An interdecadal oscillatory mode of the AMOC related to ocean dynamics and temperature variations...

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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