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The East Greenland Coastal Current: a subarctic freshwater pathway 1 Introduction

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The East Greenland Coastal Current: a subarctic freshwater pathway 1 Introduction
Woods Hole Oceanographic Institution, Woods Hole, MA
upwelling favorable winds
24
0
16
8
3
343
32
33
34
32
33
50
12
34
34
0
20
34.8
25
45
24
20
16
12
34.8
60
40
60
Distance (km)
130
110
Section 1
(60οN)
0
20
40
Freshwater flux (mSv)
Meteoric Water
100
34.8
200
−0.1 −0.08−0.06−0.04−0.02
40 oW
o
25 W
o
30 W
o
Longitude IC
35 W
0
60
Depth (m)
30
U
0
20
45
60
(cm/s)
abs
40
Distance (km)
75
60
80
90
EGCC
Adjusted EGCC
EGC
EGC + EGCC
EGC + Adj. EGCC
~ constant
5 (68N)
4 (66N) 3 (65N) 2 (63N)
JR105 Section #
1 (60N)
Distance (km)
EGCC
Adjusted EGCC
EGC
EGC + EGCC
EGC + Adj. EGCC
80
Figure 5. (top) Alongstream
transport (Sv) for the EGCC
and EGC in 2004. Thin lines
show the observed transports.
Thick lines have been adjusted to account for the
alongshelf wind forcing as described above. (bottom) Similar to the top panel, but for
freshwater flux (mSv, referenced to Sref = 34.8). The
number to the right is the net
increase in total freshwater
flux from section 5 to section
1 at Cape Farewell.
~ +38 mSv
70
60
50
40
30
80
20
10
0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
fraction
Figure 3. Fractions of Pacific Water, PW, (left panels), sea ice melt, SI, (middle panels), and
meteoric water, MW (right panels) for each JR105 section from north to south (top to
bottom). The colorbar for all plots is at the bottom. Velocity contours (green, cm/s) indicate the EGCC jet, with isohalines overlaid (black). Dots show JR105 bottle locations.
Note the vertical scale stays constant (though some shelves are deeper than 200m),
but the horizontal scale changes to reflect the shelf width at each section (see Fig. 1).
Figure 1. Map of the southeast Greenland shelf area, showing a
schematic of the observed summertime circulation. Dashed lines
indicate possible pathways of the EGC. Also plotted are the station
locations (+) from a 2004 cruise, JR105, along with section numbers (1-6) and the position of the WOCE repeat hyrography line.
15
90
0
WOCE A1E
35
3.4
3.2
3
2.8
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
120
100
80
350
80
8
34.8
34.8
20
34.8
0
Section 2
(63οN)
- By Cape Farewell, most Pacific water has been
mixed away, and only sea ice melt shows a
strong signal
34.8
40
Distance (km)
0
25
41
60
Meteoric Water
25
Distance (km)
80
60
τ = -0.016 N/m2
34
34.8
34.8
35
60
60
34.8
34.8
34.8
34.8
60
37
60
34
35
35
60
34.8
60
60
40
60
150
Sea Ice Melt
2002
20
20
34
34
35
34.8
20
33
- Fractions of sea ice melt increase from negative (indicating Arctic origin water from which
ice has formed) at section 5 to positive by section 3 and continue increasing to Cape Farewell
Alongstream transport (Sv)
16
16
16
20
20
34
25
0
200
200
The observed EGCC volume transport along the shelf ranges
from 0.5-2.2 Sv during JR105, while at Cape Farewell it ranges
from 0.5-1.0 Sv (2001-2004). Using the correlation between the
alongshelf wind stress and the volume transport, we construct an
adjusted volume transport trend along the shelf during 2004.
Similarly, we can adjust the observed freshwater transport
(relative to S = 34.8). The combined EGCC/EGC system has an approximately constant total volume transport, while the corresponding freshwater flux increases down the shelf. This increase
is presumably due to the input of sea ice melt, runoff from Greenland, and net precipitation.
