2012 Changing Arctic Ocean 506E/497E - Lecture 7 - Woodgate
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Schematic Surface and Atlantic Circulation
Jones, 2001
Typical Arctic profiles
Bottom Water
“the rest”
Western Arctic warmer
ATLANTIC WATER
T>0ºC, deeper than 200m
Tmax and layer below
Higher Salinities
Radionuclide tracers
Eastern Arctic warmer
PACIFIC WATER
High nutrients
Shallow (<200m) Tmax
Comparatively fresh (<33psu)
Mostly only in Western Arctic
MIXED LAYER
Usually thin (no wind stirring)
The “Accent”
of Sea Water - Pacific and
Atlantic Waters
in TS space
- density determined
mostly by salinity at
T < 2 deg C
The temperature and salinity of a bit of water often tells you where it is from
How to form halocline water?
Adapted from Steele and Boyd, 1998
Atlantic water
Surface Water
- cannot form Halocline
water simply by mixing
(Aagaard, 1981)
2012 Changing Arctic Ocean 506E/497E - Lecture 7 - Woodgate
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Temp Fresh Salty
ADD
COLD, SALTY
Aagaard et al, 1981
Rudels et al, 1996 Temp Salty
Co
nve
ct
ADD
COLD, FRESH
~ 50m
~ 100m
~ 50m
~ 100m
ADVECTIVE HC
CONVECTIVE HC
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
Hi S = NO CHL
Lo S = CHL
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
Guay et al., 2001
PREVIOUS – RW into Eurasian Basin – CHL in Eurasian Basin
1995 – RW along shelf instead – no CHL in Eurasian Basin
2012 Changing Arctic Ocean 506E/497E - Lecture 7 - Woodgate
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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
EASTERN ARCTIC (ATLANTIC) HALOCLINE
- less salinity range
- saltier at surface
- sharper bend in TS space
Adapted from Steele and Boyd 1998
BSW – Bering Sea Water
UHW – Upper Halocline Water
LHW – Lower Halocline Water
AW – Atlantic Water
DW – Deep Water
WESTERN ARCTIC
(PACIFIC) HALOCLINE
- greater salinity range
- fresher at surface
- general Tmax above Tmin
- very varied
- (rich in nutrients)
Western versus Eastern Arctic Halocline Image from Steele and Boyd 1998
BSW – Bering Sea Water
UHW – Upper Halocline Water
LHW – Lower Halocline Water
AW – Atlantic Water
DW – Deep 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: 34.4-34.9 psu
Atlantic Intermediate Water T: 0-3 deg C and S: 34.4-34.9 psu
Atlantic Water T: > 3 deg C and S: > 34.9 psu
Polar Water
Arctic
Surface
Water
Arctic Intermediate
Water A
tla
nti
c W
ate
r
2012 Changing Arctic Ocean 506E/497E - Lecture 7 - Woodgate
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JGR, 2004
ACW=Alaskan Coastal Water
sBSW = summer Bering Sea Water
Generic Pacific Water
circulation Steele et al, 2004
BUT
- doesn’t always match Fram Strait outflow – is there a better tracer
- how get the Pacific Water off from the Chukchi
- change in pathway with change in Atmospheric state
- shift of Pacific/Atlantic boundary from Lomonosov Ridge
Shift of Pacific/Atlantic Front
- Hi Si and low Ox = Pacific Water
- Pacific Atlantic Front retreated from Lomo
Ridge to Mendeleev Ridge by 1993
JGR, 1996
PW - Low Oxygen
NPEO
SWYD
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 Pacific
Wate
r
Falkner et al, 2005 DSR
AW
PW
2012 Changing Arctic Ocean 506E/497E - Lecture 7 - Woodgate
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PW - Hi Nutrients SILICATE, NITRATE, PHOSPHATE
- High from source in Pacific
BUT – not conservative
Try to create a “tracer” that is conservative “Quasi-conservative Tracer”
“NO” and “PO” – Broecker, 1974
- cope with growth and decay
NO = 9 NO3 + O2
PO = 135 PO4 + O2
N:P ratios
NO:PO ratios
N* (N star) – Gruber and Sarmiento, 1997
- indicates nitrogen fixation and denitrification
N* = 0.87 [N – 16 P + 2.9 µmol kg-1]
(CH20)106(NH)16(H3PO4) + 138 O2 = 106 CO2 + 122 H2O + 16 HNO3 + H3PO4
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
Redfield-Ketchum-Richards Model (Redfield et al, 1963)
PW versus AW in N:P space NITRATE:PHOSPHATE
RELATIONSHIP
different in AW and PW = For a Nitrate value, PW have more Phosphate
= Slope set by Redfield
= 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%
.... assumes ice melt, P and runoff same as AW
.... denitrification .. and other such processes) PW
AW
% PW in upper 30m
Jones, Anderson and Swift, GRL, 1998
Distribution of Atlantic and Pacific waters in the
upper Arctic Ocean: Implications for circulation
Shift of Pacific/Atlantic Front
- use TS and chemistry to show Pacific
Atlantic Front retreated from Lomo Ridge
to Mendeleev Ridge by 1993
JGR, 1996 JGR, 2005
Historic Russian Data
- silicate profiles in central Makarov
- Si max disappears in late 1980s
Near-Surface Temperature Maximum
29th Aug 2006
75N, 150W
Temperature above freezing (deg C)
Formed by local solar input
Can be trapped below surface ice melt
Heat may be removed in winter
Jackson et al, 2010, JGR
2012 Changing Arctic Ocean 506E/497E - Lecture 7 - Woodgate
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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
DSR 2010
- Top 500m Jul-Sep data
- Find FW above 34 psu
isohaline (Sref=35psu)
- Objective mapping
1992-1999 data 2006-2008 data
(2006-2008)
minus
(1992-1999)
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
the North Atlantic
Glaciers = small
Rivers = 1 paper saying increase,
rest small,
Bstrait = no trend .. maybe
Satellite evidence of wind-driven spin up
Nature GeoSci, 2012
SSH Trend
in SSH
Trend in
wind stress
curl