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  

krltracey@mail.uri.edu                      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

)

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0

64oW 62oW 60oW

62oS

60oS

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56oS

10 cm s−1

64oW 62oW 60oW

62oS

60oS

58oS

56oS

dept

h (m

)

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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  

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Line

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vel

ocity

(m s−1

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M03

C16

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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

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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.

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C10 24’ 12’

63oW 48’ 36’

32’ 28’

58oS 24.00’

20’ 16’

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Line normal velocity (m s−2)

SFZ

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Correlation

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SACCF PF SAF

South−to−North Distance (km)

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