R. Hallberg, A. Adcroft, J. P. Dunne, J. P. Krasting and R. J. Stouffer NOAA/GFDL & Princeton University

Transcription

1 Sensitivity of 21st Century Steric Sea Level Rise to Ocean Model Formulation R. Hallberg, A. Adcroft, J. P. Dunne, J. P. Krasting and R. J. Stouffer NOAA/GFDL & Princeton University Hallberg, R., A. Adcroft, J. P. Dunne, J. P. Krasting, R. J. Stouffer, 2013: Sensitivity of Twenty-First-Century Global-Mean Steric Sea Level Rise to Ocean Model Formulation. J. Climate, 26, doi:

2 Plausible 1 m ice-sheet dynamics contribution to sea level rise Projected Global Mean Sea Level Rise Sources of uncertainty in 2100 global mean sea level projections: Forcing scenario (~20 cm range at 2100 for GFDL-CM2.1) Ocean heat storage (steric rise) (13 to 32 cm in IPCC AR4) Land ice (except ice sheets) (4 to 19 cm in IPCC AR4) Ice sheet surface mass balance (-10 to 4 cm in IPCC AR4) Dams & land water ±3 cm/century? (Lettenmaier & Milly, Nature Geo. 2009) Antarctic & Greenland Ice sheet dynamics changes? Plausible range 20 to 110 cm by 2100 (Pfeffer et al., Science 2008) Reservoir sizes in Sea Level Rise equivalent: Mountain glaciers & ice caps 0.3 m Greenland Ice Sheet 7.3 m West Antarctic Ice Sheet (marine) 5 m East Antarctic Ice Sheet (land) 51.6 m (Uniform warming of ocean ~0.5 m C -1 )

3 Ocean Components of GFDL s AR5 Earth System Models ESM2G (GOLD) 1 res. (360x210), on tripolar grid. 59 Isopycnal interior layers + 4 in ML C-grid discretization Split explicit free surface ; fresh water fluxes as surface B.C. 2-layer refined bulk mixed layer with 2 buffer layers Full nonlinear equation of state except for coordinate definition Tracer diffusion rotated to s 2 surfaces Partially open faces allow explicit exchanges with marginal seas. Legg et al. BBL mixing + Simmons et al. diapycnal diffusion + Vertically constant background diffusivity w/ Henyey structure. Visbeck variable thickness diffusivity. Biharmonic Smagorinsky + Resolution scaled Laplacian viscosity. Jackson et al (2008) shear-richardson number dependent mixing. 1 hour baroclinic timestep, 2 hour tracer & coupling timesteps Continuously variable topography ESM2M (MOM4.1) 1 res. (360x200), on tripolar grid. 50 Z*-coordinate vertical levels B-grid discretization Split explicit free surface ; fresh water fluxes as surface B.C. KPP mixed layer with 10 m resolution down to 200 m Full nonlinear equation of state Multi-Dimensional PPM tracer advection Tracer diffusion rotated to neutral directions Marginal sea exchanges specified via crossland mixing Lee et al. BBL mixing + Simmons et al. diapycnal diffusion with vertically constant background w/tanh(q). Baroclinicity-dependent GM diffusivity. Biharmonic Smagorinsky + Resolution scaled Laplacian viscosity KPP specification of interior shear-richardson number dependent mixing 2 hour baroclinic and coupling timesteps. Partial cell topography

4 ESM2G 1.53 C RMS SST Bias in ESM2G & ESM2M ESM2M 1.56 C RMS Reynolds Climatology Both models were spun up for over 2000 years with preindustrial 1860 forcing. Historical forcing anomalies (greenhouse gasses, aerosols, and volcanos, solar variability, land use) start in Bias patterns are similar, but ESM2G is generally cooler at the surface. CO 2 concentrations prescribed, with ocean carbon budget inverted for net CO 2 emissions differing by 9%.

5 Potential Temperature Salinity Pacific Ocean Zonal-mean Biases ESM2G ESM2M

6 Potential Temperature Salinity Atlantic Ocean Zonal-mean Biases ESM2G ESM2M

7 Horizontal Mean Temperatures,

8 Steric Sea Level Rise Relative to [cm] Historical & Scenario-projected Steric Sea Level Rise Observed Trends in Steric SLR 2100 Both models agree with observed steric sea level rise trends. ESM2M exhibits 18% more 21 st century steric sea level rise than ESM2G.

9 Temperature Anomaly [ C] SST Anomaly [ C] Sea Surface and Volume-mean Temperature Changes Global-mean Sea Surface Temperature Anomaly Volume-mean Ocean Temperature Anomaly The models sea surface temperature trends are similar for the same scenario. ESM2M exhibits 9% more 21 st century heat uptake than ESM2G.

10 Atlantic Ocean Pacific Ocean Temperature Changes ESM2G ESM2M

11 Contributions by Depth to Steric Sea-level Rise in 2100 Temperature Change Steric SLR Contributions ESM2M-ESM2G SSLR Contrib. SSLR Difference by Expansion Term 18% SSLR difference: 9% Heat uptake; 7% Thermal expansion coefficient; 1% Nonlinearity.

12 0 400 m Depth m Depth RCP8.5 Temperature Changes by 2100 ESM2G ESM2M

13 Conclusions 21 st century steric sea level rise differs by 18% between ESM2G and ESM2M, due solely to ocean formulation. These differences are not large enough to call into question qualitative projections of climate change. The steric sea level rise differences arise primarily from: 9% differences in heat uptake (from a less stratified thermocline and greater penetration of surface-driven heating) and 7% larger thermal expansion coefficient (from a warmer spunup ocean state). Mean state ocean biases may be an indicator of coupled model reliability in predicting steric sea level rise.