Multidecadal Oscillation of the Atlantic Meridional Overturning Circulation

Multidecadal Oscillation of the Atlantic Meridional Overturning Circulation in Climate Models Bohua Huang Department of Atmospheric, Oceanic, and Earth Sciences George Mason University, Fairfax, Virginia Center for Ocean-Land-Atmosphere Studies, Maryland Collaborators: Shaoqing Zhang (GFDL) Zeng-Zhen Hu (CPC/NCEP) Jieshun Zhu (COLA)

Outline • Motivation: AMOC and AMO • Methodology: MSSA and phase composite • A quasi-30 -year AMOC oscillation link with heat content and SST an ocean internal mode effect of coupling • Summary

OBS at 26. 5 o. N (RAPID-MOCHA Array): 18. 5± 4. 9 Sv (04/2004 -10/2007, Johns et al. , 2011)

AMOC and Climate The Atlantic Ocean heat transport (OHT) is northward 60% of the OHT peak (1 PW) at 20 o. N is due to AMOC Talley 2003; Boccaletti et al. 2005 Water Hosing Experiment (HADCM 3) Surface air temperature change during years 20– 30 after collapse of AMOC. Vellenga and Wood 2002 What could happen in a world without AMOC (e. g. , all glaciers melted)?

Atlantic Multidecadal Oscillation (AMO/AMV) Same sign in North Atlantic Largest amplitude in the north Major anomalies in the subtropical Atlantic Basinwide horseshoe pattern 50 -70 year time scales AMO and AMOC needs to be linked physically AMO Index based on Enfield et al. (2001)

NAO and SST Anomaly in North Atlantic From NOAA CPC Ocean Briefing, Dec. 7, 2012 - High-latitude North Atlantic SSTA is generally closely related to NAO index (negative NAO leads to SST warming and positive NAO leads to SST cooling). Negative NAO index has persisted for 7 months, contributing to persistent positive SSTA in high-latitude N. Atlantic, and also a warming in tropical N. Atlantic in Nov 2012. - In the past three hurricane seasons, positive SSTA in MDR was strong in 2010, and became weakening in subsequent two years. Fig. NA 2. Monthly standardized NAO index (top) derived from monthly standardized 500 -mb height anomalies obtained from the NCEP CDAS in 20ºN-90ºN (http: //www. cpc. ncep. noaa. gov). Time-Latitude section of SST anomalies averaged between 80ºW and 20ºW (bottom). SST are derived from the NCEP OI SST analysis, and anomalies are departures from the 1981 -2010 base period means.

AMOC fluctuates fast in CGCMs (20 -30 yr, GFDL-CM 2. 1 Simulation) Temperature Heat Transport AMOC Frankcombe et al. 2010 Delworth and Zeng (2012)

A 30 -year signal in temperature? There is observational basis for 20 -30 yr variability in North Atlantic, faster than AMO England Temp Greenland Ice Core Surface Temperature Frankcombe et al. , 2010

Guan and Nigam 2008 Nigam et al. , 2012 AMO indices Atlantic Annual SST and Fall Precipitation Weaker subtropical anomaly Time series more wavy Is there a 30 -yr oscillation on top of 70 -yr? 700 h. Pa Z Moisture flux Precipitation

West 200 -300 meters, T East Frankcombe and Dijkstra, 2009 Subsurface Temperature 0 -400 meters, HC Observed multidecadal variability in North Atlantic sea level height 10 o. N-60 o. N Frankcombe et al. , 2010

Brief Summary of Previous Studies • Multidecadal AMOC fluctuation is a potential cause of AMO • A physical connection between AMOC fluctuation and AMO needs to be confirmed • North Atlantic temperature fluctuations may have multiple time scales • Northern North Atlantic temperature may fluctuates on an 20 -30 yr time scales • Observationally, this fluctuation may be more prominent in the subsurface temperature

Questions How robust is the 20 -30 year variability in climate models? What is its mechanism? What are its climate effects? What is its predictability?

Singular Spectrum Analysis (SSA) Characteristics of phenomenon • multiple time scales • Intermittent oscillation Method (SSA) • sensitive to time scales • identify oscillations (even if intermittent) objectively • no rigid frequency constraint • Multichannel SSA (MSSA) allows spatial propagation

Singular Spectrum Analysis (SSA) An extension of EOF analysis to time-lag Most easily explained in 1 -D case Given a time series, x(t), an M-lagged vector can be built as The lagged covariance matrix has M eigenvalues and eigenvectors: is the kth time EOF of The kth principal component is

Singular Spectrum Analysis (SSA) The kth reconstructed component (RC) is In particular, an oscillatory mode is characterized by a pair of degenerate EOFs, i. e. , and vary coherently and 90 o out of phase. is the filtered oscillatory mode This procedure can be easily extended into multivariate (or multichannel) SSA, i. e. , MSSA

Multi-channel Singular Spectrum Analysis (MSSA) Separating “Fast” and “Slow” Signals MSSA Variance: 10. 8%+10. 4% MSSA Variance: 9. 8%+ 9. 2% Anything in common among models in the “fast” modes?

1 st EOF patterns of the “fast” AMOC modes Similar spatial structures center at 40 -50 o. N

Time Scale of the “Fast” AMOC Modes A Quasi-30 -year Mode?

