Transport in the Subpolar and Subtropical North Atlantic Johannes Karstensen GEOMAR Helmholtz Centre for Ocean Research Kiel With input from: Jrgen Fischer,

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Transport in the Subpolar and Subtropical North AtlanticJohannes Karstensen GEOMAR Helmholtz Centre for Ocean Research Kiel

With input from: Jrgen Fischer, Rainer Zantopp, Martin Visbeck, Marcus Dengler

1Oceanic 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 waterThe 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

Oceanic Transports and the Thermohaline Circulation Unfortunately the THC flows are NOT swift, coherent currents easy to observeNear surface flow does not show exchange between SP/ST gyreDWBC has recirculations, interior ocean pathways, eddies & waves influence the flowProcesses 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 broadfull of small scale variability

Oceanic Transports and the Thermohaline Circulation Unfortunately the THC flows are NOT swift, coherent currents easy to observeNear surface flow does not show exchange between SP/ST gyreDWBC has recirculations, interior ocean pathways, eddies & waves influence the flowProcesses may be VERY local but with downstream effect e.g. generation of anomalies (Transport, heat, freshwater, substances) and their traceability if complexImpact of overturning flow variability on SST variability remains to be shown

Surface drifter data:virtually no gyre/gyre exchange

DWBC is broadfull of small scale variability

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.

Sea Surface Temperature trends 1900-2008 Warming rates in C pro century after removing the global average of 0.62.

Circulation of DSOW and NEADW in the SPNADifferent overflow source regions along the Greenland/Scotland ridgesDWBC 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

Circulation of LSW and upper water masses in the SPNAWarm/saline North Atlantic Water enter the SPNA from the southJoints the WBC east of GreenlandLow saline water entering the SPNA via the East Greenland Current and Davis straitDeep convection regions with impact on DWBC flowCWhat do we know about the Transport?Examples from the Cape Farewell sectionWide range of transports in the DWBC (4-16 Sv)Different methodologies to derive transportsVariability?

Sarafanov et a. 2012 (JGR)

Time scales of Transport Fluctuations in the DWBCRecent compilation by Jrgen Fischer (who unfortunately cant be here today)

Time scales of Transport Fluctuations in the DWBC

120 days10 days5 daysLabradorLabradorGreenlandGreenlandVIKING 1/20 modelvariabilityHigh resolution model captures variability well: At the boundary is at 3 to 20 daysIn the interior gyre is at 40 to 120 daysSensitive to the bottom boundary layer parameterization in the modelObservation:Interior versus boundary

Interior: 40 daysBoundary: 10 daysObservations confirm a change in spectral peak towards longer periods in the interior

53NTemperature evolution at western boundaryWhere 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?13

Center of Convection:Boundary Current:Large scale warming of Labrador Sea Dynamic response to warming? (density changes?)Diurnal Variability in DWBCDiurnal variability: 14 hours Yo-YO CTD station

SummaryMoored arrays are a key element of the international AMOC observing systemTransports 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 seasonalityVariability 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 complexOnly a comprehensive & coordinated observing system will allow to monitor the AMOC components on the multiple time and space scales of its variability Embedded in national/international programs

OSNAPVITALSRAPID RACELine WICES17

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