In the format provided by the authors and unedited.LETTERS

PUBLISHED ONLINE: 27 FEBRUARY 2017 | DOI: 10.1038/NCLIMATE3228

Increase in acidifying water in the westernArctic OceanDi Qi1,2, Liqi Chen1*, Baoshan Chen3,4, Zhongyong Gao1, Wenli Zhong5, Richard A. Feely6,Leif G. Anderson7, Heng Sun1, Jianfang Chen8, Min Chen2, Liyang Zhan1, Yuanhui Zhang1

andWei-Jun Cai3,4*The uptake of anthropogenic CO2 by the ocean decreasesseawater pH and carbonate mineral aragonite saturation state(Ωarag), a process known as Ocean Acidification (OA). This canbe detrimental to marine organisms and ecosystems1,2. TheArctic Ocean is particularly sensitive to climate change3 andaragonite is expected to become undersaturated (Ωarag < 1)there sooner than in other oceans4. However, the extent andexpansion rate of OA in this region are still unknown. Herewe show that, between the 1990s and 2010, low Ωarag watershave expanded northwards at least 5, to 85 N, and deepened100m, to 250m depth. Data from trans-western Arctic Oceancruises show that Ωarag < 1 water has increased in the upper250m from 5% to 31% of the total area north of 70 N. Tracerdata and model simulations suggest that increased PacificWinter Water transport, driven by an anomalous circulationpattern and sea-ice retreat, is primarily responsible for theexpansion, although local carbon recycling and anthropogenicCO2 uptake have also contributed. These results indicate morerapid acidification is occurring in the Arctic Ocean than thePacific and Atlantic oceans5–8, with the western Arctic Oceanthe first open-ocean region with large-scale expansion of‘acidified’ water directly observed in the upper water column.

Over the past two decades, global warming and climatechange have caused rapid changes in the Arctic, especially inthe western Arctic Ocean. These changes include rapid sea-ice retreat9 and increases in sea surface temperatures3, Pacificwater inflow10, freshwater storage11, primary production12, andsurface CO2 concentrations13. Projected climate change processesare thought to amplify OA in the Arctic Ocean, making it morevulnerable to rapid chemical changes than any other ocean basin4.Recent OA field and modelling studies in the Arctic Ocean suggestthat surface acidification due to CO2 uptake, sea-ice melt andupwelling of subsurface water14–17 and subsurface acidification dueto Pacific Winter Water (PWW) export14,18 will affect the timing offuture large-scale OA changes in the region4. The studies that reportproposedmechanisms are based on limited data collected from onlythe slope and southern basins, two cruises, or the sea surface. Thecurrent status and historical changes of OA are unknown. Herewe use Ωarag estimates derived from various cruises to identify anunprecedented rate and scale of ocean acidification in the westernArctic Ocean and evaluate links to environmental and climatechange over the past two decades.

In this study we report the upper ocean (top 300m) seawaterΩarag from marginal seas to the basins as far north as 88 N fromour summer 2010 cruise as well as four other trans-western ArcticOcean programs during 1994–2008 (Fig. 1c). We calculated Ωaragfrom total alkalinity (TA) and dissolved inorganic carbon (DIC)after a careful data quality assessment of all available historicaldata and a correction to common deep-water values (Methods andSupplementary Fig. 1 and Supplementary Tables 1 and 2). Ourresults show that the vertical section of aragonite undersaturation(Ωarag < 1) in the western Arctic Ocean expanded substantially,becoming deeper and extending into higher latitudes from 1994 to2010 (Fig. 1a,b,d–h). During the 1994 survey along the 170 Wtran-sect the undersaturated Ωarag occurred only on the Chukchi slopeand abyssal plain (approximately 55–125mdepth), with its northernboundary stopping at 78 N and never reaching 80 N where the1994 stations were still within the Canada Basin (Fig. 1a). Thisconclusion is further supported by a 1998 survey between 160 Wand 170 W where the undersaturated Ωarag waters extended to adepth of ∼75–175m at ∼75 N, but quickly narrowed northwards,with its northern boundary stopping at 80 N (Fig. 1b). In summary,the undersaturated water was largely limited to within 50–150mand its northern boundary was limited to south of 80 N in the1990s. In contrast, large-scale aragonite undersaturation occurredin the Canadian Basin in 2008 and 2010. Along the western transect(∼170 W) the undersaturated region includes waters at depthsof approximately 0–30m on the southern slope and in the basin,50–250m between 75 and 80 N within the Chukchi abyssal plainand the southern Canada Basin, 50–200m between 80 and 85 Nin the northern Canada Basin, and a slight expansion to the 50mdepth north of 85 N in theMakarovBasin (Fig. 1e,g). A similarΩaragdistribution pattern also exists along the eastern transect (160 W)(Fig. 1f,h).Overall, the percentage of aragonite undersaturated verti-cal area of the water column (0–250m and between 70 and 90 N)increased by a factor of six from 5% in 1994 to 31% in 2010, withan average rate of increase of 1.5% per year (Fig. 2 and Methods).Furthermore, if we define a critical state of Ωarag < 1.25, where thegrowth of carbonate mineral-bearing organisms could be threat-ened (see Methods), it expanded at a rate of 2.7% per year (Fig. 2).

