Characterizing the Beaufort Gyre in the Canadian Basin of the from satellite observations between 2003-2014

Heather Regan, Camille Lique Laboratoire d’Océanographie Physique et Spatiale IFREMER, Brest, France Thomas Armitage JPL, CalTech, Pasadena, USA

Ocean Salinity Science – November 2018 Background: Arctic freshwater

• The Arctic Basin stores a large amount of freshwater (FW) • Most1.3 Water of the Masses storage and Circulation occurs in the Beaufort Gyre 7

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Sea Surface Salinity Salinity FW content (Freshwater ContentSref = 34.8) (m) from MIMOC climatology from MIMOC climatology Figure 1.3: (a) Sea surface salinity and (b) liquid freshwater content (FWc) from MIMOC. The freshwater content is calculated by vertically integrating the salinity anomaly (Equation 1.1) from the surface to the depth of the 34.8 psu isohaline. The black box in (b) marks the location of the Beaufort Gyre as defined by Proshutinsky et al. (2009) and Giles et al. (2012). It is the single largest region of freshwater storage in the Arctic and will be discussed in more detail in Chapter 2. The white contour in (a) marks the location of the 34.8 psu isohaline.

Dickson et al., 2007,andRawlins et al., 2010), with the most recent budget constructed by Haine et al. (2015). It must be noted, however, that whether the budget is now fully constrained, and whether any surplus/deficit in the supply/export of freshwater to/from the Arctic is significant is still a matter of debate, especially given that the observations

of the di↵erent components of the freshwater budget vary considerably both in quality and in the period of time over which they were taken. Here, an average freshwater budget is presented based on the most robust and up-to-date observations of each component. In order of importance, freshwater in the Arctic is sourced from river runo↵,inflow through Bering Strait (Figure 1.4), and excess precipitation over evaporation. Based on the observations of Shiklomanov (2010) and the ERA-Interim reanalysis product (Dee

3 1 et al., 2011), river runo↵ accounts for an average input of 3900 390 km yr . This rep- ± resents approximately 11% of the total global continental river runo↵ (Fichot et al., 2013), and is driven by the net precipitation that occurs over the Arctic catchment area due to

the convergence of water vapour in the atmosphere above 50N(Serreze et al., 2006). JAHN AND HOLLAND: MOC IMPACTS OF ARCTIC CHANGES

