steffen m. olsen, dmi, copenhagen dk ([email protected]) center for ocean and ice interpretation of...

Steffen M. Olsen, DMI, Copenhagen DK ([email protected])

Center for Ocean and Ice

Interpretation of simulated exchange across the Iceland Faroe Ridge in a global model

Steffen M. Olsen

with contributions from

Bogi Hansen, Svein Østerhus, Detlef Quadfasel, Héðinn Valdimarsson

ChallengeSimulation of ocean exchanges across a shallow ridge

3.5±0.5 Sv. Contributes about half the ocean heat transport to the Arctic



Model limitationsIn the coarse resolution global model analyzed simulated transport across the IFR consists solely of Atlantic Inflow

-Used here are hindcast results based on the NEMO/ORCA1 code, the ocean-sea ice component of the EC-Earth coupled climate model (CMIP5)

Estimates of the IF overflow are uncertain but the total flow is likely not higher than 1 Sv



Model-data discrepancy during the 2003-event

Contribution from the two components are negatively correlated: (r=-0.71)

Inflow on the Faroe side of the Ridge correlates reasonably with observed total transport (r=0.55) but with about half the variability.



Hypothesis

Model IF-transport which per definition is the net transport compares with the true net volume transport: Atlantic inflow - dense overflow

Requires model verification of the barotropic forcing

To explain apparent discrepancy between observed and simulated inflow requires the (unresolved) IF-overflow to be weak during the event.

Need for observational support



The anomalous conditions during the

2003-event

A strong increase in the meridional (right) wind stress characterized the JFM conditions during the 2003-event.

Correlations with the local meridional (blue) and zonal (red) wind forcing are weak both for the model inflow in on the Iceland (top) and Faroe side (bottom).



Observed IF-inflow and sea-level

The IF inflow is controlled by the sea-level drop across the Ridge described by hu-hd (Hansen et al. 2010)

During the 2003 event this forcing was reduced.



The anomalous sea-level during the 2003-

event

The regional pattern of sea-level compares with observed changes.

The model do reproduce the drop in hu-hd during the 2003 event.

Only the inflow on the Faroe side (bottom) responds to the sea-level drop but with a reduced strength (55%).



Changes in the depth of the 27.8 interface during the

2003-eventNorth east of the IF-ridge the interface deepens by up to 50m sufficient to explain a large decrease in overflow (simple two-layer models).

This appears to be a baroclinic adjustment to the changes in sea-level whereby also the intermediate water circulation decreases (seen in both model and observations)

Observations (not shown) indicate that this response is delayed by up to a couple of months.



Results from a 2-year ADCP deployment in the Western Valley

200

240

280

320

360

400

Interface height (m)

Interface height

The time-series of reconstructed interface height is produced using a regression model exploiting the linkage to sea-level and fitted to synoptic hydrographic data at section K (red dot)



Reconstructed interface height variations

Evidence of a reduced interface height (above sill level) during the 2003-event as in the model simulation.

Consistent with a reduced overflow over the ridge.



Conclusions

• It is emphasized that model IF-inflow represents the net flux with realistic forcing.

• The IF-overflow is linked to variations in IF-inflow through coupled sea-level and interface height changes.

• The interface deepened sufficiently during the 2003-event to explain a significant decline in overflow.

• This linkage may partly explain the striking discrepancy between model results and observed variations in IF-inflow.

• To completely account for the discrepancy the average IF-overflow during the whole of 2003 should be ~1.5 Sv below the average.

• Such a high overall average is however not supported by the literature.

Biased ocean heat transport to the arctic across the

Iceland Faroe Ridge in global models?

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

http://www.naclim.eu/



Supplementary slides



Inflow Overflow/Outflow

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Overfl

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Inflow

Seasonality links surface inflow the the east with light surface outflow to the west- the eusturine circulation loop.

Weak seasonality in the overflows (consistent with critical flow)

Observed DS overflow distribution is inconsistent with model result – suggests a high level of noise.

Øst

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The net would account for the sum of the light EGC outflow and the denser IFR and WWTR overflows.


•The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299•NACLIM www.naclim.eu

steffen m. olsen, dmi, copenhagen dk ([email protected]) center for ocean and ice interpretation of...

Documents

model inflow

copenhagen dk

ocean heat transport

ice model limitationsin

simulated inflow

oceansea ice component

ice hypothesismodel

ice modeldata discrepancy