We now examine the factors responsible for the rapid expansionof the ‘acidified’ water in this region. In the sea surface our analysis(Fig. 3a) confirms previous reports that air–sea CO2 exchange (A)and sea-ice melt (I) are the two main processes for acidification

1Key Laboratory of Global Change and Marine-Atmospheric Chemistry of State Oceanic Administration (SOA), Third Institute of Oceanography, SOA,Xiamen 361005, China. 2College of Ocean and Earth Sciences, Xiamen University, Xiamen 361005, China. 3School of Marine Science and Policy, Universityof Delaware, Newark, Delaware 19716, USA. 4Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, USA. 5Key Lab of PhysicalOceanography, Ocean University of China, Qingdao 266100, China. 6Pacific Marine Environmental Laboratory/NOAA Seattle, Washington 98115, USA.7Department of Marine Sciences, University of Gothenburg, Box 461, 40530 Gothenburg, Sweden. 8Second Institute of Oceanography, SOA,Hangzhou 310012, China. *e-mail: chenliqi@tio.org.cn; wcai@udel.edu

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LETTERSPUBLISHED ONLINE: 27 FEBRUARY 2017 | DOI: 10.1038/NCLIMATE3228

Increase in acidifying water in the westernArctic OceanDi Qi1,2, Liqi Chen1*, Baoshan Chen3,4, Zhongyong Gao1, Wenli Zhong5, Richard A. Feely6,Leif G. Anderson7, Heng Sun1, Jianfang Chen8, Min Chen2, Liyang Zhan1, Yuanhui Zhang1

andWei-Jun Cai3,4*The uptake of anthropogenic CO2 by the ocean decreasesseawater pH and carbonate mineral aragonite saturation state(Ωarag), a process known as Ocean Acidification (OA). This canbe detrimental to marine organisms and ecosystems1,2. TheArctic Ocean is particularly sensitive to climate change3 andaragonite is expected to become undersaturated (Ωarag < 1)there sooner than in other oceans4. However, the extent andexpansion rate of OA in this region are still unknown. Herewe show that, between the 1990s and 2010, low Ωarag watershave expanded northwards at least 5, to 85 N, and deepened100m, to 250m depth. Data from trans-western Arctic Oceancruises show that Ωarag < 1 water has increased in the upper250m from 5% to 31% of the total area north of 70 N. Tracerdata and model simulations suggest that increased PacificWinter Water transport, driven by an anomalous circulationpattern and sea-ice retreat, is primarily responsible for theexpansion, although local carbon recycling and anthropogenicCO2 uptake have also contributed. These results indicate morerapid acidification is occurring in the Arctic Ocean than thePacific and Atlantic oceans5–8, with the western Arctic Oceanthe first open-ocean region with large-scale expansion of‘acidified’ water directly observed in the upper water column.

