Transport in the Subpolar and Subtropical North Atlantic

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. 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 1900-2008

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?Examples from the Cape Farewell section • Wide range of transports in the DWBC (4-16 Sv) • Different methodologies to derive transports • Variability? Sarafanov et a. 2012 (JGR)

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. Labrador Greenland VIKING 1/20° modelvariability 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 Greenland Labrador 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. 53°N 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