Sea-level fluctuations show Ocean Circulation
controls Atlantic Multidecadal Variability
GERARD D. MCCARTHY
IVAN D. HAIGH
JOEL J.-M. HIRSCHI
JEREMY P. GRIST
DAVID A. SMEED
AMO and Ocean Circulation
• The Atlantic is a place of large multi-decadal
variability esp. the Atlantic Multi-decadal
Oscillation of SSTs (AMO)
• The AMO has a range of important climate
impacts (left: from Zhang and Delworth, 2007,
GRL)
• It is widely hypothesised that the AMOC
controls the phases of the AMO through
control of ocean heat content e.g. Delworth
and Mann, 2000, Clim. Dyn.
• … but there are no direct observational
records of sufficient length to prove this
Observations to prove a link
• We use the excellent and long-running tide gauge record along the east
coast of the US to develop a proxy for ocean circulation to show the AMOCAMO link
• Sea-level is an integrated measure of the water column that can be used to
infer circulation
First attempts date
back to Montgomery
in 1938!
McCarthy et al., under
review in Nature,
Ocean control of
decadal Atlantic climate
variability revealed by
sea-level observations
Dynamic sea-level gradient
• We use a different approach from
previous attempts, which focused
on a coastal tide gauge and an
offshore sea level measurement
Mean dynamic sea-level
• We focus on the gradient along the
coast, which reduces the impact of
eddies on circulation estimates (a la
Kanzow et al., 2009)
• This responds to AMOC declines
e.g. Yin et al., Nat. Geoscience,
2009
Response to reduced AMOC
Sea-level dataset
• This coastline is subject to Glacial Isostatic adjustment and land subsidence
effects
• To study dynamic sea level we remove a linear trend and de-seasonalise
• The tide gauges are highly meridionally coherent, breaking into two groups
north and south of Cape Hatteras
Level (m) (arbitrarily offset)
Sea Level Anomalies
Site No.
3
30
2.5
25
2
20
1.5
15
1
10
0.5
5
0
1900
1920
1940
1960
Year
1980
2000
Sea-level dataset
• This coastline is subject to Glacial Isostatic adjustment and land subsidence
effects
• To study dynamic sea level we remove a linear trend and de-seasonalise
Site no.
• The tide gauges are highly meridionally coherent, breaking into two groups
north and south of Cape Hatteras
Corr.
1
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0.95
0.9
0.85
0.8
0.75
0.7
0.65
0.6
0.55
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Site no.
A sea-level index of circulation
• The sea-level index projects onto the circulation in the intergyre region, which
has links to both overturning and gyre but most importantly heat transport into
subpolar gyre
• The AMO is a subpolar focused phenomenon so heat transport into it is the
crucial factor
mean dynamic
topo contours
SST anomalies
>0.5ºC during warm
AMO phase (hatched)
Bermuda-based index
• Ezer suggested the difference of sea level from Bermuda to Atlantic city as an
AMOC proxy
• However, Sturges and Hong (1995) had shown decadal sea level oscillations at
Bermuda could be reproduced using a Rossby wave model
Bermuda
Relationship to NAO
• The NAO forces this circulation
• The NAO leads the sea-level index by a year and is significantly correlated
(r=0.71 at the 98% level, 1950-2012; r=0.61 at the 98% level, 1920-2012)
Relating to heat content changes
•
•
•
•
Our sea-level index estimates circulation
Circulation is proportional to heat transport e.g. Johns et al, 2011, J. Clim.
Therefore accumulation of the sea-level index estimates heat content
In fact, as a measure of transport in the intergyre region, it leads subpolar
heat content by 2 years
Relationship to AMO
• The 7-year sea-level index leads the 7-year rate of change of the AMO
by 2 years and is significantly correlated (r=0.51, significant at the 96%
level)
• Therefore the index offers predictability of the AMO
NAO, sea-level index and AMO
• Extending back to the 1920’s, the relationship between the accumulated NAO,
accumulated sea-level index and AMO holds
On forcing:
AMO and Ocean Circulation
• Using the meridional gradient of dynamic sea level along the US east coast, we
provide the first observational evidence that ocean circulation drive the phases
of the AMO
• The NAO forces the circulation changes, which the ocean integrates as heat
content and returns as the AMO
• Periods of accelerated sea-level rise along the US east coast (Sallenger et al.
and others) should be listed as a climate impact of the AMO
• The sea level offers a predictability to the AMO: it indicates that we are
transitioning to a negative AMO at present
McCarthy et al., under
review in Nature,
Ocean control of
decadal Atlantic climate
variability revealed by
sea-level observations