Over the past two decades, global warming and climatechange have caused rapid changes in the Arctic, especially inthe western Arctic Ocean. These changes include rapid sea-ice retreat9 and increases in sea surface temperatures3, Pacificwater inflow10, freshwater storage11, primary production12, andsurface CO2 concentrations13. Projected climate change processesare thought to amplify OA in the Arctic Ocean, making it morevulnerable to rapid chemical changes than any other ocean basin4.Recent OA field and modelling studies in the Arctic Ocean suggestthat surface acidification due to CO2 uptake, sea-ice melt andupwelling of subsurface water14–17 and subsurface acidification dueto Pacific Winter Water (PWW) export14,18 will affect the timing offuture large-scale OA changes in the region4. The studies that reportproposedmechanisms are based on limited data collected from onlythe slope and southern basins, two cruises, or the sea surface. Thecurrent status and historical changes of OA are unknown. Herewe use Ωarag estimates derived from various cruises to identify anunprecedented rate and scale of ocean acidification in the westernArctic Ocean and evaluate links to environmental and climatechange over the past two decades.

In this study we report the upper ocean (top 300m) seawaterΩarag from marginal seas to the basins as far north as 88 N fromour summer 2010 cruise as well as four other trans-western ArcticOcean programs during 1994–2008 (Fig. 1c). We calculated Ωaragfrom total alkalinity (TA) and dissolved inorganic carbon (DIC)after a careful data quality assessment of all available historicaldata and a correction to common deep-water values (Methods andSupplementary Fig. 1 and Supplementary Tables 1 and 2). Ourresults show that the vertical section of aragonite undersaturation(Ωarag < 1) in the western Arctic Ocean expanded substantially,becoming deeper and extending into higher latitudes from 1994 to2010 (Fig. 1a,b,d–h). During the 1994 survey along the 170 Wtran-sect the undersaturated Ωarag occurred only on the Chukchi slopeand abyssal plain (approximately 55–125mdepth), with its northernboundary stopping at 78 N and never reaching 80 N where the1994 stations were still within the Canada Basin (Fig. 1a). Thisconclusion is further supported by a 1998 survey between 160 Wand 170 W where the undersaturated Ωarag waters extended to adepth of ∼75–175m at ∼75 N, but quickly narrowed northwards,with its northern boundary stopping at 80 N (Fig. 1b). In summary,the undersaturated water was largely limited to within 50–150mand its northern boundary was limited to south of 80 N in the1990s. In contrast, large-scale aragonite undersaturation occurredin the Canadian Basin in 2008 and 2010. Along the western transect(∼170 W) the undersaturated region includes waters at depthsof approximately 0–30m on the southern slope and in the basin,50–250m between 75 and 80 N within the Chukchi abyssal plainand the southern Canada Basin, 50–200m between 80 and 85 Nin the northern Canada Basin, and a slight expansion to the 50mdepth north of 85 N in theMakarovBasin (Fig. 1e,g). A similarΩaragdistribution pattern also exists along the eastern transect (160 W)(Fig. 1f,h).Overall, the percentage of aragonite undersaturated verti-cal area of the water column (0–250m and between 70 and 90 N)increased by a factor of six from 5% in 1994 to 31% in 2010, withan average rate of increase of 1.5% per year (Fig. 2 and Methods).Furthermore, if we define a critical state of Ωarag < 1.25, where thegrowth of carbonate mineral-bearing organisms could be threat-ened (see Methods), it expanded at a rate of 2.7% per year (Fig. 2).

We now examine the factors responsible for the rapid expansionof the ‘acidified’ water in this region. In the sea surface our analysis(Fig. 3a) confirms previous reports that air–sea CO2 exchange (A)and sea-ice melt (I) are the two main processes for acidification

1Key Laboratory of Global Change and Marine-Atmospheric Chemistry of State Oceanic Administration (SOA), Third Institute of Oceanography, SOA,Xiamen 361005, China. 2College of Ocean and Earth Sciences, Xiamen University, Xiamen 361005, China. 3School of Marine Science and Policy, Universityof Delaware, Newark, Delaware 19716, USA. 4Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, USA. 5Key Lab of PhysicalOceanography, Ocean University of China, Qingdao 266100, China. 6Pacific Marine Environmental Laboratory/NOAA Seattle, Washington 98115, USA.7Department of Marine Sciences, University of Gothenburg, Box 461, 40530 Gothenburg, Sweden. 8Second Institute of Oceanography, SOA,Hangzhou 310012, China. *e-mail: chenliqi@tio.org.cn; wcai@udel.edu

