3.1 Energy Fluxes and Temperature

On average, surface turbulent and long‐wave energy fluxes are directed upward in high latitudes, as the ocean loses energy to the colder atmosphere. Since sea ice works as an insulator, these upward heat fluxes are reduced over ice‐covered areas. Table 1 shows the area‐averaged pan‐Arctic flux response to the recent thinning (RIT‐CTRL) for the Arctic region, here defined as the area north of 70°N. Fluxes are defined positive upward. The response shows enhanced upward energy fluxes, both for turbulent heat (sensible + latent) as well as long‐wave radiation. The strongest signal from sea ice thinning is seen throughout winter, when the temperature gradient between ocean and atmosphere is highest. The long‐wave (turbulent heat) flux response peaks in early (middle) winter.

Table 1. Seasonal Cycle of Mean Arctic (70–90°N) Heat Flux Response (RIT‐CTRL (W/m2)) for Sensible Heat (SH), Latent Heat (LH), and Long‐Wave Radiation (LW); the Root‐Mean‐Square Error (RMSE) of Both SAT Responses Respective to ERA‐I and Bimonthly Arctic Amplification Factors (AAF) for ERA‐Interim, CTRL, and RIT; and Their Relative Change Due To an Inclusion of Sea Ice Thickness Energy Fluxes SAT RMSE AAF SH LH LW CTRL RIT ERA‐I CTRL RIT Change Oct–Nov 0.11 0.15 0.57 0.51 0.23 6.6 4.2 5.4 29% Dec–Jan 0.46 0.24 0.52 0.74 0.47 11.3 2.8 3.7 32% Feb–Mar 0.11 0.08 0.40 0.52 0.22 6.9 1.6 3.1 94% Apr–May 0.07 0.16 0.02 0.74 0.44 8.4 2.0 3.7 85% Jun–Jul 0.04 0.11 0.01 0.07 0.05 1.3 1.1 1.3 19% Aug–Sep 0.03 0.04 0.02 0.02 0.01 3.4 3.0 3.0 0% Year 0.14 0.13 0.26 0.43 0.24 6.3 2.4 3.4 37%

Figure 2 shows the ensemble mean surface air temperature (SAT) response to the recent evolution of sea ice conditions, SST, and GHGs since 1982 (RIT, middle row), the response to sea ice thinning (RIT‐CTRL, bottom row), and the trend seen in the ERA‐Interim reanalysis (top row). The signal in RIT shows that most regions of the Arctic Ocean have warmed; with up to 4°C per decade highest trends are found over the Eastern Arctic basin and the Barents‐Kara Sea. Wintertime SAT trends over continents are much smaller than over the Arctic ocean. The general pattern and magnitude accord well with ERA‐Interim, although it underestimates especially the Arctic midwinter warming, a feature potentially linked to the SIC data which show different trends then in ERA‐Interim. The local December–January cooling over Siberia in ERA‐Interim can be seen in individual ensemble members (see Figure S2 in the supporting information); yet the high internal variability masks this pattern in the ensemble mean.

Figure 2 Open in figure viewer PowerPoint Surface Air temperature response for (top row) ERA‐I, (middle row) RIT, and (bottom row) RIT‐CTRL, shown as trends per decade (°C). Hatching in RIT‐CTRL indicates areas significant on the 95% confidence level. Note the different color scales.

