NORTH ATLANTIC DECADAL-TO- MULTIDECADAL VARIABILITY - MECHANISMS AND PREDICTABILITY

Transcription

1 NORTH ATLANTIC DECADAL-TO- MULTIDECADAL VARIABILITY - MECHANISMS AND PREDICTABILITY Noel Keenlyside Geophysical Institute, University of Bergen Jin Ba, Jennifer Mecking, and Nour-Eddine Omrani NTU International Science Conference, Sep. 2012

2 Multidecadal temperature fluctuations: Internal versus externally driven?

3 Pronounced year variation in observed North Atlantic Sea Surface Temperature Climatic impacts of socioeconomic importance (R Zhang) 1. What causes Atlantic Multidecadal variability Observations Models 2. Can it be predicted? 3. Do we underestimate the role of ocean-atmosphere coupling?

4 North Atlantic: wintertime ocean deep convection and a thermohaline circulation Meridional overturning circulation (MOC) Marshall & Schott, 1999 Pickart et al. 2003

5 Warm AMV and negative winter NAO Composite analysis of 10yr running mean AMV-index, SST (JFM, C) SLP (JFM, Pa)

6 Normalised Multi-decadal fluctuations in Atlantic Sector variability: ocean-stratosphere Multi decadal variability explains 48% of AMV index and 14% of NAO index

7 Observational evidence that NAO drives AMV NAO leading by several years Latif and Keenlyside, 2011

8 Bjerknes conjecture: Ocean drives mid-latitude SST and turbulent heat fluxes on multi-decadal timescales AMV SST and turbulent heat flux indices Can these changes drive multi-decadal shifts in the winter atmosphere? Gulev et al. submitted

9 What is the potential to predict the decadal Observed shifts in characteristics the global monsoons? of AMV North Atlantic supports winter-time deep convection and thermohaline overturning circulation Winter NAO and AMV tend to vary in anti-phase on multi-decadal timescales NAO drives winter convection, and may drive AMOC changes Subpolar North Atlantic SST controlled by ocean dynamics NAM in the stroposphere covaries with NAO

10 Simulated Atlantic multi-decadal variability Some robust features among climate models Delworth et al mechanism, and role of salinity Stochastic null hypothesis Other aspects Coupled feedbacks involving salinity External forcing

11 Models simulate similar decadal variability independent of external forces Kiel Climate Model, preindustrial control simulation Atlantic multi-decadal variability index Regression pattern Model year Ba et al. submitted

12 Models simulate a range of internal variability Spectra of AMOC 30N CMIP3 Pre-industrial Runs BCM IPSL ECEARTH MPIOM MPI MIUB CCSM3 GFDL MIROC CSIRO INMCM

13 AMOC fluctuations tend to drive subpolar gyre SST variations on decadal timescales Tropical North Atlantic Subpolar North Atlantic year year

14 Relation between NAO and AMOC variability not clear in models NAO spectra Cross-correlation NAO and AMOC Systematic error in winter convection sites may explain some of these differences

15 Delworth et al mechanisms for AMOC decadal variability Found in a number of models Anomalous strong SPG drives salty water to convection sites Salinity anomalies enhance oceanic convection Convection enhances AMOC Stronger AMOC reverses SPG anomaly Excited by NAO like variability through heat flux anomalies

16 Illustration from KCM: SPG and convection in Irminger Sea Ba et al. submitted

17 Illustration from KCM: Salinity variations key to mechanism Control simulation Simulation with salinity restored to climatology over the subpolar North Atlantic Ba et al. submitted

18 Stochastic null hypothesis following Hasselman (1976) Delworth & Greatbatch (2000) Latif and Keenlyside (2011)

19 Period (years) Ocean driven by stochastic atmospheric variability: No indication of oscillatory ocean only mode Wavelet spectra: ocean model simulation driven by stochastic NAO forcing Time (years) Courtesy: Jenny Mecking

20 AMOC variations best described by an auto regressive process of order 7

21 Coupled feedbacks involving ITCZ Vellinga and Wu (2004): Menary et al Salinity variations may also come from Arctic (Jungclaus et al. 2006)

