisms
Concentrate on IPCC scenario calculations, due to paucity of analyses for the 20th century

7. Geographical pattern of simulated sea level change
Ensemble average change with respect to the global mean, ( ) minus ( ) for IPCC-A1B
PCMDI, modelling groups, J. Gregory

8. Thermosteric (m) Halosteric (m)
vs. pre-industrial
Global mean halosteric change is zero
Halosteric changes increasingly important, salinity measurements needed
Thermosteric changes relative to ctrl at the end of A1B (2100)
Halosteric changes relative to ctrl at the end of A1B (2100); cause: increased atmospheric moisture transport from low to high latitudes
Landerer et al. (2006)

9. Cumulative steric change, as a function of depth
vs.
pre-industrial
thermosteric
halosteric
total steric
Landerer et al. (2006)

10. Thermosteric changes, 2xCO2 at 1% increase
Passive anomaly tracer
Surface input as heat anomaly but with constant transport processes
Thermosteric changes caused by changes in transport (e.g., circulation, stability-dependent mixing)
Caption: “Change in the temperature driven component of steric sea level rise caused by (a) anomalous heat and (b) by heat not supplied as part of the anomalous heat flux, each with global mean subtracted (units: meters)”
2xCO2 at 1% increase
Used passive anomaly tracer to decompose thermosteric changes
Redistribution has a sizeable regional pattern (but zero global mean!)
Regional distribution crucially dependent on the changes in the large-scale circulation and mixing.
Redistribution presumable also effects observations, if they do not cover the entire ocean area
Do not know how big this effect is currently (MOC changes?)
Lowe and Gregory (2006)

11. Dynamical Aspects: A Wider Perspective
Sea level as an indicator for the Thermohaline Circulation (THC)?

12. Observed sea level ( surface circulation)m
Gulf Stream, Kuro-shio, ACC. Gyres. Mention that large-scale features reflect wind field and also vertically integrated flow reasonably well, pointing to the wind as dominant forcing mechanism of ocean circulation
Rio & Hernandez (2004)

13. Regional sea level anomalies in MPI-OM A1B
Dipole/tripole structure in the North Atlantic linked to THC changes? NA sea level proxy for THC?
Landerer et al. (2006)

14. NA sea level as a proxy for the THC?
Bryan 1996:
“(North Atlantic dipole) pattern is consistent with a weakening of the upper branch of the thermohaline circulation” and “… the sea level profile should be a valuable indicator of a weakening of the Atlantic THC”
Leverman et al., 2005:
“The specific regional pattern of sea level changes found here suggests that sea level data could be useful for monitoring changes in THC.”
Slope: 4.5 cm / -1 Sv for the NA

15. SSH/Gyre transport/THC
Baroclinic gyre transport
ΔSSH
Bermuda -
Labrador Sea
THC
Atlantic 30°N
Correlation
Landerer et al. (2006)

16. NA sea level as a proxy for THC?
Basin-wide sea level anomalies (relative to the global mean)
Maximum of North Atlantic THC
Relative sea level difference between NA and Pacific changes as MOC changes, but then is reestablished without analogous MOC recovery
NA THC
Landerer et al. (2006)

17. Simulated bottom pressure changes, 2100 - 2000
Global mean sea level has risen 0.26 m for this figure!
Landerer et al. (2006)

18. Global mean sea level & length-of-day (LOD)
Poster P69
Landerer et al. (2006)

19. Conclusions
AOGCMs show an mean increase in thermosteric sea level of 0.54±0.26 mm/yr for
Discrepancies between observed and modeled variability, for poorly understood set of reasons
Lessons from IPCC scenario calculations:
Large regional variation in thermo-/halosteric changes
Heat redistribution is as important as anomalous heat uptake
Strong geographic variations in vertical heat penetration
Regional sea level changes in the North Atlantic are no valid proxies for THC changes
Steric sea level rise leads to secular bottom pressure and length-of-day changes

20. Thank you for your attention!