3.2. Arctic Freshwater Export [6] The large simulated changes in the Arctic extent and volume have aBackground: wide range of impacts,Arctic freshwater exports but here we concen- trate on the resulting changes in the Arctic FW export and the downstream impacts on the deep water formation (section 4). (a) As shown in Figures• 1eLarge–1g, the amount impact on theof simulated freshwater FW released to the N. Atlantic (ice + liquid) export to the NA through the two main pathways (Fram Strait and CAA) is a shift• towardEffect more on and SSS more downstream, FW export in the and potentially deepJAHN convection AND HOLLAND: MOC and IMPACTS AMOC OF ARCTIC CHANGES liquid phase, as well as a general increase in the total FW he export. The simulated phase change of the FW export is (a) (e) important for downstream effects, as the FW in liquid and solid form reaches different regions (see section 4). (b) CO2 increases [7] The large increase in the liquid FW export in RCP8.5 reflects a significant fresheningJAHN of the AND out HOLLAND:flow between MOC the IMPACTS OF ARCTIC CHANGES early 21st century and the early 23rd century (by 2.5 psu eshwater System with the major ocean 3.2.in Fram Arctic Strait Freshwater and 4.5 psu Export in the CAA). Between 2005 and 2200, this freshening of the Arctic Ocean (Figure 1h) is [6] The large simulated changes in the Arctic sea ice extent (b) (f) andcaused volume by have increased a wide sea range ice of melt impacts, within but here the we Arctic concen- and b FW export tratereduced on the sea-ice resulting formation changes and in the export Arctic (53%), FW export as well and as the by (c) ays in Fram Strait, the Barents Sea Opening, increases increased FW input from rivers (28%), an increase in the downstream impacts on the deep water formation (section 4). (a) FW import through Bering Strait (16%), and increased net denoting inflow and blue denoting outflow), the As shown in Figures 1e–1g, the impact on the simulated FW precipitation (2%) due to a generally enhanced hydrological he All Arctic Regions (ARR) definition of the export to the NA through the two main pathways (Fram Strait (B) summary of components of the high-latitude cycle in a warmer . and CAA) is a shift toward more and more FW export in the [8] While the salinity of the outflow decreases, the volume [this issue]. Here moisture is transported from t 89 liquid phase, as well as a general increase in the total FW fic Ocean via the Trade Winds over Central of the liquid FW export through the CAA and Fram Strait in parates the thermally stratified subarctic oceans export. The simulated phase change of the FW export is SSS and MLD RCP8.5 declines by up to 50% during the late 21st and the (c) an (lighter blue) in both(g) the Atlantic and Pacific important for downstream effects, as the FW in liquid and decreases mid 22nd century, respectively. For the CAA, the impact (d) nd Atlantic oceans to the Arctic catchment basins solid form reaches different regions (see section 4). (b) Prowse et al. k blue arrows), which subsequently drains into of the decrease in the volume export can be seen around ads initially within the Riverine Coastal Domain 2100[7] The as an large intermittent increase reductionin the liquid in the FW RCP8.5 export in liquid RCP8.5 FW reflects a significant freshening of the outflow between the waters (thin red arrows) enter the Arctic Ocean export in Figure 1g. It is caused by the shutdown of the deep rough the Barents Sea Opening (the Barents Sea early 21st century and the early 23rd century (by 2.5 psu convection in the Labrador SeaSurface circulation at this time (see section 4) Schematic Maps of: (A) the domain of the Arctic Fr in Fram Strait and 4.5 psu in the CAA). Between 2005 and MOC decreasesubsurface,ridge cyclonic, system. topographically-steered Internally modified Atlantic and the associated rise in sea surface height in the Labrador Strait along eastern Greenland. Cooler and 2200,Sea, which this freshening reduces the of sea the surface Arctic Oceanheight gradient (Figure 1h) to the is caused by increased sea ice melt within the Arctica and (d) (h) Arctic that drives this export (see Jahn et al. [2010] and Jahn & Holland ter the Arctic Ocean through Bering Strait, and reduced sea-ice formation and export (53%), as wellFigure as 2. by (e) am Strait along eastern Greenland. Within the Houssais and Herbaut [2011]Carmack for detailset al. (2016) of this mechanism).currents (long arrows), the four Arctic(c) Ocean gatew (2013) increasedThis decrease FW in input the CAAfrom riversliquid FW (28%), export anleads increase to a in sharp the een inflow, interior and outflow shelves [cf. FW import through Bering Strait (16%), and3099 increased netDavis Strait and Bering Strait (think bars with red increase in the Arctic liquid FW storage after 21003100 (Figure 1h). gyral circulation patterns (circular arrows), and t precipitationFor Fram Strait, (2%) the due volume to a generallyflux decrease enhanced is fully hydrologicalcompensated terrestrial contributing areas shown in white; and cycle in a warmer climate. 3101 by the freshening of the outflow, so it cannot be seen3102 in freshwater system as introduced in 2015]. 8 fl Figure[ ] While 1e. It the occurs salinity around of the 2145, out ow after decreases, the shutdown the volume of the3103 subtropical and tropical to the Paci of the liquid FW export through the CAA and Fram Strait in 3104 America (thick blue arrow). The subarctic front se deep convection in the Nordic Seas, and is caused by the asso- Figure 1. Time series of the (a) annual CO forcing in the CMIP5 RCP simulations, the Arctic sea ice extent in (b) (darker blue) from the salt-stratified Northern2 Oce RCP8.5ciated sea declines surface by height up to 50% gradient during change the late across 21st the and East the 3105 September(f) and (c) March, the annual Fram Strait (e) liquid and (f) solid FW export, (g) the annual CAA liquid and solid midGreenland 22nd century, current afterrespectively. deep convection For the CAA, ceases the (see impactJahn 3106FW export(d) (as markedoceans. in Moisture the panel), is transported and (h) from the the annual Pacific meana Arctic FW storage (liquid FWBluhm in the et topal., 250 m and solid by the mid-latitude (Westerlies) storm tracks (thic ofet theal. [2010], decrease for in details the volume of this mechanism). export can be seen around FW,3107 as marked in the panel). Figure 1d shows the 20 year running mean of the sea ice extent from Figures 1b and 1c versus the increasing3108 CO2 forcing inthe one Arctic ensemble Ocean (thick member white arrows) of the where RCP it simulations.spre The 20 year running mean is used to smooth 2100 as an