http://www.smi.ac.uk/images/north-atlantic-surface-currents-sams/view

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