50
100
34.8
Depth (m)
16
16
16
16
Depth (m)
Depth (m)
Depth (m)
Depth (m)
Depth (m)
8
EGC
- These high Pacific water fractions are subsurface and eroded in the core of the EGCC jet
34
Latitude
12
200
60
40
Distance (km)
150
EGCC
6 Volume and freshwater fluxes
- Maximum meteoric water fractions are ~10%
at every section, and typically decrease offshore
and in deeper water
34.8
150
150
Pacific Water
34
16
0
29
Section 3
(65οN)
34.8
20
25
100
34
50
100
200
24
0
40
Distance (km)
40
Distance (km)
34.8
8
20
0
35
- Significant fractions of Pacific-origin water are
found in the EGCC (up to 20%)
34.8
12
Sea Ice Melt
34
34
Summary of tracer analysis
46
34
16
45
20
0
34
20
60
200
50
Meteoric Water
50
150
54
35
60
24
100
58
34
Distance (km)
41
34.8
40
20
37
34.8
0
33
34
0
29
34
Pacific Water
25
200
60
Distance (km)
50
150
150
40
20
34.8
62
Section 4
(66οN)
100
Distance (km)
35
45
0
34.8
34
50
100
34.8
41
34.8
37
EGCC
100
200
60
33
20
34.8
0
50
Sea Ice Melt
32
85
Meteoric Water
0
60
50
200
89
34
34.8
200
46
50
93
20
34.8
54
97
150
34
100
16
101
24
Figure 4. Alongstream absolute velocity (color, Uabs, cm/s) and salinity contours (black) for two sections taken at Cape Farewell. Blue boxes roughly
indicate the defined regions of the EGCC (inshore) and EGC (shelfbreak)
found during each year. The alongshelf wind stress, τ (N/m2), is a two-day
average of twice-daily QuikSCAT data. (τ > 0 is upwelling favorable)
100
50
100
34.8
33
20
34
300
τ = +0.026 N/m2
0
28
250
2004
0
Section 5
(68οN)
Distance (km)
50
34
58
60
0
29
350
16
0
34
20
62
Meteoric Water
0
100
Distance (km)
34
40
Distance (km)
25
EGC
300
122
16
85
50
35
45 oW
20
0
150
Irminger Sea
89
50
50
4
0
0
34.8
2
1
34.8
200
EGCC
157
93
Sea Ice Melt
150
Pacific Water
34.8
24
200
34
150
34.8
N
2
EGCC
34
60 o
Cape
Farewell
45
3
65
150
46
50
97
100
35
62 oN
46
54
101
200
100
Distance (km)
34
25
64
58
118
150
20
Distance (km)
34
64 oN
100
50
100
114
100
50
0
50
60
Greenland
34.8
50
62
110
50
Sea Ice Melt
0
34.8 34
0
0
Strait
101
85
34.8
Pacific Water
200
25
Denmark6
89
34
200
60
KG
66 oN
Tasiilaq
5
100
34
EGCC
93
150
0
106
34
97
102
3
50
Distance (km)
101
122
16
0
118
35
102
114
16
Pacific Water
100
EGC
68 oN
110
150
50
- 170 conductivity/temperature/depth (CTD) casts
- velocities from vessel-mounted acoustic Doppler current
profiler (ADCP)
- nutrient (NO3, PO4, SiO4) and oxygen isotope samples
These data, combined with additional observations from the
WOCE-A1E line in 2001-2003, allowed the first picture of the
summertime circulation to be drawn that included the EGCC
(Sutherland and Pickart, 2007).
0
106
100
16
200
20
We present data from a 2004 summertime cruise (JR105)
aboard the ice-strengthened vessel RRS James Clark Ross that
occupied the six sections shown in Fig. 1. These data include:
102
50
16
150
0
2 Overview of circulation
122
16
100
EGCC
- to quantify the freshwater composition (sea ice melt,
meteoric water, Pacific-origin water) of the EGCC.
118
200
16 16
50
Our goals in this study are:
- to estimate the EGCC’s volume and freshwater transport,
114
EGCC
0
- to describe the alongstream evolution of the EGCC’s
hydrographic and velocity structure,
110
100
EGCC
250
Meteoric Water %
33
32
32
33
.8
34
0
106
150
32
50
3
102
Sea Ice Melt %
35
36
34.8
Pacific Water %
34
Previous research has indicated that freshwater may exit the
Arctic Ocean in one of two predominant pathways, depending
on the phase of the Arctic Oscillation (e.g. Steele, et al. 2004).
One of these pathways is through Fram Strait in the EGC, and
thus, potentially in the EGCC farther south.
4 Freshwater composition
34.8
8
34.
100
Limited hydrographic and drifter data indicate that a fresh (S <
34), intense (velocities ~ 1 m/s) current can be found over the
inner shelf off of southeast Greenland (Bacon, et al. 2002).
Named the East Greenland Coastal Current (EGCC), this flow
was initially thought to be a purely meltwater driven current.
However, new observations suggest that it is partly of Arctic
origin, as a component of the East Greenland/Irminger Current
(EGC/IC) system (Sutherland and Pickart, 2007).