intermittent reduction in the RCP8.5 liquid FW out the interannual variability, which we do not expect to be forced by CO . As we want to show 2006; the transient response of the export in Figure 1g. It is caused by the shutdown of the deep Figure 2. Wintertime3109 (February(dashed–April) green arrows). depth Warm, salty of Atlantic-origin (a–d) 2 sea ice extent to the CO2 increase, Figurethrough 1d Fram shows Strait the (the relationship Fram Strait Branch) until the and CO th 2 forcing stabilizes or reaches its maximum (see Figure 1a).3110 All panels except Figures 1a and 1d show 5 year running means and show one ensemble member as thick convection4. Impacts in the on Labrador Deep Convection Sea at this and time MOC (see section 4) maximum deep convection and (e–f) AtlanticBranch) and circulate MOC within index. the Arctic basins as s and the associated rise in sea surface height in the Labrador The spatialline, with distribution shading indicating3111 of maximum the ensemble spread deep between convection 1850 and is 2100, when multiple members are available. 4.1. Deep Convection Impacts 3112 boundary currents along the continental margin and Sea, which reduces the sea surface height gradient to the shown in Figure 2a, averaged3113 over 1981–waters2005 exit inthe Arctic one Ocean southward through Fram [9] The large increase in the liquid FW export from the ensemble member. The evolution over time (smoothedfresher Pacific-origin by waters (thin blue arrows) en Arctic that drives this export (see Jahn et al. [2010] and all practical(e) purposes, with the remaining3114 sea ice located north response is seen for all RCPs when assessing changes as func- of Greenland and in the narrow channels of the CAA. Winter- tion of the increasing CO level. Once the CO levels stabilize HoussaisArctic at and the Herbaut end of the[2011] 21st for century details in of RCP8.5 this mechanism). leads to a a 5 year running mean) averaged over3115 each of the threeexit main through the Canadian Arctic Archipelago2 and Fr 2 time sea ice extents also decline over the 21st century in all orArctic begin Ocean to decline, a topological the distinction climate beginsis made betw to adjust to the new Thisshutdown decrease of February in the CAA–April liquid (FMA) FW exportdeep convection leads to a in sharp the deep convectionRCP scenarios, regions but only in RCP8.5 the model with its (shown increasing3116 as radiative white boxesstable conditions and departs from the transient climate state 3117 Carmack and Wassmann, increaseLabrador in the Sea Arctic deep liquid convection FW storage region after at 2100 the start(Figure of 1h). the in Figureforcing 2a) past is 2150 shown shows in a drasticFigures decline 2b– in2d. the The winter continuous sea proportional to the CO2 level. As our focus is mainly on the For22nd Fram century Strait, (shown the volume as whiteflux decrease box in Figure is fully 2a, compensated maximum ensembleice extent, member leading for to 1850year-round–2300 ice-free isshown conditions as3118 by thick the transient line. response of the climate to the increasing CO2 forcing depth of deep convection is shown in Figure 2b). Before a end of the 23rd century (Figure 1c). We find that the climatic3119 in the CMIP5 scenarios, we focus on results from the RCP8.5 by the freshening of the outflow, so it cannot be seen in Shadingevolution for years of sea 1850 ice and–2100 many shows other properties the range are largely of thescenario other runs in the following, as these simulations prescribe 3120 Figurecomplete 1e. shutdown It occurs aroundof Labrador 2145, Sea after deep the convection shutdown occurs of the five ensembledetermined members.by the CO2 level, The not 5 the year trajectory running by which mean that ofCO the2 levels that encompass and far exceed the CO2 levels in deepin RCP8.5, convection several in the abrupt Nordic reductions Seas, and is of caused deep by convection the asso- Atlanticlevel MOC was reached. index As [SV] such, is a similar shown transient in Figure climate 2e.the other In scenarios. (f) ciatedoccur seain all surface six ensemble height members gradient changeof RCP8.5 across at the the end East of Figure 2f, the 20 year running mean of the Atlantic1207 MOC Greenlandthe 21st century. current They after occur deep convection in response ceases to decreased (see Jahn sea index is shown as function of the CO2 forcing used in the etsurface al. [2010], salinity for details (SSS) ofin this the mechanism). same region (see Figures 3b different RCPs (see Figure 1a), for one ensemble member and 3c), with a correlation between the maximum depth of for each of the four extended RCPs. As in Figure 1d, we deep convection and the SSS in the same region of between Figureonly show 2. theWintertime relationship (February until the–April) CO2 forcing depth stabilizes of (a–d) 4.0.90 Impacts and 0.95 on in the Deep different Convection ensemble and members MOC (p > 0.95, maximumor reaches itsdeep maximum. convection and (e–f) Atlantic MOC index. The spatial distribution of maximum deep convection is 4.1. Deep Convection Impacts 1208shown in Figure 2a, averaged over 1981–2005 in one [9] The large increase in the liquid FW export from the ensemble member. The evolution over time (smoothed by Arctic at the end of the 21st century in RCP8.5 leads to a a 5 year running mean) averaged over each of the three main shutdown of February–April (FMA) deep convection in the deep convection regions in the model (shown as white boxes Labrador Sea deep convection region at the start of the in Figure 2a) is shown in Figures 2b–2d. The continuous 22nd century (shown as white box in Figure 2a, maximum ensemble member for 1850–2300 is shown as thick line. depth of deep convection is shown in Figure 2b). Before a Shading for years 1850–2100 shows the range of the other complete shutdown of Labrador Sea deep convection occurs five ensemble members. The 5 year running mean of the in RCP8.5, several abrupt reductions of deep convection Atlantic MOC index [SV] is shown in Figure 2e. In occur in all six ensemble members of RCP8.5 at the end of Figure 2f, the 20 year running mean of the Atlantic MOC the 21st century. They occur in response to decreased sea index is shown as function of the CO2 forcing used in the surface salinity (SSS) in the same region (see Figures 3b different RCPs (see Figure 1a), for one ensemble member and 3c), with a correlation between the maximum depth of for each of the four extended RCPs. As in Figure 1d, we deep convection and the SSS in the same region of between only show the relationship until the CO2 forcing stabilizes 0.90 and 0.95 in the different ensemble members (p > 0.95, or reaches its maximum.

