Fig. 12 Composite map of (a) Heavy case SIC and (b) light case SIC. (c) Difference SIC between Light and Heavy (Light‒Heavy) (d) Time series of Okhotsk SIC for last 2,000 years of control run. red dashed
line indicate over 2 standard deviations (upper) and less 2 standard deviations (lower)
Fig. 13 shows the composite map SLP of Heavy and Light years. The minimum pressure of the AL increased from 1001.1 hPa to 1002hPa in response to Okhotsk Sea ice loss, and its position is shifted to the southwest (Fig 13). There are positive SLP anomalies in the Bering Sea and negative anomalies near the Okhotsk Sea to the North Pacific. In the case of Honda et al. (1996), there is a Rossby wave train pattern in the SLP change; anticyclonic SLP anomalies covered in the Sea of Okhotsk and cyclonic anomalies covered in the Bering sea (See their Fig 4a). However, in the KCM results, the SLP anomalies are similar to the NPO pattern; positive anomalies covered in the Bering sea and negative anomalies broadly covered sea of Okhotsk and central North Pacific (Fig 14). In this study, because we used a coupled model’s result, it is difficult to isolate atmospheric reactions to sea ice reduction as Honda et al. (1996) did in the atmosphere-only model. On the other hand, it can be suggested that there is a correlation between NPO and Sea ice variation in the Bering sea and the sea of Okhotsk.
This result is similar to positive NPO/west Pacific(WP) teleconnection height and SLP regression (Linkin and Nigam. 2008). Following Linkin and Nigam (2008), the NPO is described as the second EOF of winter SLP which is a meridional dipole in North Pacific SLP, and the WP is a similar feature in the normalized 500 hPa geopotential height anomalies. The NPO/WP represents a prominent mode of winter mid-latitude variability in the North Pacific. In the positive NPO/WP phase, there is warming in the North American continent and less precipitation over the Pacific Northwest. Moreover, the positive NPO/WP phase, which deepened AL, results in a marginal sea ice extent in the Arctic sea, the western Bering Sea and the Sea of Okhotsk. Compare with Linkin and Nigam’s (2008) result, SLP differences show a similar pattern with a positive NPO pattern.
However, there is a difference in SLP increase/decrease area. SLP difference between Light and Heavy (Light‒Heavy), while positive SLP anomaly positioned over Bernig Strait, Linkin and Nigam (2008) showed positive anomaly over the
Okhotsk to North America. Moreover, negative anomaly extended over the Okhotsk to North Pacific and the center of the negative SLP anomaly located not only the southern of Aleutian Island but also the Okhotsk sea. These results are due to use of a composite map with only the Okhotsk SIC. While the SLP change due to climate change (L30‒F30) showed the PNA pattern, the SLP change due to the Okhotsk SIC (Light‒Heavy) showed the NPO pattern. Therefore, Sea ice reduction can be a hint of the SLP variation.
Fig. 13 Winter (DJF) SLP (hPa) averaged over (a) the heavy Okhotsk SIC years (Heavy) and (b) the light Okhotsk SIC years (Light) of last 2000 years-long
pre-industrial run simulation.
Fig. 14 Difference of (a) SAT (b) SLP between Light and Heavy.
Fig. 15. shows the difference between 700 hPa zonal wind, 500 hPa geopotential height, and SLP between the L30 and the F30. The weakening of the lower tropospheric zonal winds at high latitudes is apparent overall longitudes, which is consistent with the results from Arctic sea ice loss simulation by Deser et al.
(2016). Moreover, the strengthening of westerlies wind occurs in the mid-latitudes, especially in the North Pacific. The positive U700 anomalies in the North Pacific indicate the downstream strengthening of the jet core. However, Deser et al. (2016) showed positive U700 anomalies in the eastern Atlantic sector and Pacific only and at the center of the jet in the mid-latitude region is shifted east Japan to the center of North Pacific. In the KCM, only the intensity changed without the movement of the jet core.
The difference of 500 hPa geopotential height (Z3 500hPa) increased in L30 and relatively weak increased pattern in the Bering Sea and the Aleutian Islands.
