Influence of Subtropical Air-Sea Interaction on the Multidecadal

AMOC Variability in the NCEP CFS

Bohua Huang1, Zeng-Zhen Hu2, Edwin K. Schneider1 Zhaohua Wu3, Yan Xue2

1George Mason University and COLA2Climate Prediction Center, NCEP3Florida State University

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?

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, 1ox1o (1/3o lat within 10oS-10oN),

40 levels, non-polar (70oS-65oN)no transport between Atlantic and Arctic Oceanno 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

AMOC Index

Year-to-Year

Interannual

Decadal

Multidecadal

Century

AMOC Index

Year-to-Year

Interannual

Decadal

Multidecadal

Multidecadal+

Century

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

-PC1

Cen

tury

Mod

eM

ultid

ecad

al M

ode

MSSA1

MSSA2-3

Mul

ti-D

ecad

al M

ode

Mul

ti-D

ecad

al M

ode

Mul

ti-D

ecad

al M

ode

Mul

ti-D

ecad

al M

ode

Mul

ti-D

ecad

al M

ode

cm/s

AMOC Feedback Loops

StrongAMOC

Warm HC, SSTaround 30o-45oN

Wind curl (-)Downwelling

StrongAMOC

Weak Subtr. High

Weak N.E.Trade Wind

WarmSubtr. SST

Wind Curl (+)Subtr.Upwelling

Cold Subtr.HCWeak STC

Delayed AMOC

Weakening

AM

OC

lead

sA

MO

C la

gs

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.

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%)

EOF1 EOF2

EN

SO M

ode

EN

SO M

ode