1. Influence of Subtropical Air-Sea Interaction on the Multidecadal AMOC Variability in the NCEP CFS Bohua Huang 1, Zeng-Zhen Hu 2, Edwin K. Schneider 1 Zhaohua Wu 3, Yan Xue 2 1 George Mason University and COLA 2 Climate Prediction Center, NCEP 3 Florida State University

2. Scientific Questions What role does the ocean- atmosphere interaction play in the multidecadal AMOC oscillation? Does the tropical-subtropical Atlantic Ocean also play a part in the multidecadal AMOC oscillation?

3. CFS Simulation CFS-v1, operational system for seasonal to interannual prediction at NCEP since August 2004 Atmospheric component: GFS (2003), T62 (~200 km), 64 sigma levels (prescribed sea ice cover affects surface flux into the ocean) Oceanic component: MOM3, 1 o x1 o (1/3 o lat within 10 o S- 10 o N), 40 levels, non-polar (70 o S-65 o N) no transport between Atlantic and Arctic Ocean no sea-ice formation mechanism Daily coupling over active ocean domain (without flux correction) Initial condition: January 1, 1985, Atmosphere: NCEP Reanalysis 2; Ocean: GODAS Integration is ongoing, 400-yrs done

5. AMOC Index Year-to-Year Interannual Decadal Multidecadal Century

6. AMOC Index Year-to-Year Interannual Decadal Multidecadal + Century

7. Leading MSSA (EXEOF) Modes, 5-yr Running Mean 30-Year Lags MSSA 1(22.3%) -EEMD C MSSA2-3 (7.6%+6.4%) -EEMD M -PC1 Century Mode Multidecadal Mode MSSA1 MSSA2-3

8. Multi-Decadal Mode

13. cm/s

15. AMOC Feedback Loops Strong AMOC Warm HC, SST around 30 o -45 o N Wind curl (-) Downwelling Strong AMOC Weak Subtr. High Weak N.E. Trade Wind Warm Subtr. SST Wind Curl (+) Subtr.Upwelling Cold Subtr.HC Weak STC Delayed AMOC Weakening

16. AMOC leads AMOC lags

18. Summary AMOC fluctuates on a wide range of time scales in CFS. An intermittent multidecadal (30-yr) oscillation is generated by ocean- atmosphere feedback within the Atlantic sector. Delayed response of the northern subtropical cell is crucial for the oscillation.

19. Leading MSSA (EXEOF) Modes, Seasonal mean Lag 20 seasons (5-yrs) PC2 COR 0.52 NINO3 leads 5 seasons—Normalized NINO3 index PC1 COR 0.57 NINO3 lags 1 season 28.4% ENSO Mode 68.2% MSSA1-2 (3.7%+3.6%) EOF1EOF2

20. ENSO Mode