Typical Arctic profiles. How to form halocline water? 2012 Changing Arctic Ocean 506E/497E - Lecture 7 - Woodgate

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1 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 Jones, 2001 Bottom Water the rest Western Arctic warmer The Accent of Sea Water - Pacific and Atlantic Waters in TS space How to form halocline water? Atlantic water - 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,

2 Convect Temp Fresh Salty ADVECTIVE HC Aagaard et al, 1981 Halocline formation ~ 50m ~ 100m ADD COLD, SALTY Temp Salty CONVECTIVE HC Rudels et al, 1996 ~ 50m ADD COLD, FRESH ~ 100m Steele and Boyd, 1998 source of halocline water differs (advective or convective) Woodgate et al, 2001 temperature of halocline water differs (convective must be at freezing temperature, advective may or may not be at freezing) Woodgate etal, 2001 Retreat of the Cold Halocline (Steele and Boyd, 1998) Retreat of the Cold Halocline (Steele and Boyd, 1998) In 1995, only Makarov has a cold halocline Use salinity in 40-60m band as an indicator Hi S = NO CHL Lo S = CHL Injection point of freshwater (Russian Rivers) has changed Backed up by chemical data, Ekwurzel et al., 2001 Guay et al., 2001 PREVIOUS RW into Eurasian Basin CHL in Eurasian Basin 1995 RW along shelf instead no CHL in Eurasian Basin 2

3 Atlantic Water The (partial) return of the Cold Halocline (Boyd et al, 2002) Consider (upper 80m) S over Lomo Ridge ~ 34 psu no CHL ~ psu ~ 33 psu ~ 33.3 psu CHL returning What could be causing this?? Western versus Eastern Arctic Halocline WESTERN ARCTIC (PACIFIC) HALOCLINE - greater salinity range - fresher at surface - general Tmax above Tmin - very varied - (rich in nutrients) BSW Bering Sea Water UHW Upper Halocline Water LHW Lower Halocline Water AW Atlantic Water DW Deep Water So far, this is EASTERN Arctic story, what about the Western Arctic Adapted from Steele and Boyd 1998 EASTERN ARCTIC (ATLANTIC) HALOCLINE - less salinity range - saltier at surface - sharper bend in TS space Arctic Intermediate Western versus Eastern Halocline Water Image from Steele and Boyd 1998 Polar Water Arctic Surface Water European Speak: e.g. Manley et al, 1992 Polar Water T: < 0 deg C and S: <34.4 psu Arctic Surface Water T: < 0 deg C and S: psu Atlantic Intermediate Water T: 0-3 deg C and S: psu Atlantic Water T: > 3 deg C and S: > 34.9 psu BSW Bering Sea Water UHW Upper Halocline Water LHW Lower Halocline Water AW Atlantic Water DW Deep Water 3

4 JGR, 2004 Generic Pacific Water circulation Steele et al, 2004 ACW=Alaskan Coastal Water sbsw = summer Bering Sea Water - change in pathway with change in Atmospheric state - shift of Pacific/Atlantic boundary from Lomonosov Ridge BUT - doesn t always match Fram Strait outflow is there a better tracer - how get the Pacific Water off from the Chukchi JGR, 1996 Shift of Pacific/Atlantic Front DISSOLVED OXYGEN - High at surface (ventilated from atmosphere) - Low = OLD water (long time since at surface) or = Evidence of high biological activity THUS Pacific Water has LOW Dissolved Oxygen NPEO PW - Low Oxygen Pacific Water SWYD AW - Hi Si and low Ox = Pacific Water - Pacific Atlantic Front retreated from Lomo Ridge to Mendeleev Ridge by 1993 PW Falkner et al, 2005 DSR 4

5 SILICATE, NITRATE, PHOSPHATE - High from source in Pacific BUT not conservative PW - Hi Nutrients Redfield-Ketchum-Richards Model (Redfield et al, 1963) (CH 2 0) 106 (NH) 16 (H 3 PO 4 ) O 2 = 106 CO H 2 O + 16 HNO 3 + H 3 PO 4 Biogenic matter + oxygen = Carbon Dioxide + Water + NUTRIENTS NITRATE:PHOSPHATE RELATIONSHIP different in AW and PW AW PW versus AW in N:P space = For a Nitrate value, PW have more Phosphate = Slope set by Redfield = Exact lines may change NO 3(pw) = x PO 4(pw) (Falck, 2001) Fixing of nutrients and Carbon to make biogenic matter Try to create a tracer that is conservative Quasi-conservative Tracer NO and PO Broecker, cope with growth and decay NO = 9 NO 3 + O 2 PO = 135 PO 4 + O 2 N:P ratios NO:PO ratios Decay of biogenic matter using up Oxygen, forming nutrients N* (N star) Gruber and Sarmiento, indicates nitrogen fixation and denitrification N* = 0.87 [N 16 P µmol kg -1 ] PW Jones, Anderson and Swift, GRL, 1998 Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: Implications for circulation BUT work out % influence of PW and AW (..but certainly no better than 10%... assumes ice melt, P and runoff same as AW... denitrification.. and other such processes) % PW in upper 30m Shift of Pacific/Atlantic Front Near-Surface Temperature Maximum Jackson et al, 2010, JGR JGR, 1996 JGR, th Aug N, 150W Formed by local solar input Can be trapped below surface ice melt Heat may be removed in winter Temperature above freezing (deg C) - use TS and chemistry to show Pacific Atlantic Front retreated from Lomo Ridge to Mendeleev Ridge by 1993 Historic Russian Data - silicate profiles in central Makarov - Si max disappears in late 1980s 5

6 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 data data DSR 2010 Satellite evidence of wind-driven spin up SSH Trend in SSH Trend in wind stress curl - Top 500m Jul-Sep data - Find FW above 34 psu isohaline (Sref=35psu) - Objective mapping Increase of 8400±2000 km 3 between these periods This is 20% of BG storage, & comparable to annual Arctic FW export Bering Strait ~ 2500 km 3 /yr ( ) minus ( ) Causes? - Increased river water off shelves - Regionally, also ice melt - Speculate = less export of FW to the North Atlantic Glaciers = small Rivers = 1 paper saying increase, rest small, Bstrait = no trend.. maybe Nature GeoSci,