Warm and Moist Air Intrusions into Winter Arctic: A Lagrangian view on the near-surface energy budgets

. In this study, warm and moist air intrusions (WaMAI) over the Arctic Ocean sectors of Barents, Kara, 8 Laptev, East Siberian, Chukchi and Beaufort Seas in recent 40 winters (from 1979 to 2018) are identified from 9 ERA5 reanalysis using both Eulerian and Lagrangian views. The analysis shows that WaMAIs, fuelled by Arctic 10 blockings, causes a relative surface warming and hence a sea ice reduction by exerting positive anomalies of net 11 thermal irradiances and turbulent fluxes to the surface. Over Arctic Ocean sectors with land-locked sea ice in 12 winter, such as Laptev, East Siberian, Chukchi and Beaufort Seas, total surface energy budget is dominated by 13 net thermal irradiance. From a Lagrangian perspective, total water path (TWP) increases linearly with the 14 downstream distance from the sea ice edge over the completely ice-covered sectors, inducing almost linearly 15 increasing net thermal irradiance and total surface energy-budget. However, over the Barents Sea, with an open 16 ocean to the south, total net surface energy-budget is dominated by the surface turbulent flux. With the energy in 17 the warm-and-moist air continuously transported to the surface, net surface turbulent flux gradually decreases 18 with distance, especially within the first 2 degrees north of the ice edge, inducing a decreasing but still positive 19 total surface energy budget. The boundary-layer energy-budget patterns over the Barents Sea can be categorized 20 into three classes: radiation-dominated, turbulence-dominated and turbulence-dominated with cold dome, 21 comprising about 52%, 40% and 8% of all WaMAIs, respectively. Statistically, turbulence-dominated cases with 22 or without cold dome occur along with one order of magnitude larger large-scale subsidence than the radiation- 23 dominated cases. For the turbulence-dominated category, larger turbulent fluxes are exerted to the surface, 24 probably because of stronger wind shear. In radiation-dominated WaMAIs, stratocumulus develops more strongly 25 and triggers intensive cloud-top radiative cooling and related buoyant mixing that extends from cloud top to the 26 surface, inducing a thicker well-mixed layer under the cloud. With the existence of cold dome, fewer liquid water 27 clouds were formed and less or even negative turbulent fluxes could reach the surface.

3 thermal (Flw) irradiances, the surface sensible (Fsh) and latent heat fluxes (Flh), as well as the 1-hourly temperature 75 tendencies due to different model physics extracted at model levels.

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Utilizing ERA5 reanalysis introduces uncertainty, especially for anything that comes from parameterized 77 model physics such as cloud parameters and the energy budget. Large upward residual heat flux biases exist 78 among all reanalysis and turbulent heat flux over the sea ice are also poorly simulated in all seasons (Graham et

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Although it is possible to have a warm and dry air mass intruding in the Arctic, it is quite unlikely to have an 93 intrusion that is moist and cold. We therefore identify WaMAIs by analyzing the vertically integrated northward     are also found in all other ocean basins ( Figure S1, S3, S5).

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As warm and moist air is advected into the Arctic over the Barents Sea, it interacts with the cool ice  169

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The composites of large-scale pattern discussed above are extracted from the stronger EMI events to 176 generate a clear signal, however, these may not necessarily represent the general pattern of all WaMAIs. Therefore,

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In an absolute sense the boundary layer always undergoes a gradual cooling during the advection over the sea ice.

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Over the Barents Sea, the maximum air temperature (Figure 12a, 13a, 14a) and specific humidity (Figure 12d, 290 13d, 14d) over open ocean south of the ice edge are always located right above the sea surface as a result of the 291 strong air-sea interaction and are also typically larger than those over ocean sectors with land-locked sea ice. As 292 this air-mass, considerably affected by air-sea interaction, is advected over the sea ice, different stories take place.

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Around 8% of all WaMAIs over the Barents Sea belong to category RAD (Table 2). In this category, the in category TCD (Table 3,

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Meanwhile, the value of maximum temperature and specific humidity is decreasing gradually along the 308 trajectory, indicating that the heat and moisture within the warm-and-moist air is consumed continuously by the 309 cloud formation and surface turbulent mixing. For this category, is comparable to those of category TBL and 310 TCD (Table 3), and increases almost linearly along the trajectory (Figure 16d1) due to the enhancing TWP ( Figure   311 16c1). ℎ and ℎ are generally smaller than those of category TBL since stronger mixing weakens vertical 312 gradients in the PBL and hence suppresses the surface turbulent heat flux (Table 3). The decreasing rates of ℎ  13b). In this category, the well-mixed layer is substantially thinner than in category RAD, since the turbulent 328 mixing here is mainly forced by surface friction, weaker and less effective than the buoyant mixing in category 329 RAD (Figure 12b). Turbulence is mainly forced by wind shear and buoyancy, but buoyancy is negative here in 330 the initially very stable near-surface layer. Therefore, wind shear mostly fuels the turbulent mixing. In category 40% of WaMAIs over the Barents Sea belong to this category. For this category, the boundary-layer energy-335 budget is generally similar to that in category TBL. The main difference is that there is always a layer of cold air As the warm-and-moist air-mass is advected over the cold dome, it is gradually lifted up by the cold dome and 339 consequently, ht and hsp are increasing at a faster rate than in category TBL (Figure 16b3). With faster lifting ht 340 and hsp, ℎ and ℎ would be reduced more rapidly or even become negative in the high Arctic (Figure 16a3).

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TWP is dominated by LWP in category RAD and TWP is contributed almost equally by LWP and IWP in category 342 TBL, while in category TCD, TWP is gradually more dominated by IWP; the IWP-to-TWP ratio increases linearly 343 from ~50% to ~100% (Figure 16c3); also see Figure 15d.

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In response to ten times smaller large-scale subsidence, stratocumulus develops more strongly in 367 category RAD with more intensive cloud-top radiative cooling, inducing apparently thicker well-mixed layer 368 ( Figure 15b). However, this strong radiative cooling induces intensive buoyant mixing extending from the cloud 369 top till the surface, supresses the surface turbulent mixing and decreases the lifting rate of the height to the along the trajectory since warm-and-moist air is lifted to higher altitude above the cold dome (Figure 15d).
Under the background of global warming, the rate of local process has been accelerated by 9% per year 376 (Kim et al., 2019), while the meridional heat and moisture transports (remote processes) over the Barents Sea are 377 also enhanced in recent decades (Nygå rd et al., 2020). This implies that WaMAI may play a more significant role 378 in the future Arctic warming. Therefore, the potential mechanism which enhances the occurrence and intensity of

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WaMAI deserves more attentions from atmospheric scientists.

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Data availability

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All data used can be found on the ERA5 data repository at DOI: www.ecmwf.int/en/forecasts/datasets/reanalysis-382 datasets/era5.