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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCLIMATE3228

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1 Supplementary Figure 1 2 3

AOS1994SHEBA1997-1998ODEN2005CHINARE2008CHINARE2010

TA ( mol kg-1)

2200 2250 2300 2350

Dep

th (

m)

0

1000

2000

3000

4000

TA ( mol kg-1)-Corrected

2200 2250 2300 2350

Dep

th (

m)

0

1000

2000

3000

4000

DIC ( mol kg-1)

2100 2150 2200 2250D

epth

(m

)0

1000

2000

3000

4000

DIC ( mol kg-1)-Corrected

2100 2150 2200 2250

Dep

th (

m)

0

1000

2000

3000

4000

TA ( mol kg-1)

2200 2250 2300 2350

Dep

th (

m)

0

1000

2000

3000

4000

TA ( mol kg-1)-Corrected

2200 2250 2300 2350

Dep

th (

m)

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2000

3000

4000

DIC ( mol kg-1)

2100 2150 2200 2250

Dep

th (

m)

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DIC ( mol kg-1)-Corrected

2100 2150 2200 2250

Dep

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2200 2250 2300 2350

Dep

th (

m)

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2000

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4000

TA ( mol kg-1)-Corrected

2200 2250 2300 2350

Dep

th (

m)

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1000

2000

3000

4000DIC ( mol kg-1)

2100 2150 2200 2250

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th (

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DIC ( mol kg-1)-Corrected

2100 2150 2200 2250

Dep

th (

m)

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4000

Southern Canada Basin Northern Canada Basin Markov Basin

4 Supplementary Figure 1. Comparison of TA, TA-Corrected, DIC, and DIC-Corrected between 1994, 1997-1998, 2005, 2008, and 5 2010 at stations in the Canadian Basin.6

7 Supplementary Figure 2 8 9

PWW

PSW

Pure-Pacific water (PW) line

Pure-Atlantic water (AW) line

AW

Potential Temperature (ºC)

Phosphate (μmol kg-1)

DIN

(N

O3+

NO

2+N

H4)

[μm

ol k

g-1)

Phosphate (μmol kg-1)

DIN

(N

O3+

NO

2+N

H4)

[μm

ol k

g-1)

DIN

(N

O3+

NO

2+N

H4)

[μm

ol k

g-1)

DIN

(N

O3+

NO

2+N

H4)

[μm

ol k

g-1)

Phosphate (μmol kg-1)Phosphate (μmol kg-1)

TA/Sal Potential Temperature [ºC]

Salinity

f PW (%)

a

dc

b

10 11 Supplementary Figure 2. Nitrogen versus phosphorus, Potential temperature (PT), Salinity, 12 TA/Sal and fPW(%) data for the 2010 cruise. The data points are colored by, (a) Potential 13 temperature, (b) Salinity, (c) TA/Sal and (d) f PW (%). The gray line and black dashed line 14 indicate "pure-Atlantic water line" and "pure-Pacific water line", respectively, as reported by 15 Refs1, 2. 16 17 18 19 20 21 22 23 24 25 26 27

28 Supplementary Figure 3 29 30

a b

c d e 31

Supplementary Figure 3. Map of sampling stations (red dots) and annual mean sea ice motion (black arrows) in the Arctic Ocean. 32 Here we use the sea ice motion vectors to roughly depict the surface circulation of the Arctic Ocean. Original monthly mean sea ice 33 motion data was obtained from the Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors data with a resolution of 25 km 34 (Ref 3). Annual mean sea ice motion vectors were derived from the monthly mean data. The data are available at 35 http://nsidc.org/data/nsidc-0116.html. The yellow dot represents the center of the Beaufort Gyre, which was roughly determined by 36 visually identifying the center of the anticyclonic sea ice motion. 37

