210 JOURNAL OF CLIMATE VOLUME 19

Dipole Anomaly in the Winter Atmosphere and Its Association with Ice Motion

BINGYI WU Chinese Academy of Meteorological Sciences, Beijing, China, and Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska

JIA WANG AND JOHN E. WALSH International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska

(Manuscript submitted 5 April 2004, in final form 14 August 2005)

ABSTRACT

This paper identified an atmospheric circulation anomaly–dipole structure anomaly in the Arctic atmo- sphere and its relationship with winter sea ice motion, based on the International Arctic Buoy Program (IABP) dataset (1979–98) and datasets from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) for the period 1960–2002. The dipole anomaly corresponds to the second-leading mode of EOF of monthly mean sea level pressure (SLP) north of 70°N during the winter season (October–March) and accounts for 13% of the variance. One of its two anomalous centers is stably occupied between the and Laptev Sea; the other is situated from the Canadian Archipelago through Greenland extending southeastward to the Nordic . The dipole anomaly differs from one described in other papers that can be attributed to an eastward shift of the center of action of the North Atlantic Oscillation. The finding shows that the dipole anomaly also differs from the “Barents Oscillation” revealed in a study by Skeie. Since the dipole anomaly shows a strong meridionality, it becomes an important mechanism to drive both anomalous sea ice exports out of the Arctic Basin and cold air outbreaks into the , the Nordic seas, and northern . When the dipole anomaly remains in its positive phase, that is, negative SLP anomalies appear between the Kara Sea and the Laptev Sea with concurrent positive SLP over from the Canadian Archipelago extending southeastward to Greenland, there are large-scale changes in the intensity and character of sea ice transport in the Arctic basin. The significant changes include a weakening of the Beaufort gyre, an increase in sea ice export out of the Arctic basin through Strait and the northern Barents Sea, and enhanced sea ice import from the Laptev Sea and the into the Arctic basin. Consequently, more sea ice appears in the Greenland and the Barents Seas during the positive phase of the dipole anomaly. During the negative phase of the dipole anomaly, SLP anomalies show an opposite scenario in the Arctic and its marginal seas when compared to the positive phase, with the center of negative SLP anomalies over the Nordic seas. Correspondingly, sea ice exports decrease from the Arctic basin flowing into the Nordic seas and the northern Barents Sea because of the strengthened Beaufort gyre. The finding indicates that influences of the dipole anomaly on winter sea ice motion are greater than that of the winter AO, particularly in the central Arctic basin and northward to , implying that effects of the dipole anomaly on sea ice export out of the Arctic basin become robust. The dipole anomaly is closely related to atmosphere–ice–ocean interactions that influence the Barents Sea sector.

1. Introduction warming and polar amplification. Observational evi- dence and climate modeling suggests that sea ice can Arctic sea ice plays an important role in the climate itself be an agent of (Walsh 1983; Kahl system. Arctic sea ice variation is an important indica- 1990; Mysak et al. 1990; Deser et al. 2000; Rigor et al. tor of changes in the climate system, such as global 2002; Wu et al. 2002, 2004). The presence of sea ice strongly influences air–sea interactions and high- Corresponding author address: Bingyi Wu, Chinese Academy of latitude weather and climate. Sea ice reflects a large Meteorological Sciences, Beijing, China. part of the incoming solar radiation and restricts ex- E-mail: [email protected] changes of heat, moisture, and momentum between the

