Autumn Sea Ice Cover, Winter Northern Hemisphere Annular Mode, and Winter Precipitation in Eurasia

Transcription

1 3968 J O U R N A L O F C L I M A T E VOLUME 26 Autumn Sea Ice Cover, Winter Northern Hemisphere Annular Mode, and Winter Precipitation in Eurasia FEI LI Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of the Chinese Academy of Sciences, Beijing, China HUIJUN WANG Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, and Climate Change Research Center, Chinese Academy of Sciences, Beijing, China (Manuscript received 25 June 2012, in final form 5 December 2012) ABSTRACT This paper examines the impacts of the previous autumn sea ice cover (SIC) on the winter Northern Hemisphere annular mode (NAM) and winter precipitation in Eurasia. The coherent variations among the Kara Laptev autumn SIC, winter NAM, and Eurasian winter precipitation appear after the year 1982, which may prove useful for seasonal prediction of winter precipitation. From a physical point of view, the Kara Laptev SIC and sea surface temperature (SST) anomalies develop in autumn and remain in winter. Given that winter NAM is characterized by an Arctic midlatitude seesaw centered over the Barents Sea and Kara Laptev Seas, it is closely linked to the Arctic forcing that corresponds to the Kara Laptev sea ice increase (reduction) and the associated surface temperature cooling (warming). Moreover, based on both model simulations and observations, the diminishing Kara Laptev sea ice does induce positive sea level pressure (SLP) anomalies over high-latitude Eurasia in winter, which is accompanied by a significant surface warming in northern Eurasia and cooling south of the Mediterranean. This surface air temperature (SAT) anomaly pattern facilitates increases of specific humidity in northern Eurasia with a major ridge extending southward along the East Asian coast. As a result, the anomalous Eurasian winter precipitation has a more zonal band structure. 1. Introduction It has been projected that winter precipitation in Eurasia could change and there may be more frequent and severe weather-related disasters in a warmer climate in the future (Emori and Brown 2005; Wang et al. 2011, 2012). Trenberth (1999) argued that increases in temperature and evaporation enhance the atmospheric moisture content, which is responsible for the increase of extreme precipitation, because all weather systems feed on the available moisture through storm-scale moisture convergence. Previous studies have also shown that atmospheric circulation, moisture transport, and sea surface temperatures (SSTs) in the Pacific and Atlantic Oceans are the main factors affecting the Corresponding author address: Fei Li, Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China. lifei-715@163.com changes of Eurasian winter precipitation (Yatagai and Yasunari 1994; Ye 2000; Wang and Sun 2009; Sun and Wang 2012; Ma et al. 2012). Recently, Wu et al. (2011) noted the effects of autumn and winter Arctic sea ice concentration on the winter Siberian high. Liu et al. (2012) indicated that the decline of Arctic autumn sea ice cover (SIC) is a precursor to winter snowfall in Europe and the United States. Arctic sea ice represents an important and highly variable component of the global climate system. A reduction in the amount of generally thicker perennial ice since the mid-1980s contributes to the transformation of the Arctic Ocean into a seasonally ice-covered state (Maslanik et al. 2007; Nghiem et al. 2007). The major thinning observed since the 1990s is expected to continue in the future and to make the ice cover more variable (Bader et al. 2011). As a result, the loss of SIC affects the Arctic s freshwater system and surface energy budget and could be manifested in middle latitudes as altered patterns of atmospheric circulation and DOI: /JCLI-D Ó 2013 American Meteorological Society

