An interdecadal oscillatory mode of the AMOC related to ocean dynamics and temperature variations Alexey Fedorov and Florian Sevellec Yale University June

IPCC 2001 IPCC Variability in the decadal to inter- decadal bands: years

3 A Hovmoller diagram of observed temperature anomalies averaged between m and over 10–60 o N across the North Atlantic (XBT data) Frankcombe et al 2008

4 Ocean GCM: OPA o global configuration 31 levels (ORCA2) We use linearized forward and adjoint versions of the model

5 Non-autonomous

6 The least-damped mode: AMOC variations A AMOC (Sv) Period = 24 years Damping T = 40 years quarter phase B

7 A B TEMPERATURE

8 A Hovmoller diagram for temperature anomalies averaged m,30-60 o N for the mode

9 A B TEMPERATURE

10 A B SALINITY TEMPERATURE

11 Temperature gradient Temperature Anomalies MODE MECHANISM: Westward propagation of large-scale temperature anomalies? AB AB

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13 IDEALIZED MODEL - temperature of the upper layer - Thermal wind balance + baroclinicity condition - Equivalent anomalous westward advection

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15 Upper layer depth h, m OSCILLATION PERIOD (IDEALIZED MODEL)

16 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, T damping ≈ 40 years)  AMOC variations are related to westward-propagating temperature anomalies in the upper 1000m between o N  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 17

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19 The least-damped mode: AMOC variations and temperature variations (averaged o N,0-1000m) A AMOC (Sv) Temperature ( o C) Period = 24years Damping T = 40years quarter phase B

20 Meridional density gradient is controlled by temperature Meridional density gradient is controlled by salinity MODE MECHANISM: Density ratio

21 IDEALIZED MODEL - average temperature of the upper layer - Thermal wind balance + baroclinicity condition Assumptions: - Equivalent anomalous westward advection

22 IDEALIZED MODEL - average temperature of the upper layer - Thermal wind balance + baroclinicity condition Assumptions: - Equivalent anomalous westward advection

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24 A B

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27 Figure 1. (a) Records from the eastern boundary of the North Atlantic (from Tenerife to Norway). Black circles are the individual tide gauge records and the average is shown by the red curve. (b) As for Figure 1a but for the western boundary of the North Atlantic (from Panama to Newfoundland). The time series from each tide gauge was linearly detrended before averaging. (c) Averaged SSH anomalies (SSHA) in the east and west (from Figures 1a and 1b, in mm, on the left axis), along with the AMO index (AMOI, in K, on the right axis). Frankcombe and Dijkstra 2009

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