How to form halocline water?

Transcription

1 How to form halocline water? Atlantic water - cannot form Halocline water simply by mixing (Aagaard, 1981) Surface Water Adapted from Steele and Boyd, 1998

2 ADVECTIVE HC Temp Fresh Salty Aagaard et al, 1981 ~ 50m ~ 100m ADD COLD, SALTY Temp Salty CONVECTIVE HC Rudels et al, 1996 ~ 50m Convect 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)

3 Halocline formation Convective Advective Woodgate etal, 2001

4 Basin/Shelf FSBW/BSBW origin Black= FSBW Blue = BSBW Red = AW Green = Rivers Orange = Pacific inflow.. but what about change?? Rudels et al, 2004, Polar Research

5 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

6 Retreat of the Cold Halocline (Steele and Boyd, 1998) Injection point of freshwater (Russian Rivers) has changed Backed up by chemical data, Ekwurzel et al., 2001 PREVIOUS RW into Eurasian Basin CHL in Eurasian Basin 1995 RW along shelf instead no CHL in Eurasian Basin

7 Decadal averages of Russian Data 1950s to 1980s (see also Swift et al, 2005 annual averages in boxes ftp:://odf.ucsd.edu/pub/jswift/arctic_aari_method_b

8 Levitus for the Arctic

9 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?? So far, this is EASTERN Arctic story, what about the Western Arctic

10 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 Adapted from Steele and Boyd 1998 EASTERN ARCTIC (ATLANTIC) HALOCLINE - less salinity range - saltier at surface - sharper bend in TS space

11 Arctic Intermediate Western versus Eastern Arctic Halocline Water Atlantic 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

12 Shift of Pacific/Atlantic Front JGR, 1996 JGR, 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

13 Bering Strait and the Chukchi Sea Nutrient-rich Anadyr waters Bering Shelf waters Alaskan Coastal Current (warm, fresh, seasonal) Siberian Coastal Current (cold, fresh, seasonal) Stagnation Zones over Herald and Hanna Shoals COLDER SALTIER RICHER IN NUTRIENTS WARMER FRESHER LOWER IN NUTRIENTS To first order, except for - cooling - input from coastal polynyas, Chukchi dominated by input through Bering Strait Export to Arctic ~ Input through Bering Strait Woodgate et al, DSR, 2005,

14 JGR, 2004 ACW=Alaskan Coastal Water sbsw = summer Bering Sea Water

15 Generic Pacific Water circulation Steele et al, 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

16 Chukchi Sea Outflow Long Strait = TOPOGRAPHIC CONSTRAINTS (Potential Vorticity Conservation) - Taylor columns in Chukchi - flow ~ along isobaths eastward BUT WE SEE PW GETS AWAY FROM TOPOGRAPHY = FRICTION (TOP or BOTTOM) =Four main outflows 1. Barrow Canyon 2. Central Gap 3. Herald Canyon 4. Long Strait?? = Most nutrients in West = Outflows move east & north = Seasonal & interannual variability in TS (thus density and equilibrium depth) and also in volume = DENSITY DIFFERENCES - dense water outflows = WIND EFFECTS - upwelling and downwelling - undercurrents = EDDIES = INERTIAL and TIDAL OSCILLATIONS AND MIXING

17 Dense Water Outflow e.g. from coastal polynyas (e.g. Martin et al, 2004) X Wind Ice ICE Temperature S flux as new ice Dense water on shelf ICE Thickness Dense water flows down shelf as a descending plume, entraining water. (i.e. down, but not OUT)

18 2002, JGR ~0.06Sv - can get the salinities, but volume is small

19 Wind effects X Wind X Wind isopycnal Upwelling of deeper water - can come up canyon onto the shelf - cf Chukchi slope canyons, Barrow Canyon, and many others If initial stratification enough, can get undercurrent opposite to the wind (Yoshida Undercurrent) - cf Beaufort slope

20 3 rd October Results of a strong westward wind 5 th October Ship s ADCP of the Beaufort slope current system (red= towards you) (Andreas Muncheow, UDel)

21 Chukchi slope velocity red=73 20N blue=73 37N

22 Chukchi Slope TS-properties Temperature Maximum is December March, i.e. advective from the south Intrusions of Atlantic Water in Autumn 73 20N - red 60m/70m water 73 37N - cyan 60m/110m water - navy 100m/110m water

23 Upwelling versus polynyas?? Use Silicate to track Pacific Water in the Chukchi Borderland

24 Can we get this TS from Bering Strait?? NO... salinities are only near 34 psu in extreme winters, and then the waters are at freezing, not warmer Bering Strait TS Woodgate et al, 2005

25 Influence of shelf waters?? Along the Chukchi Shelf, upwelling and diapycnal mixing of lower halocline waters and Pacific waters (Note ventilation by polynya waters couldn t give this T-S structure) (Woodgate et al, 2005)

26 The Eddy Band-wagon

27 Eddies in the Beaufort Sea e.g.,hunkins and Manley, Plueddemann and MANY others halocline depth Arctic Rossby Radius Predominantly Anticyclonic

28 Eddy Census 1 eddy = (10 km radius, 50m) - XBT lines ~ 2 x m 3 Flux through Bering Strait ~ 1 Sv ~ 3 x m 3 per yr If all eddies, makes ~1000 eddies a year. Do we see 1000 eddies??? - no

29 Eddies in the non-beaufort Arctic (Woodgate et al, 2001) LM2 LM3 LM1 TWO EDDY TYPES Cold (Tf), Fresh, near surface, AC, - likely from shelf polynyas Warm, Salty, ~ 1000m deep, AC - instabilities on upstream front (e.g. St Anna) 40 cm/s; ~ 10km radius, but volume flux ~ 0.1 Sv or less

30 DISSOLVED OXYGEN - High at surface (ventilated from atmosphere) - Low = OLD water (long time since at surface) or = Evidence of high biological activity HUS Pacific Water has LOW Dissolved Oxygen NPEO PW - Low Oxygen Pacific Water SWYD AW PW Falkner et al, 2005 DSR

31 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 Fixing of nutrients and Carbon to make biogenic matter Decay of biogenic matter using up Oxygen, forming nutrients 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 N* (N star) Gruber and Sarmiento, indicates nitrogen fixation and denitrification N* = 0.87 [N 16 P µmol kg -1 ]

32 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) PW 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 Jones, Anderson and Swift, GRL, 1998 Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: Implications for circulation