37
0
35
1 Introduction
20
downwelling favorable winds
.8
Robert S. Pickart
34
MIT/WHOI Joint Program, Woods Hole, MA ([email protected])
Characterized by a wedge-like salinity shape, the EGCC’s depth
and width scales can change dramatically on short time scales,
with a strong dependence on the alongshelf wind stress. This
behavior has been seen before in smaller scale coastal currents
(Lentz and Largier, 2006), but has never been observed in large
rscale flows. However, on longer time scales (seasonal to interannual), not much is known.
35
David A. Sutherland
5 EGCC variability
34
The East Greenland Coastal Current:
a subarctic freshwater pathway
5 (68N)
4 (66N) 3 (65N) 2 (63N)
JR105 Section #
1 (60N)
7 Summary
3 Nutrient and isotope data
- The EGCC is a robust feature of the summertime circulation on
the southeast Greenland shelf, and together with the EGC carries
a significant volume (~ 2 Sv) and freshwater transport (up to ~90
mSv) that is similar in magnitude to the freshwater transport leaving Fram Strait
We use the JR105 tracer data to differentiate between the freshwater sources in the EGCC. First, we solve a three end-member balance (see box) to get fractions (f) of sea ice melt (SI), meteoric water (MW), and a combined oceanic water (Taylor, et al., 2003). Then, we separate the oceanic water fraction into Pacific (PW) and Atlantic components (AW) based upon the PO4:NO3 relationship (Jones, et al. 2003), shown in Fig. 2, right.
- Alongshelf wind forcing alters the structure of the EGCC, suggesting it is highly variable on synoptic time scales
fOW SOW + f MW SMW + f SI SSI = Sobs
(2)
OOW + f MW
18
OMW + f SI
OSI =
18
Oobs
18
Table 1. End member values and uncertainties for
the freshwater fraction calculation in equations 1-3.
These all are estimated based on the published literature (e.g. Taylor, et al. 2003).
Salinity
18
O
Oceanic
water
34.85 (±0.1)
0.3 (±0.1)
Sea ice
melt
4 (±1)
1 (±0.2)
Meteoric
water
0
-21 (±2)
40
35
−0.5
30
25
−1
20
−1.5
−2.5
28
18
45
0
−2
55
50
0.5
(3)
δO18 (per mil)
fOW
18
1
20
15
10
freeze
melt
29
30
31
32
Salinity
33
34
35
Figure 2. (left) Salinity vs. δO18 for JR105. Color indicates
the depth at which the sample was taken. The dashed
line is the mixing line between oceanic water, at top
right, with meteoric water (S = 0). The arrows indicate
what effects sea ice melting/freezing processes have on
the relationship. (right) PO4 (μmol/kg) vs. NO3 (μmol/kg)
JR105 data (blue dots). The solid lines show the PO4:NO3
relationship for waters taken from Atlantic Water and
Pacific Water source regions. Variability in these source
regions leads to the dashed error bars.
- Significant fractions of Pacific-origin water (up to 20%) are
found in the EGCC, although they are less than observed in the
EGC at similar latitudes (but in different years, so this may be due
to interannual variability)
AW source
16
14
NO3 (μmol/kg)
(1)
60
Depth (m)
fOW + f MW + f SI = 1
1.5
12
PW source
10
8
Acknowledgments and references
6
My thanks go to the crew of the JCR; Dan Torres, Terry McKee, and Paula Fratantoni for help processing the
cruise data; and to Peter Jones and Kumiko Azetsu-Scott for help on the nutrient analyses.
4
5
2
0
0
0
0.5
1
1.5
PO4 (μmol/kg)
2
2.5
Bacon S., et al., 2002. A freshwater jet on the east Greenland shelf. JGR, 107.
Jones, E.P., et al., 2003. Tracing Pacific water in the North Atlantic Ocean. JGR, 108.
Lentz, S.J., and J. Largier, 2006. The influence of wind forcing on the Chesapeake Bay buoyant coastal current. JPO, 36.
Steele, M., et al., 2004. Circulation of summer Pacific halocline water in the Arctic Ocean. JGR, 109.
Sutherland, D.A. and R.S. Pickart, 2007. The East Greenland Coastal Current: structure, variability, and forcing. Submitted to PiO.
(available at http://www.whoi.edu/hpb/Site.do?id=522)
Taylor, J.R., et al., 2003. Quantitative considerations of dissolved barium as a tracer in the Arctic Ocean. JGR, 108.
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