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, 20146 Hamburg, Germany

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

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?

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

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

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

Geographical pattern of simulated sea level change Ensemble average change with respect to the global mean, (2070-2099) minus (1970-1999) for IPCC-A1B PCMDI, modelling groups, J. Gregory

Thermosteric (m) Halosteric (m) 2090-2099 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)

Cumulative steric change, as a function of depth 2090-2099 vs. pre-industrial thermosteric halosteric total steric Landerer et al. (2006)

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)

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

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)

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)

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

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

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)

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

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

Conclusions AOGCMs show an mean increase in thermosteric sea level of 0.54±0.26 mm/yr for 1950-2000 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

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