2 1JUNE 2013 L I A N D W A N G 3969 FIG. 1. Correlations of autumn Arctic SIC with winter NAM for the periods (a) , (b) , and (c) The dotted regions have correlations above the 95% confidence level. precipitation (Fan 2007; Serreze et al. 2007). The Arctic forcing linked to the SIC variability thus has become more important but has not been well recognized so far. The Northern Hemisphere annular mode (NAM; Wallace 2000), or as it is sometimes called the Arctic Oscillation (AO; Thompson and Wallace 1998), is one of the leading modes of the Northern Hemisphere s climate variability, which is similar to the North Atlantic Oscillation (NAO; Wallace and Gutzler 1981; Barnston and Livezey 1987) at the surface, but with more zonal symmetry, especially at high latitudes. At the upper level, the signature of the NAM indicates a strong or weak polar vortex (Baldwin and Dunkerton 1999). In light of recent reports of retreating (Bjorgo et al. 1997; Parkinson et al. 1999) and thinning (Rothrock et al. 1999; Wadhams and Davis 2000) ice, it is important to understand the interrelationships between the autumn SIC and winter NAM. Of the studies mentioned above, the work of Liu et al. (2012) is the only study so far that examines the association between winter precipitation and the autumn Arctic SIC based on both models and observations. How the variations in autumn regional SIC influence the large-scale winter precipitation in entire Eurasia, however, is unclear. The objective of the present study is to build upon the work of Liu et al. by (i) examining the unstable relationship between the autumn SIC and winter NAM, with both having an important influence on the Eurasian winter precipitation; (ii) proposing a mechanism that may be responsible for the covariability among the autumn SIC, winter NAM, and Eurasian winter precipitation; and (iii) examining the interdecadal shift of Eurasian winter precipitation after the early 2000s, which is associated with the recent decline of the Arctic SIC. 2. Data The datasets employed in this research included the following: 1) the National Centers for Environmental Prediction (NCEP) atmospheric reanalysis with aresolutionof2.58 (Kalnay et al. 1996), using variables including sea level pressure (SLP), surface air temperature (SAT), 850-hPa wind vector (UV850), 200-hPa wind vector (UV200), 300-hPa geopotential height (Z300), and specific humidity; 2) the Hadley Centre sea ice and sea surface temperature dataset, version 1 (HadISST1), with a resolution of 1.08 (Rayner et al. 2003); 3) the National Oceanic and Atmospheric Administration (NOAA) precipitation reconstruction (PREC) data with a resolution of 1.08 (Chen et al. 2002, 2004); and 4) the NAM index obtained from the NOAA/Climate Prediction Center (CPC) ( gov/products/precip/cwlink/daily_ao_index/ao.shtml). In this study, SIC refers to the actual area covered by sea ice with 15% and greater ice concentration. The common time period is set to The winter of 1950 refers to the 1950/51 winter. The months of December, January, and February are used in calculating the winter mean for all variables (e.g., atmosphere, sea ice, SST, and precipitation). 3. The connection between the Kara Laptev autumn SIC and winter NAM As described in the introduction, the winter NAM may be related to the SIC during the previous autumn. Here, we calculated the correlation coefficients between the Arctic autumn SIC and winter NAM to identify the region of interest (Fig. 1). For the entire period , an area with a positive sign is obvious along the Kara Laptev

