Longterm Variability of the Atlantic Meridional Overturning Circulation
Long-term Variability of the Atlantic Meridional Overturning Circulation in the NCEP Climate Forecast System Bohua Huang Acknowledgment: Z. Hu, E. Schneider, Z. Wu, K. Pegion, J. Manganello, Y. Xue
Salty Moving southward within 2 -4 km Po. Temp North Atlantic Deep Water (NADW) Salinity Evidence of Deep Flows in Atlantic Hydrographic Cross Section Fresher, colder Moving northward near bottom From Tomczak and Godfrey, Regional Oceanography Oxygen Antarctic Bottom Water (AABW) W. Atlantic
NADW is Coupled with Upper Ocean Circulation Atlantic Meridional Overturning Circulation (AMOC) From Latif et al. (2009), 1 st US AMOC Meeting
Multidecadal AMOC Oscillation is the leading candidate for the Atlantic Multidecadal Oscillation (AMO) AMO index defined by Enfield et al. (2001)
Potential Mechanisms of AMOC Variability Most evidence points towards the “ocean-only” oscillator (Latif) From Latif et al. (2009), 1 st US AMOC Meeting
Scientific Questions • What role does the oceanatmosphere 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-v 1, operational system for seasonal to interannual prediction at NCEP since August 2004 • Atmospheric component: GFS (2003), T 62 (~200 km), 64 sigma levels (prescribed sea ice cover affects surface flux into the ocean) • Oceanic component: MOM 3, 1 ox 1 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
AMOC Index Year-to-Year Interannual Decadal Multidecadal Century
AMOC Index Year-to-Year Interannual Decadal Multidecadal + Century
Leading MSSA (EXEOF) Modes, 5 -yr Running Mean Multidecadal Mode Century Mode 30 -Year Lags -EEMD C MSSA 1 -PC 1 MSSA 1(22. 3%) -EEMD M -PC 1 MSSA 2 -3 (7. 6%+6. 4%) MSSA 2 -3
Multi-Decadal Mode
Multi-Decadal Mode
Multi-Decadal Mode
Multi-Decadal Mode
Multi-Decadal Mode
cm/s
AMOC Feedback Loops Strong AMOC Warm Subtr. SST Delayed AMOC Weakening Warm HC, SST around 30 o-45 o. N Weak N. E. Trade Wind Cold Subtr. HC Weak STC Strong NAO Downwelling Strong AMOC Weak Subtr. High Weak wind Curl Subtr. Upwelling
AMOC lags AMOC leads
65% First EOF and spectrum of accompanying principal component of the annual mean AMOC stream function, SST and Northern Hemispheric 500 h. Pa geopotential as simulated by a 1000 -yr integration of the ECHAM 3 -LSG model. The standard deviations of the corresponding principal components are 6. 71 Sv, 1. 67 K, and 114 gpm, respectively (From Grötzner et al. , 1999, J. Climate).
The pattern of AMOC EOF 1 from CCSM 3 and its time series with power spectrum and autocorrelation (from Danabasoglu, 2008, J. Climate).
Summary • AMOC fluctuates on a wide range of time scales in CFS. • An intermittent multidecadal (30 -yr) oscillation is generated by oceanatmosphere 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) MSSA 1 -2 (3. 7%+3. 6%) ENSO Mode —Normalized NINO 3 index EOF 1 EOF 2 68. 2% 28. 4% PC 1 COR 0. 57 NINO 3 lags 1 season PC 2 COR 0. 52 NINO 3 leads 5 seasons
ENSO Mode
ENSO Mode
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