2.Mean barotropic transport calculated from near-‐bo6om currents
5. Discussion
Karen L. Tracey, Kathleen A. Donohue, and D. Randolph Wa8s
950
1. cDrake experiment
[email protected] www.cDrake.org
3. Barotropic transport variability calculated from bo6om pressures
References Chidichimo et al. 2014. Baroclinic transport time series of the Antarctic Circumpolar Current in Drake Passage. J. Phys. Oceanogr. (submitted). Cunningham et al. 2003. Transport of the Antarctic Circumpolar Current in Drake Passage. J. Geophys. Res. 108: 8084. Firing et al. 2011. Vertical structure and transport of the Antarctic Circumpolar Current in Drake Passage from direct velocity measurements. J. Geophys. Res. 116: C08015. Koshlyakov et al. 2012. Currents in Drake Passage based on observations in November of 2010. Oceanology. 52: 299-308. Mazloff et al. 2010. An eddy-permitting Southern Ocean State Estimate. J. Phys. Oceanogr. 40: 880-899. Whitworth. 1983. Monitoring the transport of the Antarctic Circumpolar Current at Drake Passage. J. Phys. Oceanogr. 13: 2045-2057. Whitworth and Peterson. 1985.Volume transport of the Antarctic Circumpolar Current from bottom pressure measurements. J. Phys. Oceanogr. 15: 810-816.
C17
C16
C15
C14
C13
C12
C11
C10
C09
C08
C07
C06
C19
C05
C20
C04
C03
C02
C01
C17C16C15C14C13C12C11C10C09C08C07C06C19C05C20C04C03C02C01
Correlation
0 0.5 1
Barotropic Transport of the AntarcDc Circumpolar Current Measured in Drake Passage
Region
Mean (Sv)
NS 7.5 (a)
NDP 7.7 (a)
SFZ 15.9 (b)
SDP 15.1 (a)
SS -‐2.3 (c)
Total 43.8
Visit other cDrake posters: 951 Foppert et al. Mean baroclinic structure of Polar Front in stream coordinates near Shackleton Fracture Zone. 849 Millar et al. Four-year observations of interfacial form stress in the northern Drake Passage.
Energetic transport fluctuations with periods of 50-180 days in SFZ and NDP regions are associated with the main ACC fronts shifting between regions. These fluctuations are nearly equal and opposite, and the variability of total barotropic transport in that period band is reduced. Transport fluctuations with periods in the 50-180 day band are weak at the southern and northern subregions of the passage.
C11-C17 pressures, south of the Shackleton Fracture Zone, exhibit less variance than those north of the topographic ridge. Southern pressure records (C17-C11) are highly correlated with one another.
• 4-year observational period from Nov 2007 to Nov 2011.
• 42 Current and Pressure recording Inverted Echo Sounders (CPIES) in transport line (solid triangles) and local dynamics array (open triangles).
• C01-C08 spacing: ~30 km • C08-C17 spacing: ~60 km • 5 closely-spaced CPIES (H01-
H05) along Shackleton Fracture Zone in year 4.
• 3 short current meter moorings (M01-M03) deployed at continental margins for first 2 years.
Bo8om pressures
C01
C02
C03
C04
C20
C05
C19
C06
C07
2008 2009 2010 2011−0.2
00.2 C08
C09
C10
C11
C12
C13
C14
C15
C16
2008 2009 2010 2011
C17
• Leveled and dedrifted using near-bottom currents. • Unresolved residual drift (< 1 cm per 1000 days) corresponds
to 4 Sv barotropic transport error over a 4-year experiment.
Line-‐normal currents
dbar
• Measured 50 m above the seafloor. • Rotated to be perpendicular to the transport line.
3 dlp currents are poorly correlated from site to site, while neighboring 90 dlp currents are positively or negatively correlated.
All measurements: • 1 hour sampling interval. • Tides removed: Semi-diurnal and diurnal
using response analysis, fortnightly and lunar monthly tides with TPXO7.2.
• 3 day low-pass filter applied to hourly samples and subsampled to twice daily.
dept
h (m
)
−5000
−4000
−3000
−2000
−1000
0
64oW 62oW 60oW
62oS
60oS
58oS
56oS
10 cm s−1
64oW 62oW 60oW
62oS
60oS
58oS
56oS
dept
h (m
)
−5000
−4000
−3000
−2000
−1000
0
64oW 62oW 60oW
62oS
60oS
58oS
56oS
SS
SDP
SFZ
NDP
NS
0.1 m s−1
64oW 62oW 60oW
62oS
60oS
58oS
56oS
Annual means are remarkably stable from year to year across the whole Drake Passage. This permits variable-record-length means to be used to estimate the barotropic transport and its error.