3 3970 J O U R N A L O F C L I M A T E VOLUME 26 FIG. 2. (a) The interannual variations of the Kara Laptev autumn SIC and winter NAM (i.e., AO) indices for the period (solid line) and their 11-yr moving averages (dashed line), which are detrended and standardized. (b) The 21-yr sliding correlation coefficient between the Kara Laptev autumn SIC and winter NAM indices. The dotted lines denote the correlations above the 95% confidence level. Seas ( N, E), with a peak value of 0.46 (Fig. 1a). After 1982, this positive area becomes more significant and extends farther into the Barents Sea, with a peak value of 0.72 (Fig. 1c). However, before 1982, there is no significant correlation (Fig. 1b). These results indicate that the Kara Laptev Seas is a key region influencing the winter NAM particularly after the year Figure 2a displays the temporal variations of the autumn SIC index along the Kara Laptev Seas and the winter NAM index for the period The Kara Laptev autumn SIC has strong variability on interannual and interdecadal time scales, particularly after the year Besides, coherent declining tendencies in the autumn SIC and winter NAM indices are observed after It is apparent that the autumn SIC winter NAM relationship is low over the entire period, with a correlation coefficient of However, a more detailed investigation indicates that the autumn SIC winter NAM connection varies with time. After 1982, they are highly correlated with one another with a coefficient of In contrast, before 1982, they are relatively independent and there is no correlation. Figure 2b shows the 21-yr sliding correlation coefficient between the Kara Laptev autumn SIC and winter NAM indices. The configuration reveals a consistent rise from 1960 to 1981; thereafter it is relatively level, with the year 1982 appearing to be a turning point. Significantly in-phase correlations between the autumn SIC and winter NAM exist only after the year Taken together, the Kara Laptev autumn SIC and winter NAM are two closely related components particularly after the year The decline of Kara Laptev autumn SIC does occur after 2004 along with the weakening of winter NAM. There is evidently a significant temporal relationship between the autumn SIC and winter NAM, but to what extent are the autumn SIC and winter NAM related large-scale circulation anomalies spatially analogous? Figure 3 shows the linear regressions of SLP and UV850, and Z300 and UV200 in winter associated with the enhanced Kara Laptev autumn SIC (Figs. 3a f) and the positive polarity of the winter NAM (Figs. 3g l). Hereafter, we focus on the three periods , , and During all three periods, the positive polarity of the winter NAM is characterized by a deeper Arctic midlatitude seesaw pattern centered over the Barents Sea and Kara Laptev Seas, which is consistent with Thompson and Wallace (1998). The associated midlatitude anomalies consist of two cells in the Pacific and Atlantic Oceans (Figs. 3g i). At 300 hpa, the signature describes a stronger polar vortex centered over Greenland (Baldwin and Dunkerton 1999). The associated midlatitude anomalies have become more apparent with three cells in the Pacific, Siberia, and the Atlantic (Figs. 3j l). In the vertical direction, the structure features an anomalous equivalent barotropic zonal wind dipole that shows anomalous westerlies at roughly 558N and anomalous easterlies at roughly 358N (Thompson and Wallace 2000). However, when Kara Laptev autumn SIC is enhanced, the deeper Arctic midlatitude seesaw pattern, the stronger polar vortex, and the anomalous equivalent barotropic zonal wind dipole only occur after the year 1982 (Figs. 3c,f). The autumn SIC and winter NAM related large-scale circulation anomalies share analogous features particularly after The anomalous winter precipitation in Eurasia associated with the Kara Laptev autumn SIC and winter NAM To understand the anomalous winter precipitation in Eurasia associated with the Kara Laptev autumn SIC

4 1 JUNE 2013 LI AND WANG FIG. 3. Linear regressions of (a) (c) SLP (colors; hpa) and UV850 (arrows; m s21) and (d) (f) Z300 (colors; 10 gpm) and UV200 (arrows; m s21) in winter with respect to the Kara Laptev autumn SIC for the periods , , and (g) (l) As in (a) (f), but for the winter NAM. The dotted regions have correlations above the 95% confidence level. 3971

