1. Transport in the Subpolar and Subtropical North Atlantic
Johannes Karstensen GEOMAR Helmholtz Centre for Ocean Research Kiel
With input from: Jürgen Fischer, Rainer Zantopp, Martin Visbeck, Marcus Dengler

2. Oceanic Transports and the Thermohaline Circulation
The Atlantic meridional overturning circulation consists of a poleward net transport of warm water at/near the surface and a southward net flow of cold deep water
The flow is a key component of the Earth’ s climate system and therefore the strength of the “flow”, its characteristic, and its pathways must be determined and understood

3. Oceanic Transports and the Thermohaline Circulation
Unfortunately the THC “flows” are NOT swift, coherent currents easy to observe
Near surface flow does not show exchange between SP/ST gyre
DWBC has recirculations, interior ocean pathways, eddies & waves influence the flow
Processes may be VERY local but with downstream effect – e.g. generation of anomalies (Transport, heat, freshwater, substances) and their traceability if complex
Surface drifter data: virtually no gyre/gyre exchange
DWBC is “broad” full of small scale variability

4. Oceanic Transports and the Thermohaline Circulation
Unfortunately the THC “flows” are NOT swift, coherent currents easy to observe
Near surface flow does not show exchange between SP/ST gyre
DWBC has recirculations, interior ocean pathways, eddies & waves influence the flow
Processes may be VERY local but with downstream effect – e.g. generation of anomalies (Transport, heat, freshwater, substances) and their traceability if complex
Impact of overturning “flow” variability on SST variability remains to be shown
Surface drifter data: virtually no gyre/gyre exchange
DWBC is “broad” full of small scale variability

5. Sea Surface Temperature trends 1900-2008
Regional Warming of the Oceans (Wu et al 2012)
Regional difference are quite apparent even when averaging over 100 years. The combined model-data analysis suggests that the main boundary currents might have shifted poleward.
Warming rates in °C pro century after removing the global average of 0.62.
Sea Surface Temperature trends

6. Circulation of DSOW and NEADW in the SPNA
Different overflow source regions along the Greenland/Scotland ridges
DWBC manifests itself along the eastern continental slop of Greenland
Interaction of the Deep water and surface waters at multiple places – maybe most intense in the Overflow regions

7. Circulation of LSW and upper water masses in the SPNA
Warm/saline North Atlantic Water enter the SPNA from the south
Joints the WBC east of Greenland
Low saline water entering the SPNA via the East Greenland Current and Davis strait
Deep convection regions with impact on DWBC flow C

8. What do we know about the Transport
What do we know about the Transport? Examples from the Cape Farewell section
Sarafanov et a (JGR)
Wide range of transports in the DWBC (4-16 Sv)
Different methodologies to derive transports
Variability?

9. Time scales of Transport Fluctuations in the DWBC
Recent compilation by Jürgen Fischer (who unfortunately can’t be here today)

10. Time scales of Transport Fluctuations in the DWBC

11. VIKING 1/20° model variability
Labrador
Greenland
VIKING 1/20° model variability
High resolution model captures variability well:
At the boundary is at 3 to 20 days
In the interior gyre is at 40 to 120 days
Sensitive to the bottom boundary layer parameterization in the model
Labrador
Greenland
5 days
10 days
120 days

12. Observation: Interior versus boundary
Observations confirm a change in spectral peak towards longer periods in the interior
Interior: 40 days
Boundary: 10 days

13. Temperature evolution at western boundary
Where does this warming it originates from?
How does this warming trend propagate and what is the role of the DWBC in communicating the warming to the rest of the deep ocean?

14. Large scale warming of Labrador Sea
Dynamic response to warming? (density changes?)
Center of Convection:
Boundary Current:

15. Diurnal Variability in DWBC
Diurnal variability: 14 hours Yo-YO CTD station

16. Summary
Moored arrays are a key element of the international AMOC observing system
Transports of deep water masses show variability on different time scales but overall have been remarkably constant over the last decade (and within the uncertainty of our estimates)
Variability is strongest at the core of the deep flow with periods in the range of weeks rather than months an no significant seasonality
Variability within the interior is at much lower frequencies (about 120days) indicating that flow/topography interaction play an important role in generating this fluctuations (implications for models?)
Through local recirculation and other processes (e.g. feeding cold, fresh water from the East Greenland Current into the DWBC) traceability of anomalies is complex
Only a comprehensive & coordinated observing system will allow to monitor the AMOC components on the multiple time and space scales of its variability

17. Embedded in national/international programs
ICES
VITALS
OSNAP
RACE
Line W
RAPID