Arctic Ocean Freshwater: past, present, & future Michael Steele Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle WA

P-E “Global” sea surface salinity ? 40 35 30 25 A: Uhhh… ice melting?! Ferrel Cell “westerlies” (N. Hem.) P-E Q: Why is the Arctic Ocean fresh? A: Uhhh… ice melting?!

Mackenzie River discharge” Mackenzie River discharge” SHEBA: Beaufort Sea, 1997-98 SHEBA 1997 JAS SAT anom (NCDC) Freshening of the upper ocean in the Arctic: Is perennial sea ice disappearing? McPhee et al. (1998) “It was mostly sea ice melt” October, 1997 Connections among ice, runoff and atmospheric forcing in the Beaufort Gyre Macdonald et al. (1999) “It was mostly Mackenzie River discharge” Steele et al., 2006; Rawlins et al., 2009. “It was mostly Mackenzie River discharge”

Arctic Ocean freshwater inputs …adapted from A. Jahn, Jan’09 AOMIP FW flux (km3/year) Woodgate et al., 2005 Russian ice camp data FW flux (km3/year) Total = 7700 Serreze et al., 2006 For the Arctic Ocean, net sea ice GROWTH is a FW SINK  0.4 m/yr  7 million km2 = 2800 km3/yr “river runoff → sea ice growth” River Runoff Bering Strait liquid P-E Total = 5870 Q: Where’s the ice term? Arctic Ocean GIN Sea Steele & Flato (2000) Aagaard and Carmack, 1989

Mackenzie River discharge” SHEBA: Beaufort Sea, 1997-98 SHEBA 1997 JAS SA anom (NCDC) Distance north from Alaska (km) River water Melt water net melt net growth Extends to 150-200 m depth! ...but mostly 0-50 m (mixed layer) October, 1997 halocline surface layer Atlantic Water layer Surface + halocline layers  FW layer: 0 - 250 m S  34.8 Connections among ice, runoff and atmospheric forcing in the Beaufort Gyre Macdonald et al. (1999) “It was mostly Mackenzie River discharge”

Getting the right stratification: The effect of “background mixing” fresh layer Kdiff = 0.01 cm2/s PHC (Canada Basin) Kdiff = 1.25 cm2/s Depth (meters) Atlantic Water layer Zhang & Steele, 2006 Salinity “Typical” Kdiff  0.1 cm2/s; Need to reduce mixing to levels 10 times less! …it’s ok: sea ice suppresses mixing! (D’Asaro & Morison, 1992) …for now, anyway…

What is Freshwater (FW)? …adapted from A. Jahn, Jan’08 CCSM Sref = 34.8 (Aagaard & Carmack’s mean salinity of the Arctic Ocean)

What is Freshwater Content (FWC)? FWC   (Sref – S)/Sref dz surface layer #1 Sref Units: meters layer #2 zref How deep to integrate to? …to where S ~ constant? What about “negative freshwater?” …depends on your focus S(z) bottom

FWC: How to integrate?  (Sref – S)/Sref dz Sref surface  (Sref – S)/Sref dz layer #1 Sref Option 1: Integrate down to the bottom, allowing “negative freshwater” layer #2 …using PHC annual mean zref 50 40 30 20 10 FWC (m) S(z) Chukchi Borderland Canadian Basin Eurasian Basin bottom Sref The result: FW + bathymetry!

FWC: How to integrate?  (Sref – S)/Sref dz Sref zref surface bottom S(z) freshwater layer Atlantic Water layer  (Sref – S)/Sref dz Option 2: Integrate down to a fixed depth zref, allowing “negative freshwater” 50 40 30 20 10 FWC (m) Zref = 500 m Zref = 1000 m freshwater layer + Atlantic Water layer

FWC: How to integrate?  (Sref – S)/Sref dz Sref zref surface  (Sref – S)/Sref dz freshwater layer Sref Atlantic Water layer zref Option 3: Integrate down to Sref (no neg FW) S(z) 50 40 30 20 10 FWC (m) bottom freshwater layer only: Salinity Thickness

FWC: Seasonal salinity vs. thickness changes …uses option #3 (no neg FW) (JGR, 2009) There’s 2 seasonal maxima: June/July: FWC peak #1 from ice melt  upper layer freshening Nov/Dec/Jan: FWC peak #2 from Ekman convergence  upper layer deepening

FWC: Sref vs. zref Sref zref S(z) 1970-1990s: incr. AW inflow surface FW layer FWC > 0 Sref AW layer FWC < 0 zref S(z) FW volume (km3) Sref level rises depth (m) 1970-1990s: incr. AW inflow more “neg FW” Sref level rises incr. FW outflow less “pos FW”  Zref method yields a higher amplitude FW volume (km3) Polyakov et al., 2008

The Mean FW Budget …Serreze et al. (2006) Atmos. moisture transport is the major source of FW! ? sea surface salinity

The Mean FW Budget FWC (m) ?? 50 40 30 20 10 FWC (m) Atmos. moisture transport is the major source of FW! …ie, storms  arctic drainage basins rivers  ocean ( ocean  sea ice ) ocean/sea ice  North Atl. Ocean ??

