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An interdecadal oscillatory mode of the AMOC related to ocean dynamics and temperature variations Alexey Fedorov and Florian Sevellec Yale University June 2010. IPCC 2001. Variability in the decadal to inter-decadal bands: 10-30 years. IPCC 2007. Frankcombe et al 2008. - PowerPoint PPT Presentation
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An interdecadal oscillatory mode of the AMOC related to ocean dynamics and temperature
variations
Alexey Fedorov and Florian Sevellec
Yale University
June 20101
IPCC 2001
IPCC 2007
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Variability in the decadal to inter-decadal bands: 10-30 years
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A Hovmoller diagram of observed temperature anomalies averaged between 300-400m and over
10–60oN across the North Atlantic (XBT data)
Frankcombe et al 2008
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Ocean GCM:
OPA 8.22o global configuration31 levels (ORCA2)
We use linearized forward and adjoint versions of the model
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Non-autonomous
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The least-damped mode: AMOC variations
A
AM
OC
(Sv)
Period = 24 yearsDamping T = 40 years
quarter
phase
B
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A B
TEMPERATURE
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A Hovmoller diagram for temperature anomalies averaged 0-
1000m,30-60oN for the mode
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A B
TEMPERATURE
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A B
SALINITY
TEMPERATURE
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Tem
pera
ture
gra
dien
t
Temperature AnomaliesMODE MECHANISM:
Westward propagation of large-scale temperature
anomalies? A
B
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IDEALIZED MODEL
- temperature of the upper layer
- Thermal wind balance + baroclinicity condition
- Equivalent anomalous westward advection
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Upp
er la
yer
dept
h h,
m
OSCILLATION PERIOD (IDEALIZED MODEL)
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Adjoint mode: Non-normality!
A B
Summary:
We have rigorously shown the existence of an interdecadal, weakly-damped oscillatory mode of the AMOC (T≈ 24 years, Tdamping≈ 40 years)
AMOC variations are related to westward-propagating temperature anomalies in the upper 1000m between 30-60oN
This westward propagation results from a competition between(1) Mean zonal eastward advection(2) Equivalent anomalous westward advection due to the mean
meridional temperature gradient(3) Westward advection typical of Rossby waves (the -effect)
The system is non-normal: atmospheric noise can efficiently excite this mode
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The least-damped mode: AMOC variations and temperature variations (averaged 30-60oN,0-1000m)
A
AM
OC
(Sv)
Tem
perature ( oC)
Period = 24yearsDamping T = 40years
quarter
phase
B
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Meridional density gradient is controlled by temperature
Meridional density gradient is
controlled by salinity
MODE MECHANISM:
Density ratio
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IDEALIZED MODEL
- average temperature of the upper layer
- Thermal wind balance + baroclinicity condition
Assumptions:
- Equivalent anomalous westward advection
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IDEALIZED MODEL
- average temperature of the upper layer
- Thermal wind balance + baroclinicity condition
Assumptions:
- Equivalent anomalous westward advection
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A B
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Figure 1. (a) Records from the eastern boundary of theNorth Atlantic (from Tenerife to Norway). Black circles arethe individual tide gauge records and the average is shownby the red curve. (b) As for Figure 1a but for the westernboundary of the North Atlantic (from Panama to Newfoundland).The time series from each tide gauge was linearlydetrended before averaging. (c) Averaged SSH anomalies(SSHA) in the east and west (from Figures 1a and 1b, inmm, on the left axis), along with the AMO index (AMOI,in K, on the right axis).
Frankcombe and Dijkstra 2009
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