What controls the historical timeseries of shortwave fluxes in the North Atlantic?

Abstract

Abstract. Both aerosol radiative forcing and cloud-climate feedbacks have large effects on climate, mainly through modification of solar shortwave radiative fluxes. Here we determine what causes the long-term trends in the shortwave (SW) top-of-the-atmosphere (TOA) fluxes (FSW) over the North Atlantic region. The UK Earth System Model (UKESM1) and the Hadley Centre General Environment Model (HadGEM) simulate a positive FSW trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We find that the pre-1970 positive FSW trend is mainly driven by an increase in cloud droplet number concentrations due to increases in aerosol and the 1970–2014 trend is mainly driven by a decrease in cloud fraction, which we attribute mainly to cloud feedbacks caused by greenhouse gas-induced warming. Using nudged simulations where the meteorology can be controlled we show that in the pre-1970 period aerosol-induced cooling and greenhouse gas warming in coupled atmosphere-ocean simulations roughly counteract each other so that aerosol forcing is the dominant effect on FSW, with only a weak temperature-driven cloud feedback effect. However, in the post-1970 period the warming from greenhouse gases intensifies and aerosol radiative forcing falls, leading to a large overall warming and a reduction in FSW that is mainly driven by cloud feedbacks. Our results show that it is difficult to use satellite observations in the post-1970 period to evaluate and constrain the magnitude of the aerosol-cloud interaction forcing, but that cloud feedbacks might be evaluated. Comparisons to observations between 1985 and 2014 show that the simulated reduction in FSW and the increase in temperature are too strong. However, analysis shows that this temperature discrepancy can account for only part of the FSW discrepancy given the estimated model feedback strength (λ = ∂FSW/∂T). This suggests a model bias in either λ or in the strength of the aerosol forcing (aerosols are reducing during this time period) is required to explain the too-strong decrease in FSW. Both of these biases would also tend to cause temperature increases over the 1985–2014 period that are too large, which would be consistent with the sign of the model temperature bias reported here. Either of these model biases would have important implications for future climate projections using these models.