38 Supplementary Figure 4 39 40

Year

1950 1960 1970 1980 1990 2000 2010

AO

O in

dex

-6

-4

-2

0

2

4

6

1998

2015

1994

2010

Abnormal Accumulation

of PWW

Divergence of PWW

41 Supplementary Figure 4. Arctic Ocean Oscillation (AOO) index, a measure of circulation regime (clockwise/anticyclonic or 42 counterclockwise/cyclonic) of the Arctic Ocean wind-driven circulation (refer to http://www.whoi.edu/page.do?pid=66578). Positive 43 (blue bars) AOO indices correspond to years with anticyclonic circulation regimes (ACCR) and negative (red) bars show cyclonic 44 regimes (CCR). During an ACCR, PWW/freshwater is accumulated in the Beaufort Gyre of the Canada Basin due to Ekman Pumping 45 convergence, which reduces PWW/freshwater transport toward the North Atlantic Subpolar Region4-6. By contrast, during a CCR 46 PWW/freshwater exports to the Northern Atlantic Ocean increases7. 47 48 49

50 Supplementary Figure 5 51

52 Supplementary Figure 5. Latitudinal distributions of salinity, potential temperature, TA/Sal ratio and fraction of Pacific water (f 53 PW(%)) during the cruises from 1994 to 2010, including the Arctic Ocean Section 1994 (AOS 1994), Surface Heat Budget of the 54 Arctic Ocean 1998 (SHEBA 1998), Beringia 2005 (ODEN 2005), and Chinese National Arctic Research Expedition 2008 and 2010 55 (CHINARE 2008 and CHINARE 2010) cruises. As δ18O data are only available for 1994 and 2010, f PW (%) are only calculated for 56 1994 and 2010. Note that f PW (%) and TA/Sal are similar for 1994 and 2010. Thus TA/Sal is used as a qualitative tracer for Pacific 57 Winter Water (PWW) (see Methods). 58

59

60 Supplementary Figure 6 61 62 63

Slope SCB MB

upwelling downwelling

PSW

AW

PWW

NCB

Ekman transport

64 65

Supplementary Figure 6. Potential temperature (ºC) (color contour and solid contours) and potential density anomaly (kg m-3) (white 66 dashed contours) along the eastern cruise track (~150-160ºW) of CHINARE 2010. In the year of 2010, two water masses are apparent, 67 including Pacific Summer Water (PSW) characterized by a T maximum (~-0.5ºC), and PWW mode at nearby freezing temperature 68 (-1.5ºC), σθ= 26.5 (26.0-27.0). The Ekman transport zone, the upwelling-affected zone and the downwelling zone are based on Refs8 69 and9, respectively. 70 71 72 73 74 75 76 77 78 79

80 Supplementary Table 1: The five zones of AOS 1994 and CHINARE 2010. 81

Region Station Date Longitude Latitude Depth Station Date Longitude Latitude Depth °W °N m °E °N m SR10 8/29/2010 -169.00 73.00 77 5 7/29/1994 73.99 -168.74 175 SR11 8/29/2010 -168.99 73.99 184 6 7/29/1994 74.50 -168.84 186 SR12 8/29/2010 -169.00 74.50 187

Slope 7 7/29/1994 75.01 -169.99 252 M06 8/28/2010 -172.03 74.99 393 8 7/30/1994 75.45 -170.57 862 M07 8/28/2010 -172.00 75.33 809 9 7/31/1994 75.75 -171.24 1670 SR16 8/29/2010 -168.97 77.98 657

Chukchi abyssal plain 10 7/31/1994 75.94 -171.67 1877 M01 8/26/2010 -172.06 77.50 2310 11 8/01/1994 76.62 -173.32 2218 M02 8/26/2010 -171.99 77.00 2300 12 8/02/1994 77.33 -175.03 1843 M03 8/26/2010 -171.83 76.50 2298 13 8/03/1994 77.79 -176.30 1208 M04 8/28/2010 -171.98 76.00 2016 14 8/03/1994 78.15 -176.90 947 M05 8/28/2010 -172.13 75.65 1617 15 8/04/1994 78.14 -174.28 1836

Southern Canada Basin S24 2010/7/26 -153.21 72.25 2567 S25 2010/7/26 -152.50 72.34 2830 S26 2010/7/27 -153.55 72.70 3521 S26 2010/7/27 -153.55 72.70 3521 MS01 2010/7/27 -154.71 73.17 3814 MS02 2010/7/28 -156.37 73.68 3743 MS03 2010/7/28 -157.30 74.07 3890