© 2006 American Meteorological Society

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JCLI3619 15 JANUARY 2006 W U E T A L . 211 ocean and atmosphere. Through latent heat release 2002). Consequently, sea ice flux from the Arctic basin during the frozen season and the heat absorbed during into the is weaker than normal. the melting of sea ice, sea ice strongly affects the atmo- Although previous studies have revealed the re- spheric energy budget. Wu et al. (2004) show that sea sponse of sea ice motion to the NAO (AO) (Kwok ice variations can influence stratification and stability of 2000; Rigor et al. 2002), the NAO (AO) cannot explain the boundary layer and further produce dynamic influ- all of the phenomena present in sea ice motion and ences on the atmosphere, which in turn feeds back on export. A few studies have suggested that it is not the sea ice and the ocean. NAO (AO) that is forcing sea ice export anomalies Processes related to sea ice variations, including the through Fram Strait (Jung and Hilmer 2001; Holland growth and melting (producing freshwater) of sea ice, 2003). Thus, the question is raised whether there is influence seawater salinity flux and sea ice export out of other forcing beyond the NAO (AO); that is, there are the Arctic basin into the Nordic seas and the Barents likely other processes in the atmospheric circulation Sea. Sea ice export through Fram Strait and its inter- state that also influence sea ice motion and export out action with the North Atlantic give sea ice a potential of the Arctic basin. The objective of this study is to role in the North Atlantic climate variations and deep- identify a meridional anomaly of atmospheric variabil- water formation (Aagaard and Carmack 1989), and the ity over the Arctic and its effects on sea ice variability of thermohaline circulation (THC). Evi- motion and sea ice export through Fram Strait. The dence shows that the Great Salinity Anomaly (GSA) meridional anomaly shows a dipole structure that occurred in the northern North Atlantic during 1968–82 strongly influences sea ice motion and sea ice export. and was connected with an anomalously large export of sea ice from the Arctic (Dickson et al. 1988; Häkkinen 1993). Recent evidence (Dong and Sutton 2002) indi- 2. Data and methods cates that atmospheric feedbacks could spread globally the influence of a change in the THC much more The primary datasets used in this study are monthly quickly and efficiently than ocean processes alone. mean sea level pressure (SLP), surface wind, surface air With respect to atmospheric forcing regimes of the temperature, and geopotential heights at 500 hPa - Arctic sea ice, many previous studies focused mainly on tained from the National Centers for Environmental the North Atlantic Oscillation (NAO) (Wang et al. Prediction (NCEP) and the National Center for Atmo- 1994; Kwok and Rothrock 1999; Dickson et al. 2000; spheric Research (NCAR) reanalysis datasets for the Wu et al. 2000; Zhang et al. 2000; Kwok 2000; Jung and period from 1960 to 2002 and monthly sea ice motion Hilmer 2001; and many others) and the Arctic Oscilla- (SIM) obtained from International Arctic Buoy Pro- tion (AO) (Wang and Ikeda 2000; Vinje 2001a; Rigor et gram (IABP) for the period 1979–98. For detailed in- al. 2002; Zhang et al. 2003; Holland 2003). The response formation about the SIM dataset, see Rigor et al. of sea ice to the NAO (AO) shows out of phase varia- (2002). The monthly mean sea ice concentration tions of sea ice concentration between the Labrador dataset (on a 1° latitude ϫ 1° longitude grid for the and Greenland–Barents Seas (Walsh and Johnson period 1871–2002) was obtained from the British At- 1979; Wang et al. 1994; Slonosky et al. 1997; Deser et al. mospheric Data Centre (BADC; http://badc.nerc.ac.uk/ 2000; Wu et al. 2000) and a cyclonic (anticyclonic) cir- data/list_all_datasets.html). In this study, a sea ice con- culation anomaly in sea ice motion in the centration dataset after 1960 was used to calculate the (Kwok 2000; Rigor et al. 2002). However, it has become climatology and anomalies. apparent that the relationship between the NAO (AO) We applied empirical orthogonal function (EOF) and sea ice export out of the Arctic basin through Fram analysis to monthly mean SLP north of 70°N to reveal Strait is complicated, depending on the sea ice flux dominant modes of variations in SLP. In this study, we dataset (Kwok and Rothrock 1999; Vinje 2001a) and focus our attention on the winter season (October to the specific period (Hilmer and Jung 2000). March). Following the procedure of Vinje (2001a), we Proshutinsky and Johnson (1997) revealed an anticy- used monthly mean SLP grid points (80°N, 10°W and clonic and a cyclonic circulation regime over the Arctic 72.5°N, 20°E) data to approximately calculate the wind- basin, with each regime persisting about 5 to 7 years. induced ice volume flux through Fram Strait. In this During the former, anticyclonic winds are prevalent in study, more attention is paid to the seasonal mean av- the Arctic basin so that sea ice and the water flow to- eraged for the six winter months (October–March). Si- ward Fram Strait is reduced (Proshutinsky and Johnson multaneously, as a comparison, this paper also prelimi- 1997; Tremblay and Mysak 1998; Proshutinsky et al. narily explores intraseasonal variations.

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FIG. 1. Spatial distributions of the first two leading EOF modes of winter monthly mean SLP (October– March): (a) EOF1 and (b) EOF2, accounting for 61% and 13% of total variance, respectively.

