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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 2010. IPCC 2001. Variability in the decadal to inter-decadal bands: 10-30 years. IPCC 2007. Frankcombe et al 2008.

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

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  1. An interdecadal oscillatory mode of the AMOC related to ocean dynamics and temperature variations Alexey Fedorov and Florian Sevellec Yale University June 2010

  2. IPCC 2001 Variability in the decadal to inter-decadal bands: 10-30 years IPCC 2007

  3. Frankcombe et al 2008 A Hovmoller diagram of observed temperature anomaliesaveraged between 300-400m and over 10–60oN across the North Atlantic (XBT data)

  4. Ocean GCM: OPA 8.2 2o 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 quarter phase AMOC (Sv) Period = 24 years Damping T = 40 years A B

  7. AB TEMPERATURE

  8. A Hovmoller diagram for temperature anomalies averaged 0-1000m,30-60oN for the mode

  9. AB TEMPERATURE

  10. AB TEMPERATURE SALINITY

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

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

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

  14. AB Adjoint mode: Non-normality!

  15. 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 • Mean zonal eastward advection • Equivalent anomalous westward advection due to the mean meridional temperature gradient • Westward advection typical of Rossby waves (the b-effect) • The system is non-normal: atmospheric noise can efficiently excite this mode

  16. The least-damped mode: AMOC variations and temperature variations (averaged 30-60oN,0-1000m) quarter phase Temperature (oC) AMOC (Sv) Period = 24years Damping T = 40years A B

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

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

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

  20. AB

  21. 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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