The across-passage mean currents are well resolved by the closely-spaced cDrake array. Standard error ellipses are small compared to the record-length mean currents.
Gray contours: 4-year mean SSH from Aviso (c.i. 10 cm).
(a) Standard trapezoidal integration. (b) Distance-weighted trapezoidal integration with l = 15 km for H01 and H03. Error estimation allowed l (distance from the Shackleton Fracture Zone) to vary between 10, 15, and 20 km based on LADCP casts taken on cDrake cruises and by Koshlyakov et al. (2012). (c) Distance-weighted trapezoidal integration with l = 24 km for C16. Error estimation was based on l = 24 km (distance from the South Shetland Trench) and l = 36 km (no distance weighting).
4. Total transport through Drake Passage
6. Summary
−0.2
−0.1
0
0.1
0.2
Line
nor
mal
vel
ocity
(m s−1
)
C17
M03
C16
C15
C14
C13
C11
C10
C09
C08
C07
C06
C19
C05
C20
C04
C03
M02
C02
M01
C01
yr 1yr 2yr 3yr 44 yrs
62°S 60°S 58°S 56°S
0
2000
4000Dep
th (m
)
Error EsDmaDon Transport (Sv)
Array-‐wide resoluJon for 90 day low-‐passed currents 7.7
CorrelaJon scales near topography in subregion SFZ (b) 2.8
CorrelaJon scales near topography in subregion SS (c) 1.8
Possible bo8om trapping bias (under invesJgaJon) -‐4 ± 4
Total 9.3
Barotropic transport is calculated using trapezoidal integration of the near-bottom velocity times the site-dependent water depth. The average site depth in the deep passage is about 4000 meters. Five subregions were defined based on topographic features: Northern and Southern Slopes (NS and SS), Shackleton Fracture Zone (SFZ), and Northern and Southern Deep Passages (NDP and SDP). H01-H03 sites were shifted along topography to the transport line to resolve the deep flow near the Shackleton Fracture Zone. The mean eastward transport accumulates beneath the SAF and PF. The mean eastward barotropic transport is 43.8 Sv with an uncertainty of 9.3 Sv. This mean transport translates to a mean across-passage eastward bottom-reference velocity of 1.3 cm s-1.
The cDrake moored array provided measurements across the Drake Passage with high spatial and temporal resolution for four years. Annual mean currents are remarkably stable from year to year and provide robust estimates of the barotropic transport. The mean barotropic transport is 43.8 Sv ± 9.3 Sv. Variability of the barotropic transport is dominated by the variability northward and downstream of the Shackleton Fracture Zone. Summing the barotropic transport with the bottom-referenced baroclinic transport of 127.7 Sv gives a total mean eastward transport of 171.4 Sv through Drake Passage. The variability of the barotopic component accounts for the largest fraction (80%) of the total transport variability. cDrake observations have been assimilated into the Southern Ocean State Estimate (Mazloff et al. 2010). This effort will provide perspective on how the cDrake measurements and derived quantities fit into the context of local, regional and large-scale variability, and their associated forcing.
C01
M01
C02
M02
C03
C04
C20
2008 2009 2010 2011−0.4
00.4
m s−1 C05
C19
C06
C07
C08
C09
C10
C11
2008 2009 2010 2011
C12
C13
C14
C15
C16
M03
2008 2009 2010 2011
C17
Record-length mean near-bottom currents perpendicular to the transport line were used to determine the mean barotropic transport.
J A J O J A J O J A J O J A J O
SS
SDP
SFZ
NDP
NS
Total Barotropic
50 S
v
2008 2009 2010 2011
3 dlp and 90 dlp Transports By Region
Transports (Sv)
4 year Mean
SEM
Std dev
Yr 1 Mean
Yr 2 Mean
Yr 3 Mean
Yr 4 Mean
Baroclinic + Barotropic
171.4 3.1 18.7 170.1 175.8 177.2 162.7
Baroclinic 127.7 1.0 8.1 128.0 128.5 126.8 127.4
Barotropic 43.8 3.0 18.3 42.2 47.3 50.4 35.3
J A J O J A J O J A J O J A J O−50
0
50
100
150
200
250Baroclinic + Barotropic
Baroclinic
Barotropic
Tran
spor
t (Sv
)2008 2009 2010 2011
Full-depth baroclinic transport referenced to zero at the bottom was calculated from the cDrake CPIES records by Chidichimo et al. (2014).