1208 Background: Recent accumulation of FW in the Beaufort Gyre

• In situ measurements show that FW has accumulated since 2003

Freshwater Content = ∫ �� ()

141 FWC. Numbers at the bottom of each panel indicate total FWC in the region (103 km3) and root mean 142 square error FWC (103 km3). (with Sref = 34.8)

143 138 Andrey Proshutinsky (personal comm.) - BGEP dataset 144 Figure 3 Time-series of BGR FWC volume (103 km3) in different layers defined by salinity (S) ranges: 139 Figure 2 FWC in the BGR based on hydrographic measurements conducted in August-October.145 Colors deep layer (S > 33.8, yellow bars), middle layer (30 < S < 33.8, cyan bars), and surface layer (S < 30, 140 show FWC (m) and black dots indicate locations of observational sites. White lines duplicate contours146 orangeof bars). Note that the bars are not stacked, but that the green bars represent total FWC. Error bars 147 depict uncertainties in FWC estimates. Linear trends (km3a-1) are shown by dotted lines. The left-most 148 wide bars are FWC volume derived from summer climatology 1950-1989 (see Timokhov and Tanis, 7 149 1998). Trend errors are estimated at 95% confidence interval.

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Background: Functioning the Beaufort Gyre (1) o W 180

• The anticyclonic gyre is forced on average by the

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o • Momentum input from the wind is strongly W modulated by sea ice (e.g Martin et al. 2016)

o • Taking into account the surface current when 0 60 o W computing the surface stress modulates (or even 1 1.005 1.010 1.015 1.020 1.025 1 1.005 1.01 1.015 1.02 1.025 cancels out) the Ekman pumping (a.k.a the Ice- Mean SLP (from ERA40) ocean governor, Menegehello et al. 2018)

Meneghello et al. (2018) 1NOVEMBER 2014 D A V I S E T A L . 8179

section we will discuss some of the broader implications of our results and their relevance to the wider Arctic system.

a. Adjustment timescales The results of both the modified seasonality experi- ments and the modified net forcing experiments shed some light on the time scales over which the Arctic Ocean adjusts to a change in forcing, as well as on which adjustment processes are responsible. Figures 6b and 6e show that over each season the layer thickness in the center of the domain continues to increase or decrease in response to the change in ocean surface stress, and never fully adjusts to a new equilibrium state. In con- trast, Figs. 6c and 6f suggest that the magnitude of the transport through the channel adjusts more quickly to the seasonal changes in ocean surface stress, asymptot- ing toward a new equilibrium value over the period of each season. Consequently, it appears the processes that dominate the adjustment to a change in forcing around the boundary of the domain are very different from those that determine the adjustment in the center. The response time scale (e-folding time scale) of the trans- port through the channel is approximately 1 month, which is in good agreement with the time required for a boundary trapped Kelvin wave to propagate around

