What controls the historical timeseries of shortwave fluxes in the North Atlantic?
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 ( F SW ) over the North Atlantic regi...
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ftcopernicus:oai:publications.copernicus.org:acpd105864 2023-05-15T17:32:33+02:00 What controls the historical timeseries of shortwave fluxes in the North Atlantic? Grosvenor, Daniel Peter Carslaw, Kenneth S. 2022-08-26 application/pdf https://doi.org/10.5194/acp-2022-583 https://acp.copernicus.org/preprints/acp-2022-583/ eng eng doi:10.5194/acp-2022-583 https://acp.copernicus.org/preprints/acp-2022-583/ eISSN: 1680-7324 Text 2022 ftcopernicus https://doi.org/10.5194/acp-2022-583 2022-08-29T16:22:54Z 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 ( F SW ) over the North Atlantic region. The UK Earth System Model (UKESM1) and the Hadley Centre General Environment Model (HadGEM) simulate a positive F SW trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We find that the pre-1970 positive F SW 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 F SW , 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 F SW 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 F SW and the increase in temperature are too strong. However, analysis shows that this temperature discrepancy can account for only part of the F SW discrepancy given the estimated model feedback strength (λ = ∂ F SW /∂ 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 ... Text North Atlantic Copernicus Publications: E-Journals |
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Copernicus Publications: E-Journals |
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English |
description |
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 ( F SW ) over the North Atlantic region. The UK Earth System Model (UKESM1) and the Hadley Centre General Environment Model (HadGEM) simulate a positive F SW trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We find that the pre-1970 positive F SW 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 F SW , 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 F SW 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 F SW and the increase in temperature are too strong. However, analysis shows that this temperature discrepancy can account for only part of the F SW discrepancy given the estimated model feedback strength (λ = ∂ F SW /∂ 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 ... |
format |
Text |
author |
Grosvenor, Daniel Peter Carslaw, Kenneth S. |
spellingShingle |
Grosvenor, Daniel Peter Carslaw, Kenneth S. What controls the historical timeseries of shortwave fluxes in the North Atlantic? |
author_facet |
Grosvenor, Daniel Peter Carslaw, Kenneth S. |
author_sort |
Grosvenor, Daniel Peter |
title |
What controls the historical timeseries of shortwave fluxes in the North Atlantic? |
title_short |
What controls the historical timeseries of shortwave fluxes in the North Atlantic? |
title_full |
What controls the historical timeseries of shortwave fluxes in the North Atlantic? |
title_fullStr |
What controls the historical timeseries of shortwave fluxes in the North Atlantic? |
title_full_unstemmed |
What controls the historical timeseries of shortwave fluxes in the North Atlantic? |
title_sort |
what controls the historical timeseries of shortwave fluxes in the north atlantic? |
publishDate |
2022 |
url |
https://doi.org/10.5194/acp-2022-583 https://acp.copernicus.org/preprints/acp-2022-583/ |
genre |
North Atlantic |
genre_facet |
North Atlantic |
op_source |
eISSN: 1680-7324 |
op_relation |
doi:10.5194/acp-2022-583 https://acp.copernicus.org/preprints/acp-2022-583/ |
op_doi |
https://doi.org/10.5194/acp-2022-583 |
_version_ |
1766130729016623104 |