Northern Canada Basin 17 8/06/1994 79.99 -174.31 1652 BN05 8/01/2010 -161.47 80.48 3156 18 8/06/1994 80.15 -173.25 2635 BN06 8/02/2010 -164.94 81.46 3566 19 8/08/1994 80.16 -176.77 2106 BN07 8/03/2010 -166.47 82.48 3627 20 8/08/1994 80.33 -178.63 1427 BN09 8/05/2010 -167.13 84.19 2500 21 8/09/1994 80.71 179.99 1631 SR17 8/25/2010 -169.00 79.00 3067 SR18 8/25/2010 -169.10 79.99 3347 SR20 8/24/2010 -169.03 81.95 3409 SR22 8/23/2010 -170.67 83.74 2459

Makarov Basin 23 8/10/1994 81.58 176.88 2515 BN10 8/05/2010 -178.64 85.50 2434 25 8/12/1994 83.17 173.94 3063 BN11 8/06/2010 -176.10 86.08 3903 26 8/13/1994 84.06 175.07 3110 BN13 8/20/2010 -176.63 88.39 3995 27 8/14/1994 84.85 170.69 3295 28 8/15/1994 85.89 166.70 3419 29 8/17/1994 87.16 160.71 3891

Supplementary Table 2: Average concentrations and standard deviations (SD) of NDIC and NTA 82 (μmol kg-1) in deep waters (>2000 m) of the Canadian Basin (CB), including the southern 83 Canada Basin (SCB), northern Canada Basin (NCB), and Makarov Basin (MB), during the 84 different cruises. Data were normalized to salinity 35. See cruise names and data sources in the 85 footnotes. 86

NDIC NTA Cruisea Year Meanb SD n Mean SD n

ARKXII 1996 SCB NCB MB 2151 3 17 2300 3 17

SHEBA 1996 SCB 2153 1 4 2313 3 4 NCB MB

JOIS 1997 SCB 2156 3 44 2300 4 42 NCB MB

SHEBA 1997-1998 SCB 2158 2 2 NCB 2154 5 2 2312 3 2

MB SBI spring 2002 SCB 2150 2 3 2324 1 NCB MB

SBI summer 2002 SCB 2150 2 8 2319 4 8 NCB MB

ODEN 2005 SCB 2164 5 40 2301 4 44 NCB 2173 3 22 2301 3 32 MB 2163 4 19 2307 3 26

ARKXII 2007 SCB NCB MB 2156 1 15 2303 1 14

CHINARE 2008 SCB 2155 6 5 2313 3 10 NCB MB

HLY 2010 SCB 2304 1 4 NCB 2301 4 8 MB

HLY 2011 SCB 2152 1 NCB 2156 3 2 2320 4 2 MB 2158 2 2

HLY 2012 SCB 2152 1 2 2301 1 2 NCB MB

Supplementary Table 2: (continued) 87

AOS 1994 SCB NCB 2162 3 9 2310 2 5 MB 2163 3 34 2304 3 33

CHINARE 2010 SCB 2154 3 5 2309 3 7 NCB 2156 4 4 2308 4 9 MB 2148 3 6 2306 2 7

mean SCB 2154 4 10 2309 8 9 NCB 2160 8 5 2309 7 6 MB 2156 6 6 2304 3 5

Total mean CB 2157 2307 88 a Cruise names: ARKXII is the Polarstern Arctic’96 cruise. SHEBA is the Surface Heat Budget 89 of the Arctic Ocean cruise in 1997-1998. JOIS is the Joint Ocean Ice Study cruise in 1997. SBI 90 indicates the Shelf-Basin Interactions Project in 2002. ODEN is the Beringia 2005 cruise. 91 ARK-XXII/2 is the Polarstern 2007 cruise to the Arctic. HLY is the R/V Healy cruises from 92 2010 to 2012. AOS is the Arctic Ocean Section cruise in 1994. CHINARE is the Chinese Arctic 93 and Antarctic Research Expedition in 2008 and 2010. CHINARE data are available at 94 http://www.chinare.org.cn/index/. The other cruises data are downloaded from CDIAC at 95 http://cdiac.ornl.gov/oceans/. 96 b Values exceeding ± 2 standard deviations (SDs), approximately 5% of the total data size, could 97 be outliers and were excluded for the calculation of average values. 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119

120 Supplementary Table 3: End-member properties of Pacific winter water (PWW, σϴ≈26.5) at various locations and times. 121

Cruise Year Salinity

Potential

Temperature

[°C]

Sigma-θ

[kg m-3]

DO

[µmol kg-1]

AOU

[µmol kg-1]

NO3-

+NO2-[µmol

kg-1]

PO43-

[µmol kg-1]

TAlk

[µmol kg-1]

DIC

[µmol kg-1] TA/Sal

f (PW)

(%) Ωarag

AOS 1994

Slope 33.03±0.12 -1.44±0.04 26.57±0.10 260±13 107±12 16.22±2.17 2.15±0.16 2255±12 2221±6 68.3±0.1 99±3 0.75±0.07

CPA 33.01±0.20 -1.36±0.06 26.55±0.17 244±4 122±4 16.62±0.84 2.18±0.09 2245±12 2217±7 68.0±0.1 99±2 0.71±0.03

SCB

NCB

Mean 32.97 -1.37 26.52 252 115 16.42 2.17 2250 2219 68.1 99 0.73

SHEBA 1998

Slope

CPA 33.00±0.22 -1.56±0.12 26.56±0.18 294±14 75±13 16.48±1.49 2.02±0.14 2262±13 2200±15 68.5±0.3 99±5 0.92±0.08

SCB 32.96±0.24 -1.54±0.08 26.52±0.20 298±11 70±10 15.12±1.29 1.81±0.08 2275±14 2202±14 69.0±0.3 97±6 0.99±0.07

NCB 32.93±0.18 -1.63±0.04 26.50±0.14 290±14 79±14 15.38±1.64 1.96±0.19 2251±13 2194±15 68.3±0.1 97±5 0.89±0.07

Mean 32.96 -1.58 26.53 294 75 15.66 1.93 2263 2199 68.6 98 0.93

ODEN 2005

Slope

CPA

SCB 32.90±0.15 -1.56±0.10 26.62±0.11 283±16 84±15 15.10±1.15 1.88±0.11 2263±8 2214±14 68.8±0.3 98±5 0.84±0.07

NCB 32.75±0.16 -1.43±0.04 26.73±0.10 251±6 115±6 15.39±0.81 1.93±0.07 2246±11 2213±13 68.6±0.1 99±1 0.74±0.03

Mean 32.83 -1.50 26.68 267 267 15.52 1.91 2255 2214 68.7 99 0.79

CHINARE

2008

Slope 32.99±0.22 -1.49±0.03 26.55±0.18 2276±14 2185±20 69.0±0.3 1.15±0.15

CPA 33.05±0.12 -1.55±0.04 26.60±0.10 2269±9 2213±7 68.6±0.2 0.90±0.08

SCB 32.97±0.28 -1.47±0.28 26.54±0.23 2276±14 2185±20 68.6±0.4 0.91±0.10

NCB 32.93±0.23 -1.43±0.07 26.50±0.18 2256±11 2204±15 68.5±0.3 0.86±0.06

Mean 32.99 -1.49 26.55 2269 2197 68.4 0.95

2010

Slope 32.97±0.15 -1.61±0.16 26.53±0.12 280±32 88±33 13.90±2.71 1.85±0.21 2270±6 2237±30 68.8±0.5 101±1 0.76±0.16

CPA 32.91±0.13 -1.48±0.01 26.47±0.10 282±5 86±4 15.50±0.81 1.88±0.05 2261±5 2211±7 68.7±0.2 97±7 0.83±0.05

SCB 32.93±0.08 -1.48±0.03 26.49±0.06 276±3 92±3 15.20±0.53 1.80±1.30 2267±3 2218±5 68.8±0.3 100±0 0.83±0.03

NCB 32.96±0.19 -1.46±0.07 26.51±0.16 255±16 112±15 16.41±1.06 2.04±0.13 2249±22 2227±18 68.2±0.4 96±6 0.68±0.10

Mean 32.94 -1.51 26.50 273 95 15.25 1.89 2262 2223 68.6 99 0.78

122 123

124 125 Supplementary Table 4: Endmember properties of AW (27.80≤σϴ≤27.90) in the years between 1994 and 2010 126

127 128 129 130 131 132 133 134 135 136 137

138

Cruise Year Salinity Potential

Temperature [°C]

Sigma-θ [kg m-3]

DO [µmol kg-1]

AOU [µmol kg-1]

NO3- +NO2

- [µmol kg-1]

PO43-

[µmol kg-1] TAlk

[µmol kg-1] DIC

[µmol kg1]

TAlk/Sal

f (PW) (%)

Ωarag

AOS 1994 34.74 ±0.06 0.58 ±0.36 27.86 ±0.03 301 ±2 42 ±6 11.41 ±0.23 0.82 ±0.02 2291 ± 4 2145 ± 5 66.0 ± 0.1 0 ± 1 1.55 ± 0.06

SHEBA 1998 34.66 ± 0.02 0.27± 0.16 27.82 ± 0.01 295± 8 51± 9 11.49± 0.26 0.86± 0.03 2297± 7 2156± 8 66.3± 0.2 4 ± 2 1.51± 0.09

ODEN 2005 34.61 ± 0.07 0.63±0.19 27.86 ± 0.03 295 ± 5 52 ± 6 11.74 ± 0.39 0.84 ± 0.02 2288 ± 4 2158 ± 5 66.1± 0.1 0 ± 2 1.42 ± 0.06

CHINARE 2008 34.76± 0.04 0.63± 0.14 27.88± 0.02

2291± 6 2163 ± 11 65.9± 0.1 1.37± 0.10

2010 34.75±0.06 0.63 ± 0.19 27.87 ± 0.03 291 ± 9 52 ± 9 12.11 ± 0.44 0.86 ± 0.05 2291 ± 3 2163 ± 8 65.9± 0.1 -2± 4 1.38 ± 0.07

Supplementary References 1 Jones, E. P., Anderson, L. G., Jutterström, S., Mintrop, L., & Swift, J. H. Pacific

freshwater, river water and sea ice meltwater across Arctic Ocean basins: Results from the 2005 Beringia Expedition. J. Geophys. Res. 113, 179-185 (2008).

2 Yamamoto‐Kawai, M, Mclaughlin, F. A., Carmack, E. C., Nishino, S., & Shimada, K. Freshwater budget of the Canada Basin, Arctic Ocean, from salinity, δ18O, and nutrients. J. Geophys. Res. 113, doi:10.1029/2006jc003858 (2008).

3 Tschudi, M., Fowler, C., Maslanik, J., Stewart, J. S., & Meier,W. Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors, Version 3. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi:http://dx.doi.org/10.5067/O57VAIT2AYYY. (2016).

4 Giles, K. A., Laxon, S. W., Ridout, A. L., Wingham, D. J., & Bacon, S. Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre. Nat. Geosci. 5, 194-197 (2012).

5 Proshutinsky, A., Dukhovskoy, D., Timmermans, M. L., Krishfield, R., & Bamber, J. L. Arctic circulation regimes. Philos. Trans. A Math. Phys. Eng. Sci. 373, 10.1098/rsta.2014.0160 (2015).

6 Rabe, B. et al. Arctic Ocean basin liquid freshwater storage trend 1992-2012. Geophys. Res. Lett. 41, 961-968 (2014).

7 Stewart, K. D. & Haine, T. W. N. Wind-driven Arctic freshwater anomalies. Geophys. Res. Lett. 40, 6196-6201 (2013).

8 Mathis, J. T. et al. Storm-induced upwelling of high pCO2 waters onto the continental shelf of the western Arctic Ocean and implications for carbonate mineral saturation states. Geophys. Res. Lett. 39, doi:10.1029/2012gl051574 (2012).

9 McLaughlin, F. A. & Carmack, E. C. Deepening of the nutricline and chlorophyll maximum in the Canada Basin interior, 2003-2009. Geophys. Res. Lett. 37, doi:10.1029/2010gl045459 (2010).