5 3972 J O U R N A L O F C L I M A T E VOLUME 26 and winter NAM, it is necessary to examine the anomalous SAT field, which can feed on or starve the atmospheric moisture content and consequently change precipitation (Trenberth 1999). Figure 4 shows the linear regressions of SAT and vertically integrated specific humidity in winter associated with the enhanced Kara Laptev autumn SIC (Figs. 4a f) and the positive polarity of the winter NAM (Figs. 4g l) for three periods , , and During all three periods, a significant surface warming in northern Eurasia and cooling south of the Mediterranean are considered to be largely tied to the positive polarity of the winter NAM (Figs. 4g i), as discussed in Thompson and Wallace (2001). This SAT anomaly pattern facilitates increases of specific humidity in northern Eurasia with a major ridge extending southward along the East Asian coast (Figs. 4j l). In contrast, the enhanced Kara Laptev autumn SIC influences the anomalous surface temperature and vertically integrated specific humidity in the middle and high latitudes in nearly the same way only after the year 1982 (Figs. 4c,f). Figure 5 shows the linear regressions of vertically integrated moisture transport and precipitation in winter with the enhanced Kara Laptev autumn SIC (Figs. 5a f) and the positive polarity of the winter NAM (Figs. 5g l) for the three periods , , and Consistent with the SAT fields mentioned above, during all three periods, the anomalous moisture transports are closely linked to the winter NAM, which is attributed to the north south vacillation of the midlatitude westerly jet. Accompanying the positive polarity of the winter NAM, the midlatitude westerly jet has moved northward and Eurasia has two main branches of anomalous water vapor transport. The first branch is the strong transport by the anomalous westerly flow at roughly 558N, which brings abundant moisture from the Atlantic Ocean, crossing northern Europe into Russia. The second branch is the weak transport by the anomalous southwesterly flow straddling E, which brings warmer tropical moisture from the Indian Ocean into South Asia (Figs. 5g i). In contrast, the anomalous moisture transports associated with the enhanced Kara Laptev autumn SIC align with the node of the winter NAM only after the year 1982 (Fig. 5c). As a result, the anomalous winter precipitation has a more zonal band structure with higher-than-normal precipitation anomalies in northern ( N, E) and southern Eurasia ( N, E) but lower-than-normal precipitation anomalies in central Eurasia ( N, E). Besides, the spatial scales of southern Eurasian precipitation anomalies are relatively small compared with the other two (Figs. 5f,j l). To better illustrate the changes of Eurasian winter precipitation associated with the autumn SIC and winter NAM from one area to another, Fig. 6a displays the temporal variations of northern, central, and southern Eurasian winter precipitation (PrecipN, PrecipM, and PrecipS, respectively) for the period The configurations record a consistency of movement and fluctuation in the curves of PrecipN and PrecipS but an opposite sign in the curve of PrecipM. Particularly, PrecipN and PrecipS show a coherent decline after the year 2004, while PrecipM shows an obvious increase. The 21-yr sliding correlation coefficients between PrecipN, PrecipM, and PrecipS and the Kara Sea Laptev autumn SIC and winter NAM indices are also calculated for the quantitative evaluation (Fig. 6b). Consistent with the linear regressions shown in Figs. 5j l, for the entire period , PrecipN and PrecipS, and the simultaneous winter NAM shows highly in-phase correlations, with coefficients of 0.63 and 0.39, respectively (Table 1), whereas PrecipM and winter NAM show a highly out-ofphase correlation, with a coefficient of In contrast, significantly in-phase correlations between the PrecipN and the previous autumn SIC exist only after the year 1982, with a coefficient of The correlations between PrecipM and PrecipS and the Kara Laptev autumn SIC are relatively weak. Based on the results shown in Figs. 4 6, for the entire period, the precipitation-related variables (including SAT, vertically integrated specific humidity, vertically integrated moisture transport, and precipitation) show high correlations with the simultaneous winter NAM, whereas significant correlations with the previous Kara Laptev autumn SIC exist only after the year In light of these findings, it is necessary to address why there is a shift in the relationships between the autumn SIC and winter NAM/Eurasian winter precipitation around the year First, the changes in observational sea ice estimates are significant (i.e., the introduction of more reliable satellite-based estimates in approximately 1979). Second, the Arctic Ocean is transforming to a seasonally ice-covered state, with a reduction in the amount of generally thicker perennial ice observed since the mid-1980s (Maslanik et al. 2007; Nghiem et al. 2007), and hence the Kara Laptev autumn SIC has strong variability on interannual and interdecadal time scales as shown in Fig. 2a. This is speculated to contribute to the high correlations with the winter NAM after 1982 through fostering large heat fluxes to the atmosphere and further contribute to the high correlations with the Eurasian winter precipitation through the impacts on the winter NAM. In addition, Fig. 7 shows the linear regressions of Arctic SIC and SST in autumn and winter associated

6 1 JUNE 2013 LI AND WANG FIG. 4. Linear regressions of (a) (c) SAT (8C), (d) (f) specific humidity (vertically integrated from the surface to 700 hpa; kg kg21) in winter with respect to the Kara Laptev autumn SIC for the periods , , and (g) (l) As in (a) (f), but for the winter NAM. The dotted regions have correlations above the 95% confidence level. 3973

7 3974 JOURNAL OF CLIMATE VOLUME 26 FIG. 5. Linear regressions of (a) (c) moisture transport fluxes [arrows; vertically integrated from the surface to 700 hpa; kg (m s)21] and its divergence [colors; kg (m2 s)21] and (d) (f) precipitation (mm day21) in winter with respect to the Kara Laptev autumn SIC for the periods , , and (g) (l) As in (a) (f), but for the winter NAM. The dotted regions have correlations above the 95% confidence level.