Following Deser et al. (2010) and Screen et al. (2015), the elevated 500 hPa geopotential height values over the polar cap are a direct consequence of the warming of the lower troposphere in response to Arctic sea ice loss. However, the Z3 500hPa decreased in the southern Aleutian Islands in Deser et al. (2016), while the overall increase in KCM dominated. Both experiments showed a different pattern of geopotential height in the Bering Sea and the Aleutian Islands.
The SLP response to global warming exhibits PNA pattern, with negative values over the Okhotsk Sea, the Bering Sea, southeast of the Aleutian Islands and over Canada, and positive values over the southeast of Japan. The SLP also showed differences between this study and Deser et al. (2016). The AL strengthened around the Bering Sea in this study, unlike the result of Deser et al. (2016), which is a negative anomaly of SLP over Canada. Moreover, the SLP anomalies showed a continental wave-one pattern in Deser et al. (2016), but in KCM the SLP anomalies exhibit a PNA pattern. This PNA pattern may be connected with ENSO.
Further work is needed to assess the impact of the dynamic mechanism.
Fig. 15 Difference of DJF (a) 700hPa zonal wind(m/s), (b) 500 hPa geopotential height (m), and (c) SLP (hPa) response to global warming (L30-F30). Contour lines show F30 value (contour intervals of 5m/s for U700, 200m for geopotential height and 8 hPa for SLP).
Fig 16. DJF 700 hPa zonal wind (U700; m/s), 500 hPa geopotential height (Z500; m), and SLP (hPa) responses to Arcticsea ice loss from Deser et al. (2016). Stippling indicates where the response is statistically significant at the 95% confidence level. Contours indicate the twentieth century climatology (contour interval of 5m/s for U700 with the
zero contour thickened, 500m for Z500 and 10 hPa for SLP)
The DJF SAT, precipitation, and SST responses to global warming are shown in Fig. 16.
SAT anomalies show warming over the high-latitude continents, with maximum values exceeding 20~25℃ near the Arctic border. There are no negative temperature anomalies in KCM. However, the negative anomaly covered across Eurasia in Deser et al. (2016), which is related to positive SLP anomalies over Siberia. Moreover, there are no positive SLP anomalies in Siberia and negative anomalies led to only positive temperature anomalies existed in KCM. Deser et al. (2016) suggested that thermodynamic effects associated with increased SST.
The response of precipitation pattern by Arctic sea ice loss is similar to Deser et al.
(2016), with regional differences in magnitude. In the Arctic, there is a positive anomaly of precipitation existed directly over regions of sea ice loss, with magnitudes over 0.5~1mm/day.
These increased precipitation pattern results are consistent with the result of Deser et al.
(2016). However, the largest increased precipitation pattern existed over the mid-latitudes in the North Pacific especially in coastal regions of Canada and Alaska likely driven by the deepened AL. And decreased precipitation pattern existed over the low-latitude in the North Pacific. Normally, increased precipitation associated with the unbalanced heat flux due to the warmed SST. The warming of SST is apparent over North Hemisphere significantly in the Bering Sea and eastern tropical Pacific in response to global warming.
According to Deser et al. (2016), Arctic sea ice loss weakens the high-latitude westerlies and it is consistent with thermal wind balance in response to warming of the Arctic lower troposphere. Also, strengthen jet over the North Pacific is related to deepened AL. Increased air temperature, precipitation is the responses to Arctic sea ice loss. However, Deser et al.
(2016) examined only the sea ice loss induced portion of climate change so there is a different reaction with KCM, which considered the full response to the global warming effect.
As a result, we can conclude that not only the sea ice loss effect but also other factors can affect deepened AL and other atmospheric change.
Fig. 17 Difference of DJF (a) Surface Air Temperature (℃) (b) precipitation (mm/day), and (c) SST (℃) response to global warming (L30‒F30)
Fig 18. DJF surface air temperature (°C; land only), precipitation (mm/day), and SST (°C) responses to Arctic sea ice loss from Deser et al.(2016). Stippling indicates where the response is statistically significant at the 95% confidence level