Progress in Characterizing AMOC Structure
and Variability from Observations
Bill Johns
RSMAS, University of Miami, Miami FL
Outline:
1.
An AMOC tour from the subpolar gyre to the South
Atlantic
2.
How to move forward in directly observing the AMOC
3.
What have we learned in the last ~5 years?
AMOC Observational Network
U.S. Programs
International Programs
Nordic Seas Overflows
Quadfasel and Käse (2007)
Based on Hansen
et al. (2007),
Macrander et al.
(2005), and
updates
Nordic Seas Overflows (modeled)
Faroe Bank Channel
Olsen et al. (2008)
Blue = obs.
Red = model
Model hindcast of Faroe
Bank overflow during the
observational record
(top), and for the last 50
years (right). Total
Nordic Sea overflow
shown in green (right).
Total overflow
Faroe Bank
Entrainment into the overflows
LSW
4 Sv
Dickson and Brown (1994)
DWBC Measurements at Cape Farewell
Mean transport = 8.6 Sv (σθ > 27.80)
Bacon and Saunders (2010)
Historical DWBC Measurements at Cape Farewell
Baroclinic transport
relative to 1000 db
Mean:
5.5 Sv
Bacon (1998)
Historical DWBC Measurements at Cape
Farewell (updated)
Sarafanov et al. (2009)
Baroclinic
transport
anomaly
relative to
mean value
of 5.5 Sv
Summary of LNADW (DSOW/ISOW) Transports
------- 9 Sv
Haine at el. (2008)
Labrador Sea Water Formation Rates
Kieke et al. (2006)
LSW Production estimates: 2-10+ Sv (Haine et al., 2008)
CFC 11/12 Tracer Inventories (1970-1997):
Average cLSW production: 4.4 – 5.6 Sv
Average uLSW production: 3.2 – 3.3 Sv
Average total LSW production: 7.6 – 8.9 Sv
Labrador Sea Water Formation Rates (’97-’03)
uLSW
cLSW
97-99 99-01 01-03
Kieke et al. (2007)
Export to the Subtropics (48ºN)
Lumpkin et al. (2008)
MOC: 16.2 Sv
Mean UNADW: 7.1 Sv
Mean LNADW: 9.1 Sv
Summary for Subpolar region:
1.
Increasing evidence that overflows are stable (over the modern
record, last 50 yrs)
2.
DWBC at Cape Farewell (overflows+entrainment) is variable on
decadal timescales; mean transport of ISOW/DSOW ~9 Sv (not
13 Sv). Varies by ±30%.
3.
LSW production constrained by tracer inventories. Recent biennial
surveys resolve temporal variability of formation. Mean LSW
production 7.6-8.9 Sv (1970-97), Highly variable. Cycling between
cLSW/uLSW, linked to NAO forcing.
4.
Export to subtropics in MOC (at 48N): 16.2 ± 2.0 Sv (1993-2000).
Relatively stable (recently). LSW: 7.1± 1.4 Sv; LNADW: 9.1±
1.7 Sv.
RAPID/MOCHA 26.5°N Array
Mid-ocean
Array
(Cunningham
et al., 2007)
Florida Current Monitoring by undersea
cable (Baringer and Larson, 2001)
Western Boundary array
(Johns et al., 2008)
MOC streamfunction and layer transports
(Rapid Array 26.5ºN)
MOC: 18.5 Sv
UNADW: 8.2 Sv
LNADW: 12.3 Sv
uLSW
2.8 Sv
cLSW
5.4 Sv
ISOW
4.8 Sv
DSOW
7.5 Sv
AABW
2.1 Sv
AMOC Variability at 26.5ºN
Contributions
to northward
flowing (upper
ocean) part of
AMOC cell
MOC
variability
range:
5-30 Sv
Kanzow et al. (2010)
MOC and Heat Transport Variability
3.5 year mean MOC: 18.5 ± 4.9 (3.8*) Sv (σerr = 2.1 Sv)
mean MHT: 1.33 ± 0.40 (0.24*) PW (σerr = 0.12
PW)
*with contribution by Ekman transport variability removed
AMOC variability spectrum at 26.5ºN
Contributions
to northward
flowing (upper
ocean) part of
AMOC cell
Kanzow et al. (2010)
AMOC seasonal cycle at 26.5ºN
AMOC seasonal
cycle and
seasonal
contributions to
upper ocean
part of AMOC
cell
The interior transport
(TUMO) cycle can be
explained by linear,
forced Rossby wave
response to wind
stress curl, contained
mostly in eastern
basin
Kanzow et al. (2010)
26.5ºN in perspective
CCSP (2008): Abrupt Climate Change
Bryden (2005)
MOC values
after application
of seasonal
correction
Kanzow et al.
(2010)
Synthesis model ensemble
South Atlantic
“SAMOC”
Program
MOC and
MHT
estimates at
35ºS (AX18)
South Atlantic (35ºS)
Dong et al. (2009)
17 transects (2002-2007):
Mean MOC: 17.9 ± 2.2 Sv
Mean MHT: 0.55 ± 0.14 PW
AMOC Pathways/Processes
Complex AMOC
structure in South
Atlantic (relative to N.