8 1JUNE 2013 L I A N D W A N G 3975 FIG. 6. (a) The interannual variations of PrecipN, PrecipM, and PrecipS for the period (solid line) and their 11-yr moving averages (dashed line), which are detrended and standardized. (b) The 21-yr sliding correlation coefficient between PrecipN, PrecipM, and PrecipS and the Kara Laptev autumn SIC (solid line) and winter NAM (dashed line) indices. The dotted lines denote the correlations above the 95% confidence level. with the enhanced Kara Laptev autumn SIC for the period During the previous autumn, an area with enhanced SIC is observed over most of the eastern Arctic Ocean (Fig. 7a), with a decrease of SST in the Barents Sea (Fig. 7b), where the center of the NAMrelated Arctic midlatitude seesaw is located. During the following winter, the areas with both enhanced SIC and decreased SST are observed in the Barents Sea (Figs. 7c,d). These results suggest that the Kara Laptev SIC and SST anomalies develop in autumn and remain in winter. Given that winter NAM is characterized by an Arctic midlatitude seesaw centered over the Barents and Kara Laptev Seas, it is closely linked to the Arctic forcing that corresponds to the Kara Laptev sea ice increase (reduction) and the associated surface temperature cooling (warming) through fostering large heat fluxes to the atmosphere. 5. The interdecadal shift of Eurasian winter precipitation after the early 2000s Our assessments of the interdecadal shift of Eurasian winter precipitation focus on a period accompanied by a strong loss of autumn and winter SIC along the Kara Laptev Seas. As shown in Fig. 8a, the Arctic autumn SIC was significantly reduced from to by km 2, a 17.9% loss from km 2 to km 2, with the largest sea ice loss in the eastern Arctic Ocean (including the Barents Sea, Kara Laptev Seas, East Siberian Sea, and Beaufort Sea). The winter sea ice loss covers just in the Barents Sea (Fig. 8b), which can be considered as the loss of autumn SIC persisting into the winter. Figure 9 shows the anomalies of SLP and UV850, Z300 and UV200, SAT, and vertically integrated specific humidity in winters after the year As shown in Fig. 9a, the main feature is a positive anomaly belt extending in the west east direction, with three centers over Greenland, the Kara Laptev Seas, and the Pacific Ocean. At 300 hpa, the signature describes a weakened polar vortex centered over Greenland and the Kara Laptev Seas (Fig. 9b). In the vertical direction, the structure features an equivalent barotropic zonal wind dipole of anomalous easterlies at roughly 558N and westerlies at roughly 358N. Besides, the SAT field records a significant surface cooling in northern Eurasia TABLE 1. Correlation coefficients between PrecipN, PrecipM, and PrecipS, and the Kara Laptev autumn SIC and winter NAM for the periods , , and The values in boldface are above the 99% confidence level. Autumn SIC ( ) Autumn SIC ( ) Autumn SIC ( ) Winter NAM ( ) Winter NAM ( ) Winter NAM ( ) PrecipN PrecipM PrecipS

