How to form halocline water?
Atlantic water
- cannot form Halocline water simply by mixing (Aagaard, 1981)
Surface Water
Adapted from Steele and Boyd, 1998 ADVECTIVE HC
Temp Fresh Salty Aagaard et al, 1981
~ 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 (convective must be at freezing temperature, advective may or may not be at freezing) Halocline formation
Woodgate etal, 2001 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 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 Decadal averages of Russian Data 1950s to 1980s
http://nsidc.org/data/g01961.html
(see also Swift et al, 2005 annual averages in boxes 1948-1993 - ftp:://odf.ucsd.edu/pub/jswift/arctic_aari_method_B “Levitus” for the Arctic http://psc.apl,washington.edu/Climatology.html The (partial) return of the Cold Halocline (Boyd et al, 2002)
Consider (upper 80m) S over Lomo Ridge - 1995 ~ 34 psu – no CHL - 1997 ~ 33.55 psu - 1999 ~ 33 psu - 2000 ~ 33.3 psu – CHL returning
What could be causing this??
So far, this is EASTERN Arctic story, what about the Western Arctic Western versus Eastern Arctic Halocline
WESTERN ARCTIC (PACIFIC) HALOCLINE - greater salinity range - fresher at surface - general Tmax above Tmin - very varied - (rich in nutrients)
Adapted from Steele and Boyd 1998 EASTERN ARCTIC (ATLANTIC) HALOCLINE - less salinity range - saltier at surface - sharper bend in TS space Shift of Pacific/Atlantic Front
JGR, 1996 JGR, 2005
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 Bering Strait and the Chukchi Sea
COLDER WARMER Nutrient-rich SALTIER FRESHER Anadyr waters RICHER IN NUTRIENTS LOWER IN NUTRIENTS To first order, Bering Shelf except for waters - cooling - input from Alaskan coastal Coastal Current polynyas, (warm, fresh, Chukchi seasonal) dominated by input through Siberian Bering Strait Coastal Current (cold, fresh, Export to Arctic seasonal) ~ Input through Stagnation Bering Strait Zones over Herald and Hanna Shoals Woodgate et al, DSR, 2005, http://psc.apl.washington.edu/Chukchi.html JGR, 2004
ACW=Alaskan Coastal Water sBSW = summer Bering Sea Water Generic Pacific Water circulation
Steele et al, 2004
- 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 Chukchi Sea Outflow
Long = TOPOGRAPHIC CONSTRAINTS Strait (Potential Vorticity Conservation) - Taylor columns in Chukchi - flow ~ along isobaths eastward BUT WE SEE PW GETS AWAY FROM TOPOGRAPHY
= FRICTION (TOP or BOTTOM) = DENSITY DIFFERENCES - dense water outflows =Four main outflows = WIND EFFECTS 1. Barrow Canyon 2. Central Gap - upwelling and downwelling 3. Herald Canyon 4. Long Strait?? - undercurrents = Most nutrients in West = EDDIES = Outflows move east & north = INERTIAL and TIDAL = Seasonal & interannual variability OSCILLATIONS AND MIXING in TS (thus density and equilibrium depth) and also in volume Chukchi slope velocity 2002-2003 red=73 20N blue=73 37N Chukchi Slope 2002-2003 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 Dense Water Outflow – e.g. from coastal polynyas (e.g. Martin et al, 2004)
X Wind Ice
S flux as new ice Dense water on shelf ICE Temperature
Dense water flows down shelf as a ICE descending plume, Thickness entraining water.
(i.e. down, but not OUT) 2002, JGR
- can get the salinities, but volume is small Wind effects
X Wind X Wind isopycnal
Upwelling of deeper water If initial stratification enough, - can come up canyon can get undercurrent onto the shelf opposite to the wind - cf Chukchi slope (Yoshida Undercurrent) canyons, Barrow Canyon, - cf Beaufort slope and many others Results of a strong westward wind 3rd October
5th October
Ship’s ADCP of the Beaufort slope current system (red= towards you) (Andreas Muncheow, UDel) Upwelling versus polynyas??
Use Silicate to track Pacific Water in the Chukchi Borderland 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 1990-1991 Woodgate et al, 2005 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) Do the volumes work out?
Volume PW at 33.1 psu per year ~ 6x 10^12 m3/yr (0.6 Sv for 4 months) In Arctic ~ half pure, half mixed with AW (1:1) Thus need 3 x 10^12 m3/yr AW to be raised onto shelf
Observed upwelling events in Barrow Canyon = 2-3 x 10^11 m3 per event Therefore need 10 events and there are multiple wind events, and multiple canyons
So – plausible
Also, this ventilation rate is an order of magnitude higher than estimates of polynya water formation
Woodgate et al, 2005, GRL The Eddy Band-wagon Eddies in the Beaufort Sea e.g.,Hunkins and Manley, Plueddemann and MANY others http://www.whoi.edu/science/PO/arcticgroup/projects/eddies.html
halocline depth
Arctic Rossby Radius
Predominantly Anticyclonic Eddies in the non-Beaufort Arctic
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 Upper Arctic Ocean Circulation and Ventilation
CIRCULATION CHANGES HALOCLINE BASICS Shift in rivers outflow Formation possibilities Retreat and recovery of CHL Western versus Eastern Arctic Shift of Pacific/Atlantic Front Different branches of PW
MECHANISMS OF SHELF-BASIN EXCHANGE Winddriven upwelling and mixing of AW with PW Potential vorticity conservation, following topography Dense water outflow from polynyas Eddies (AC, ~ scale of Rossby radius)
Rudels et al,1996 Winddriven undercurrents (Yoshida jet) Tides and inertial oscillations
Photo PSC/UW