Atlantic subtropics):
• Eddy transport
processes important in
both upper and lower
limbs
• Contributions of
warm/salty (Agulhas)
and cool/fresh
(AAIW/SAMW) to the
upper limb
• Interaction of DWBC
with equator; interior
NADW pathways; deep
water mass
transformation
Lumpkin and Speer (2007)
Schematic: R. Lumpkin NOAA/AOML
NADW Transformation in S. Atlantic
AMOC streamfunction in density coordinates
Lumpkin and Speer (2007)
AMOC Monitoring Strategy
Establish discrete set of
trans-basin arrays
(moorings +
autonomous profiling)
for continuous AMOC
estimates
Value:
• Accurate multi-year
mean AMOC estimates, for
comparison with future
(and past) AMOC states
• Understanding of
processes underlying
short-term (intraseasonal
to annual) variability
• Benchmarks for
evaluation of modeled
AMOC variability (GCMs,
data synthesis models)
O-SNAP
RAPID
MOVE
SAMOC
Lumpkin and Speer (2007)
Profiling Floats and AMOC Monitoring
AMOC
Variability at
41ºN from
ARGO/altimetry
Willis (2010)
What have we learned in the past ~5 years?
1.
Increasing evidence that overflows are stable (over the modern
record, last 50 yrs). -> Denmark Strait and Iceland-Faroes Ridge
monitoring remains challenging.
2.
Mean transport of ISOW/DSOW at Cape Farewell appears to be
~9 Sv (not 13 Sv). Varies by ±30% on decadal timescales.
-> Entrainment variability? LSW “blocking” at Gibbs?
3.
LSW production can be temporally monitored by transient tracers.
Mean LSW production 7.6-8.9 Sv (1970-97). Cycling between
cLSW/uLSW, w/ link to NAO forcing. -> How to monitor going
forward (SF6 )? Pathways of export to the subtropics?
4.
LSW makes up nearly half of the deep limb of the AMOC.
48ºN: LSW: 7.1 Sv; DSOW/ISOW: 9.1 Sv.
26ºN: LSW: 8.2 Sv; DSOW/ISOW (minus AABW): 10.2 Sv.
-> How are variations in LSW production reflected in export to
subtropics? Modulating/buffering processes?
What have we learned in the past ~5 years?
1.
Large short-term (intraseasonal to annual) MOC variability in
subtropics. Ekman forcing dominates at intraseasonal; geostropic
variability dominates on longer time scales (annual+). Annual MOC
cycle documented and its fundamental mechanism explained.
2.
AMOC snapshots derived from single hydrographic sections can be
subject to considerable aliasing. The interior baroclinic flow cannot
be assumed steady. The Bryden (2005) “trend” can be largely
explained by seasonal aliasing.
3.
MOC strength is fairly uniform throughout the basin. (16-18 Sv).
Minor “internal” closure. -> How does the partitioning of internal
components vary? uLSW/CLSW? Agulhas leakage vs. AAIW?
4.
Complex NADW transformation processes in the S. Atlantic. DWBC
eddies; interior pathways -> eastern boundary “DWBC”. Significant
upward shift in mean density of NADW limb. -> Equatorial
mixing/deep jets?
Questions/Issues on Variability:
1. Response of MOC to variation in deep water formation rates
(convective + overflow)?
- LSW formation rates have large interannual variation (and still
disagreement on the “mean” formation rate). How are these variations
connected to export, and MOC variability?
- New results (Bacon and Saunders, 2010) call into question accepted
magnitude of DSOW/ISOW entrainment and DWBC flux at Cape Farewell –
while sill overflows appear ~steady. Does this imply a reduction in LNADW limb
of AMOC? What is the “transfer function” between overflows and LNADW part
of MOC? Is it stable/robust?
Questions/Issues on Variability:
2. Inter-gyre AMOC coherence/connectivity?
- Models suggest “breakpoints” in AMOC coherence at certain latitudes.
Where and for what time scales? Mechanisms?
HadCM3 (coupled)
OCCAM (forced)
Bingham
et al.
(2007)
Questions/Issues on Variability:
3. Inter-hemispheric (and global) AMOC coherence/connectivity?
- Role of “equatorial buffer” in AMOC meridional coherence?
- Influences arising from S. Atlantic (e.g. change of partitioning between
warm/cold routes)? Changes external to Atlantic?
HadCM3
l
Johnson and Marshall (2002)
Dong and Sutton (2003)
Questions/Issues on Variability:
HadCM3
4. Can we devise a long-term strategy for
observing coherent modes of
interannual/decadal variability?
Forced
Coupled
Bentsen et al. (2004)
Bingham et al. (2007)
Questions/Issues on Variability:
5. Are we prepared to observe and document anthropogenicallyforced AMOC changes?
IPCC/AR4
A1-B
scenario
runs
(Schmittner
et al.,
2005)
Discussion…
Atlantic MOC
ECCO-50y
ECCO-SIO
ECCO-GODAE
ECCO-JPL
GFDL
INGV
SODA