9 3976 J O U R N A L O F C L I M A T E VOLUME 26 FIG. 7. Linear regressions of (a) autumn and (b) winter Arctic SIC (10 2 km 2 ) with respect to the Kara Laptev autumn SIC for the period (c),(d) As in (a),(b), but for autumn and winter Arctic SST (8C). The dotted regions have correlations above the 95% confidence level. and warming southeast of the Mediterranean (Fig. 9c). Because, as expected, the specific humidity anomaly pattern shares the same signs as the SAT field, it reveals a decrease in northern Eurasia with a major ridge extending southward along the East Asian coast and an increase southeast of the Mediterranean (Fig. 9d). Figure 10 shows the anomalies of vertically integrated moisture transport and precipitation in winter after the year Because the midlatitude westerly jet has moved southward, Eurasia has one main branch of anomalous water vapor transport by the anomalous southwesterly flow straddling E, which brings abundant moisture from the Atlantic Ocean into southern Europe (Fig. 10a). As a result, the anomalous Eurasian winter precipitation has a more zonal band structure with lowerthan-normal precipitation anomalies in northern and southern Eurasia but higher-than-normal precipitation anomalies in central Eurasia. In summary, the characteristics of anomalies in the large-scale circulation and the precipitation-related variables after 2004 (Figs. 9, 10) are in good agreement with those of the linear regressions associated with the enhanced Kara Laptev autumn SIC (Figs. 3 5), but with the opposite sign. Besides, the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) also suggests that diminishing

10 1JUNE 2013 L I A N D W A N G 3977 FIG. 8. Anomalies in (a) autumn and (b) winter Arctic SIC (10 2 km 2 ) after the year The anomalies are calculated as the period minus the period The dotted regions have values above the 99% confidence level. Arctic sea ice will promote increases in SAT and specific humidity in the Arctic region through fostering large heat fluxes to the atmosphere, which further enhances moisture transport flux during (Screen and Simmonds 2010). It is thus reasonable to expect that the recent decline of the autumn SIC should affect the interdecadal shift of winter precipitation in Eurasia. 6. The numerical simulation To verify the mechanism mentioned above, we conduct simulations with the National Center for Atmospheric Research Community Atmosphere Model, version 3.1 (Collins et al. 2006), in which SST and sea ice concentrations are specified as boundary conditions based on a merged product of the HadISST dataset and the NOAA weekly optimum interpolation SST analysis (Hurrell et al. 2008). The experimental design is similar to that of Liu et al. (2012). The simulation configuration has a horizontal resolution of approximately 2.88 and 26 vertical levels extending up to 3.5 hpa. The impact of the diminishing Arctic sea ice during the freeze up on atmospheric circulation is assessed by comparing two experiments with different seasonally varying sea ice distributions, with all other external variables held fixed. The control experiment is run with seasonally varying Arctic sea ice based on the climatology of the Hadley Centre sea ice concentrations for The perturbed experiment is integrated with seasonally varying Kara Laptev Seas ( N, E) ice loss after The sea ice losses are calculated as the period minus the period Global SSTs in both experiments are set to their climatological monthly values based on the merged SST dataset for the same period of record used for the sea ice climatology in the control experiment. In addition, in the perturbed experiment, in those areas where sea ice is removed, SST is set to the freezing point of seawater, 21.88C. To help gauge confidence in the model s response to sea ice losses, each experiment consists of 20 ensemble members with slightly different initial conditions. As shown in Fig. 11b, the diminishing Kara Laptev sea ice does generate a southward-moving NAM mode, which is accompanied by much broader meridional structure in midlatitude Eurasia. Because the SIC is closely associated with the NAM after 1982, the signals of a continued and rapid reduction of sea ice can extend southward toward Eurasia through the southwardmoving NAM mode. These interannual signals can further lead to the NAM-like long-term changes in Eurasia. Besides, the response of the model to the Kara Laptev sea ice losses is examined by differencing SLP and SAT between the ensemble mean of the perturbed and control experiments. As shown in Figs. 12a,b, the diminishing Kara Laptev sea ice does induce positive SLP anomalies over high-latitude Eurasia in winter, which is accompanied by a significant surface warming in the Kara Laptev Seas and Europe and cooling over Siberia. Besides, the regions showing the largest increase