3. Results and the western Arctic: one center is over northern and the Arctic marginal seas, and the other is a. Dipole anomaly at an intraseasonal time scale over and Greenland, a dipole struc- We focus our attention on the Arctic basin and the ture (hereafter the dipole anomaly). The zero isoline Arctic marginal seas because one of the objectives of cuts through the central Arctic basin and Barents Sea. this study is to investigate the response of sea ice mo- Such distributions of SLP anomalies are favorable for tion to the meridional anomaly of the atmospheric cir- both sea ice export out of the Arctic basin and cold air culation, considering only the area north of 70°N. To outbreaks over the Nordic seas and the Barents Sea. reveal dominant modes of SLP variations, we applied Owing to a strong annular structure, the AO (NAO) the EOF analysis to monthly mean SLP (October to obstructs cold air outbreaks over the Barents Sea and March) north of 70°N for the period from 1960 to 2002 northern Eurasia, and there is a weak connection be- (sample number: 252). The spatial distributions for the tween the AO (NAO) and sea ice efflux through Fram first two modes are shown in Fig. 1, and they account Strait (Häkkinen and Geiger 2000; Vinje 2001a; Jung for 61% and 13% of the total variance, respectively. We and Hilmer 2001). Considering sea ice and freshwater calculated autocorrelations of the principle component export out of the Arctic basin and its potential effect on of EOF2: the thermohaline circulation in the North Atlantic and R͑0͒ ϭ 1, R͑1͒ ϭ 0.15, R͑2͒ ϭ 0.008, further on the global climate system, the dipole mode of atmospheric variations could play an important role in R͑3͒ ϭ 0.026, R͑4͒ ϭ 0.019, .... the effect of sea ice export on climate, although it only Based on those autocorrelations, the number of de- accounts for 13% of the total variance. ϭ grees of freedom was found to be Neffective 243. Ac- cording to the literature of North et al. (1982), the b. Differences from the “Barents Oscillation” EOF2 is well separated from the other modes. Wang et al. (1995) had derived the similar dipole anomaly It should be pointed out that, although the dipole using SLP data of 45°N for the period of 1953–93, ac- anomaly is similar to the “Barents Oscillation” (Skeie counting for 12% of the total variance. Nevertheless, no 2000) in the Arctic (strong meridionality) (Fig. 2a), it further analysis was conducted. differs from the Barents Oscillation: 1) The Barents The leading mode represents the Arctic Oscillation, Oscillation is not well separated from the third and with Ϫ0.95 correlation between the principle compo- fourth EOFs, as Skeie (2000) suggested. Consequently, nent of the leading mode and Thompson and Wallace’s in the literature of Tremblay (2001), the Barents Oscil- (1998) monthly mean AO index (Fig. 1a). Figure 1b lation was characterized by the EOF3 of SLP fields shows two centers with opposite signs over the eastern rather than EOF2. In addition, because the Barents

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FIG. 2. Reproduced (a) “Barents Oscillation” and (b) its north side from 70°N (Skeie 2000).

Oscillation did not appear in the first four EOFs of SLP (AO), which occurred circa the late 1970s. However, fields during the period from 1977 to 1999, as Tremblay Fig. 3 implies that the dipole anomaly cannot be attrib- (2001) suggested, it is not a stable mode. However, the uted to an eastward shift of the northern center of ac- dipole anomaly is a stable mode (see the following sec- tion associated with the NAO (AO). Furthermore, tion). 2) The Barents Oscillation is not a dipole struc- Fig. 4d of Hilmer and Jung (2000) clearly shows that ture because there are four centers in SLP anomalies no strong meridionality can be found over the Arctic from the Arctic basin extending southward to the Nor- basin. dic seas, as shown in Fig. 1b of Skeie (2000). Skeie (2000) only mentioned that the surface air temperature anomalies associated with the Barents Oscillation dis- play a dipole pattern between the Nordic seas and cen- tral . He never suggested the existence of a di- pole in SLP anomalies. 3) The spatial distribution of the dipole anomaly differs from the Barents Oscillation over the Siberian marginal seas (Fig. 2b), and the cor- relation between the dipole anomaly and the Barents Oscillation is only marginally significant (0.26 at the 0.1 significance level). To confirm stability of the dipole anomaly in this study, an EOF analysis is again carried out for the win- ters from 1977/78 to 2001/02 (October–March). The spatial distribution of EOF2 is shown in Fig. 3, and the mode accounts for 12.78% of the total variance. Ap- parently, Fig. 3 closely resembles Fig. 1b. This demon- strates that the dipole anomaly in this study is a stable mode, differing from the Barents Oscillation. Addition- ally, it is should be pointed out that Hilmer and Jung (2000) and Tremblay (2001) also respectively revealed the dipole structure of SLP anomalies, which centered over the Greenland Sea. They attributed the dipole FIG. 3. Spatial distributions of EOF2 of winter monthly mean structure in SLP anomalies to an eastward shift of the SLP for the period 1977/78–2001/02 (October–March). The mode northern center of action associated with the NAO accounts for 12.78% of the total variance.

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lies related to the dipole anomaly at an interannual time scale. On an interannual time scale, the dipole anomaly accounts for 10.4% of total variance. Mean- while, for comparison and as a complement, we pre- liminarily discuss intraseasonal variation of the dipole anomaly. The dipole anomaly displays a strong inter- annual variability with no apparent interdecadal trend (Fig. 4), which differs from the AO. Figures 5a and 5b show SLP and 500-hPa geopotential height anomalies FIG. 4. Interannual variations of PC2. All the data have been normalized. obtained from a linear regression analysis (regression on the monthly mean principal component PC2), re- spectively. Figure 5a shows a –4 hPa isoline covering c. Regression and composite analyses of the dipole the northern Asian coast and between the Kara Sea and anomaly at interannual time scale the Laptev Sea, with a concurrent 2-hPa isoline cover- In this section, regression and composite analyses are ing the area from via Greenland south- performed to illustrate atmospheric circulation anoma- eastward to the Nordic seas. At 500 hPa, negative geo-

FIG. 5. Regression maps of (a) SLP (hPa) and (b) 500-hPa geopotential height north of 20°N (gpm), regression on the monthly PC2. Contour interval: 1 hPa in (a); 10 gmp in (b); (c), (d) as in (a) and (b), respectively, but for regression on the winter mean PC2. Contour interval: 0.5 hPa in (c) and 5 gmp in (d).

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the Nordic seas and Greenland, consistent with the cen- ter of positive SLP anomalies shown in Figs. 5a and 5c. Consequently, the two centers of action associated with the dipole anomaly are negatively correlated, and posi- tive SLP anomalies over the Aleutian and Alaska area (Fig. 5c) could not influence the existence of the dipole anomaly. Based on the time series of the winter PC2 shown in Fig. 4, we chose those cases with winter-averaged stan- dard deviation Ն 1.0 (1961/62, 1967/68, 1968/69, 1980/ 81, 1988/89, 1992/93, 1994/95, and 1995/96) or Յ Ϫ1.0 (1969/70, 1976/77, 1982/83, 1984/85, 1989/90, 1993/94, 1998/99, 1999/2000, and 2000/01) to construct a compos- ite analysis. Compared to the positive phase of the di- pole anomaly, the Siberian high becomes robust and FIG. 6. Correlations of the region-averaged (70°–80°N, 90°– the area occupied by the 1020-hPa isoline extends to 120°E) residual SLP with residual SLP at each grid point after the Arctic basin, with a weakened Icelandic low (not removing the influence of the winter AO on SLP. The blue and shown). yellow areas represent correlations exceeding the 0.05 and 0.01 significance levels, respectively. During the high-index state of the dipole anomaly, negative SLP anomalies occupy nearly the whole Arc- tic, with a center close to the Laptev Sea and concurrent potential height anomalies occupy the northern Kara positive SLP anomalies in the Canadian and the northeastern Barents Sea, and anomalies via Greenland extending southeastward to the North exceeding 20 gpm extends from the Canadian Archi- Atlantic (Fig. 7a). Thus, SLP anomalies show a strong pelago southeastward to the Nordic seas (Fig. 5b). Con- meridional structure over the Arctic basin and Green- sequently, the dipole anomaly shows a quasi-barotropic land Sea. Compared to the high-index state, the low- structure. It is apparent that the dipole anomaly is a index state produces near-opposite SLP anomalies over “local scale” phenomenon rather than a Northern the Arctic Ocean and the Siberian marginal seas with Hemisphere phenomenon. Interannual variations of the center of positive SLP anomalies between the Kara the variables averaged for the winter months shows a Sea and Laptev Sea (Fig. 7b). Although there are nega- large similarity to their intraseasonal variations, partic- ularly in the locations of the centers of action (Figs. 5c tive SLP anomalies over northern Canada and Green- and 5d). Certainly, there are some apparent differences land, its center is situated in the northeastern North between them; that is, interannual variations are obvi- Atlantic. Obviously, the low-index state corresponds to ously weaker than intraseasonal variations. Addition- a strengthened Icelandic low (Fig. 7b). Significant dif- ally, positive anomalies in SLP and 500-hPa geopoten- ferences in mean SLP fields between the positive and tial heights become robust over the Aleutian area when negative phase occurs over northern Eurasia and the compared to intraseasonal variations, and the center of eastern Arctic, with the center of negative SLP anoma- Ϫ negative SLP anomalies seems to shift toward the lies (below 7 hPa) situated in the Laptev Sea (Fig. 7c). Laptev Sea. However, no significant SLP differences can be found Because a new center in SLP anomalies exists over over northern Canada and Greenland. the Aleutian area (Fig. 5c), it is natural to ask whether Removing the influence of the AO on SLP by using or not the dipole anomaly still exists at an interannual a linear regression method, the dipole anomaly be- time scale? To answer this question, we first removed comes robust, as shown in Fig. 8. It is apparent that influences of the winter AO on SLP by means of a Figs. 8a and 8b closely resemble Figs. 7a and 7b, re- linear regression method. Then, a regionally averaged spectively. As can be seen, over the Arctic basin and (70°–80°N, 90°–120°E) residual SLP is used to repre- the Siberian marginal seas anomalies in SLP become sent approximately the intensity index of the center of weaker; however, over northern Canada, Greenland, action associated with the dipole anomaly, which is situ- and the Nordic seas anomalies become robust, when ated in the area between the Kara Sea and Laptev Sea. compared to Fig. 7. Differences between Fig. 7c and Figure 6 shows correlations of the intensity index with Fig. 8c are significant SLP differences appearing over the residual SLP at each grid point. As can be seen, the the Canadian Archipelago extending via Greenland center of significant negative correlations emerges over southeastward to the Nordic seas. The area where cor-

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FIG. 7. Composites of SLP anomalies: (a) the positive phase, (b) the negative phase, and (c) differences between (a) and (b). The blue and yellow areas in (c) represent differences exceeding the 0.05 and 0.01 significance levels, respectively. Contour interval: 1 hPa. relations of the winter PC2 with the residual SLP after Surface wind over much of the Arctic basin flows removing the influence from the AO on SLP are sig- toward Fram Strait and the Barents Sea during the nificant (Fig. 9) is consistent with the shaded area positive phase, which is favorable for sea ice export out shown in Fig. 8c. of the Arctic basin into the Greenland and Barents Seas At 500 hPa, the center of the polar vortex appears (Fig. 10a). Compared to the positive phase in the Arctic over the Arctic basin close to the northern Kara Sea basin, the surface wind displays a strong anticyclonic and Laptev Sea during the positive phase of the dipole circulation pattern during the negative phase (Fig. 10b). anomaly (not shown). In contrast, when the negative As a response of the ocean to the anticyclonic circula- phase prevails, the center of the polar vortex moves to tion, the oceanic circulation including sea ice motion the Canadian Archipelago and northwestern Green- (shown later) also tends to strengthen an anticyclonic land (not shown). The 500-hPa geopotential height motion in the Arctic basin, which is unfavorable for sea anomalies after removing the influence of the winter ice export out of the Arctic basin into the Greenland AO on SLP show a large similarity to Fig. 8 (not and Barents Seas because the strengthened Beaufort shown). gyre accumulates sea ice in the Arctic basin (Proshutin-

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FIG. 8. As in Fig. 7 but for composites of the residual SLP anomalies after removing the influence of the winter AO on SLP. sky et al. 2002). In Fig. 10c, there is a strong cyclonic Eurasia and the Arctic marginal seas, surface wind dif- circulation anomaly covering the Arctic Ocean and ferences also exceed statistical significance levels. Con- much of the Arctic marginal seas, with the center of the sequently, the dipole anomaly strongly influences sur- cyclonic circulation anomaly close to the Laptev Sea. It face wind and further influences SIM (Thorndike and is clear that the center of the cyclonic circulation Colony 1982). Correlation coefficients of the surface anomaly is consistent with the center of negative SLP wind with the winter AO show that no significant cor- anomalies (Fig. 7c). It is apparent that the positive relation (in this study, we only discuss the significant phase of the dipole anomaly would drive more sea ice levels over the 0.01 and 0.001) can be found over the export out of the central Arctic basin than its negative Barents Sea, the Greenland Sea, and the vicinity of phase. Fram Strait (not shown), implying that the winter AO is Statistical significance tests for mean surface wind not a major factor to directly drive sea ice into the differences between the positive phase (␴ Ն 1) and Greenland and Barents Seas. In contrast, the dipole negative phase (␴ Յ Ϫ1) demonstrate that over the anomaly shows a close relationship with the surface Arctic basin, particularly over most of the Barents Sea wind over most of the Barents Sea, the vicinity of Fram and the vicinity of Fram Strait, surface wind differences Strait, and the Arctic basin, particularly with a meridi- are significant (not shown). In addition, over northern onal wind (not shown).

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changes include a weakening of the Beaufort gyre; an increase in sea ice export out of the Arctic basin through Fram Strait and the northern Barents Sea; and enhanced sea ice import from the Laptev Sea, the East Siberian Sea, and the into the Arctic ba- sin. The tends to parallel to the Greenwich meridian (Fig. 11c). Holland’s (2003) nu- merical simulation results also support that ice export through Fram Strait is closely related to reduced SLP along the Eurasian coast and increased SLP in northern Canada and Greenland (see her Fig. 14), which is simi- lar to Fig. 5c. With a coupled model Jung and Hilmer (2001) investigated the linkage between the NAO and the Arctic sea ice export via Fram Strait and they sug- gested that in this coupled GCM it is not the NAO that is forcing sea ice export anomalies through Fram Strait. FIG. 9. Correlations of the winter mean PC2 with the residual SLP after removing the influence of the AO on SLP. The blue and While the dipole anomaly is in its negative phase (Fig. yellow areas represent correlations exceeding the 0.05 and 0.01 11d), compared to Fig. 11a, the strengthened Beaufort significance levels, respectively. gyre is apparent, and sea ice export from the Laptev Sea into the Arctic Ocean is weaker. Since the negative phase of the dipole anomaly corresponds to an anticy- d. Response of SIM to the dipole anomaly at an clonic circulation in the Arctic basin (Fig. 10b), exports interannual time scale of sea ice and freshwater from the Arctic basin into the Certainly, there should be different responses of Nordic seas and the Barents Sea are weaker than usual SIM to different atmospheric forcing. In this section, we because the Beaufort gyre accumulates sea ice and discuss the response of the winter mean SIM to the freshwater in the Arctic basin (Proshutinsky and dipole anomaly. Composite maps of the winter mean Johnson 1997; Tremblay and Mysak 1998; Proshutinsky SIM fields during the high-index (␴ ϭ 1) and low-index et al. 2002). Consequently, the negative phase of the (␴ ϭϪ1) states of the dipole anomaly, constructed by dipole anomaly obstructs sea ice export out of the simply adding and subtracting the regression result to Arctic basin into the Nordic seas and the Barents Sea the winter SIM climatology (1979–98), are shown in (Fig. 11b). Fig. 11. Figure 11b shows the regression map of the Additionally, we constructed case composite maps of winter mean SIM to the dipole anomaly, and a coherent winter SIM according to the time series of the winter cyclonic circulation anomaly covering nearly the whole PC2 (␴ Ն 1: 1988/89, 1992/93, 1994/95, and 1995/96 and Arctic Ocean and the Siberian marginal seas with its ␴ Յ Ϫ1: 1982/83, 1984/85, 1989/90, and 1993/94 during center in the Kara and Laptev Seas. One exception is in the period 1979–98). Composite maps of SIM for the the Canadian and Greenland marginal seas where an positive and negative phase of the dipole anomaly anomalous anticyclonic sea ice motion is visible, reflect- closely resemble Figs. 11c and 11d, respectively (not ing the effect of positive SLP anomalies over the Ca- shown). nadian Arctic and Greenland on sea ice motion. This The circulation patterns in the dipole anomaly ex- implies that changes in sea ice motion in the Arctic tremes are unlike the two regimes of wind-forced ice basin and sea ice export through Fram Strait not only circulation discussed by Proshutinsky and Johnson are associated with SLP anomalies over the Kara Sea (1997). The correlation coefficient between the time and the Laptev Sea but also depend on SLP anomalies series of the dipole anomaly and the sea level gradients over northern Canada and Greenland. Comparing to in the central Arctic basin is poor (0.16); positive sea Fig. 5a with Fig. 5c, it is found that the cyclonic circu- level gradients correspond to an anticyclonic regime lation anomaly shown in Fig. 11b can be attributed to a and negative gradients correspond to a cyclonic regime northward shift of the center of the dipole anomaly of circulation (Proshutinsky and Johnson 1997, see over the Siberian marginal seas. their Fig. 18). Compared to the climatology (Fig. 11a), during the e. Sea ice export and the dipole anomaly positive phase of the dipole anomaly there are coherent large-scale changes in the intensity and character of Most of the sea ice formed in the Arctic Ocean is sea ice transport in the Arctic basin. The significant transferred into the Greenland Sea and the Barents

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FIG. 10. Composites of surface wind: (a) the positive phase, (b) the negative phase, and (c) differences between (a) and (b). Unit: m sϪ1.

Sea. Estimated winter maximum sea ice volume flux present study, the correlation between the winter AO through Fram Strait is 300–400 km3 per month (Vinje and winter sea ice volume flux through Fram Strait is 2001a). Many previous studies have explored the rela- only 0.04 (Fig. 12), which is very low and consistent with tionship between sea ice export from Fram Strait and the previous studies (Vinje 2001a; Jung and Hilmer atmospheric forcing (NAO or AO). Using the satellite 2001). According to simulation results from a coupled observations for 1979–96, Kwok and Rothrock (1999) ice–ocean model, Häkkinen and Geiger (2000) con- indicated that there is a high correlation (0.86) between cluded that the AO does not have much influence on positive phases of the NAO and sea ice area transport sea ice export through Fram Strait. Jung and Hilmer through Fram Strait. However, Vinje (2001a) revealed (2001) also show that the NAO cannot directly force a weak connection between the AO and sea ice volume sea ice export anomalies through Fram Strait. In con- flux from Fram Strait (Ϫ0.15). Hilmer and Jung (2000) trast, the dipole anomaly shows a higher correlation showed that a high correlation (0.7) between ice export coefficient with the winter sea ice export (0.33 at the through Fram Strait and the NAO exists since the mid- 0.05 significance level) (Fig. 12). 1970s but not earlier because the spatial structure of the Additionally, Fig. 12 shows that during the winters NAO has been unusual since the mid-1970s. In the from 1960/1961 to 1968/1969, a positive phase prevailed

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FIG. 11. (a) Winter (October–March) mean sea ice motion averaged for the period 1979–98; (b) regression of winter mean sea ice motion on the winter mean PC2; (c) composite of winter sea ice motion for the positive phase of the dipole anomaly (winter sea ice motion climatology adding the regression analysis shown in Fig. 11b); (d) as in (c) except for the negative phase composite (winter sea ice motion climatology subtracting the regression analysis shown in Fig. 11b). Unit: cm sϪ1.

(mean value: 0.7); during the following winters, from positive phase of the dipole anomaly directly causes 1969/1970 to 1979/1980, the negative phase was domi- increases in sea ice concentration in the Nordic seas and nant (mean value: Ϫ0.4). Thus, the positive phase ep- the Barents Sea with concurrent decreases of sea ice och corresponded to more winter sea ice in the Nordic concentration in much of the Arctic basin (Fig. 13). seas and the Barents Sea, which is consistent with the Decreases of sea ice concentration in the Laptev Sea result of Deser et al. (2000, see their Fig. 3). A com- are most apparent compared to that in the other Sibe- posite of sea ice concentration further verified that the rian marginal seas, consistent with the preceding com- posite map of the winter SIM. Simulation results pro- duced by Arfeuille et al. (2000) also show that there was more sea ice export out of the Arctic basin during 1967/68, which corresponds to the positive phase of the dipole anomaly (Fig. 4). While the response of SIM to atmospheric forcing (Arfeuille et al. 2000, see their Fig. 8) is in agreement with that shown in Fig. 11c, it seems that there is more sea ice export into the Barents Sea than into the Greenland Sea.