While the barotropic component accounts for only 25% of the total transport, with a temporal standard deviation of 18.3 Sv, it accounts for 80% of the total variability observed over the four years.
NSF Office of Polar Programs supported this work under grants ANT-0635437 and ANT-1141802.
Steady annual mean transports confirm that the 4-year means are robust.
Baroclinic component of 127.7 Sv accounts for 75% of the total eastward transport of 171.4 Sv through Drake Passage.
Transport fluctuations with periods of 20-180 days account for 50% of the variance.
Bottom pressure anomaly differences provide estimates of the barotropic transport variability. Time series were calculated for 4 subregions using the pressure differences of the end points. For the NS region, the time series was calculated using the near-bottom currents which provided more continuous coverage.
Standard deviation of the total barotropic transport is 18.3 Sv.
Using single pressure records from the southern end misses a large fraction of the variability.
The transport variability correlates better with pressures from the northern gauge.
Most of the barotropic transport variability is captured by using the deepest northern gauge with any of the southern gauges as end points.
cor = 0.81 C03 − C16
cor = 0.76 C03 − C17
cor = 0.66 C03
cor = −0.38 C16
cor = −0.24 C17
2008 2009 2010 2011−50
050
100150 Total Barotropic
1. The canonical transport estimate from ISOS (Whitworth 1983; Whitworth and Peterson 1985) of 134 Sv with the revised error estimate of 27 Sv from Cunningham et al. (2003) gives a maximum transport of 161 Sv, which is in good agreement with the cDrake minimum estimate of 162 Sv.
2. Using a 4.5 year time series of directly-measured SADCP velocity data across Drake Passage, Firing et al. (2011) estimated the mean transport in the upper 1000 m of 95 ± 2 Sv.
The ISOS array lost 2 critical moorings in the high transport region of the SAF and PF which would result in the transport being underestimated. A mean reference velocity of 0.04 m s-1 in this 125 km region could account for an additional 20 Sv of transport.
For cDrake the baroclinic and barotropic contributions in the upper 1000 m are 77 ± 4.4 Sv and 11 ± 2.2 Sv, respectively. These combine to give a total of 88 ± 5 Sv, which is in good agreement with the Firing et al. estimate.
cDrake mean transports can be compared to mean values from other time series in Drake Passage:
cDrake Objectives: • Determine horizontal and vertical structure of time-varying transport. • Describe the mesoscale eddy field. • Provide guidance for future monitoring. • Assess the skill of model simulations.
dept
h (m
)
−5000
−4000
−3000
−2000
−1000
0
C01C02C03C04C05
C06C07C08
C09
C10
C11
C12
C13
C14C15
C16
C17
C19
C20
64oW 62oW 60oW
62oS
60oS
58oS
56oS
M01M02
M03
64oW 62oW 60oW
62oS
60oS
58oS
56oS
H01H02
H04H05 H03
C10 24’ 12’
63oW 48’ 36’
32’ 28’
58oS 24.00’
20’ 16’
10−3 10−2 10−1 1000
50
100
150
200
250
300
Frequency (cpd)
Pow
er s
pect
rum
(Sv2
)
TotalSSNDP
l
l
l
−0.1 0 0.1
0
20
40
60
80
100
C17
M03C16
C15
Dis
tanc
e (k
m)
SS
−0.1 0 0.1
360
380
400
420
440
460
C11
H01C10
H02H03
C09
Line normal velocity (m s−2)
SFZ
C17
M03
C16
C15
C14
C13
C12
C11
C10
C09
C08
C07
C06
C19
C05
C20
C04
C03
M02
C02
M01
C01
C17M03C16C15C14C13C12C11C10C09C08C07C06C19C05C20C04C03M02C02M01C01
Correlation
−1 −0.5 0 0.5 1
−20
0
20
40
60Tr
ansp
ort (
Sv)
SS SDP SFZ NDP NS
SACCF PF SAF
South−to−North Distance (km)
Dep
th (m
)
0 100 200 300 400 500 600 700 800
0
2000
4000
90 dlp currents
3 dlp pressures