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Estimating the Atlantic
overturning at 26N using
satellite altimetry
Eleanor Frajka-Williams (Univ of Southampton)
RRS Dis...
[Kulbrodt et al, 2007]
The Meridional Overturning Circulation
(MOC)
RAPID-MOCHA project:
Observations of the time-varying
...
Single value (the MOC) or components (Gulf Stream, mid-ocean…)?
• Components & their variability help us understand where ...
1. Introduce a proxy for the MOC at 26N that recovers over 90% of the
interannual variability of the RAPID time series fro...
Data: RAPID transbasin transport
MOC = EK + GS + UMO
For details of the method, 

see McCarthy et al. 2015, Measuring the ...
Regress RAPID UMO against SLA
Temporal:

Remove seasonal cycle

1.5 year Tukey filter
Method:
RAPID upper mid-ocean
transpo...
Method:
RAPID upper mid-ocean
transport time series (UMO):
Regress RAPID UMO against SLA
AVISO Sea level anomaly (SLA):
[Frajka-Williams 2015]
UMO transport is proportional to thermocline
depth at the west.
Deeper (more negative) thermocline ...
[Frajka-Williams 2015]
From SLA:
Using SLA for UMO, determine MOC
From RAPID:
MOC = EK + GS + UMO
MOC* since 1993
MOC* = E...
Why does this work?
at 26N, the west dominates interannual
variability of dynamic height, and
transbasin transport.
SLA pr...
This MOC* recovers over 90% of the variability of the RAPID MOC.
(note: the two are not independent since both use the sam...
To date, MOC interannual variability dominated by wind-forcing
(debatable, but evidence suggests yes).
• This is consisten...
Caveats aside…
[Frajka-Williams 2015]
SLA proxy for MOC suggests
• Moderate reduction (1 Sv) in the AMOC between 1994- & 2...
Caveats aside…
[Frajka-Williams 2015]
SLA proxy available at: http://eleanorfrajka.com/moc-from-space
SLA proxy for MOC su...
MOVE 16N: 15 years of observations,
w/slightly different measurement
principle than 26N:
• Western basin only: from easter...
Why does this work?
at 16N, the west (mostly) dominates
interannual variability of dynamic height,
and transbasin transpor...
Using the MOVE time series between the islands and the MAR, filter and
compare with spatially-smoothed, filtered SSH.
SLA pr...
[Frajka-Williams
SLA proxy available at: http://eleanorfrajka.com/moc-from-space
Potential to use SLA (& GRACE) to
help co...
[Frajka-Williams
SLA proxy available at: http://eleanorfrajka.com/moc-from-space
Potential to use SLA (& GRACE) to
help co...
[Frajka-Williams
SLA proxy available at: http://eleanorfrajka.com/moc-from-space
Potential to use SLA (& GRACE) to
help co...
Potential to use SLA (& GRACE) to
help connect the dots between
latitudes in the Atlantic.
What about:
• longer timescales...
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Using satellite observations to broaden our spatial view of AMOC variability [RAPID mtg]

This is longer version of a similar talk from IUGG in Prague. The main differences are the addition of results for 16N and more evidence for why the altimetry-transport relationship works at these latitudes.

Climate simulations predict a slowing of the AMOC in the coming years, while present day observations from boundary arrays demonstrate substantial variability on weekly- to interannual timescales. These arrays are necessarily limited to individual latitudes. How well does satellite altimetry replicate transbasin, full-depth ocean transports? Can we use satellite altimetry to extend our estimate of AMOC variability back in time (at 26N)? Do the spatial patterns of SSH variability help to broaden our view of AMOC strength beyond individual latitudes? This analysis complements in situ observational efforts to measure the MOC at multiple latitudes.

http://www.rapid.ac.uk/ic15/programme.php

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Using satellite observations to broaden our spatial view of AMOC variability [RAPID mtg]

  1. 1. Estimating the Atlantic overturning at 26N using satellite altimetry Eleanor Frajka-Williams (Univ of Southampton) RRS Discovery [RAPID/USAMOC meeting in Bristol, July 2015] Questions? @EleanorFrajka
  2. 2. [Kulbrodt et al, 2007] The Meridional Overturning Circulation (MOC) RAPID-MOCHA project: Observations of the time-varying
 large-scale ocean circulation in the Atlantic 10 years of MOC (2004-2014)
 Funded by UK NERC, NSF and NOAA
  3. 3. Single value (the MOC) or components (Gulf Stream, mid-ocean…)? • Components & their variability help us understand where and why the MOC is changing • But the actual value of the MOC is also useful Volume vs Heat transport [Johns et al., 2011] Different forcing mechanisms for different timescales • Eddies on 20-100 day timescales (e.g. Clement et al. 2014; Frajka-Williams et al. 2013) • Wind-variability on seasonal - interannual timescales 
 (e.g. Zhao & Johns 2014, Yang 2015, Pillar et al. 2015, Yeager 2015) • Buoyancy-driven variability … longer timescales? What do we really want to know?
  4. 4. 1. Introduce a proxy for the MOC at 26N that recovers over 90% of the interannual variability of the RAPID time series from 2004-2014.
 