the domain at a speed given by g0h. This suggests that FIG. 7. Results of the modified net forcing experiments. (a) The boundary waves are the dominant process behind the pffiffiffiffiffiffiffi increase in mean layer thickness in the center of the domain plotted fast adjustment on the boundary. Waves propagating as a function of the increase in net forcing. The blue dot represents out of the sponge region (where we apply a strong re- the control run, the red dots represent model runs with an in- storing to the initial layer thickness of 400 m) effectively creased ocean surface stress in summer, and the green dots rep- resent model runs with a longer melt season (as per the colors in fix the layer thickness around the boundary in our model Figs. 5c and 5d). (b) The cumulative volume and freshwater on time scales longer than 1 month. transports into the domain over the full 40 years ofBackground: each model runFunctioning the Beaufort Gyre (2)Away from the boundaries, however, Kelvin waves as a function of the increase in net forcing. have little effect. Here, the layer thickness as a function • The leading orderof balance time is has determined been determined by the from balance theory betweenand simple wind- process and 2012, the Arctic-wide annual mean oceanmodels surface induced Ekman pumping and the -induced volume stress has increased by 0.007 N m22, which corresponds to • Main balance betweenflux toward Ekman the convergence boundary and [i.e., eddy Eq.-induced(8)]: volume flux: an accumulation of freshwater in our model of ap- proximately 3500 km3. This is around 50% of the in- › 1 t(l) 1 h dA 52 dA 2 k(h 2 h ) dl, (12) crease observed by Giles et al. (2012) over broadly the ›t r r f r c b ðð ðð 0 þ same period. Ekman (from wind and ice, Eddies where themodulated by surface current) first term on the right-hand side represents wind-induced Ekman pumping and the second term rep- 6. Discussion resents the relaxation effect of the eddy flux.Scale analysis suggests Scale analysis T adjustment timescale Using a simple process model and idealized perturba-Ekman suggests that the relaxation timeTEkman scale due to the eddy term due to eddies is tions to the ocean surface stress, we have demonstrated is given by rABG/klBG,whereABG and lBG are the area that the dynamical response of the Beaufort Gyre to the and circumference of the Beaufort Gyre respectively,rA r2 and ⌧ ~ 10 years decline in Arctic sea ice cover, and the related increase in Teddiesis approximatelyh equalTeddies to 14 yr. As a result,⇠  we can⇠  con- the efficiency of momentum transfer into the upper clude that eddies play a limited role in balancing Ekman ocean, is a plausible mechanism behind the accelerated pumping on the seasonal time scale. Consequently, the accumulation of freshwater inferred by Giles• etCaveats al. (2012): no. bathymetry,layer thickness constant in and the symmetrical centerofthedomaincontinually forcing, the Arctic is not just a Despite the simple nature of our process model,fresh insurface this layeradjusts… to the change in ocean surface stress over each Davis et al. 2014, Lique et al. 2015, Manucharyan & Spall 2016, Meneghello et al. 2018 Objectives & Methods

Objectives • Characterization of the time and space variability of the Beaufort Gyre

• Roles of the forcing and bathymetry

Datasets DOT average in 2003

• Dynamic Ocean Topography (DOT) produced from satellite altimetry retrieved through sea ice leads Armitage et al., 2016 -> spans 2003-2014, monthly means -> Resolution: 0.25° x 0.75°

• Sea ice concentration from NSIDC

• SLP and winds from ERA Interim A short aside: SSH (or DOT) and FW

• In the (cold) Arctic, the density is determined by salinity • Steric height is thus a indication of FW content StH = DOT – OBP • Most of the FW variationsLETTERS can be reconstructed from DOT only NATURE GEOSCIENCE DOI: 10.1038/NGEO1379

8,000 show a freshening of the Nordic Seas and Subpolar Basins during Change in freshwater content (using GRACE data where available) 16,19

) Change in freshwater content (not including GRACE data) this period . Our results indicate an increase in the transfer of 3 6,000 momentum between the atmosphere and the ocean after 2002, which could enhance the spin-up and spin-down of the Arctic 4,000 Ocean. Although the increase in fresh water might increase the vertical stratification of the water column in the Beaufort Gyre, we 2,000 120° E

160 E note too that increased spin-up of the Arctic Ocean might, through ° 8 0° E 0 increased turbulence, enhance the vertical transport of heat from W °

160 warm, deeper Atlantic-sourced waters to the cold upper ocean and

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¬2,000 4 12 lead to a reduction in winter ice growth, creating an additional 0° W positive feedback to the ice-albedo effect as the ice cover retreats. ° ¬4,000 0

