Ocean Heat Content responses to changing Anthropogenic Aerosol Forcing
Strength: regional and multi-decadal variability
Abstract
The causes of decadal variations in global warming are poorly
understood, however it is widely understood that variations in ocean
heat content are linked with variations in surface warming. To
investigate the forced response of ocean heat content (OHC) to
anthropogenic aerosols (AA), we use an ensemble of historical
simulations, which were carried out using a range of anthropogenic
aerosol forcing magnitudes in a CMIP6-era global circulation model. We
find that the centennial scale linear trends in historical ocean heat
content are significantly sensitive to AA forcing magnitude
($-3.0\pm0.1$ x10$^{5}$ (J m$^{-3}$
century$^{-1}$)/(W m$^{-2}$), R$^2$=0.99), but
interannual to multi-decadal variability in global ocean heat content
appear largely independent of AA forcing magnitude. Comparison with
observations find consistencies in different depth ranges and at
different time scales with all but the strongest aerosol forcing
magnitude, at least partly due to limited observational accuracy. We
find broad negative sensitivity of ocean heat content to increased
aerosol forcing magnitude across much of the tropics and sub-tropics.
The polar regions and North Atlantic show the strongest heat content
trends, and also show the strongest dependence on aerosol forcing
magnitude. However, the ocean heat content response to increasing
aerosol forcing magnitude in the North Atlantic and Southern Ocean is
either dominated by internal variability, or strongly state dependent,
showing different behaviour in different time periods. Our results
suggest the response to aerosols in these regions is a complex
combination of influences from ocean transport, atmospheric forcings,
and sea ice responses.