11 3978 JOURNAL OF CLIMATE VOLUME 26 FIG. 9. Winter anomalies of (a) SLP (colors; hpa) and UV850 (arrows; m s21), (b) Z300 (colors; 10 gpm) and UV200 (arrows; m s21), (c) SAT (8C), and (d) specific humidity (vertically integrated from the surface to 700 hpa; kg kg21) after the year The anomalies are calculated as the period minus the period The dotted regions have values above the 99% confidence level. of specific humidity are found in Europe (Fig. 12c). The largest increase of precipitation is mainly located north of the Mediterranean, whereas decrease of precipitation is over Siberia and south of the Mediterranean (Fig. 12d). While the regional details differ somewhat between the response of the modeled SAT (Fig. 12b) and the observations (Fig. 9c), the model simulations do not show above-normal temperatures southeast of the Mediterranean in general. The encouraging consistency between model simulations and observations support our conclusion that the Kara Laptev autumn SIC may influence the Eurasian winter precipitation through the impacts on the winter NAM. 7. Summary The impacts of the autumn SIC on the following winter NAM and winter precipitation in Eurasia are

12 1JUNE 2013 L I A N D W A N G 3979 FIG. 10. Winter anomalies in (a) moisture transport fluxes [arrows; vertically integrated from the surface to 700 hpa; kg (m s) 21 ] and its divergence [colors; kg (m 2 s) 21 ] and (b) precipitation (mm day 21 ) after the year The anomalies are calculated as the period minus the period The dotted regions have values above the 99% confidence level. analyzed in this paper. The key results are summarized as follows: 1) The Kara Laptev autumn SIC and winter NAM are two closely related components particularly after the year From a physical point, the Kara Laptev SIC and SST anomalies develop in autumn and remain in winter. Given that winter NAM is characterized by an Arctic midlatitude seesaw centered over the Barents and Kara Laptev Seas, it is closely linked to the Arctic forcing that corresponds to the Kara Laptev sea ice increase (reduction) and the associated surface temperature cooling (warming). 2) A significant surface warming in northern Eurasia and cooling south of the Mediterranean are considered largely tied to the enhanced SIC particularly after the year This SAT anomaly pattern facilitates increases of specific humidity in northern Eurasia, with a major ridge extending southward along the East Asian coast. As a result, the anomalous winter precipitation has a more zonal band structure with higher-than-normal precipitation anomalies in northern and southern Eurasia but lower-than-normal precipitation anomalies in central Eurasia. 3) The characteristics of anomalies in the large-scale circulation and the precipitation-related variables after FIG. 11. The leading EOF patterns of winter SLP in (a) the control and (b) the perturbed experiments.

13 3980 J O U R N A L O F C L I M A T E VOLUME 26 FIG. 12. Differences in (a) SLP (colors; hpa) and UV850 (arrows; m s 21 ), (b) SAT (8C), (c) specific humidity (vertically integrated from the surface to 700 hpa; kg kg 21 ), and (d) precipitation (mm day 21 ) in winter between the perturbed and control experiments. The dotted regions have values above the 99% confidence level (Figs. 9, 10) are in good agreement with those of the linear regressions associated with the enhanced Kara Laptev autumn SIC (Figs. 3 5), but with the opposite sign. It is thus reasonable to expect that the recent decline of autumn SIC should affect the interdecadal shift of winter precipitation in Eurasia. 4) To further interpret the observational data analyses, we conduct simulations whose experimental design is similar to that of Liu et al. (2012). The diminishing Kara Laptev sea ice does generate a southwardmoving NAM mode, which is accompanied by much broader meridional structure in midlatitude Eurasia. Besides, the response of the model to the Kara Laptev sea ice losses is examined by differencing SLP, SAT, vertically integrated specific humidity, and precipitation between the ensemble mean of the perturbed and control experiments. The regional details differ somewhat between the response of the modeled SAT (Fig. 12b) and the observations (Fig. 9c). The encouraging consistency between model simulations and observations support our conclusion that the Kara Laptev SIC autumn may influence the Eurasian winter precipitation through the impacts on the winter NAM. 5) The enhanced correlations among the autumn SIC, winter NAM, and Eurasian winter precipitation after

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