How much of the modeled warming is due to the thinning of sea ice? The difference RIT‐CTRL shows that prescribing a realistic sea ice thickness enhances warming trends locally by up to 1.5°C per decade over the Arctic Sea. As expected, the warming signal linked to sea ice thinning is most pronounced in winter and thus lags behind the time of strongest sea ice thinning, as the higher‐temperature differences between ocean and atmosphere give rise to higher conductive heat fluxes and hence stronger surface heating. Comparing the spatial pattern with sea ice thickness changes (Figure 1) shows that strongest warming trends coincide with regions where the Arctic sea ice (i) has been declining most in extent and thickness and (ii) is relatively thin (i.e., in marginal ice areas). These regional warmings are congruent with a thickness loss of 1–2 m over the 32 year period. Apart from few minor remote relative cooling trends (weaker warming than in the control run, see, e.g., the Eastern U.S. in December–January, Figure 2, bottom row), no robust signals are found in lower latitudes. A comparison between RIT and RIT‐CTRL shows that about a third to a half of the modeled wintertime warming over the Eastern Arctic Ocean is due to changes in ice thickness. On the other hand, sea ice thinning contributes only little to the regionally strongest warming in the Barents Sea. This is likely due to the relatively strong impact of ice extent reductions and the comparatively small change in ice thickness due to predominantly seasonal ice in this area. Linking the SAT response with the heat flux response stresses that the temperature signal is primarily of thermodynamic nature rather than forced via the large‐scale circulation. The enhanced warming due to an inclusion of a variable thickness generally leads to a better match with ERA‐Interim and thus increases the IFS' model skill (see Table 1).

The vertical profile of Arctic warming has received much attention concerning the debate about the drivers of Arctic amplification, as it can give insights into the role of different mechanisms through their respective “fingerprints” [Screen et al., 2010]. The total response to sea ice, SST, and GHG forcing (RIT; Figure 3, middle row) shows the well‐known structure of Arctic amplification: strongest warming is found in the lowermost troposphere up to 850 hPa (a result of the stably stratified boundary layer in the Arctic) at roughly 70–80°N, i.e., the latitudes where sea ice reduced most in extent and thickness. The spatial pattern stresses the role of sea ice in the surface‐based Arctic amplification. ERA‐Interim (Figure 3, top row) shows a similar pattern as seen in RIT but with overall stronger warming trends. The position of strongest modeled warming is shifted farther south in RIT, a feature related to the absence of Siberian wintertime cooling (see Figure 2). However, there is considerable uncertainty in the validity of the Arctic vertical temperature profile as seen in reanalysis sets like ERA‐Interim, and different sets can give different results (as, e.g., in Graversen et al. [2008] and Screen and Simmonds [2010]). The response to a realistic sea ice thinning (RIT‐CTRL; Figure 3, bottom row) shows a significant warming of near‐surface layers only up to 950 hPa and latitudes north of 70°N, congruent with regions of sea ice thinning. That is, changes in sea ice thickness only affect lower levels of the troposphere and response signals quickly vanish with height. This supports the understanding that sea ice loss is linked to surface‐based warming [Kumar et al., 2010; Screen et al., 2013], while the Arctic amplification in the free atmosphere cannot directly be linked to sea ice loss, even if the recent sea ice thinning is considered. This stresses that there are likely still other processes involved.

Figure 3 Open in figure viewer PowerPoint Vertical profile of temperature response for (top row) ERA‐I, (middle row) RIT, and (bottom row) RIT‐CTRL, shown as trends per decade (°C). Contour lines indicate areas significant on the 95% confidence level.

To set the local warming response due to sea ice thinning in context with the recent Arctic amplification, we compare Arctic amplification factors (AAF) between RIT and CTRL, calculated as the SAT trend in the Arctic region (70°N–90°N) divided by the global mean trend. As seen in Table 1, CTRL underestimates the AAF, especially in winter, a feature many CMIP3 models exhibit [Mahlstein and Knutti, 2012]. In our case this mainly stems from larger (smaller) global (Arctic) SAT trends in IFS compared to ERA‐Interim. However, reanalysis sets such as ERA‐Interim may be afflicted with systematic errors due to the lack of in situ observations and the handling of sea ice data and hence tend to shift toward their own climate. The inclusion of realistic ice thickness distribution greatly improves the simulation of AAF and leads to an increase in the year‐round AAF over the last 32 years by 37% from 2.45 to 3.36, which can mainly be attributed to the enhanced winter warming. The strongest relative impact of sea ice thinning on recent Arctic amplification is found in late winter (AAF increase by 94%), while the total modeled Arctic amplification is strongest in early winter. Hence, the recent Arctic sea ice thinning has significantly contributed to and amplified the surface‐based Arctic amplification.