Phase Composite The 1 st principal component of the RC field and its derivative, both normalized, can be written as A cycle can be characterized by eight chosen phase intervals, The average of the original data for any variable A over all occurrences in phase m is called the phase composite Am (m=1, …, 8)

Both models show similar AMOC phase evolution

Weaker modes also follow a similar pattern

Both models show similar AMOC phase evolution

North. Atlantic. SSTisisincreasedfollowingstronger. AMOC North

North Atlantic HCA forces SSTA. Both are induced by AMOC heat transport

GFDL-CM 3. 0 Mean Heat Content (HC, 0 -500 m) Surface Current Surface Drifter OBS WOA 09 CCSM 3. 0 HCA/SSTA along Gulf Stream Extension and North Atlantic Current A “deformation” of subpolar gyre

Potential Temperature (θ) and Density (σθ) at 300 m, 40 o-50 o. N σθ

An Internal Ocean Mode? Warm anomaly in north-central induces a negative zonal overturning Upwelling (downwelling) causes warm (cold) anomaly in the west (east) Negative zonal ΔT induces a negative MOC Upwelling in north-central reverses zonal overturning Uncoupled Ocean Model Simulations Westward propagation of thermal anomalies Sustained oscillation in idealized thermal boundary condition Thermal overturning: Salinity is secondary Te Raa and Dijkstra (2002), see also, Huck et al. (1999)

Geostrophic Self-Advection Sévellec and Fedorov 2012, J Clim. , in press Phase B Phase A Phase B

Effects of Ocean. Atmosphere Interaction • Surface heat flux • Atmospheric response • Surface evaporation

Surface latent heat flux anomalies damp SSTA

Surface. SST latent heat flux following anomaliesstronger damp SSTA North Atlantic is increased AMOC

The AMOC oscillation is associated with NAO (“delayed in CCSM 3”)

Surface. SST latent heat flux following anomaliesstronger damp SSTA North Atlantic is increased AMOC

SSTA forces divergence/convergence near the surface (SSTA is “delayed in CCSM 3”)

Air temperature is warm after a strong AMOC

Strong AMOC induces sea ice melting

SST SSS σt Surface salinity is dominant, due to evaporation

Potential Density Salinity Temperature

σt SSS SST

Summary A quasi-30 -year oscillation appears in climate models, centered at 40 o-50 o. N. GFDL-CM 3 and CCSM 3 show vigorous oscillation with similar lifecycles. Associated with a strong AMOC, eastern subpolar gyre is warmed up while its south cools down. The gyre is “deformed”. HCA forces SSTA. Warm SSTA in northern North Atlantic weakens NAO, reduces sea ice, and expands warm air to North America. There is a westward propagation of subsurface temperature around 40 o-50 o. N. Evaporation damps SSTA but enhances SSSA, making salinity dominant near surface.

Further Questions • • • Any feedback from the NAO? What roles do evaporation play? Is AMOC active or passive? What roles do the AMOC-heat transport play? How strongly does mean state affect oscillation? Is there any tropical-extratropical feedback? Is there an optimal perturbation? How predictable is the oscillation? How to initialize prediction in real world? ……

Is there any potential feedback from NAO to AMOC? Can the extratropical processes affect the tropics? Some CFSv 1 results

Leading MSSA Patterns CFSv 2 300 -yr simulation Mode 1(22. 3%) EEMD Century 99. 5% Mode 2 (7. 6%), 3 (6. 4%) EEMD Matidecadal 74. 2% 23. 0% 80. 1% 18. 6% Mode 4 (5. 0%), 5 (4. 8%) EEMD Decadal

Multidecadal Mode Is there a Subtropical Connection? Huang et al. Clim. Dyn. , 2012

Summary II • The multidecadal AMOC oscillation may be affected by ocean-atmosphere feedback within the Atlantic sector. • Low-frequency NAO fluctuation plays a more active role. • Southward expansion of the SLP anomalies affects the subtropical gyre circulation • Delayed heat transport of the subtropical shallow cell affects the oscillation.

Atlanitc Meridional overturning circulation scheme (Schott 1902) (From Richardson 2008)

Schematic Meridional Circulation along 30 W (Merz 1925) northward spreading of intermediate water in about 800 m (45 S - 20 N) southward spreading of deep water at 1500– 3500 m (30 N -> 55 S) From Richardson (2008)

Observed Transport at 26. 5°N (3. 5 Years) RAPID-MOCHA observing system Joint UK/US Monitoring Array Mean and Standard Deviation Gulf-Stream: 31. 7 ± 2. 8 Sv (Florida Strait) Ekman: 3. 5 ± 3. 4 Sv Upper Mid-Ocean: -16. 6 ± 3. 2 Sv (mooring densities) MOC: 18. 5 ± 4. 9 Sv (from http: //www. noc. soton. ac. uk)

Long-term AMOC Variability A potential source of decadal predictability • Forced change associated with global warming • Multi-decadal oscillations due to internal dynamics Confidence in potential to predict decadal variability in AMOC reduced by model-dependence of both these features. From Schmittner et al. (2005) with additions (IPCC report).

AMO Global Effects OBS Model EOF Patterns of Precipitation From Zhang and Delworth 2006 OBS Model

Quasi-30 -yr oscillation is intermittent and model dependent Danabasolgu 2008 Danabasolgu et al. 2012 Leading AMOC mode, CCSM 3

NAO and SST Anomaly in North Atlantic From NOAA CPC Ocean Briefing, Oct 5, 2012 - High-latitude North Atlantic SSTA are closely related to NAO index (negative NAO leads to SST warming and positive NAO leads to SST cooling). -Negative NAO index persisted over five months, contributing to the strong warming in the highlatitude N. Atlantic. Fig. NA 2. Monthly standardized NAO index (top) derived from monthly standardized 500 -mb height anomalies obtained from the NCEP CDAS in 20ºN-90ºN (http: //www. cpc. ncep. noaa. gov). Time-Latitude section of SST anomalies averaged between 80ºW and 20ºW (bottom). SST are derived from the NCEP OI SST analysis, and anomalies are departures from the 1981 -2010 base period means.