Change in freshwater content Change in freshwater (km 8 0° W 40° W ¬6,000 Methods 1995 1997 1999 2001 2003 2005 2007 2009 2011 Sea surface height. Although SSH is measured by radar altimeters over the world Year ocean, different processing techniques must be used over ice. ERS-2 provided the first map of Arctic SSH variability20 and the ICESat laser altimeter provided the Figure 3 Change in Western Arctic freshwater contentGiles et al. (2012) 1995–2010. The Arctic dynamic topography for February/March, 2004–2008 (ref. 21). Our method | uses the fact that the radar observes specular echoes over leads and diffuse echoes asterisks show the change in the freshwater content if the GRACE data are over ice20. The Supplementary Information describes the process of calculating not used in the calculation. Error bars are the 1 uncertainty. Map inset elevations from echoes, and the calibration between data from leads and ocean, shows the Western Arctic region, marked by the grey area. and data from ERS-2 and Envisat. The monthly average SSH was calculated by subtracting the EGM08 directly over leads, and deforms and moves the sea-ice, which drives geoid (Pavlis and colleagues, unpublished data) from the elevation data and filtering to remove outliers; data were then averaged on a 200 km grid. For the water beneath. Buoy observations show a large ice deformation each grid cell, we averaged the monthly data to calculate the mean sea surface rate in summer 2007 compared with previous summers (1979– (MSS; Fig. 1a) and the SSH variability (Fig. 2) was calculated by computing 2006), suggesting that the mechanical strength of the ice decreased, annual MSS (September–August the following year) and subtracting the total making it easier to move9. An increased ice drift speed has also been MSS. The trend in the SSH was calculated using LINFIT (IDL), which fits data to the model, y a bx, by minimizing the chi-square error statistic observed from 2004 onwards, which cannot be fully explained by = + 10 (http://star.pst.qub.ac.uk/idl/LINFIT.html). It is possible that during June, July changes in wind speed . Arctic sea-ice extent and thickness are and August, elevation estimates might include measurements from melt ponds, declining11–13 and this decrease in ice thickness is a likely cause of which would bias our elevations high. However, excluding these months from the increase in ice deformation rate and drift speed9,10. Increasing our data biases our trend high (by 20%), as the annual SSH cycle is not uniform. ice deformation also results in more leads9 and ridges, increasing The fact that this bias is positive demonstrates that increasing melt pond fraction the area of vertical surfaces the wind can blow against, which cannot contribute to the trend. 14 The uncertainty in the SSH is due to measurement, orbit, tidal, instrument increases the momentum transfer to the sea-ice . The atmospheric noise and atmospheric propagation error, along with the uncertainties in correcting momentum flux is also influenced by the turbulent fluxes of for the biases between the two satellites and between measurements from the ocean sensible and latent heat from the surface15, which depend on the and leads (see Supplementary Information). presence/thickness of the sea-ice. These potential influences on the Wind field curl. transfer of momentum between the atmosphere and the ocean @ (v u ) @ (u u ) might also explain why we see more interannual variability in the u u | | | | ˆz (1) r⇥ | | = @x @y wind field curl than the SSH between 2002 and 2010. ✓ ◆ Our results provide a basin-wide, time-continuous view of where u p(u2 v2) and u and v are surface zonal and meridional winds from | |= + changes to the SSH, revealing an increase in the freshwater content NCEP/NCAR Reanalysis data22 (National Oceanic & Atmospheric Administration, between 1995 and 2010 of 8,000 2,000 km3 over the Western Arc- Earth System Research Laboratory, Physical Sciences Division (NOAA/OAR/ESRL ± 3 PSD), Boulder, Colorado, USA, http://www.esrl.noaa.gov/psd/). Monthly averages tic (similar to the 8,500 and 8,400 2,000 km observed from in situ were calculated on a 200 km grid. For each grid cell, the total mean curl was 4,5 ± measurements ). The geostrophic velocity is almost three times calculated by averaging all months of data and the annual anomaly was computed greater in 2010, compared with the 1990s, and the spatial pattern by subtracting the total mean curl from annual means of the monthly data. . We estimate an uncertainty of 10% in equation (1) from comparison with in of the trend in SSH is correlated (r 0 9) to the spatial pattern of 23,24 the trend in the wind field curl, providing= observational evidence situ validation of wind speed estimates (see Supplementary Information). that has driven the storage of freshwater in the Geostrophic velocity. The geostrophic balance is25 Beaufort Gyre between 1995 and 2010. Our results also provide a @⌘ @⌘ detailed picture of the year-to-year variability in the SSH and wind fu g fv g (2) field curl and suggest that other factors beyond simply the change = @y = @x in the wind might contribute to the spin-up of the Beaufort Gyre. where f is the Coriolis parameter, g is the acceleration due to gravity, ⌘ is the SSH Although these data only address changes in the Western Arctic, and u and v are the geostrophic velocities. We assume the geostrophic balance it is striking that our calculated increase of freshwater is similar is the same in all directions and calculate the velocity in the x-direction. For the 3 MSS for 1996–2002, we take the difference between the gyre’s maximum SSH to the approximately 10,000 km of freshwater that entered the (74 N,145 W) and the SSH at the edge ( 70 N), on the same meridian, to find 16 ⇠ 1 Nordic Seas from the Arctic during the late 1960s and early 1970s, @⌘/@x. Substituting into equation (2) gives v 1.90 0.1 cm s . The position causing the Great Salinity Anomaly17, influencing the production of the maximum SSH is the same in the MSS= for 2002–2010± as in 1996–2002, of Labrador Sea Water, which becomes upper North Atlantic Deep therefore we calculated the gradient in the trend between 2002 and 2010 between 18 the same points defined above, to compute the change in the geostrophic velocity Water . Our results suggest that a reversal of the wind field to per year. Multiplying by eight (the number of years in the latter half of our 1 more cyclonic conditions would result in the spin-down of the time period) gives an increase in the geostrophic velocity of 3.60 0.03 cm s . Beaufort Gyre and the consequent release of this freshwater into Adding this to the geostrophic velocity during the first half of the± time period 1 gives 5.50 0.1 cm s . the rest of the Arctic Ocean and/or its exchange with adjacent ± oceans. Indeed, when we extend the wind field curl anomaly over The uncertainty in the velocity is estimated by propagating the uncertainty in SSH through equation (2) (see Supplementary Information). the Western Arctic back in time (not shown here) it reveals that the became increasingly cyclonic between the Freshwater volume change. To calculate the freshwater volume change (1FW ) mid-1980s and the mid-1990s, and hydrographic observations also we represent each grid cell by a column of water composed of two homogeneous

