Eleanor Frajka-Williams (Univ of Southampton)

Grace (NASA/JPL)

RRS Discovery

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Estimating the Atlantic overturning at 26N using satellite altimetry

[IUGG general assembly in Prague, Jun 2015]

Questions? @EleanorFrajka

[Kulbrodt et al, 2007]

Overturning circulation

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RAPID-MOCHA project: Observations of the time-varying large-scale ocean circulation

Funded by UK NERC, NSF and NOAA

Single value (the MOC) or components? • Components help us understand where and why the MOC is changing • But the actual value of the MOC is also important

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What do we really want to know?

Volume or Heat transport?

MOC timescales of variability: • Eddies on 20-100 day timescales (Clement et al. 2014; Frajka-Williams et al. 2013) • Wind-variability on interannual timescales (Yang & Johns 2014)

• Buoyancy-driven variability …?[Johns et al., 2011]

[Frajka-Williams 2015]

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In this talk:Introduce a proxy for the MOC at 26N

that recovers over 90% of the interannual variability of

the RAPID time series from 2004-2014.

Tell you why it doesn’t replace the in situ observations.

Data: RAPID transbasin transport

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MOC = EK + GS + UMO

For details of the method, see McCarthy et al. 2015, Measuring the MOC

EK (meridional Ekman) from ERA-Interim GS (Gulf Stream) from Florida Cable UMO (upper mid-ocean transport, Bahamas to Africa) from current meter & dynamic height moorings

Data: RAPID transbasin transport

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MOC = EK + GS + UMO

For details of the method, see McCarthy et al. 2015, Measuring the MOC

EK (meridional Ekman) from ERA-Interim GS (Gulf Stream) from Florida Cable UMO (upper mid-ocean transport, Bahamas to Africa) from current meter & dynamic height moorings

Method

Temporal:Remove seasonal cycle1.5 year Tukey filter

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AVISO Sea level anomaly (SLA):

RAPID upper mid-ocean transport time series (UMO):

Focus on the interannual variability…

Remove eddies…Spatial:Smooth (5x10 deg):

Regress RAPID UMO against SLA

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AVISO SLA: RAPID UMO transport:

[Frajka-Williams 2015]

Regress RAPID UMO against SLA

Method

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[Frajka-Williams 2015]

[Frajka-Williams 2015]

UMO transport is proportional to thermocline depth at the west.

Deeper (more negative) thermocline depth means stronger (more negative) UMO transport.

SLA vs transbasin transport UMO

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[Frajka-Williams 2015]

UMO transport is proportional to thermocline depth at the west.

2 cm change in SLA results in a 1 Sv change in UMO

SLA vs transbasin transport UMO

[Frajka-Williams 2015]

[Frajka-Williams 2015]

From SLA: MOC* = EK + GS + UMO*

Using SLA for UMO, determine MOC

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From RAPID: MOC = EK + GS + UMO

EK from ERA-Interim since 1979 GS from Florida Cable since 1982 UMO* from SLA since 1993

[Frajka-Williams 2015]MOC* since 1993

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This MOC* recovers over 90% of the variability of the RAPID MOC. (note: the two are not independent since both use the same GS and Ek.)

Can we just use SLA to investigate longer term MOC changes?

[Frajka-Williams 2015]

Using SLA for UMO, determine MOC

Single value (the MOC) or components? • Components help us understand where and why the MOC is changing • But the actual value of the MOC is also important

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Recall: What do we really want to know?

Volume or Heat transport?

MOC timescales of variability: • Eddies on 20-100 day timescales (Clement et al. 2014; Frajka-Williams et al. 2013) • Wind-variability on interannual timescales (Yang & Johns 2014)

• Buoyancy-driven variability …?[Johns et al., 2011]

To date, MOC interannual variability has been dominated by wind-forcing (debatable, but evidence suggests yes).

This is consistent with model-based studies (e.g., Yeager 2015; Pillar et al. 2015)

• RAPID observations demonstrate that most of the interannual variability originates in Ekman & UMO transport. • SLA reconstruction works because UMO-SLA relationship is strong.

Buoyancy-driven variability occurs on longer time scales (e.g., Yeager 2015; Pillar et al. 2015)

• Under buoyancy forcing/on longer timescales, not clear that the UMO-SLA relationship would be as strong.

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Why not just use SLA proxy?

The SLA proxy provides a 20-year proxy for MOC variability.

IF the SLA-UMO relationship is stationery,then we can use it to look at lower frequency MOC changes.

Suggests that: • Trend over 2004-2014 does not

continue back in time • Moderate reduction (1 Sv) between

1994 decade & 2004 decade[Frajka-Williams 2015]

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Even so…

Thank you! See: http://eleanorfrajka.com/moc-from-space Questions? @EleanorFrajka