Well, no… “The Arctic is the freshest ocean!” FWC (m) 45 m! 15-20 m Sea surface salinity Well, no… 50 40 30 20 10 FWC (m) 45 m! FWC  factor * steric height Bering Strait flow DH: Pac-Nordic = over a meter 15-20 m

The Mean FW Budget In this budget, river discharge > ice growth/export i.e., ~23% of discharge  liquid ocean export Distance north from Alaska (km) River water Remember the SHEBA data… Serreze et al. (2006)

The Mean FW Budget River discharge mean is big! …Serreze et al. (2006) River discharge mean is big! River discharge variability is small!

Interannual Variability … & now for some … Interannual Variability

Fall 2003: Hey Mike: “You’re invited to NASA HQ for a press conference “Arctic sea ice melting at worrying rate: NASA” Posted: Oct. 24, 2003 Agence France-Presse (Papers by Comiso, others) “Increasing River Discharge to the Arctic Ocean” Peterson et al., Science, Dec 2002 …Russian! Canadian decr, but total still incr “20th Century Trends of Arctic Precipitation from Observational Data & a Climate Model Simulation” Kattsov & Walsh, J. Climate, 2000 ….increasing “Interannual Changes in the Bering Strait Fluxes of Volume, Heat and Freshwater between 1991 and 2004” Woodgate et al., GRL, 2006 ….umm, actually, no FW incr Mike: “What should I talk about?” NASA Program Manager: “How about the freshening of the Arctic Ocean?” Mike: Uhhhhhh……

Swift et al. , JGR 2005 (presented by K Swift et al., JGR 2005 (presented by K. Aagaard at SEARCH mtg Seattle Oct’03) Long-term variability of Arctic Ocean waters: Evidence from a reanalysis of the EWG data set … i.e., classified Russian data Upper 175 m gets saltier in mid-1970s …& stays that way…

Arctic Ocean FWC anomalies …using another form of the same data… Arctic Ocean FWC anomalies Steele & Ermold, 2007 “fw capacitor” Proshutinsky et al., 2002) annual 5-yr R.M. freshwater export in mid-1970s …liquid ocean export after GSA? Short time series… POSSIBLE flywheel Cite Proshutinsky’s FW flywheel FW loss  30 cm

The “Great Salinity Anomaly”: 1968-1980 arctic sea ice Dickson et al., 1988 nordic seas subpolar gyre

Arctic Ocean FWC anomalies Steele & Ermold, 2007 freshwater export in mid-1970s …liquid ocean export after GSA? Short time series… POSSIBLE flywheel Cite Proshutinsky’s FW flywheel GSA sea ice export Belkin (1998): “Great Salinity Anomalies”

Häkkinen and Proshutinsky, 2004 Arctic FW storage & release: 1970s vs. 1990s …from A. Jahn, Jan’09 AOMIP Köberle and Gerdes, 2007 ~7000 km3 ~10,000 km3 ~6500 km3 ~2500 km3 Häkkinen and Proshutinsky, 2004 Jahn et al, 2009 ~6500 km3 ~1500 km3 Polyakov et al. 2008 ~6000 km3 1990s: some agreement 1970s: little agreement!

Arctic liquid FW release in the 1990s: Origins Karcher et al. GRL 2005: “Arctic Ocean change heralds North Atlantic freshening” Laptev Sea  Transpolar Drift  East Greenland Current  Subpolar Gyre? FWC anom (m) 1995

OK… so why did the Arctic Ocean get saltier over the 2nd half of the 20th century?

North Atlantic Oscillation 18 19 20 North Atlantic Oscillation LOW HIGH Arctic Oscillation HIGH Atmospheric Forcing LOW

Arctic Ocean get saltier? was a much smaller effect N/AO & Arctic Ocean FW Steele et al., 2004 …similar figs in A. Jahn et al. (2009) High High Why did the Arctic Ocean get saltier? N/AO-: FW storage N/AO+: FW release It was the atmospheric forcing! (i.e., winds  ice, ocean FW release) (non-anthropogenic?) Incr. river discharge was a much smaller effect 18 19 20 LOW HIGH storage release

What’s New? …in the 2000’s

Increasing Beaufort Gyre The Beaufort Gyre April, 2008 climatology Increasing Beaufort Gyre FWC S  26! 2003 2004 2005 2006 2007 2008

Where’d this Beaufort Gyre FW come from? April, 2008 Alaska April, 2008 Alkire et al. (2009): It’s mostly Eurasian River Water! …how’d it get there? Alaska

Where’s this Beaufort Gyre FW going? deSteur et al., 2009 Davis St. FW flux FWC (m) BGEP NABOS NPEO + SY Fram St. Davis St. Fram St. FW flux Incr? Decr? …short time series! It’s moving through the Lincoln Sea  a new GSA coming?

The Longer-Term Future? (2006): CCSM ensembles   + 0.03 Sv   + 0.01 Sv (1) FW transformation: ice  ocean (0.1 Sv) (2) New FW: R + (P-E)  ocean export (0.04 Sv)   - 0.1 Sv Rawlins et al. 2009 Arctic Ocean (liquid) FWC Note: The models show late 20th century FWC increase!   + 0.14 Sv

Mysteries: Circulation (ice? ocean?) of river water? Future Arctic Ocean stratification / mixing? Fate of Bering St. inflow? When will the Arctic Ocean start freshening?

Thank You