berx et al., ocean sci., 9, 639-654 , 2013
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Bee Berx1, Bogi Hansen2, Svein Østerhus3, Karin Margaretha Larsen2, Toby Sherwin4 and Kerstin Jochumsen5
1 Marine Scotland Science (b.berx@marlab.ac.uk); 2 Faroe Marine Research Institute; 3 University of Bergen; 4 Scottish Association for Marine Science; 5 Universität Hamburg
Integrating altimetry into the estimates of Atlatnic water transport in the Faroe Shetland Channel
Berx et al., Ocean Sci., 9, 639-654, 2013
Why are we interested in the Faroe Shetland Channel (FSC)?
Oceanographic observations in the FSC
Observing Circulation in the FSC
Temperature and Salinity in the FSC
Incorporating altimetry in transport estimates
The assumption of geostrophy allows us to convert sea level height measurements into
current speeds.
http://www.seos-project.eu/modules/oceancurrents/oceancurrents-c06-
s02-p01.html
SEOS Project
Estimating average transport of AW
• Define the AW velocity in the ADCP data as “surface 325m”
• Correlate monthly ADCP velocities with SSH difference
• Calculate mean AW velocities from altimetry in all available months
• Adjust geostrophic profiles from mean temperature & salinity in surface 325m equals altimetry-adjusted AW velocities
Mean velocity cross-section in the FSC
Average volume, relative heat and salt transports in the FSC
NE SW Net (Atl. Inflow)Surface-to-bottom 3.5 Sv -3.1 Sv 0.4 Sv
Warmer than 5°C 3.5 Sv -0.8 Sv 2.7 Sv
Colder than 5°C 0.0 Sv -2.3 Sv -2.3 Sv
Relative Heat Transport 131 TW -24 TW 107 TW
Salt Transport 125 x 106 kg s-1 -27x 106 kg s-1 98 x 106 kg s-1
Compared to the total transport of Atlatnic Water across the GSR = 7.0 Sv
Obtaining monthly estimates of AW Transport: 1. ADCP-based
• ADCP sites given “associated widths”
• Define AW layer as mean depth of 5°C isotherm
• Use average altimetry-adjusted velocities at the boundaries
• Depending on period, either 4 or 7 ADCPs across ~150km wide section
Obtaining monthly estimates of AW Transport: 2. altimetry-based
• Investigated these assumptions with ADCP data
• Adjusted the altimetry-based estimate so the time series mean equals the long-term average calculated previously
Builds on two assumptions: 1. Vertical-average AW-velocity is proportional to
the surface2. The product of (1) with the depth of AW is
approx. constant throughout the section.
Transport of Atlantic water in the FSC
Jan-98 Jan-00 Jan-02 Jan-04 Jan-06 Jan-08 Jan-100
1
2
3
4
5
6
Vol
ume
Tra
nspo
rt (
Sv)
AltimetryADCP
Seasonal cycle in volume transport
Jan Mar May Jul Sep Nov0
1
2
3
4
5
6
Vol
ume
Tra
nspo
rt (
Sv)
Monthly Avg. ADCP-Tr.Seas. Fit ADCPAvg. Alt.-Tr Std. Alt.TrSeas. Fit Alt.
Seasonal amplitude of Atlantic water transport in FSC ~ 0.7-0.9 Sv
This is ~ 25% of the average transport
Maximum transport in winter-time, lowest transport in summer.
Different amplitude based on calculation method
Transport of Atlantic Water in the FSC 1992-2012
Jan-92 Jan-94 Jan-96 Jan-98 Jan-00 Jan-02 Jan-04 Jan-06 Jan-08 Jan-100.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Vol
ume
Tra
nspo
rt (
Sv)
No
long
term
tren
d !
Conclusions
• Combined water property observations (Temperature and Salinity) with current meter and sea elevation observations to estimate transport of Atlantic water through the FSC.
• On average 2.7±0.5 Sv is transported into the Nordic Seas• Net volume transport shows consistent seasonality: maximum
Dec-Jan, and amplitude of 0.7 Sv. • No significant trend in volume transport between 1992 and 2011.• We have observed increases in temperature and salinity, and may
therefore expect trends in relative heat and salt transports (but difficult to verify statistically based on this time series).
• Currently studying whether less variable observations can be made in an area to the south-west of the FIM section
2014 Initiatives close to NACLIM in the FSC
and end-user partners
Brahan HF Radar Demonstration
UK Ocean-Shelf Exchange Study
Thank you.
The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299NACLIM www.naclim.eu
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