22 AMV may also be driven by external forcing But controversy exists See also Ottera et al Booth et al. 2012

23 Mechanisms for AMV based on models AMOC variations drive significant part of AMV AMOC changes driven by salinity modulated convection Stochastic atmospheric forcing excites oceanic variability Coupled feedbacks in the tropics could play a role External forcing may also contribute

24 To what extent is AMV predictable: IPCC AR5 near-term predictions

25 Skill of (7) CMIP5 initialized nearterm forecasts Indian Ocean and western Pacific North Atlantic Guemas et al. 2012

26 North Atlantic subpolar gyre very predictable Bias corrected Raw CCSM4 CCSM4 predictions predictions of SPG of SPG heat heat content content anomalies anomalies 10 year long; 10 members Budget analysis shows predictability comes from ocean circulation changes Yeager et al. 2012

27 AMV predictable up to 5 years in advance Kim et al. 2012

28 Uncertainties in initial conditions Keenlyside & Ba (2010)

29 Large uncertainty model projections of AMOC AMOC at 30N, CMIP3 models, 20C/A1B Schmittner et al. (2005)

30 Quantifying uncertainty in decadal AMOC change Following Hawkins and Sutton (2009)

31 Near-term predictions of AMV AMV appears potential predictable, particularly in the subpolar region Challenges Model uncertainty Ocean initial conditions uncertain

32 Stratosphere drives decadal shifts in winter NAO Changes forced by a strengthening of stratospheric polar votex SLP (DJF, hpa) Zonal average zonal wind (DJF, ms - 1 ) Scaife et al. (2005) Can AMV drive the stratospheric changes, and in turn the NAO?

33 Ocean-atmosphere coupling: Potential role of stratosphere Observed SST anomaly NCEP/NCAR 1000hPa GPH anomaly Observed anomalies Lower stratosphere resolving Entire stratosphere resolving Simulated (ECHAM5)resp onse to Atlantic SST hPa GPH Omrani et al., submitted

34 Simulated and observed weakening of early winter stratospheric polar vortex 20 hpa geopotential height (NDJ) High-top NCEP/NCAR

35 Normalised Investigating the stratosphere s role Warm ( ) Reference ( ) Experiments: High top: MAECHAM5 at T63L39, surface to 0.01hPa (80km) Parameterisation for gravity wave momentum flux Low top: standard ECHAM5 at T63L19, surface to 10hPa (30km) Enhanced horizontal diffusion in upper layers (sponge layer)

36 m/s Stratosphere resolving model has dynamic polar vortex High Top 80 Daily zonally averaged wind 60N, 10hPa Control Warm phase -60 Aug Jun Low-top Control Warm phase

37 Stratosphere resolving coupled model support warm AMV drives negative NAO ECHAM6/MPIOM T63L95 atmos. res, model top 0.01hPa (~80km) 1.5deg/L40 ocean res 500-year-long preindustrial coupled control integration Composite analysis of unfiltered winter AMV index Stand alone ECHAM6 simulations driven by coupled model SST

38 Coupled model results warm phase associate with negative NAO like SST (JFM) pattern 500 hpa GPH (Feb)! "#$%*%

39 Warm phase Atmospheric pattern largely driven by North Atlantic SST! "#$%*% 500 hpa GPH (Feb) Uncoupled Coupled

40 Warm phase Stratospheric polar vortex weakening largely driven by North 30 hpa GPH (Jan) Uncoupled Atlantic SST! "#$%) % Coupled

41 AMV contributes to low-frequency NAO variations stratosphere key Observations and model indicate warm AMV drives negative NAO, with stratosphere playing a key role: Upward propagating waves weaken the vortex in early winter Weakening of the westerlies propagates down and causes a negative NAO in late winter Robustness for cold case less clear Potenial implications for understanding multidecadal variability in the North Atlantic sector Omrani et al. 2012a,b in revision

42 Power Power Not 10clear if resolving stratosphere interaction 3 enhances simulated AMV E20 C02 Spectra of AMOC at 30N from 500 yr long high- and low top coupled models 10 3 High top ECHAM5 T63L47/MPIOM E20 C Low top ECHAM5 T63L31 /MPIOM Year

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44 Systematic error in winter convection sites Standard deviation of winter mixed layer depth

45 Cold phase results less clear! "#$%*% 500 hpa GPH (Feb) Uncoupled Coupled

46 Figure from Holger at 48N