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

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

[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

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?

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

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

Regress RAPID UMO against SLA

Temporal: Remove seasonal cycle1.5 year Tukey filter

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

Focus on interannualvariability…

Spatial:Smooth (5x10 deg):

Remove eddies…

AVISO Sea level anomaly (SLA):

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 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


From SLA:

Using SLA for UMO, determine MOCFrom 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

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

http://eleanorfrajka.com/moc-from-space

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?


Using SLA for UMO, determine MOC

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…

Caveats aside…


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



Caveats aside…



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


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?


Note: At RAPID we used the transbasin UMO transport (top 1000m, after applying barotropic compensation)


Why does this work?

at 16N, the west (mostly) dominates interannual variability of dynamic height, and transbasin transport.


Relatively simple vertical structure at MOVE

MAR 16N

West 16N


Using the MOVE time series between the islands and the MAR, filter and compare with spatially-smoothed, filtered SSH.


Does this work elsewhere? MOVE 16N


[Frajka-Williams


Potential to use SLA (& GRACE) to help connect the dots between latitudes in the Atlantic.

Synergies between latitudes/datasets


[Frajka-Williams



Maybe even on longer timescales?


1990 1995 2000 2005 2010−3

−2

−1

0

1

2

Using the SSHA at 30N and −75W

Tran

spor

t [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


[Frajka-Williams



What about: • longer timescales? • more depth structure? • outside the subtropics? • regions with less steep

topography? • transports that rely less on

dynamic height?

Thank you!




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



New project: MerMEED at 26N Mechanisms responsible for mesoscale eddy energy dissipation

Small boat fieldwork, mixing, gliders

Will be looking for a postdoc!


Science

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