The Roles of the Atmosphere and Ocean in Driving Arctic Warming Due to European Aerosol Reductions

Abstract

Clean air policies can have significant impacts on climate in remote regions. Previous modeling studies have shown that the temperature response to European sulfate aerosol reductions is largest in the Arctic. Here we investigate the atmospheric and ocean roles in driving this enhanced Arctic warming using a set of fully coupled and slab-ocean simulations (specified ocean heat convergence fluxes) with the Norwegian Earth system model (NorESM), under scenarios with high and low European aerosol emissions relative to year 2000. We show that atmospheric processes drive most of the Arctic response. The ocean pathway plays a secondary role inducing small temperature changes mostly in the opposite direction of the atmospheric response. Important modulators of the temperature response patterns are changes in sea ice extent and subsequent turbulent heat flux exchange, suggesting that a proper representation of Arctic sea ice and turbulent changes is key to predicting the Arctic response to midlatitude aerosol forcing.Plain Language Summary Aerosols are liquid or solid particles suspended in air, which may have adverse air quality and health impacts. Sulfate aerosols also have a cooling influence on climate and can mask some of the greenhouse gas-induced global warming. While aerosol emissions are variable in space and time, their impacts are not limited to where they are emitted. In fact, studies using global climate models have shown that changing sulfur dioxide emissions in Europe can have significant impacts on Arctic climate. Here we investigate the roles of changes in atmospheric and ocean heat transport in driving these changes in the Arctic by conducting a series of climate model simulations with specified anthropogenic sulfur dioxide emissions and different ocean heat transport fluxes. We find that changes through the atmosphere play a primary role in affecting the Arctic climate. These changes are modulated by changes in sea ice extent and the energy exchange between ocean and atmosphere in the sub-Arctic. Aerosol-driven changes in ocean heat transport play a smaller, secondary role in the Arctic and tend to reduce the impacts. Our results show that the proper representation of Arctic sea ice is crucial for accurately modeling the Arctic response to changes in midlatitude aerosol forcing.

The Arctic has warmed nearly four times faster than the globe since 1979

“…During the last decade, multiple factors have been proposed to explain the potential causes of AA: enhanced oceanic heating and ice-albedo feedback due diminishing sea ice [6][7][8][9] , Planck feedback 10 , lapse-rate feedback 11 , near-surface air temperature inversion 12 , cloud feedback 13 , ocean heat transport 14 and meridional atmospheric moisture transport [15][16][17] . Furthermore, the reduced air pollution in Europe may have contributed to the Arctic warming during the last decades 18,19 , and possible reductions of Asian aerosols under a strong mitigation policy may increase the future AA 20 . In climate models, it has been shown 21 that AA occurs rapidly in response to external forcings due to atmospheric lapse rate feedback, with sea ice-related feedbacks becoming more important later on.…”

The recent emergence of Arctic Amplification

“…We note that the Arctic Amplification factor, commonly defined as the Arctic SAT trend divided by the global SAT trend, is only 2.2 for greenhouse gases (a driver of Arctic and global warming) over this period, but 3.0 for industrial aerosols (a driver of Arctic and global cooling). This difference likely arose because aerosol emissions primarily occurred over North America and Northern Europe Krishnan et al, 2020;Navarro et al, 2016). This helps to explain why the simulated forced response (i.e., the CESM1-LE ensemble mean) features a small cooling trend in the Arctic SAT and a weak warming trend in the global SAT (compare ensemble mean lines in Figure 2c).…”

“…10.1029/2021GL094086 2 of 10 sea ice (Ding et al, 2017(Ding et al, , 2019England, 2021;England et al, 2019;Krishnan et al, 2020;Landrum & Holland, 2020;Polvani et al, 2020).…”

The Arctic has warmed four times faster than the globe since 1980

“…During the last decade, multiple factors have been proposed to explain AA: reduced surface albedo due to sea-ice loss and snow retreat (Screen and Simmonds, 2010;Taylor et al, 2013;Dai et al, 2019), Planck feedback (Pithan and Mauritsen, 2014), lapse-rate feedback (Stuecker et al, 2018), surface thermal inversion (Bintanja et al, 2011), cloud feedback (Taylor et al, 2013), ocean heat transport (Beer et al, 2020) and meridional atmospheric moisture transport (Graversen and Burtu, 2016;Woods and Caballero, 2016;Kim et al, 2017). Furthermore, the reduced pollution in Europe may have contributed to the Arctic warming during the last decades (Navarro et al, 2016;Krishnan et al, 2020), and possible reductions of Asian aerosols under a strong mitigation policy may increase the future AA (Merikanto et al, 2021). Also, there is little consensus on the magnitude of AA.…”