 
 
 
 
 
 
 
 
 
 2. Highlight a few important caveats In this talk: Paper in GRL 2015
  5. 5. Data: RAPID transbasin transport 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
  6. 6. Regress RAPID UMO against SLA Temporal:
 Remove seasonal cycle
 1.5 year Tukey filter Method: RAPID upper mid-ocean transport time series (UMO): Focus on interannual
 variability… Spatial:
 Smooth (5x10 deg): Remove eddies… AVISO Sea level anomaly (SLA):
  7. 7. Method: RAPID upper mid-ocean transport time series (UMO): Regress RAPID UMO against SLA AVISO Sea level anomaly (SLA):
  8. 8. [Frajka-Williams 2015] UMO transport is proportional to thermocline depth at the west. Deeper (more negative) thermocline depth means stronger (more negative) UMO transport. 2 cm rise in SLA results in a 1 Sv southward intensification of UMO SLA vs transbasin transport UMO
  9. 9. [Frajka-Williams 2015] From SLA: Using SLA for UMO, determine MOC From RAPID: MOC = EK + GS + UMO MOC* since 1993 MOC* = EK + GS + UMO* EK from ERA-Interim since 1979 GS from Florida Cable since 1982 UMO* from SLA since 1993
  10. 10. Why does this work? at 26N, the west dominates interannual variability of dynamic height, and transbasin transport. SLA proxy available at: http://eleanorfrajka.com/moc-from-space Relatively simple vertical structure at RAPID East 26N West 26N
  11. 11. 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 use SLA to investigate MOC changes beyond 2004-2014? Does SLA inform our understanding of the spatial structure of MOC variability? [Frajka-Williams 2015] Using SLA for UMO, determine MOC
  12. 12. To date, MOC interannual variability dominated by wind-forcing (debatable, but evidence suggests yes). • This is consistent with model-based studies (e.g., Yeager 2015; Pillar et al. 2015; Yang et al. 2015; Zhao and Johns 2014) RAPID observations demonstrate that most of the interannual variability originates in Ekman and UMO transport. • Sea level reconstruction works because the 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 UMO-SLA relationship would work. • Details in the SLA - steric height relationship more likely to change on longer timescales First, a couple caveats…
  13. 13. Caveats aside… [Frajka-Williams 2015] SLA proxy for MOC suggests • Moderate reduction (1 Sv) in the AMOC between 1994- & 2004- decades • Strong (0.5 Sv/year) trend from 2004- decade is not continued back in time SLA proxy available at: http://eleanorfrajka.com/moc-from-space
  14. 14. Caveats aside… [Frajka-Williams 2015] SLA proxy available at: http://eleanorfrajka.com/moc-from-space SLA proxy for MOC suggests • Moderate reduction (1 Sv) in the AMOC between 1994- & 2004- decades • Strong (0.5 Sv/year) trend from 2004- decade is not continued back in time
  15. 15. MOVE 16N: 15 years of observations, w/slightly different measurement principle than 26N: • Western basin only: from eastern edge of Caribbean to mid-Atlantic ridge • Primary observations below 1000 m • Geostrophic reference level of 1200 m Does this work elsewhere? SLA proxy available at: http://eleanorfrajka.com/moc-from-space Note: At RAPID we used the transbasin UMO transport (top 1000m, after applying barotropic compensation)
  16. 16. Why does this work? at 16N, the west (mostly) dominates interannual variability of dynamic height, and transbasin transport. SLA proxy available at: http://eleanorfrajka.com/moc-from-space Relatively simple vertical structure at MOVE MAR 16N West 16N
  17. 17. Using the MOVE time series between the islands and the MAR, filter and compare with spatially-smoothed, filtered SSH. SLA proxy available at: http://eleanorfrajka.com/moc-from-space Does this work elsewhere? MOVE 16N
  18. 18. [Frajka-Williams SLA proxy available at: http://eleanorfrajka.com/moc-from-space Potential to use SLA (& GRACE) to help connect the dots between latitudes in the Atlantic. Synergies between latitudes/datasets
  19. 19. [Frajka-Williams SLA proxy available at: http://eleanorfrajka.com/moc-from-space Potential to use SLA (& GRACE) to help connect the dots between latitudes in the Atlantic. Maybe even on longer timescales? Synergies between latitudes/datasets 1990 1995 2000 2005 2010 −3 −2 −1 0 1 2 Using the SSHA at 30N and −75W Transport[Sv] SSHA proxy UMO −2 0 2 −0.26 −0.24 −0.22 −0.2 R=0.81 UMO transport [Sv] SSHA[m] 30 year NEMO 1/12 run
  20. 20. [Frajka-Williams SLA proxy available at: http://eleanorfrajka.com/moc-from-space Potential to use SLA (& GRACE) to help connect the dots between latitudes in the Atlantic. What about: • longer timescales? • more depth structure? • outside the subtropics? • regions with less steep topography? • transports that rely less on dynamic height? Thank you! Synergies between latitudes/datasets
  21. 21. Potential to use SLA (& GRACE) to help connect the dots between latitudes in the Atlantic. What about: • longer timescales? • more depth structure? • outside the subtropics? • regions with less steep topography? • transports that rely less on dynamic height? Thank you! [Frajka-Williams SLA proxy available at: http://eleanorfrajka.com/moc-from-space Synergies between latitudes/datasets New project: MerMEED at 26N Mechanisms responsible for mesoscale eddy energy dissipation Small boat fieldwork, mixing, gliders Will be looking for a postdoc!

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