2012 Changing Ocean 506E/497E - Lecture 7 - Woodgate

Schematic Surface and Atlantic Circulation Typical Arctic profiles

MIXED LAYER Usually thin (no wind stirring)

PACIFIC WATER High nutrients Shallow (<200m) Tmax Comparatively fresh (<33psu) Mostly only in Western Arctic

ATLANTIC WATER T>0ºC, deeper than 200m Tmax and layer below Higher Salinities Radionuclide tracers Eastern Arctic warmer

Bottom Water “the rest” Jones, 2001 Western Arctic warmer

The “Accent” How to form halocline water?

of Sea Water Atlantic water - Pacific and Atlantic Waters in TS space - cannot form Halocline water simply by mixing (Aagaard, 1981)

- density determined mostly by salinity at T < 2 deg C Surface Water

The temperature and salinity of a bit of water often tells you where it is from Adapted from Steele and Boyd, 1998

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ADVECTIVE HC Temp Fresh Salty Aagaard et al, 1981 Halocline formation

~ 50m

ADD COLD, SALTY ~ 100m

CONVECTIVE HC Temp Salty Rudels et al, 1996

ADD

~ 50m COLD, FRESH Convect

~ 100m

Steele and Boyd, 1998 – source of halocline water differs (advective or convective)

Woodgate et al, 2001 – temperature of halocline water differs Woodgate etal, 2001 (convective must be at freezing temperature, advective may or may not be at freezing)

Retreat of the Cold Retreat of the Cold Halocline Halocline (Steele and Boyd, 1998) (Steele and Boyd, 1998)

In 1995, only Makarov has a cold halocline Injection point of freshwater (Russian Rivers) has changed Use salinity in 40-60m Backed up by chemical data, band as an indicator Ekwurzel et al., 2001 Guay et al., 2001 Hi S = NO CHL Lo S = CHL PREVIOUS – RW into Eurasian Basin – CHL in Eurasian Basin 1995 – RW along shelf instead – no CHL in Eurasian Basin

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The (partial) return of Western versus Eastern Arctic Halocline

the Cold Halocline WESTERN ARCTIC (Boyd et al, 2002) (PACIFIC) HALOCLINE - greater salinity range - fresher at surface Consider (upper 80m) S over Lomo Ridge - general Tmax above Tmin - 1995 ~ 34 psu – no CHL - very varied - 1997 ~ 33.55 psu - (rich in nutrients) - 1999 ~ 33 psu - 2000 ~ 33.3 psu – CHL returning BSW – Bering Sea Water UHW – Upper Halocline Water LHW – Lower Halocline Water AW – Atlantic Water What could be causing this?? DW – Deep Water

So far, this is EASTERN Arctic story, Adapted from Steele and Boyd 1998 what about the Western Arctic EASTERN ARCTIC (ATLANTIC) HALOCLINE - less salinity range - saltier at surface - sharper bend in TS space

Arctic Intermediate Western versus Eastern ArcticWater Halocline

Image from Steele and Boyd 1998 Atlantic Water Atlantic

Polar Water

Arctic Surface Water

European Speak: e.g. Manley et al, 1992 BSW – Bering Sea Water Polar Water T: < 0 deg C and S: <34.4 psu UHW – Upper Halocline Water Arctic Surface Water T: < 0 deg C and S: 34.4-34.9 psu LHW – Lower Halocline Water Atlantic Intermediate Water T: 0-3 deg C and S: 34.4-34.9 psu AW – Atlantic Water Atlantic Water T: > 3 deg C and S: > 34.9 psu DW – Deep Water

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Generic Pacific Water JGR, 2004 circulation

Steele et al, 2004

- change in pathway with change in Atmospheric state - shift of Pacific/Atlantic boundary from

ACW=Alaskan Coastal Water BUT sBSW = summer Bering Sea Water - doesn’t always match outflow – is there a better tracer - how get the Pacific Water off from the Chukchi

DISSOLVED OXYGEN Shift of Pacific/Atlantic Front - High at surface (ventilated from atmosphere) PW - Low Oxygen - Low = OLD water (long time since at surface) JGR, 1996 or = Evidence of high biological activity

THUS – Pacific Water has LOW Dissolved Oxygen PacificWater

NPEO

SWYD

AW

- Hi Si and low Ox = Pacific Water

- Pacific Atlantic Front retreated from Lomo PW Ridge to Mendeleev Ridge by 1993

Falkner et al, 2005 DSR

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SILICATE, NITRATE, PHOSPHATE NITRATE:PHOSPHATE - High from source in Pacific PW - Hi Nutrients RELATIONSHIP PW versus AW in N:P space BUT – not conservative different in AW and PW = For a Nitrate value, PW have more Phosphate Redfield-Ketchum-Richards Model (Redfield et al, 1963) AW (CH20)106(NH)16(H3PO4) + 138 O2 = 106 CO2 + 122 H2O + 16 HNO3 + H3PO4 = Slope set by Redfield

Biogenic matter + oxygen = Carbon Dioxide + Water + NUTRIENTS = Exact lines may change

NO3(pw) = 14.828 x PO4(pw) – 12.16 (Falck, 2001)

BUT work out % influence of PW and AW

(..but certainly no better than 10% Fixing of nutrients and Carbon Decay of biogenic matter .... assumes ice melt, P and runoff same as AW to make biogenic matter using up Oxygen, forming nutrients .... denitrification .. and other such processes) PW Try to create a “tracer” that is conservative “Quasi-conservative Tracer” % PW in upper 30m

“NO” and “PO” – Broecker, 1974 - cope with growth and decay

NO = 9 NO3 + O2 PO = 135 PO4 + O2

N:P ratios NO:PO ratios Jones, Anderson and Swift, GRL, 1998 N* (N star) – Gruber and Sarmiento, 1997 Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: Implications for circulation - indicates nitrogen fixation and denitrification N* = 0.87 [N – 16 P + 2.9 µmol kg-1]

Shift of Pacific/Atlantic Front Near-Surface Temperature Maximum Jackson et al, 2010, JGR

JGR, 1996 JGR, 2005 29th Aug 2006 75N, 150W

Formed by local solar input Can be trapped below surface ice melt Heat may be removed in winter

Temperature above freezing (deg C)

Historic Russian Data - use TS and chemistry to show Pacific - silicate profiles in central Makarov Atlantic Front retreated from Lomo Ridge - Si max disappears in late 1980s to Mendeleev Ridge by 1993

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JGR 2008 Beaufort Gyre Freshwater Storage Proshutinsky et al, 2009, JGR 1950s to1980s Climatology - line = 50m salinity - colors = freshwater relative to 34.8psu

Influenced by - Ekman Pumping (largest) - also sea ice melt/formation and mixing

Satellite evidence of wind-driven spin up DSR 2010 Trend Trend in SSH in SSH wind stress curl 1992-1999 data 2006-2008 data - Top 500m Jul-Sep data - Find FW above 34 psu isohaline (Sref=35psu) - Objective mapping

Increase of 8400±2000 km3 between these periods This is 20% of BG storage, & comparable to annual Arctic FW export Bering Strait ~ 2500 km3/yr

Causes? - Increased river water off shelves - Regionally, also ice melt - Speculate = less export of FW to Nature GeoSci, 2012 (2006-2008) the North Atlantic minus (1992-1999) Glaciers = small Rivers = 1 paper saying increase, rest small, Bstrait = no trend .. maybe

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