196 NATURE GEOSCIENCE VOL 5 MARCH 2012 www.nature.com/naturegeoscience | | | Variability

-> Gyre defined as the largest closed contour around the max DOT

-> large seasonal, interannual variability + trend! Variability

-> The position of the gyre’s center shifts to Northwest over 2003-2014 … gets closer and then breaches over the Chukchi plateau Longitude Latitude DOT Max /

Min -> The variations of size and intensity of the gyre are not correlated! area. Gyre Atmospheric forcing 2003 2004 2005

-> the position and size of the gyre is partly determined by the forcing

-> Depends on both the intensity 2006 2007 2008 and the position of the atmospheric forcing (BSH)

+ memory of the forcing over a decade or more 2009 2010 2011 (Johnson et al. 2018)

2012 2013 2014

SLP - max: Max DOT: Regan, Lique and Armitage (in review for JGR) Interactions with bathymetry

-> the position and size of the gyre are partly determined by the forcing

-> Depends on both the intensity and the position of the atmospheric forcing (BSH)

-> DOT gradient gets steeper when the gyre encounters the slope/shelf.

Gradient of DOT [x 106 m.m-1] Regan, Lique and Armitage (in review for JGR) Interactions with bathymetry

DOT [m] Gradient of DOT [x 106 m.m-1] N Along 74 °

Chukchi plateau N

Along 76 ° Gradient of SSH [x 106 m.m-1] Mendeleev Ridge Regan, Lique and Armitage (in review for JGR) Confidential manuscript submitted to JGR-Oceans

Implications for the monitoring of FW changes

Trend of DOT (2003-2014)

141 FWC. Numbers at the bottom of each panel indicate total FWC in the region (103 km3) and root mean 142 square error FWC (103 km3). Trend of FW content

143

144 Figure 3 Time-series of BGR FWC volume (103 km3) in different layers defined by salinity (S) ranges: 145 deep layer (S > 33.8, yellow bars), middle layer (30 < S < 33.8, cyan bars), and surface layer (S < 30,

959 Figure 7. Map showing the linear trend (metres/year) of the DOT146 timeseriesorange bars). at Note each that location the bars are not stacked, but that the green bars represent total FWC. Error bars 147 depict uncertainties in FWC estimates. Linear trends (km3a-1) are shown by dotted lines. The left-most 960 from 2003-2014. Timeseries of the DOT where the maximum trend occurs (black dot, over the 148 wide bars are FWC volume derived from summer climatology 1950-1989 (see Timokhov and Tanis,

961 Northwind Ridge) is shown below. The maximum trend found by 149Giles 1998). et al. Trend[2012] errors is shownare estimated by at 95% confidence interval.

