Constraining the uncertainty associated with sea salt aerosol parameterizations in global models using nudged UKESM1-AMIP simulations

Abstract

Sea salt is the largest source of natural aerosol in the atmosphere by mass. Formed when ocean waves break and bubbles burst, sea salt aerosols (SSA) influence Earth’s climate via direct and indirect processes. Models participating in the sixth Coupled Model Intercomparison project (CMIP6) demonstrate a negative effective radiative forcing when SSA emissions are doubled. However, the magnitude of the effective radiative forcing ranges widely from -0.35 +/- 0.04 W/m^2 to -2.28 +/- 0.07 W/m^2, with the largest difference over the Southern Ocean. Differences in the response to doubled SSA emissions arise from model uncertainty (e.g. individual model physics, aerosol size distribution) and parameterization uncertainty (e.g. how SSA is produced in the model). Here, we perform single-model experiments with UKESM1-AMIP incorporating all of the SSA parameterizations used by the current generation of CMIP6 Earth system models. Using a fixed SSA size distribution, our experiments show that the parameterization uncertainty causes large inter-model diversity in SSA emissions in the models, particularly over the tropics and the Southern Ocean. The choice of parameterization influences the ambient aerosol size distribution, cloud condensation nuclei and cloud droplet number concentrations, and therefore direct and indirect radiative forcing. We recommend that modelling groups evaluate their SSA parameterizations and update them where necessary in preparation for future model intercomparison activities

Authors 8

1. Abhijith U. Venugopal corresponding

   University of Canterbury

   Affiliation as printed

   University of Canterbury

   School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand

2. Yusuf Bhatti Aachen SRON Netherlands Institute for Space Research Now at SRON Netherlands Institute for Space Research

   University of Canterbury · National Institute of Water and Atmospheric Research · Space Research Organisation Netherlands · University of Reading

   Affiliation as printed

   SRON Netherlands Institute for Space Research

   National Institute of Water and Atmospheric Research, Wellington, New Zealand

   Now at Reading University, Department of Meteorology, UK

   Now at SRON Netherlands Institute for Space Research, Leiden, the Netherlands &

   School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand

   University of Canterbury

3. Olaf Morgenstern Aachen SRON Netherlands Institute for Space Research

   Met Office · National Institute of Water and Atmospheric Research · Space Research Organisation Netherlands

   Affiliation as printed

   NIWA

   Met Office, Exeter, United Kingdom

   National Institute of Water and Atmospheric Research, Wellington, New Zealand

   SRON Netherlands Institute for Space Research

4. Jonny Williams Aachen SRON Netherlands Institute for Space Research

   National Institute of Water and Atmospheric Research · Space Research Organisation Netherlands · University of Reading

   Affiliation as printed

   University of Reading

   National Institute of Water and Atmospheric Research, Wellington, New Zealand

   SRON Netherlands Institute for Space Research

5. Nicholas J. Edkins

   Met Office · National Institute of Water and Atmospheric Research

   Affiliation as printed

   NIWA

   Met Office, Exeter, United Kingdom

6. Catherine J. Hardacre

   University of Canterbury

   Affiliation as printed

   University of Canterbury

   School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand

7. Anthony Crawford Jones

   Met Office · University of Exeter

   Affiliation as printed

   University of Exeter

   Met Office, Exeter, United Kingdom

8. Laura E. Revell

   University of Canterbury

   Affiliation as printed

   University of Canterbury

   School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand