LETTER doi:10.1038/nature10705 Changing Arctic Ocean freshwater pathways James Morison',Ron Kwok2,Cecilia Peralta-Ferriz,Matt Alkire',Ignatius Rigor,Roger Andersen'&Mike Steele Freshening in the Canada basin of the Arctic Ocean began in the thousand by weight)lower than values from pre-1990s climatology2,a 1990s12 and continueds to at least the end of 2008.By then,the difference that is about five times the climatological root-mean-square Arctic Ocean might have gained four times as much fresh water as interannual variability (Supplementary Fig.4b).Geostrophic water comprised the Great Salinity Anomaly of the 1970s,raising the velocities at 50-60 m computed from density-determined dynamic spectre of slowing global ocean circulation.Freshening has been heights's relative to the depth where ocean pressure equals 500 dbar attributed to increased sea ice melting'and contributions from (1 dbar corresponds to about 1 m water equivalent pressure)for 2008 runoff,but a leading explanation has been a strengthening of (Fig.1)show the anticyclonic Beaufort Gyre current pattern as an the Beaufort High-a characteristic peak in sea level atmospheric intense southern core with westward motion along the Alaskan coast, pressure2-which tends to accelerate an anticyclonic(clockwise) but with broader eastward return flows farther north wind pattern causing convergence of fresh surface water.Limited In contrast to the Canada basin,the Makarov basin upper-ocean observations have made this explanation difficult to verify,and salinities in 2008 are 1-2p.s.u.greater than values from climatology2 observations of increasing freshwater content under a weakened (Fig.1).The corresponding trough in dynamic heights forces the Beaufort High suggest that other factors?must be affecting fresh- geostrophic upper-ocean currents to sweep cyclonically around the water content.Here we use observations to show that during a time southeastern part of the Makarov basin and across the Chukchi of record reductions in ice extent from 2005 to 2008,the dominant borderland (see location in Fig.1)into the Canada basin's anticyclonic freshwater content changes were an increase in the Canada basin gyre. balanced by a decrease in the Eurasian basin.Observations are Hydrochemistry sampling in 2008 indicates that,relative to a drawn from satellite data (sea surface height and ocean-bottom reference salinity of 34.87 p.s.u.,Pacific water and Eurasian runoff pressure)and in situ data.The freshwater changes were due to a provide the dominant fractions of freshwater in the upper 200 m of cyclonic(anticlockwise)shift in the ocean pathway of Eurasian the Beaufort Sea.The sea ice melt fraction is comparatively small and runoff forced by strengthening of the west-to-east Northern almost always negative,indicating the dominance of sea ice production Hemisphere atmospheric circulation characterized by an increased and export over melt.We have compared spring 2008(ref.16)and Arctic Oscillation index.Our results confirm that runoff is an summer 2003(ref.17)hydrochemistry data,and adjusting for seasonal important influence on the Arctic Ocean and establish that the differences2 of the order of 1m(Supplementary Information 3 and spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation,rather than the strength of o NP CTD o Exp.CTD.Switchyard oBeaufort oBuoy (spring)oBuoy (fall) the wind-driven Beaufort Gyre circulation. A comparison between the results of large-scale trans-Arctic hydro- 70N graphic sections in 1993 (ref.10)and 1994 (ref.11)and data from climatology revealed a large-scale cyclonic shift in the boundary Alask between Atlantic-derived and Pacific-derived water masses across the Arctic deep basins.This cyclonic shift was related to an increase in the cyclonic atmospheric circulation of the Northern Hemisphere asso- ciated with low Arctic sea level atmospheric pressure and characterized by an increased AO index'3(the AO is the strength of the Northern Hemisphere Annular Mode;Supplementary Information 2). Arctic regional indices have also been proposed to characterize Salinity anomaly (p.s.u. Arctic Ocean change,including the doming of the sea surface char- acterized by the sea surface height gradient in a wind-forced model of the Arctic Ocean.The doming is related to the strength ofthe Beaufort High and has been linked to changes in Arctic Ocean freshwater content24,because anticyclonic wind stress drives convergence of Ekman transport in the ocean surface boundary layer,thickening the fresh surface layer and increasing the doming. Figure 1 2008 Arctic Ocean salinity anomaly and geostrophic velocity at In contrast,we use a new combination of satellite altimetry and 50-60 m depth.The salinity anomaly(colour shading)is relative to the pre- gravity,along with traditional hydrography,to show that from 2005 1990 winter climatology2given in practical salinity units.The velocities to 2008,an increased AO index caused the circulation to become more (vectors)are derived from dynamic heights relative to the 500-dbar pressure cyclonic and the Eurasian river runoff to be increasingly diverted surface5.Red arrows highlight S-shaped pathways from the Russian shelves eastward to the Canada basin at the expense of the Eurasian basin, into the Canada basin anticyclonic gyre.Dashed lines highlight the Alpha- Mendeleyer and Lomonosor ridges.'NP CTD','Exp.CTD'and'Switchyard'are with a nearly negligible increase in average Arctic Ocean fresh water. the hydrographic stations done in spring 2008,'Beaufort'represents the The recent Canada basin fresheningis illustrated by in situ salinity hydrographic stations done in summer 2007,and 'Buoy(spring)'and 'Buoy observations taken in 2008(Fig.1)at ocean depths of 50 to 60 m that (fall)'indicate hydrographic data's from Ice Tethered Profilers gathered in are 1-3 p.s.u.(practical salinity units,essentially equivalent to parts per autumn 2007 and spring 2008. PoarScience Center.Applied Physics Laboratory.University of Washington.1013 Northeast 40th Street,Seattle,Washington9105,USAet Propulsion Laboratory,California Institute of Technology. 4800 Oak Grove Drive,Pasadena,Califomia 91109,USA 66INATURE I VOL 481I5 JANUARY 2012 2012 Macmillan Publishers Limited.All rights reserved
LETTER doi:10.1038/nature10705 Changing Arctic Ocean freshwater pathways James Morison1 , Ron Kwok2 , Cecilia Peralta-Ferriz1 , Matt Alkire1 , Ignatius Rigor1 , Roger Andersen1 & Mike Steele1 Freshening in the Canada basin of the Arctic Ocean began in the 1990s1,2 and continued3 to at least the end of 2008. By then, the Arctic Ocean might have gained four times as much fresh water as comprised the Great Salinity Anomaly4,5of the 1970s, raising the spectre of slowing global ocean circulation6 . Freshening has been attributed to increased sea ice melting1 and contributions from runoff7 , but a leading explanation has been a strengthening of the Beaufort High—a characteristic peak in sea level atmospheric pressure2,8—which tends to accelerate an anticyclonic (clockwise) wind pattern causing convergence of fresh surface water. Limited observations have made this explanation difficult to verify, and observations of increasing freshwater content under a weakened Beaufort High suggest that other factors2 must be affecting freshwater content. Here we use observations to show that during a time of record reductions in ice extent from 2005 to 2008, the dominant freshwater content changes were an increase in the Canada basin balanced by a decrease in the Eurasian basin. Observations are drawn from satellite data (sea surface height and ocean-bottom pressure) and in situ data. The freshwater changes were due to a cyclonic (anticlockwise) shift in the ocean pathway of Eurasian runoff forced by strengthening of the west-to-east Northern Hemisphere atmospheric circulation characterized by an increased Arctic Oscillation9 index. Our results confirm that runoff is an important influence on the Arctic Ocean and establish that the spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation, rather than the strength of the wind-driven Beaufort Gyre circulation. A comparison between the results of large-scale trans-Arctic hydrographic sections in 1993 (ref. 10) and 1994 (ref. 11) and data from climatology12 revealed a large-scale cyclonic shift in the boundary between Atlantic-derived and Pacific-derived water masses across the Arctic deep basins. This cyclonic shift was related to an increase in the cyclonic atmospheric circulation of the Northern Hemisphere associated with low Arctic sea level atmospheric pressure and characterized by an increased AO index13 (the AO is the strength of the Northern Hemisphere Annular Mode; Supplementary Information 2). Arctic regional indices have also been proposed to characterize Arctic Ocean change, including the doming of the sea surface characterized by the sea surface height gradient in a wind-forced model of the Arctic Ocean6 . The doming is related to the strength of the Beaufort High and has been linked to changes in Arctic Ocean freshwater content2,8,14, because anticyclonic wind stress drives convergence of Ekman transport in the ocean surface boundary layer, thickening the fresh surface layer and increasing the doming. In contrast, we use a new combination of satellite altimetry and gravity, along with traditional hydrography, to show that from 2005 to 2008, an increased AO index caused the circulation to become more cyclonic and the Eurasian river runoff to be increasingly diverted eastward to the Canada basin at the expense of the Eurasian basin, with a nearly negligible increase in average Arctic Ocean fresh water. The recent Canada basin freshening2,4 is illustrated by in situ salinity observations taken in 2008 (Fig. 1) at ocean depths of 50 to 60 m that are 1–3 p.s.u. (practical salinity units, essentially equivalent to parts per thousand by weight) lower than values from pre-1990s climatology12, a difference that is about five times the climatological root-mean-square interannual variability (Supplementary Fig. 4b). Geostrophic water velocities at 50–60 m computed from density-determined dynamic heights15 relative to the depth where ocean pressure equals 500 dbar (1 dbar corresponds to about 1 m water equivalent pressure) for 2008 (Fig. 1) show the anticyclonic Beaufort Gyre current pattern as an intense southern core with westward motion along the Alaskan coast, but with broader eastward return flows farther north. In contrast to the Canada basin, the Makarov basin upper-ocean salinities in 2008 are 1–2 p.s.u. greater than values from climatology12 (Fig. 1). The corresponding trough in dynamic heights forces the geostrophic upper-ocean currents to sweep cyclonically around the southeastern part of the Makarov basin and across the Chukchi borderland (see location in Fig. 1) into the Canada basin’s anticyclonic gyre. Hydrochemistry sampling in 2008 indicates that, relative to a reference salinity of 34.87 p.s.u., Pacific water and Eurasian runoff provide the dominant fractions of freshwater in the upper 200 m of the Beaufort Sea16. The sea ice melt fraction is comparatively small and almost always negative, indicating the dominance of sea ice production and export over melt. We have compared spring 2008 (ref. 16) and summer 2003 (ref. 17) hydrochemistry data, and adjusting for seasonal differences2 of the order of 1 m (Supplementary Information 3 and 1 Polar Science Center, Applied Physics Laboratory, University of Washington, 1013 Northeast 40th Street, Seattle, Washington 98105, USA. 2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA. Alaska –4 –3 –2 –1 0 1 2 3 Canada 70° N 5 cm s–1 90° W Greenland 80° N Canada basin Eurasian basin Makarov basin NP CTD Exp. CTD Switchyard Beaufort Buoy (spring) Buoy (fall) AlphaMendeleyev ridge Lomonosov ridge Beaufort Sea Chukchi borderland Salinity anomaly (p.s.u.) Figure 1 | 2008 Arctic Ocean salinity anomaly and geostrophic velocity at 50–60 m depth. The salinity anomaly (colour shading) is relative to the pre- 1990 winter climatology12 given in practical salinity units. The velocities (vectors) are derived from dynamic heights relative to the 500-dbar pressure surface15. Red arrows highlight S-shaped pathways from the Russian shelves into the Canada basin anticyclonic gyre. Dashed lines highlight the AlphaMendeleyer and Lomonosor ridges. ‘NP CTD’, ‘Exp. CTD’ and ‘Switchyard’ are the hydrographic stations done in spring 2008, ‘Beaufort’ represents the hydrographic stations done in summer 2007, and ‘Buoy (spring)’ and ‘Buoy (fall)’ indicate hydrographic data15 from Ice Tethered Profilers gathered in autumn 2007 and spring 2008. 66 | NATURE | VOL 481 | 5 JANUARY 2012 ©2012 Macmillan Publishers Limited. All rights reserved
LETTER RESEARCH Supplementary Figs 5 and 6),we find that the change in average freshwater inventories in the top 195 m of the Beaufort Sea are 0.6m Alask anada of sea ice melt,-0.8 m of Pacific water and 3.6 m of Eurasian runoff Siberia with a total increase of3.4 m.Only the increases in Eurasian runoffand the total inventories are substantially greater than the standard errors (s.e.)of these averages,owing to the variability among stations (s.e. value is 0.93 m for sea ice melt,0.3 m for Pacific water and 0.99 m for Eurasian runoff,with an s.e.value for the total of 1.12 m;see Supplementary Information 3).From 2003 to 2008,the dominant source of freshening is increased Eurasian runoff,especially in the depth range of 50 to 115m (Supplementary Information 3 and Supplementary Fig.5),consistent with transport by geostrophic cur- rents extending down into the upper halocline that come cyclonically around the Makarov basin into the Canada basin(Fig.1). Combined with spot verification by hydrographyise,dynamic ocean topography(DOT,deviation of the sea surface from the geoid) from the Ice Cloud and Land Elevation Satellite (ICESat)laser alti meter's and ocean bottom pressure(OBP)from the Gravity Recovery and Climate Experiment Gravity(GRACE)satellites(Supplementary cms Information 4)provide the spatial and temporal coverage needed to understand the 2005-2008 Arctic Ocean changes.OBP is the sum of DOT and the steric pressure anomaly (SPA,due to changes in water density).Comparisons among satellite-derived DOT and OBP and in situ observations of SPA have been done for temperate oceans but Siberia this is the first such effort for the Arctic Ocean.Models indicate that at shorter than seasonal timescales,Arctic OBP variations are barotropic (Supplementary Information 4)and reflect DOT variations2021.At interannual and longer timescales the deep ocean response is baroclinic, OBP variations are smaller than DOT variations,and the SPA is com- parable to-DOT (Supplementary Fig.7).Comparisons of ICESat DOT with hydrography's (Supplementary Information 4)and with GRACE OBP confirm this for the Arctic Ocean;multiyear variations in DOT,with a fractional correction by OBP,yield the multiyear varia- tions in SPA DOT from 2005 to 2008 increased at~5-8 cm yr(uncertainty is 1 standard deviation (s.d.)of 0.9cmyr;Supplementary Informa- 4 tion 4)in the Canada basin (Fig.2a),resulting in anticyclonic spin- up of the surface velocity.However,DOT decreasedby3-4cmyrin a trough aligned with the Eurasian continental shelf-break(seaward edge of the shallow continental shelf),resulting in increasing DOT- Greenland gradient-driven eastward surface velocities and transport of Eurasian river water along the Russian shelf.Variations in the pattern include 10 cyclonic cells that allow for the runoff-rich coastal water to be carried across the shelf and into the eastern Makarov basin and Chukchi Figure 2 Rates of change between 2005 and 2008 of DOT,SPA and freshwater content.a,DOT rate of change from ICESat altimetry (s.d. borderland regions.Furthermore,the increase in DOT towards the Russian coast could be driving a seaward secondary flow in the bottom 0.9cmyr-Supplementary Information 4).Arrows show rate of change in near-surface velocity driven by DOT.The DOT rate of change equal to GRACE boundary layer over the shelf that injects runoff-enriched water into OBP rate of change minus SPA rate of change from hydrography are shown as the upper halocline of the central basin,where we find increases in the colour-coded triangles(s.d.ofdifference is 1.2 cm yr,Supplementary Fig.10). Eurasian runoff. b,SPA rate of change(+1.2cmyr)equal to the OBP rate of change ICESat DOT rates of change agree with those inferred from the dif- (Supplementary Fig.9)minus the DOTrate ofchange(Fig.2a)for water depths ference between GRACE OBP and the SPA from repeat hydrographic over 50m(Supplementary Information 5).The scale labelled in red indicates stations (Fig.2a,correlation 0.84,for number of samples N=19;99% the change in freshwater content,-35.6 X SPA(+0.42 m yr,Supplementary confidence limits:0.5<correlation coefficient <0.95);the s.d.of DOT Information 6). relative to OBP minus DOT,1.2 cmyr,is also applicable to SPA rate of freshwater content is approximately -35.6X SPA (Fig.2b). change from the difference between GRACE OBP and ICES at DOT rates Freshwater content changes are dominated by strong increases in of change(Supplementary Fig.10).Taking the difference of the DOTand the Canada basin balanced by decreases in the Eurasian basin and OBP rates of change (Supplementary Information 4,OBP 4-year, along the Russian shelf-break,reflecting change in Eurasian runoff 42-sample,rate-of-change uncertainty=+0.37 cmyr;ref.22)yields pathways. estimates of SPA changes over the whole Arctic Ocean (Fig.2b)-to The area-averaged 2005-2008 freshwater rate of change(Fig.2b)of our knowledge the first suchestimates.TheincreasingSPA(3-5cmyr)0.04myr-is almost insignificant (s.e.0.034myr-,Supplemen- in the Eurasian basin and along the Russian shelf-break balances the tary Information 6).If we consider the deep basin only (water decreasing(-4 to-6cmyr)SPA in the Canada basin associated with depths exceeding 500m),the average is greater at 0.18 myr(s.e. declining salinity. 0.039myr,Supplementary Information 6),because the 500-m SPA is negatively related to freshwater content in cold polar depth contour runs near the middle of the freshwater minimum along oceans23.The correlation of SPA and freshwater content calculated the Russian shelf-break(Fig.2b).This is essentially equal to the rate directly for the Beaufort Sea(Supplementary Fig.12)suggests that of change estimated for the deep basin over the previous decade'.The 5 JANUARY 2012 VOL 481I NATURE 67 2012 Macmillan Publishers Limited.All rights reserved
Supplementary Figs 5 and 6), we find that the change in average freshwater inventories in the top 195 m of the Beaufort Sea are 0.6 m of sea ice melt, 20.8 m of Pacific water and 3.6 m of Eurasian runoff, with a total increase of 3.4 m. Only the increases in Eurasian runoff and the total inventories are substantially greater than the standard errors (s.e.) of these averages, owing to the variability among stations (s.e. value is 0.93 m for sea ice melt, 0.3 m for Pacific water and 0.99 m for Eurasian runoff, with an s.e. value for the total of 1.12 m; see Supplementary Information 3). From 2003 to 2008, the dominant source of freshening is increased Eurasian runoff, especially in the depth range of 50 to 115 m (Supplementary Information 3 and Supplementary Fig. 5), consistent with transport by geostrophic currents extending down into the upper halocline that come cyclonically around the Makarov basin into the Canada basin (Fig. 1). Combined with spot verification by hydrography15,16, dynamic ocean topography (DOT, deviation of the sea surface from the geoid) from the Ice Cloud and Land Elevation Satellite (ICESat) laser altimeter15 and ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment Gravity (GRACE) satellites (Supplementary Information 4) provide the spatial and temporal coverage needed to understand the 2005–2008 Arctic Ocean changes. OBP is the sum of DOT and the steric pressure anomaly (SPA, due to changes in water density). Comparisons among satellite-derived DOT and OBP and in situ observations of SPA have been done for temperate oceans18,19, but this is the first such effort for the Arctic Ocean. Models indicate that at shorter than seasonal timescales, Arctic OBP variations are barotropic (Supplementary Information 4) and reflect DOT variations20,21. At interannual and longer timescales the deep ocean response is baroclinic, OBP variations are smaller than DOT variations, and the SPA is comparable to 2DOT (Supplementary Fig. 7). Comparisons of ICESat DOT with hydrography15 (Supplementary Information 4) and with GRACE OBP confirm this for the Arctic Ocean; multiyear variations in DOT, with a fractional correction by OBP, yield the multiyear variations in SPA. DOT from 2005 to 2008 increased at ,5–8 cm yr21 (uncertainty is standard deviation (s.d.) of 0.9 cm yr21 ; Supplementary Information 4) in the Canada basin (Fig. 2a), resulting in anticyclonic spinup of the surface velocity. However, DOT decreased by 324 cm yr21 in a trough aligned with the Eurasian continental shelf-break (seaward edge of the shallow continental shelf), resulting in increasing DOTgradient-driven eastward surface velocities and transport of Eurasian river water along the Russian shelf. Variations in the pattern include cyclonic cells that allow for the runoff-rich coastal water to be carried across the shelf and into the eastern Makarov basin and Chukchi borderland regions. Furthermore, the increase in DOT towards the Russian coast could be driving a seaward secondary flow in the bottom boundary layer over the shelf that injects runoff-enriched water into the upper halocline of the central basin, where we find increases in the Eurasian runoff. ICESat DOT rates of change agree with those inferred from the difference between GRACE OBP and the SPA from repeat hydrographic stations (Fig. 2a, correlation 0.84, for number of samples N 5 19; 99% confidence limits: 0.5 , correlation coefficient , 0.95); the s.d. of DOT relative to OBP minus DOT, 1.2 cm yr21 , is also applicable to SPA rate of changefrom the difference between GRACE OBP and ICES at DOT rates of change (Supplementary Fig. 10). Taking the difference of the DOT and OBP rates of change (Supplementary Information 4, OBP 4-year, 42-sample, rate-of-change uncertainty5 60.37 cm yr21 ; ref. 22) yields estimates of SPA changes over the whole Arctic Ocean (Fig. 2b)—to our knowledge thefirst such estimates. Theincreasing SPA (3–5 cm yr21 ) in the Eurasian basin and along the Russian shelf-break balances the decreasing (24 to 26 cm yr21 ) SPA in the Canada basin associated with declining salinity. SPA is negatively related to freshwater content in cold polar oceans23. The correlation of SPA and freshwater content calculated directly for the Beaufort Sea (Supplementary Fig. 12) suggests that freshwater content is approximately 235.6 3 SPA (Fig. 2b). Freshwater content changes are dominated by strong increases in the Canada basin balanced by decreases in the Eurasian basin and along the Russian shelf-break, reflecting change in Eurasian runoff pathways. The area-averaged 2005–2008 freshwater rate of change (Fig. 2b) of 0.04 m yr21 is almost insignificant (s.e. 0.034 m yr—1, Supplementary Information 6). If we consider the deep basin only (water depths exceeding 500 m), the average is greater at 0.18 m yr21 (s.e. 0.039 m yr21 , Supplementary Information 6), because the 500-m depth contour runs near the middle of the freshwater minimum along the Russian shelf-break (Fig. 2b). This is essentially equal to the rate of change estimated for the deep basin over the previous decade3 . The Alpha ridge Lomonosov ridge Mendeleyev ridge Canada basin Makarov basin Eurasian basin Greenland Siberia Alaska Canada 2 cm s–1 yr–1 −10 –8 –6 –4 –2 0 2 4 6 8 10 a Alpha ridge Lomonosov ridge Mendeleyev ridge Canada basin Makarov basin Eurasian basin Greenland Siberia Alaska Canada SPA (cm yr–1) Freshwater content change (m yr–1) DOT trend (cm yr–1) –10 –8 -6 -4 –2 0 2 4 6 8 10 3 2 1 –1 –2 –3 b 1,000 m 1,000 m 1,000 m Figure 2 | Rates of change between 2005 and 2008 of DOT, SPA and freshwater content. a, DOT rate of change from ICESat altimetry (s.d. 0.9 cm yr21 , Supplementary Information 4). Arrows show rate of change in near-surface velocity driven by DOT. The DOT rate of change equal to GRACE OBP rate of change minus SPA rate of change from hydrography are shown as colour-coded triangles (s.d. of difference is 1.2 cm yr21 , Supplementary Fig. 10). b, SPA rate of change (61.2 cm yr21 ) equal to the OBP rate of change (Supplementary Fig. 9) minus the DOT rate of change (Fig. 2a) for water depths over 50 m (Supplementary Information 5). The scale labelled in red indicates the change in freshwater content, 235.6 3 SPA (60.42 m yr21 , Supplementary Information 6). LETTER RESEARCH 5 JANUARY 2012 | VOL 481 | NATURE | 67 ©2012 Macmillan Publishers Limited. All rights reserved
RESEARCH LETTER rate of change of deep-basin freshwater volume and its difference from The SLP pattern is similar to the mean SLP pattern(Supplementary the larger area average are comparable to recent rates of change in ice Fig.2a)but strengthened and expanded westward.Eurasian runoff volume2and variations in liquid freshwater exports2s(Supplementary leaves the Arctic directly across the Eurasian basin.In the high-AO- Information 7),illustrating the importance of observations with suf- index pattern(Fig.4b),cyclonic motion occurs on the Russian side of ficiently broad spatial coverage(Supplementary Information 9). the Arctic Ocean,and the anticyclonic cell shifts to the southeast in the Maps of 2006-2008 sea level pressure(SLP),DOT and SPA anomalies Canada basin.Eurasian runoff is diverted eastward and off the East (Fig.3)are consistent with variations in the AO.Just as highs in SLP Siberian shelf into the Canada basin circulation,where it can increase force convergence of near-surface Ekman transport,doming of DOT freshwater content through Ekman transport at the surface and by and deepening of isohaline surfaces,SLP lows cause divergence, geostrophic currents at depth.The dipole character of the cyclonic development of a trough in DOT and shoaling of isohalines.Whereas mode and its connection to the AO cannot be captured by the doming the Beaufort High dominates the mean SLP pattern,the AO manifests indexe because the doming criterion considers only a single DOT itself over the Arctic Ocean as a trough of low pressure extending from feature. the Greenland-Norwegian seas into the Eurasian and Makarov basins The 2005-2008 high-AO-index shift exemplifies a change in Arctic (Supplementary Fig.2).When the winter AO index increased in 2007, Ocean characteristics that began in 1989 and largely characterized the the SLP anomaly decreased over the Eurasian and Makarov basins, next 20 years(Supplementary Information 2).Then,as in 2005-2008, reflecting the AO pattern(Fig.3a).The trough in SLP anomaly forces the AO index increased relative to its pre-1989 average,and the trans- a trough in DOT aligned with the Russian shelf-break(Fig.3b and polar drift of sea ice and surface water shifted cyclonically(Sup- Fig.2a).The trough pattern includes upwelling of isohaline surfaces plementary Fig.13a-f).Salinity increased in the Makarov and under the centre of the trough,as indicated by increased SPA (Fig.3c, Eurasian basins026 and decreased in the Beaufort Sea(Supplemen- Fig.2b).It also includes increased DOT and downwelling of isohalines tary Fig.4a)owing to an increase in the fraction of runoff,specifically across the Russian shelf.Raised DOT towards the coast moves fresher caused by a diversion of Eurasian runoff to the east2.An important runoff-rich water eastward.The average upper-ocean circulation difference is that,although the 2005-2008 Canada basin circula- patterns for 2004-2005(Supplementary Fig.13e)and 2007-2009(Sup- tion was increasingly anticyclonic,it became less anticyclonic and plementary Fig.13f)confirm in absolute terms the increased cyclonic doming decreased in the early 1990s2(Supplementary Figs 13a-f). circulation on the Russian side of the Arctic Ocean(Figs 2a and 3b). Clearly,increased doming was not the cause of the 1990s Beaufort Our observations suggest idealized modes of Arctic Ocean circula- Sea freshening. tion(Fig.4).In the low-AO-index mode(Fig.4a),an expanded high in The climate implications of cyclonic AO-induced shifts in fresh- SLP drives an anticyclonic surface circulation over most of the basin. water pathways include increasing deep thermohaline convection in 2006.A0=-02 2007,A0=1.3 2008.A0=0.8 10 Lena R 6 42024680 4080008640000000006 _50 Figure 32006-2008 anomalies relative to 2004-2005 averages of SLP, contribution.b,DOT (February-March)(in cm).c,SPA,equal to the DOTand SPA.a,Winter sealevel atmospheric pressure from the International February-March-average OBP minus DOT.The SLP is the winter (previous Arctic Buoy Program (http://iabp.aplwashington.edu/data_slp.html).Black November-April)average.The text AO value is the winter (November-April) contours are the mean SLP anomaly relative to 2004-2005 plus the AO average anomaly relative to the 1950-1989 average winter AO. 68 INATURE I VOL 481I5 JANUARY 2012 2012 Macmillan Publishers Limited.All rights reserved
rate of change of deep-basin freshwater volume and its difference from the larger area average are comparable to recent rates of change in ice volume24 and variations in liquid freshwater exports25 (Supplementary Information 7), illustrating the importance of observations with sufficiently broad spatial coverage (Supplementary Information 9). Maps of 2006–2008 sea level pressure (SLP), DOT and SPA anomalies (Fig. 3) are consistent with variations in the AO. Just as highs in SLP force convergence of near-surface Ekman transport, doming of DOT and deepening of isohaline surfaces, SLP lows cause divergence, development of a trough in DOT and shoaling of isohalines. Whereas the Beaufort High dominates the mean SLP pattern, the AO manifests itself over the Arctic Ocean as a trough of low pressure extending from the Greenland–Norwegian seas into the Eurasian and Makarov basins (Supplementary Fig. 2). When the winter AO index increased in 2007, the SLP anomaly decreased over the Eurasian and Makarov basins, reflecting the AO pattern (Fig. 3a). The trough in SLP anomaly forces a trough in DOT aligned with the Russian shelf-break (Fig. 3b and Fig. 2a). The trough pattern includes upwelling of isohaline surfaces under the centre of the trough, as indicated by increased SPA (Fig. 3c, Fig. 2b). It also includes increased DOT and downwelling of isohalines across the Russian shelf. Raised DOT towards the coast moves fresher runoff-rich water eastward. The average upper-ocean circulation patterns for 2004–2005 (Supplementary Fig. 13e) and 2007–2009 (Supplementary Fig. 13f) confirm in absolute terms the increased cyclonic circulation on the Russian side of the Arctic Ocean (Figs 2a and 3b). Our observations suggest idealized modes of Arctic Ocean circulation (Fig. 4). In the low-AO-index mode (Fig. 4a), an expanded high in SLP drives an anticyclonic surface circulation over most of the basin. The SLP pattern is similar to the mean SLP pattern (Supplementary Fig. 2a) but strengthened and expanded westward. Eurasian runoff leaves the Arctic directly across the Eurasian basin. In the high-AOindex pattern (Fig. 4b), cyclonic motion occurs on the Russian side of the Arctic Ocean, and the anticyclonic cell shifts to the southeast in the Canada basin. Eurasian runoff is diverted eastward and off the East Siberian shelf into the Canada basin circulation, where it can increase freshwater content through Ekman transport at the surface and by geostrophic currents at depth. The dipole character of the cyclonic mode and its connection to the AO cannot be captured by the doming index6 because the doming criterion considers only a single DOT feature. The 2005–2008 high-AO-index shift exemplifies a change in Arctic Ocean characteristics that began in 1989 and largely characterized the next 20 years (Supplementary Information 2). Then, as in 2005–2008, the AO index increased relative to its pre-1989 average, and the transpolar drift of sea ice and surface water shifted cyclonically10 (Supplementary Fig. 13a–f). Salinity increased in the Makarov and Eurasian basins10,26,27 and decreased in the Beaufort Sea1,2 (Supplementary Fig. 4a) owing to an increase in the fraction of runoff7 , specifically caused by a diversion of Eurasian runoff to the east26,27. An important difference is that, although the 2005–2008 Canada basin circulation was increasingly anticyclonic, it became less anticyclonic and doming decreased in the early 1990s2 (Supplementary Figs 13a–f). Clearly, increased doming was not the cause of the 1990s Beaufort Sea freshening. The climate implications of cyclonic AO-induced shifts in freshwater pathways include increasing deep thermohaline convection in −8 −6 −4 −2 0 2 4 6 8 10 –10 −2.5 −2 −2 −2 −2 −1.5 −1.5 −1.5 −1.5 −1 −1 −1 −1 −1 −0.5 −0.5 −0.5 0 0 0.5 − −4 −4 −4 −4 −3 −4 −3 −3 −3 − −2 −2 −2 −2 −2 −1 −1 −1 0 0 1 −0.4 −0.2 0 0.2 0.2 0.4 0.4 0.4 0.4 . 0 6 0.6 0.6 0.6 0.6 0.8 0.8 0.8 0.8 0.8 1 1 1 1 1.2 1.2 1.2 1.2 1.2 1.4 1.4 1.4 1.4 1.4 1.6 AO = –0.2 −40 −30 −20 –10 0 10 20 30 40 −50 −40 −30 −20 −10 0 10 20 30 40 −50 Russia Alaska Canada Greenland 2006, 2007, AO = 1.3 2008, AO = 0.8 SLP (hPa) DOT (cm) SPA (cm of water equivalent) Lomonosov R Canada basin Eurasian basin 90° W 0° 70° N 80° N Lena R Ob R 180° Yenisey R Alpha-Mendeleyev ridge Makarov basin a b c Figure 3 | 2006–2008 anomalies relative to 2004–2005 averages of SLP, DOT and SPA. a,Winter sea level atmospheric pressure from the International Arctic Buoy Program (http://iabp.apl.washington.edu/data_slp.html). Black contours are the mean SLP anomaly relative to 2004–2005 plus the AO contribution. b, DOT (February–March) (in cm). c, SPA, equal to the February–March-average OBP minus DOT. The SLP is the winter (previous November–April) average. The text AO value is the winter (November–April) average anomaly relative to the 1950–1989 average winter AO. RESEARCH LETTER 68 | NATURE | VOL 481 | 5 JANUARY 2012 ©2012 Macmillan Publishers Limited. All rights reserved
LETTER RESEARCH 6 -Low AO anticyclonic mode High AO- cyclonic mode Idealized ocean circulation Atlantic Atlantic Derived Derived7 Convergence of Ekman transport High SLP A High SLP Low SLP Atmosphere High DOT Eurasian runoff Hiah Do】 Low DOT Density Upper geostrophic transport Density ocean High SPA SPA Continental Runoff transport Lower Continental by secondary circulation ocean shelt shemt Figure 4 Schematic views of the idealized Arctic Ocean circulation prevailing surface geostrophic ocean circulation.The Ob,Yenisey and Lena patterns under low and high AO anomalies.At the top are plan views and at rivers are the dominant sources of runoff to the Arctic Ocean.The red,green the bottom are section views for the anticyclonic pattern for low AO index and purple arrows show the Eurasian runoff freshwater paths as indicated. (a)and the cyclonic pattern for high AO index(b).The blue arrows indicate the the Greenland Sea at the expense of the Labrador Sea,and enhancing Received 9 February;accepted 9 November 2011. sea ice melt by weakening the cold halocline layer of the Eurasian basin2.Climate models suggest an increasing AO with greenhouse 1. McPhee,M.G.Stanton,T.P.Morison,J.H.&Martinson,D.G.Freshening of the warming?s,but the atmospheric models used typically produce low- upper ocean in the Arctic:is perennial sea ice disappearing?Geophys.Res.Lett 25, 1729-1732(1998) pressure anomalies centred over the central Arctic Ocean2-or show Proshutinsky.A.et al Beaufort Gyre freshwater reservoir:state and variability from a negative AO bias(Supplementary Information 1).Climate models observations.J.Geophys Res.114,C00A10,http://dx.doi.org/10.1029/ need to capture the asymmetric effect of the AO on SLP in the Arctic to 2008JC005104(2009) 3. Rabe,B.et al.An assessment of Arctic Ocean freshwater content changes from the predict the role of the Arctic Ocean in our changing climate. 1990 s to the2006-2008 Deriod.Deep Sea Res./58.173-185(2011) 4. McPhee,M.G.Proshutinsky,A.,Morison,J.,Steele,M.Alkire,M.Rapid change in METHODS SUMMARY freshwater content of the Arctic Ocean.Geophys.Res.Lett 36,L10602,http:// The 2005-2008 repeat hydrographic stations(Figs 2 and 3)are from the North Pole dx.doi..org/10.1029/2009GL037525(2009). Environmental Observatory (NPEO,http://psc.apl.washington.edu/northpole/) 5. Dickson,R.R.,Meincke,J.,Malmberg,S.A.Lee,A.J.The 'Great Salinity Anomaly in the northern North Atlantic 1968-1982.Prog.Oceanogr.20,103-151(1988) and Lincoln Sea Switchyard (http://psc.apl.washington.edu/switchyard/index.html) 6 Proshutinsky,A.Y.Johnson,M.A.Two circulation regimes of the wind-driven airborne surveys,plus the Beaufort Gyre Exploration Project (http://www.whoi. Arctic Ocean.J.Geophys.Res.102,12493-12514 (1997) edu/beaufortgyre/)ship-borne surveys.Data from near the Laptev Sea shelf-break 7. Macdonald,R.W.Carmack,E.C.,McLaughlin,F.A,Falkner,K.K.Swift,J.H are from the Nansen and Amundsen Basin Observational System(http://nabos.iarc. Connections among ice,runoff and atmospheric forcing in the Beaufort Gyre. uaf.edu/)and Fram Strait data comes from the Norwegian Polar Institute (http:// Geophys.Res.Let26,2223-2226(1999). www.npolar.no/en/). 8. Proshutinsky.A..Bourke,R.H.McLaughlin,F.A The role of the Beaufort Gyre in The spring 2008 NPEO programme(Fig.1)included conductivity-temperature- Arctic climate variability:seasonal to decadal climate scales.Geophys Res.Lett 29, 2100.http://dx.doi.org/10.1029/2002GL015847(2002) depth and hydrochemistry (Supplementary Information-3)stations in the 9. Thompson,D.W.J.Wallace,J.M.The Arctic Oscillation signature in the North Pole and Beaufort Sea regionsaugmented by Ice Tethered Profile buoy wintertime geopotential height and temperature fields.Geophys.Res.Lett 25 conductivity-temperature-depth data (http://www.whoi.edu/science/PO/ 1297-1300(1998) arcticgroup/projects/ipworkshop.html).For Fig1,salinity anomaliesand dynamic 10.Morison,J.Steele,M.Andersen,R.Hydrography of the upper Arctic Ocean heights relative to 500 dbar at each station are linearly interpolated,and the measured from the nuclear submarine USS Pargo.Deep Sea Res./45,15-38 gradients in dynamic height determine geostrophic currents'. (1998. We use GRACE monthly fields of OBP from the University of Texas Center for 11.Carmack,E.C.etal.Changes in temperature and tracer distributions within the Arctic Ocean:results from the 1994 Arctic Ocean Section.Deep Sea Res.I/44, Space Research release 4(dpc201012),from August 2002 to December 2009 1487-1502(1997) (http://grace.jpl nasa-gov/data)processed from spherical harmonic gravity co- 12.Environmental Working Group (EWG).Joint U.S-Russian Atfas of the Arctic Ocean efficients by the Center for Space Research following ref.31.The values represent Oceanography Atlas for the Winter Period (National Ocean Data Center (NODC). anomalies relative to the mean from January 2003 to December 2007.We use data 1997). filtered with a 300-km half-amplitude radius Gaussian smoother.The GRACE 13.Morison,J.Aagaard.K.Steele,M.Recent environmental changes in the Arctic. Arctic OBP has been validated with in situ pressure at the North Pole22. Arctic53,359-371(2000). 14.Lique,C.et al Evolution of the Arctic Ocean Salinity,2007-08:contrast between We derive DOT (here filtered with a 100-km radius Gaussian smoother)as the the Canadian and the Eurasian Basins.J.Clim.24,1705-1717(2011). difference between ICESat laser altimeter measurements of sea surface height in 15.Kwok,R.Morison,J.Dynamic topography of the ice-covered Arctic Ocean from open water leads relative to the WGS84 ellipsoid and the EGM2008 geoid'.These ICESat Geophys.Res.Lett 38,L02501,http://dxdoiorg/10.1029/ data are available for download at http://rkwokjpl.nasa.gov/icesat/data_topogra 2010GL046063(2011). phy.html.The DOT measured with ICESat in February-March 2008 are well 16.Alkire,M.B.et al.Sensor-based profiles of the NO parameter in the central Arctic correlated (correlation coefficient,092,N=176,9%confidence limits: and southern Canada Basin:new insights regarding the cold halocline.Deep Sea 0.8840945)with dynamic height relative to 500dbar calculated from Res./57,1432-1443(2010). 2008 hydrographic data(Supplementary Fig.8).The resulting geostrophic surface 17.Yamamoto-Kawai,M,McLaughlin,F.ACarmack EC.Nishino,.&Shimada,K. Freshwater budget of the Canada Basin,Arctic Ocean,from salinity,8180,and velocities show the same features derived from the dynamic heights relative to nutrients.J.Geophys.Res.113,C01007,http://dx.doi.org/10.1029/ 500 dbar(Fig.1)'5. 2006JC003858(2008). 5 JANUARY 2012 VOL 481I 2012 Macmillan Publishers Limited.All rights reserved
the Greenland Sea at the expense of the Labrador Sea, and enhancing sea ice melt by weakening the cold halocline layer of the Eurasian basin27. Climate models suggest an increasing AO with greenhouse warming28, but the atmospheric models used typically produce lowpressure anomalies centred over the central Arctic Ocean28–30 or show a negative AO bias30 (Supplementary Information 1). Climate models need to capture the asymmetric effect of the AO on SLP in the Arctic to predict the role of the Arctic Ocean in our changing climate. METHODS SUMMARY The 2005–2008 repeat hydrographic stations (Figs 2 and 3) are from the North Pole Environmental Observatory (NPEO, http://psc.apl.washington.edu/northpole/) and Lincoln Sea Switchyard (http://psc.apl.washington.edu/switchyard/index.html) airborne surveys, plus the Beaufort Gyre Exploration Project (http://www.whoi. edu/beaufortgyre/) ship-borne surveys. Data from near the Laptev Sea shelf-break are from the Nansen and Amundsen Basin Observational System (http://nabos.iarc. uaf.edu/) and Fram Strait data comes from the Norwegian Polar Institute (http:// www.npolar.no/en/). The spring 2008 NPEO programme (Fig. 1) included conductivity–temperature– depth and hydrochemistry (Supplementary Information-3) stations in the North Pole and Beaufort Sea regions4,16 augmented by Ice Tethered Profile buoy conductivity–temperature–depth data (http://www.whoi.edu/science/PO/ arcticgroup/projects/ipworkshop.html). For Fig. 1, salinity anomalies and dynamic heights relative to 500 dbar at each station are linearly interpolated, and the gradients in dynamic height determine geostrophic currents15. We use GRACE monthly fields of OBP from the University of Texas Center for Space Research release 4 (dpc201012), from August 2002 to December 2009 (http://grace.jpl.nasa.gov/data) processed from spherical harmonic gravity coefficients by the Center for Space Research following ref. 31. The values represent anomalies relative to the mean from January 2003 to December 2007. We use data filtered with a 300-km half-amplitude radius Gaussian smoother. The GRACE Arctic OBP has been validated with in situ pressure at the North Pole22. We derive DOT (here filtered with a 100-km radius Gaussian smoother) as the difference between ICESat laser altimeter measurements of sea surface height in open water leads relative to the WGS84 ellipsoid and the EGM2008 geoid15. These data are available for download at http://rkwok.jpl.nasa.gov/icesat/data_topography.html. The DOT measured with ICESat in February–March 2008 are well correlated (correlation coefficient, rcorr 5 0.92, N 5 176, 99% confidence limits: 0.884 , rcorr , 0.945) with dynamic height relative to 500 dbar calculated from 2008 hydrographic data (Supplementary Fig. 8). The resulting geostrophic surface velocities show the same features derived from the dynamic heights relative to 500 dbar (Fig. 1)15. Received 9 February; accepted 9 November 2011. 1. McPhee, M. G., Stanton, T. P., Morison, J. H. & Martinson, D. G. Freshening of the upper ocean in the Arctic: is perennial sea ice disappearing? Geophys. Res. Lett. 25, 1729–1732 (1998). 2. Proshutinsky, A. et al. Beaufort Gyre freshwater reservoir: state and variability from observations. J. Geophys. Res. 114, C00A10, http://dx.doi.org/10.1029/ 2008JC005104 (2009). 3. Rabe, B. et al. An assessment of Arctic Ocean freshwater content changes from the 1990s to the 2006–2008 period. Deep Sea Res. I 58, 173–185 (2011). 4. McPhee, M. G., Proshutinsky, A., Morison, J., Steele, M. & Alkire, M. Rapid change in freshwater content of the Arctic Ocean. Geophys. Res. Lett. 36, L10602, http:// dx.doi.org/10.1029/2009GL037525 (2009). 5. Dickson, R. R., Meincke, J., Malmberg, S. A. & Lee, A. J. The ‘Great Salinity Anomaly’ in the northern North Atlantic 1968–1982. Prog. Oceanogr. 20, 103–151 (1988). 6. Proshutinsky, A. Y. & Johnson, M. A. Two circulation regimes of the wind-driven Arctic Ocean. J. Geophys. Res. 102, 12493–12514 (1997). 7. Macdonald, R. W., Carmack, E. C., McLaughlin, F. A., Falkner, K. K. & Swift, J. H. Connections among ice, runoff and atmospheric forcing in the Beaufort Gyre. Geophys. Res. Lett. 26, 2223–2226 (1999). 8. Proshutinsky, A., Bourke, R. H. & McLaughlin, F. A. The role of the Beaufort Gyre in Arctic climate variability: seasonal to decadal climate scales. Geophys. Res. Lett. 29, 2100, http://dx.doi.org/10.1029/2002GL015847 (2002). 9. Thompson, D. W. J. & Wallace, J. M. The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett. 25, 1297–1300 (1998). 10. Morison, J., Steele, M. & Andersen, R. Hydrography of the upper Arctic Ocean measured from the nuclear submarine USS Pargo. Deep Sea Res. I 45, 15–38 (1998). 11. Carmack, E. C. et al. Changes in temperature and tracer distributions within the Arctic Ocean: results from the 1994 Arctic Ocean Section. Deep Sea Res. II 44, 1487–1502 (1997). 12. Environmental Working Group (EWG). Joint U.S.-Russian Atlas of the Arctic Ocean, Oceanography Atlas for the Winter Period (National Ocean Data Center (NODC), 1997). 13. Morison, J., Aagaard, K. & Steele, M. Recent environmental changes in the Arctic. Arctic 53, 359–371 (2000). 14. Lique, C. et al. Evolution of the Arctic Ocean Salinity, 2007–08: contrast between the Canadian and the Eurasian Basins. J. Clim. 24, 1705–1717 (2011). 15. Kwok, R. & Morison, J. Dynamic topography of the ice-covered Arctic Ocean from ICESat. Geophys. Res. Lett. 38, L02501, http://dx.doi.org/10.1029/ 2010GL046063 (2011). 16. Alkire, M. B. et al. Sensor-based profiles of the NO parameter in the central Arctic and southern Canada Basin: new insights regarding the cold halocline. Deep Sea Res. I 57, 1432–1443 (2010). 17. Yamamoto-Kawai, M., McLaughlin, F. A., Carmack, E. C., Nishino, S. & Shimada, K. Freshwater budget of the Canada Basin, Arctic Ocean, from salinity, d18O, and nutrients. J. Geophys. Res. 113, C01007, http://dx.doi.org/10.1029/ 2006JC003858 (2008). Russia Alaska Lomonosov R Canada basin Eurasian basin Alpha-Mendeleyev R 180° 90° E Makarov basin 80° N 70° N Canada Grnlnd Atlantic Derived Pacific Derived A A′ Atmosphere Upper ocean High SLP Density High DOT Low SPA A A′ Lower ocean Continental shelf 90° W 0° a b Yenisey R. Lena R. Ob R. Russia Alaska Lomonosov R Canada basin Eurasian basin Alpha-Mendeleyev R Makarov basin 80° N 70° N Canada Greenland Atlantic Derived Pacific Derived A A′ High SLP Low SLP High DOT Low SPA Low DOT A A′ Continental shelf Yenisey R. Lena R. 180° 90° E 90° W 0° Ob R. Runoff transport by secondary circulation Convergence of Ekman transport Eurasian runoff geostrophic transport Idealized ocean circulation High AO cyclonic mode Low AO anticyclonic mode High SPA Density Figure 4 | Schematic views of the idealized Arctic Ocean circulation patterns under low and high AO anomalies. At the top are plan views and at the bottom are section views for the anticyclonic pattern for low AO index (a) and the cyclonic pattern for high AO index (b). The blue arrows indicate the prevailing surface geostrophic ocean circulation. The Ob, Yenisey and Lena rivers are the dominant sources of runoff to the Arctic Ocean. The red, green and purple arrows show the Eurasian runoff freshwater paths as indicated. LETTER RESEARCH 5 JANUARY 2012 | VOL 481 | NATURE | 69 ©2012 Macmillan Publishers Limited. All rights reserved
RESEARCH LETTER 18.Chambers,D.P.Observing seasonal steric sea level variations with GRACE and 30.Walsh,J.E.,Chapman,W.L.,Romanovsky,V Christensen,J.H.Stendel,M.Global satellite altimetry..Geophys.Res 111,C03010.http://dxdoi.org/10.1029/ Climate Model performance over Alaska and Greenland.J.Clim.21,6156-6174 2005jC002914(2006). (2008). 19.Willis,J.K.Chambers,D.P.Nerem,R.S.Assessing the globallyaveraged sea level 31.Chambers,D.P.Evaluation of new GRACE time-variable gravity data over the budget on seasonal to interannual timescales.J.Geophys.Res.113,C06015, ocean.Geophys.Res.Lett 33,L17603(2006). http:/dx.doi.org/10.1029/2007JC004517(2008. 20.Vinogradova,N.,Ponte,R.M.&Stammer,D.Relation between sea level and bottom Supplementary Information is linked to the online version of the paper at pressure and the vertical dependence of oceanic variability.Geophys.Res.Lett www.nature.com/nature. 113,L03608,http:/dx.doi..org/10.1029/2006GL028588(2007) 21.Bingham,R.J.Hughes,C.W.The relationship between sea-level and bottom Acknowledgements This work was supported chiefly by NSF grants OPP 0352754, pressure variability in an eddy permittingocean model.Geophys.Res.Lett 35. ARC-0634226,ARC-0856330 and NASA grant NNXO8AH62G.R.K.was supported at L03602,http:/dx.doi.org/10.1029/2007GL032662(2008) the Jet Propulsion Laboratory.California Institute of Technology,under contract with 22.Morison,Wahr,J.Kwok R.&Peralta-Ferriz,C.Recent trer nds in Arctic Ocean mass distribution revealed by GRACE.Geophys.Res.Lett 34,L07602,http:// Uen NAS ICaCrm dx.doiorg/10.1029/2006GL029016(2007). R.Collier,M.McPhee,W.Ermold,L de Steur,A.Proshutinsky and the Beaufort Gyre 23.Steele,M.Ermold,W.Steric sea level change in the Northern Seas.J.Clim.20, Exploration Project,J.Toole and R.Krishfield and the lce Tethered Profiler project at 403-417(2007). WHOI,and W.Smethie of the Switchyard project for the observations that made this 24.Kwok,R.eial.Thinning and volume loss of the Arctic Ocean sea ice cover:2003- vork possible. 2008.J.Geophys..Res114,C07005,http/dk.doi.org/10.1029/2009J0005312 Author Contributions The main idea was developed by J.M.and R.K.J.M.wrote most of (2009). the text and with R.A.and C.P.-F.drew most of the figures.R.K.developed the DOT 25.Serreze,M.C.et al.The large-scale freshwater cycle of the Arctic.J.Geophys.Res. 111,C11010,http://dx.doi.c rg/10.1029/2005JC003424(2006). records.The SLPand OBP anomaly plotswereoriginallydeveloped by C.P.-F.The2008 hydrography observations were made by J.M.M.A,RA and M.S.The AOspatial pattem 26.Ekwurzel,B.,Schlosser,P.,Mortlock,R.A.,Fairbanks,R.G.Swift,J.H.River runoff, data,figures and insight were provided by LR.The hydrographic data processing was sea ice meltwater,and Pacific water distribution and mean residence times in the done by R.A.and the chemistry analysis was done by M.A.Switchyard data and Arctic Ocean.J.Geophys.Res.106,9075-9092 (2001). 27.Steele.M.Boyd.T.Retreat of the cold halocline layer in the Arctic Ocean. freshwater insight was provided by M.S.All authors discussed the results and commented on the manuscript J.Geophys.Res.103,10419-10435(1998). 28.Shindell.D.T.Miller.R.L.Schmidt.G.A Pandolfo.L Simulation of recent Author Information Reprints and permissions information is available at northem winter climate trends by greenhouse-gas forcing.Nature 399,452-455 www.nature.com/reprints.The authors declare no competing financial interests. (1999). Readers are welcome to comment on the online version of this article at 29.Koldunov,N.V.,Stammer,D.Marotzke,J.Present-day Arctic sea ice variability in www.nature.com/nature.Correspondence and requests for materials should be the coupled ECHAM5/MPI-OM model.J.Clim.23,2520-2543 (2010). addressed to J.M.(morison@apl.washington.edu). 70 NATUREI VOL 481|5 JANUARY 2012 2012 Macmillan Publishers Limited.All rights reserved
18. Chambers, D. P. Observing seasonal steric sea level variations with GRACE and satellite altimetry. J. Geophys. Res. 111, C03010, http://dx.doi.org/10.1029/ 2005JC002914 (2006). 19. Willis, J. K., Chambers, D. P. & Nerem, R. S. Assessing the globally averaged sea level budget on seasonal to interannual timescales. J. Geophys. Res. 113, C06015, http://dx.doi.org/10.1029/2007JC004517 (2008). 20. Vinogradova, N., Ponte, R. M. & Stammer, D. Relation between sea level and bottom pressure and the vertical dependence of oceanic variability. Geophys. Res. Lett. 113, L03608, http://dx.doi.org/10.1029/2006GL028588 (2007). 21. Bingham, R. J. & Hughes, C. W. The relationship between sea-level and bottom pressure variability in an eddy permitting ocean model. Geophys. Res. Lett. 35, L03602, http://dx.doi.org/10.1029/2007GL032662 (2008). 22. Morison, J., Wahr, J., Kwok, R. & Peralta-Ferriz, C. Recent trends in Arctic Ocean mass distribution revealed by GRACE. Geophys. Res. Lett. 34, L07602, http:// dx.doi.org/10.1029/2006GL029016 (2007). 23. Steele, M. & Ermold, W. Steric sea level change in the Northern Seas. J. Clim. 20, 403–417 (2007). 24. Kwok, R. et al. Thinning and volume loss of the Arctic Ocean sea ice cover: 2003– 2008. J. Geophys. Res. 114, C07005, http://dx.doi.org/10.1029/2009JC005312 (2009). 25. Serreze, M. C. et al. The large-scale freshwater cycle of the Arctic. J. Geophys. Res. 111, C11010, http://dx.doi.org/10.1029/2005JC003424 (2006). 26. Ekwurzel, B., Schlosser, P., Mortlock, R. A., Fairbanks, R. G. & Swift, J. H. River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean. J. Geophys. Res. 106, 9075–9092 (2001). 27. Steele, M. & Boyd, T. Retreat of the cold halocline layer in the Arctic Ocean. J. Geophys. Res. 103, 10419–10435 (1998). 28. Shindell, D. T., Miller, R. L., Schmidt, G. A. & Pandolfo, L. Simulation of recent northern winter climate trends by greenhouse-gas forcing. Nature 399, 452–455 (1999). 29. Koldunov, N. V., Stammer, D. & Marotzke, J. Present-day Arctic sea ice variability in the coupled ECHAM5/MPI-OM model. J. Clim. 23, 2520–2543 (2010). 30. Walsh, J. E., Chapman, W. L., Romanovsky, V., Christensen, J. H. & Stendel, M. Global Climate Model performance over Alaska and Greenland. J. Clim. 21, 6156–6174 (2008). 31. Chambers, D. P. Evaluation of new GRACE time-variable gravity data over the ocean. Geophys. Res. Lett. 33, L17603 (2006). Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Acknowledgements This work was supported chiefly by NSF grants OPP 0352754, ARC-0634226, ARC-0856330 and NASA grant NNX08AH62G. R.K. was supported at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. GRACE ocean data were processed by D. P. Chambers, supported by the NASA MEASURES Program. We thank the NASA ICESat and GRACE programmes, K. Falkner, R. Collier, M. McPhee, W. Ermold, L. de Steur, A. Proshutinsky and the Beaufort Gyre Exploration Project, J. Toole and R. Krishfield and the Ice Tethered Profiler project at WHOI, and W. Smethie of the Switchyard project for the observations that made this work possible. Author Contributions The main idea was developed by J.M. and R.K. J.M. wrote most of the text and with R.A. and C.P.-F. drew most of the figures. R.K. developed the DOT records. The SLP and OBP anomaly plots were originally developed by C.P.-F. The 2008 hydrography observations weremade by J.M., M.A., R.A. andM.S. The AO spatial pattern data, figures and insight were provided by I.R. The hydrographic data processing was done by R.A. and the chemistry analysis was done by M.A. Switchyard data and freshwater insight was provided by M.S. All authors discussed the results and commented on the manuscript. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of this article at www.nature.com/nature. Correspondence and requests for materials should be addressed to J.M. (morison@apl.washington.edu). RESEARCH LETTER 70 | NATURE | VOL 481 | 5 JANUARY 2012 ©2012 Macmillan Publishers Limited. All rights reserved