962 a cyan dot. White contours indicate 0.005 m/yr intervals, and areas150which are not significant at

963 the 95% level are hatched. The minimum and maximum gyre extent (in August 2004 and March 8

964 2013 respectively) are shown in green.

–37– Confidential manuscript submitted to JGR-Oceans

25 contributions from Japanese, Korean and Chinese expeditions to the western gyre. These data

Confidential manuscript submitted26 provide to JGR-Oceans an unprecedented view of the summertime structure and inter-annual variability of the 27 Beaufort Gyre atmospheric, sea ice, thermohaline stratification and geochemical characteristics. 28 Over our 15 years of data collection, the globally-significant freshwater content of the gyre 29 increased by 40% due to: Ekman convergence of the low salinity surface waters, from unusually- 30 persistent, anticyclonic, wind- and ice-driven surface-stress, and extensive ice melt. The long 31 time series now allows an empirical estimation of the end-of summer freshwater content as a Implications for the monitoring of FW changes 32 function of the annual Ekman convergence and summertime melt. Inter-annual variation in the 33 gyre can thus be viewed as a result of these two simple surface processes. -> which characteristics of the gyre captures best the FW content variations ? 34

35 1 Introduction

36 The Beaufort Gyre Observational System (BGOS, Figure 1a) initiated measurements and 37 analysis of atmospheric, sea ice, oceanic and geochemical parameters in the Beaufort Gyre 38 Region (BGR) of the Arctic Ocean in 2003 (Proshutinsky et al., 2002, 2009a,b, 2015; Krishfield 39 et al., 2014). Over the subsequent years, an unprecedented dataset has been amassed 40 (http://www.whoi.edu/website/beaufortgyre/data) that constitutes a 15-year time-series of BGR 41 characteristics with observations at more than 30 “standard” locations (Figure 1a). Analysis of 42 these data quantifiesBGEP standard changes in the BGR system under conditions of global warming. stations and moorings

43

44 Figure 1 a) BGOS assets configuration with A-D mooring locations depicted as stars, and sites of 45 “standard” CTD stations where observations of sea ice and water physical and geochemical parameters 983 Figure 11. Timeseries showing a) maximum DOT, b) mean DOT within the gyre, c) strength 46 have been conducted over the period 2003-2017; b) Time-averaged summer freshwater content (relative 6 984 (x 10 ) and d) area of the gyre, along with their 12-month running means (blue). Maximum 2

985 freshwater (FW) content during the summer of each year (BGEP; Proshutinsky et al. [2009],

986 updated each year) is shown in green, while total summer freshwater content within the BG box

987 (Figure 1, black box; Proshutinsky et al. [2009]) is show in red. Note that the freshwater content

988 lines are plotted in the August of each year.

–41–

983 Figure 11. Timeseries showing a) maximum DOT, b) mean DOT within the gyre, c) strength

6 984 (x 10 ) and d) area of the gyre, along with their 12-month running means (blue). Maximum

985 freshwater (FW) content during the summer of each year (BGEP; Proshutinsky et al. [2009],

986 updated each year) is shown in green, while total summer freshwater content within the BG box

987 (Figure 1, black box; Proshutinsky et al. [2009]) is show in red. Note that the freshwater content

988 lines are plotted in the August of each year.

–41– Summary

• Freshwater content variations in the Arctic can be monitored using satellite altimetry • Arctic freshwater is mostly stored in the Beaufort Gyre ... and the freshwater content has largely increased over the past 20 years

• Over 2003-2014, the Beaufort Gyre has: ü Increased its size almost linearly ü Intensified (max DOT) after 2008 ü Shifted Northwest • The intensity is mostly sensitive to forcing strength, while gyre area is also sensitive to Beaufort Sea High location • Shallow bathymetry of Chukchi Plateau restricts westward expansion, resulting in asymmetry and potential changes to gyre dynamics

• Regan, Lique & Armitage (in rev for JGR): The Beaufort Gyre extent, shape, and location between 2003 and 2014 from satellite observations • DOT dataset available CPOM - UCL(www.cpom.ucl.ac.uk/dynamic topography )