Use of CCSM3 and CAM3 Historical Runs: Estimation of Natural and Anthropogenic Climate Variability and Sensitivity

Bruce T. Anderson, Boston University (brucea@bu.edu)Clara Deser, NCAR

Introduction

Recent analysis of coupled-climate model simulations and observations suggests that presently there is an energy imbalance within the Earth’s climate system on the order of 0.75-0.85W/m2

This imbalance, associated with ocean-heat uptake, results in a time-lag within the climate system e.g. globally-averaged temperatures represent a lagged response to climate forcingHere we want to use various runs of atmosphere-only and atmosphere-ocean general circulation models to:

Estimate the historical evolution of the full anthropogenic radiative forcing over the last 50+yearsEstimate historical ocean heat uptakeSee what effect ocean heat uptake has had upon the effective radiative forcing and realized (vs. unrealized) surface temperature changes

Data Sets

NCAR’s Coupled Community System Model (CCSM3)T85-resolution (approximately 75km)Three “Forced Simulations” including historical greenhouse gas (GHG) concentrations, sulfate aerosols, volcanic particulates, stratospheric and tropospheric ozone, and solar activity for the period 1870-1999One “Control Simulation” run with constant GHG concentrations (set to 1990 levels) for a 400+ year integration period

NCAR’s Community Atmosphere Model (CAM3.1)T85-resolutionFive “AMIP Simulations” of the CAM3.1 forced only by historical changes in global SSTs Five “AMIP-ATM Simulations” forced by historical changes in SSTs, GHG concentrations, sulfate aerosols, volcanic particulates, stratospheric and tropospheric ozone, and solar activity

Data Sets Con’t

ObservationsUpper-air data• NCEP Reanalysis-I data product • The European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40 product

Surface and sub-surface data• Climate Research Unit (CRU) globally-averaged surface temperatures

• Ocean-heat content from Levitus et al. (2005)

Energy Budget Considerations

Comparison of Top-of-Atmosphere (TOA) Radiation Estimates

Use a method equivalent to the archetypical “cloud radiative forcing” methodology (Cess et al., 1990) in which the the difference between “cloudy” and “clear” net incoming TOA radiation gives the radiative forcing associated with changes in the radiatively-active chemical composition of the atmosphereOn a global- and time-averaged basis the net incoming radiation through the top of the atmosphere balances energy fluxes into the atmosphere from the underlying surface (Trenberth et al., 2002):

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ΔRnetTOA + ΔFs = 0

For AMIP Simulations change in net radiation can be used to estimate radiative heating, G’, assuming SST changes are in equilibrium with radiative forcing (Cess et al., 1990):

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ΔRAMIPTOA = −ΔFs = −ΔG'

Energy Budget Considerations

Comparison of Top-of-Atmosphere (TOA) Radiation Estimates

However, given subsurface ocean heat up-take, H0, the SSTs are not in equilibrium with total radiative forcing, G (e.g. Hansen et al., 2005)

By taking the difference of the top of atmosphere net radiation from the AMIP-ATM Simulations and AMIP Simulations we can estimate the total radiative forcing, G:

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ΔRAMIP−ATMTOA − ΔRAMIP

TOA = ΔH0 − −ΔG'( ) = ΔG€

ΔRAMIP−ATMTOA = ΔG − ΔFs = ΔG − ΔG'= ΔH0

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ΔG = ΔG'+ΔHO

AMIP-ATM Simulations provide an estimate of the difference between total radiative heating, G, and effective radiative heating associated with SST changes, Fs:

Simulated Top-of-Atmosphere Radiation Est.

ΔH0

ΔG’

Simulated Estimate of Total Radiative Forcing

ΔGtotal

Actual and Expected Global Temp. Change

Observed and Estimated Heat Content Changes

Conclusions

Forcing an AGCM both with observed changes in global-scale SST anomalies, and then forcing it with both global-scale SSTs combined with radiatively-active atmospheric constituents can provide estimates of ocean-heat uptake, total radiative forcing associated with changing Greenhouse gases, and overall climate sensitivity

Total anthropogenic radiative forcing has increased 1.57W/m2 over the last 50 yearsOcean heat-uptake has increased approximately 0.47W/m2 over the last 50 yearsClimate sensitivity is approximately 0.40K/(W/m2)

The estimates for ocean-heat uptake, climate senstivity and effective radiative forcing are model-dependent!!!Comparison with observed ocean-heat uptake can be used to evaluate AGCM response and hence climate sensitivity

Schematic

ΔFs

ΔRAMIP ΔFs Greenhouse Gases

ΔG

For:ΔG = ΔFs ΔH0 = 0ΔG > ΔFs ΔH0 > 0ΔG < ΔFs ΔH0 < 0

Simulated Ocean Heat Uptake and ENSO Events

ΔH0

Simulated and Observed Temperatures

Simulated and Obs. Sea-Surface Temperatures

Simulated Top-of-Atmosphere Radiation Est.