Changes in Summer Sea Ice Area
Increasing greenhouse gases in the RCP8.5 experiment result in an ice-free Arctic Ocean in September by the end of the 21st century [Vavrus et al., 2012; Jahn and Holland, 2013]. In the regional solar dimming experiments, the area of retained summer sea ice is strongly correlated with the amount of solar dimming (Figure 1). A local dimming poleward of 60°N of 1 or 2 times the amount of the global insolation reduction saves less than half of the present-day sea ice area. Four times the amount of the global insolation reduction is required, reaching 180 W/m2(approximately 13% of TSI) by the end of the century, to save 82% of the summer ice, a figure similar to the 1xGlobal experiment (Figure 1). In this case, the summer sea ice volume is still reduced by 34% compared to the control, and the summer snow volume on the sea ice is increased by half (Table S1 in the supporting information) due to enhanced moisture transport from lower latitudes, as discussed below. Changes in the 4x70N experiment are comparable to the 3x60N experiment, resulting in a summer Arctic sea ice area that is 64% of the control.
Figure 1. Sea ice area for different model experiments. Evolution of Northern Hemisphere (a) annual average and (b) summer average sea ice area between 2020 and 2100 for different experiments (see legend); a 5 year running mean is applied. The standard deviations of changes with regard to the 5 year running mean are shown on the right of each line, marking the average value derived for the last 20 years of each of the simulations. (c) Scatterplot of NH Ice Area and the downward SW clear sky forcing north of 60°N, for different experiments between 2080 and 2099, (left) annual average and (right) summer average. The forcing of the 4x70N experiment is averaged over 60–90°N.
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Changes in Heat Fluxes
Both local radiative changes and adjustments in the northward heat transport (NHT) by the atmosphere and ocean are important drivers of the Arctic energy budget. Our simulations show that, with growing regional solar insolation reduction, both local feedback processes and NHT counteract the initial decrease in surface energy flux and therefore require a much larger amount of solar dimming locally than for global interventions.
In summer, changes in the net SW radiation are largely controlled by local feedback, which lead to a net increase in the absorbed SW radiation and cooling of the atmosphere (supporting information, section 1, and Figure 1). In fall and winter, changes in sensible and latent heat fluxes from the surface to the atmosphere are the dominant forces in the surface energy budget, resulting in intensified heat exchange from the ocean surface to the atmosphere. Poleward of 60°N, the change in net downward surface energy flux in RCP8.5 is about 10 W/m2 in early summer (June) and −10 W/m2 in October/November (Figure 2). This results in the seasonality of surface air temperature and the largest warming in autumn and winter [Manabe and Stouffer, 1980].
Figure 2. Surface energy budget for different experiments. Seasonal cycle of the Arctic surface heat budget change relative to the control for net shortwave (SW), net long wave (LW), net turbulent (colored bars), total fluxes (black plus signs), and temperature changes relative to the control (blue line), averaged between 60 and 90°N for different experiments (different panels). Flux changes are positive downward.
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The artificial reduction in incoming SW radiation on top of the atmosphere in regional Arctic dimming experiments leads to a reduction in the downwelling surface SW energy flux mostly in summer (Figure 2, bottom). However, continuous loss of sea ice in spring and early summer induces a positive surface albedo feedback, especially for the experiments with a smaller amount of dimming. For regional dimming experiments where temperatures are closer to being stabilized (3x60N and 4x60N), more summer Arctic sea ice is retained. With increasing regional dimming, changes of clouds and turbulent fluxes counteract the artificial insolation reduction (Figure 2, bottom), requiring additional dimming to sufficiently maintain summer sea ice area close to control conditions.
In winter, when SW radiation is mostly irrelevant to the local energy budget, a reduction of net longwave fluxes in the strongest dimming experiments (due in part to changes in moisture transport, as discussed below) contribute to the warming of the atmosphere of 4–6 K in high latitudes (Figure 2). Winter warming reduces ice growth rates, leading to a continuous reduction of Arctic ice volume. Changes of surface energy fluxes for the 1xGlobal experiment are smaller than regional interventions but still lead to modest high-latitude warming in winter and a reduction of Arctic ice volume.
A net downward surface SW flux change of −20 W/m2 in summer high latitudes is required for the regional dimming experiments to balance changes in the horizontal atmospheric and oceanic NHT (Figure 3). In response to increased greenhouse gas concentrations, the atmospheric latent NHT increases proportionally to the global temperature change. In contrast, the sensible NHT decreases with a magnitude related to a reduction in the temperature gradient between high and low latitudes [Hwang et al., 2011]. Despite the high-latitude temperature reduction in the Arctic dimming experiments, the warming equatorward of 60°N (and hence globally) is similar to RCP8.5. Latent NHT into the Arctic is therefore similar or even slightly increased for 4x60N in comparison to the RCP8.5 case (Figure 3). In contrast, an increased meridional temperature gradient in the regional dimming experiments, due to the relative cooling of high latitudes, leads to a smaller reduction of the sensible NHT compared to RCP8.5. Indeed, in the 4x60N experiment, sensible NHT actually increases relative to the control (Figure 3). Therefore, the total atmospheric NHT increases strongly in the regional dimming experiments compared to RCP8.5 and control conditions.
Figure 3. Northward heat transport (NHT) change in Pettawatts at 60°N, relative to the control, for sensible NHT, latent NHT, ocean NHT (colored bars), and total NHT (black plus signs) and for different experiments.
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High-latitude ocean heat transport also changes with rising greenhouse gases. In the RCP8.5 simulation, the ocean transports less heat poleward as the meridional overturning circulation weakens over the 21st century [Winton, 2008]. In both the global and regional dimming experiments, the reduction in ocean NHT in RCP8.5 is mitigated by a smaller MOC response; in fact, in 4x60N (Figure 3), there is an increase in ocean NHT compared to the control. For the 4x70N experiment, more than half of the Arctic sea ice area is retained compared to RCP8.5, while the ocean NHT decreases much more than in all the other regional dimming experiments (Figure 3). This is due to a large increase in net precipitation in the 4x70N experiment (Table S1 in the supporting information), resulting in a stronger MOC response than the other dimming experiments.
Impact on the Meridional Overturning Circulation
Relative to the control, all the regional dimming experiments show a freshening of the northern North Atlantic and consequent reductions in Labrador Sea convection, with the largest changes in the weaker dimming experiments (Table S1 in the supporting information). This weakens the MOC. These changes are smaller than for RCP8.5, due to the reduced melting of sea ice and a cooling in the Labrador Sea region, but are still considerable. In addition to sea ice melting, the enhanced hydrological cycle in a warmer global climate leads to increased river input from midlatitudes into the Arctic Ocean and increased net precipitation over the Arctic Ocean. Together with the sea ice melt, this in turn leads to increased liquid Arctic freshwater export through the Fram Strait and enhanced stratification in the Labrador Sea. The runoff is even larger in the regional dimming experiments than in RCP8.5, due to the increased temperature gradient and the resulting increased northward atmospheric latent heat and moisture fluxes.