FIG. 12. Normalized winter sea ice volume flux through Fram f. Influence of sea ice on surface air temperature Strait (solid line) and the winter PC1 (dashed line) and PC2 (dashed line with dots). The year refers to the winter (October– Vinje (2001b) suggested that variations in the ocean March) season, taken as the year for October. temperature and its positive or negative correlation

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FIG. 13. Composite of sea ice concentration: (a) the positive phase, (b) the negative phase, and (c) differences between (a) and (b). with wind direction seems to be of crucial importance large differences over the Barents and the Greenland for the variation in sea ice and that air temperature Seas during the positive and negative phases of the di- plays a minor role. On the other hand, changes in SIM pole anomaly, as shown in Fig. 14. It should be pointed and sea ice extent produce significant influences on sur- out that, as suggested by Deser et al. (2000), air tem- face air temperature. For example, the divergence of perature changes in the Barents Sea, part of the Green- sea ice motion produces more open water and thin-ice land Sea, the vicinity of Fram Strait, and some areas of area where heat flux from the ocean directly heats the the Arctic basin shown in Fig. 14a may be exaggerated atmosphere in the boundary layer. Wu et al. (2004) because of the crude binary representation of the mar- show that changes in winter sea ice extent in the Green- ginal ice zonal in the NCEP–NCAR reanalysis. In fact, land and Barents Seas produce significant influences on the marginal sea ice zone is a mixture of ice types and the local air temperature and stratification. Based on open water. Figure 14c shows that the most significant observations, Rigor et al. (2002) come to similar con- differences in mean air temperature between the posi- clusions regarding the impact of sea ice transport on the tive and negative phase of the dipole anomaly occur heating of the atmosphere. Influenced by sea ice export over the northern Barents Sea, parts of the Greenland out of the Arctic basin, surface air temperatures show Sea, the vicinity of Fram Strait, and some areas of the

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FIG. 14. As in Fig. 7 but for composites of surface air temperature anomalies. Units: °C.

Arctic basin north of . The largest differ- mean AO forcing and the dipole monthly mean forcing ences are approximately 8°C. Additionally, influenced using a linear regression method, to the accumulated by divergence (convergence) of SIM, surface air tem- total variance of SIM velocity. The accumulated total perature anomalies in the Laptev Sea are obviously dif- variance of SIM velocity is defined as the following: ferent for different phases of the dipole anomaly. As 1998 3͑next͒ shown in Figs. 13 and 14, variations in sea ice concen- ͓ Ј 2 ϩ ␷Ј 2͔ ͑ ͒ ͚ ͚ ui,j i,j 1 tration and surface air temperature produced by the iϭ1979 jϭ10 dipole anomaly show a seesaw structure in the Laptev uЈ ϭ u Ϫ u , ␷Ј ϭ ␷ Ϫ ␷ , Sea and the Barents Sea. i,j i,j j i,j i,j j where i and j stand for year and month, respectively, 4. Discussion and the over bar corresponds to climatology averaged over 1979–99. The winter AO or the dipole-forcing- Because the dipole anomaly only accounts for 13% driven accumulated variance of SIM velocity is the Ј ␷Ј of the total variance, it is natural to ask how to the role same as (1) except for ui,j and i,j, which are obtained by of the dipole anomaly in influencing SIM compared a linear regression analysis method. with that of the winter AO. To answer this question, we Figure 15a shows that the contribution from the win- respectively calculated ratios of the accumulated vari- ter AO forcing exceeds 40% of the total variance, with ance of SIM velocity, driven by the winter monthly the maximum center appearing in the eastern Arctic,

