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

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

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

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

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 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

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

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)

Time scales of Transport Fluctuations in the DWBC

Recent compilation by Jürgen Fischer (who unfortunately can’t be here today)

Time scales of Transport Fluctuations in the DWBC

120 days

10 days

5 days

Labrador

Labrador

Greenland

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

Observation:Interior versus boundary

Interior: 40 days Boundary: 10 days

Observations confirm a change in spectral peak towards longer periods in the interior

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?

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

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

Embedded in national/international programs

OSNAPVITALS

RAPID

RACE

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