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of the total variance and causes alongshore gradients along the Canadian Archipelago. Compared to Fig. 15b, the maximum center moves toward the eastern Arctic (Fig. 15c). Figure 15 further demonstrates that the dipole anomaly plays a more important role than the winter AO in driving sea ice export out of the Arc- tic basin through the northern Barents Sea and Fram Strait. Jung and Hilmer (2001) and Holland (2003), respec- tively, showed the linkage between SLP variations and sea ice export through Fram Strait based on simulation results, as mentioned previously. They showed the monopole and dipole structure in SLP anomalies, re- spectively. Although the dipole anomaly identified in this study also relates to sea ice export via Fram Strait, it differs from the two previous studies owing to the following facts. First, the monopole and dipole struc- ture in SLP anomalies are obtained from simulation results, and thus one cannot affirm that they really exist in observations. Second, the two studies cannot answer how sea ice motion in the Artic basin responds to the monopole and dipole structure, and how to describe their variations. Third, the center of SLP anomalies over northern Canada and Greenland, missed in Jung and Hilmer (2001), would influence sea ice motion in the Canadian and Greenland marginal seas and further sea ice export through Fram Strait. In fact, sea ice ex- port through Fram Strait is more dependent on SLP gradient. Thus, Vinje (2001a) used SLP gradient data to approximately calculate the wind-induced ice volume flux through Fram Strait. In this study, an anomalous anticyclonic sea ice motion is visible in the western Arc- tic basin (Fig. 16b), reflecting the effect of positive SLP FIG. 15. Ratios of the accumulated variance of SIM velocity driven by (a) the monthly mean AO, (b) the monthly mean dipole anomalies over northern Canada and Greenland on sea anomaly, and (c) combination of (a) plus (b), the accumulated ice motion (Fig. 5a). The monopole structure of Jung total variance of SIM velocity. and Hilmer (2001) may be one of atmospheric modes of variability in the coupled model; however, it does not consistent with that shown in the regression map of exist in the first two modes of atmospheric variability in SIM on the AO index (Fig. 16a). The contribution from observations. The dipole anomaly plays a more impor- the dipole forcing also exceeds 40% of the total vari- tant role in influencing sea ice motion than the AO. ance in the Arctic basin, showing an annular structure However, Jung and Hilmer (2001) did not show any (Fig. 15b). The maximum value is in the central Arctic role of the monopole structure except its connection basin (Fig. 15b), consistent with that shown in Fig. 16b. with sea ice export through Fram Strait. Apparently, the contribution from the dipole forcing is What mechanism is responsible for the existence of greater than that driven by the winter AO, particularly the dipole anomaly in the Arctic atmosphere? Compos- in the central Arctic basin, the northern Barents Sea, ite and regression analysis of SLP and geopotential and the vicinity of Fram Strait. The contribution from heights at 500 hPa demonstrate that anomalies related the two modes to the total variance (Fig. 15c) qualita- to the dipole anomaly show a quasi-barotropic struc- tively resembles that shown in Fig. 15b. Consequently, ture in the atmosphere. Consequently, the Siberian the strong annular structure shown in Fig. 15c has high could not give a reasonable explanation for this masked the effects of the AO on the winter SIM. It is anomaly due to its strong baroclinicity (Wu and Wang clear that the AO strengthens the north–south gradient 2002). The atmospheric dynamic process should be a

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FIG. 16. Regression maps of monthly mean sea ice motion on (a) the monthly AO and (b) the monthly PC2. Unit: cm sϪ1. major factor in maintaining the existence of the dipole ent cyclonic SIM circulation anomaly covering nearly anomaly. the whole Arctic basin and the Siberian marginal seas, with its center in the Laptev Sea. One exception is in the Canadian and Greenland marginal seas where an 5. Conclusions anomalous anticyclonic sea ice motion is visible, reflect- This study identified the dipole anomaly in the Arctic ing the effect of positive SLP anomalies over the Ca- atmosphere and its relationship with the winter SIM in nadian Arctic and Greenland on sea ice motion. Cor- the Arctic basin, based on the NCEP–NCAR reanalysis respondingly, there are negative SLP anomalies ap- dataset, the IABP dataset, and a sea ice concentration pearing over the northern Eurasian coast and the dataset. After the winter AO, the dipole anomaly cor- Arctic Ocean with its center in the Laptev Sea, concur- responds to the second dominant mode in the Arctic rent with the center of positive SLP anomalies over the atmosphere, accounting for 13% of the variance. One south of Greenland. The negative phase of the dipole of its two anomalous centers is stably located between anomaly directly causes a decrease in sea ice export out the Kara Sea and the Laptev Sea; the other is situated of the Arctic basin because the strengthened Beaufort over the Canadian Archipelago extending through gyre accumulates sea ice and freshwater in the Arctic Greenland southeastward to the Nordic seas. The di- basin. pole anomaly differs from the one described in Hilmer Influenced by the dipole anomaly, variations in sea and Jung (2000) and Tremblay (2001), and they attrib- ice concentrations and surface air temperatures exhibit uted the dipole to an eastward shift of the center of a seesaw structure in the Laptev Sea and the Barents action of the North Atlantic Oscillation. The finding Sea. The winter (October–March) sea ice volume flux shows that the dipole anomaly also differs from the through Fram Strait shows a higher correlation (0.33, 42 “Barents Oscillation” revealed by Skeie (2000). samples) with the dipole anomaly than the winter AO The results show that the dipole anomaly plays an forcing (0.04). The finding also demonstrates that influ- important role in driving sea ice export out of the Arc- ences of the dipole anomaly on winter SIM are greater tic basin into the Greenland Sea and the Barents Sea. than that of the winter AO. We suggest that the dipole When the dipole anomaly remains in its positive phase, anomaly is closely related to the atmosphere–ice–ocean it contributes to coherent large-scale changes in the in- interactions that influence the Barents Sea sector. tensity and character of sea ice transport in the Artic basin. The significant changes include a weakening of Acknowledgments. We thank the anonymous review- the Beaufort gyre; an increase in sea ice export out of ers for helpful comments and constructive suggestions. the Arctic basin through Fram Strait and the northern The first author is grateful to Prof. Mark A. Johnson Barents Sea; and enhanced sea ice exports from the and Roger L. Colony for constructive discussions. Jia Laptev Sea and the East Siberian Sea into the Arctic Wang thanks the Coastal Marine Institute, University basin (Transpolar Drift Stream tends to parallel to the of Alaska Fairbanks for financial support (MMS/CMI). Greenwich meridian). Simultaneously, there is a coher- This study was also supported by the National Nature

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