Dynamics of annual cycle/enso interactions

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1 Dynamics of annual cycle/enso interactions A. Timmermann (IPRC, UH), S. McGregor (CCRC), M. Stuecker (Met Dep., UH) F.-F. Jin (Met Dep., UH), K. Stein (Oce Dep., UH), N. Schneider (IPRC, UH),

2 ENSO and its interactions Noise ENSO PDO, IPO Annual cycle Combination Tones

3 ENSO and its interactions Greenhouse warming Noise ENSO PDO, IPO Annual cycle Combination Tones

4 ENSO and its interactions Ice Ages Noise ENSO PDO, IPO Annual cycle Combination Tones

5 ENSO and its interactions Orbital Cycles Noise ENSO PDO, IPO Annual cycle Combination Tones

6 ENSO-WWB interaction (Multiplicative Noise) Interannual ENSO mode (Recharge physics) Annual cycle in western tropical Pacific Termination of El Nino events after DJF Seasonally modulated wind anomalies Philippine Anticyclone/ SE Asian Monsoon Zonal SPCZ events 2:1 Phase locking Seasonal variance modulation Combination Tones of EOF modes Taimasa events

7 Classical linear oscillator theory Interannual ENSO mode (Recharge physics) EOF1 Z20 EOF1 EOF2 Z20 EOF2 PC1 Discharging What causes discharging of heat? PC2; quadrature to PC1 McGregor, Timmermann, Schneider (2012), JCL

8 Interannual ENSO mode (Recharge physics) Annual cycle in western tropical Pacific P. (Anti)cyclone Southward wind anomaly shift PC2 Stuecker, Jin, Timmermann, McGregor, Ren, submitted (2012) Stuecker, Timmermann, Jin, Ren (2013) NGC

9 What does EOF2 do? Why is important for the El Nino termination? Southward shift of El Nino-related wind anomaly weakens downwelling Kelvin wave SWM PC2 related wind stress curl accelerates discharge in a meridionally asymmetric way (see Kug et al. 2003) Equatorial zonal mean thermocline depth Effect of PC1 winds (recharge) Effect of PC2 winds McGregor, Timmermann, Schneider (2012)

10 Effects of wind EOF2 Combination of EOF1, EOF2 Describes southward shift of El Nino-related wind anomalies, akin to: McGregor, Timmermann, Schneider, Jin, JCL (2012) Harrison 87, Harrison and Larkin 96,98, Harrison and Vecchi 99, Lengaigne et al. 06, Lengaigne and Vecchi 09, Spencer 04, Wang et al PC2 (wind-shift) = PC1 cos (w a t) = ENSO x annual cycle Stuecker, Timmermann, Jin, McGregor, Ren (2013) NGC

11 Where does EOF2 come from? Stuecker,, Timmermann, Jin, McGregor, Ren, (2013), NGC [A cos(w a t)+b cos(w e t)] 2 = 0.5 A 2 cos(2w a t) + AB[cos((w a -w e )t)+cos((w a +w e )t)] +0.5 B 2 cos(2w e t) A hint is provided by the power spectrum of EOF2. It shows nonlinear combination tones at 1-f and 1+f with f being the ENSO frequency EOF mode 2 emerges from the nonlinear interaction between ENSO and annual cycle

12 What is the nonlinearity that generates EOF2 via El Nino/ annual cycle interactions? Option 1: Nonlinearity provided by nonlinear momentum damping During DJF SPCZ develops, weak surface wind stress south of equator Turbulent momentum damping seasonally modulated McGregor, Timmermann, Schneider (2012), JCL

13 What is the nonlinearity that generates EOF2 via El Nino/ annual cycle interactions? Using of Fu-Wang intermediate atmosphere model (Lindzen Nigam coupled to Gill atmosphere) Minimum windspeed zone allows Best coupling between Lower troposphere and Boundary layer, best imprint of Gill model Wave response to surface Simulated zonal wind speed between 160E-150W Fixed windspeed in F x, F y Latitude of maximum of zonal wind response to constant El Nino SST, CTR McGregor, Timmermann, Schneider (2012)

14 Interannual ENSO mode (Recharge physics) Annual cycle in western tropical Pacific Parametric Resonance Combination Tones Seasonal variance modulation 2:1 Phase locking

15 Approximate analytical solution by An and Jin (2010), Tellus => Combination Tones l 0 ENSO/annual cycle interactions = C [ cos(w 0 t) + l 0 (w 2 a-4w 2 0) -1 [0.5 w 0 (sin((w 0 -w a )t) +sin((w 0 +w a )t)) (w 2 a-2w 2 0) w a (sin((w 0 +w a )t) + sin((w a -w 0 )t) )] Spectral peaks at and +/- (combination tones)

16 Seasonal variance modulation Approximate analytical solution by An and Jin (2010), Tellus => Seasonal variance modulation Stein, Timmermann, Schneider, Stuecker, Jin, JCL (2014)

17 Combination mode dynamics of the Northwestern Pacific Anticyclone ERA40 Streamfunction, precipitation Theoretical C-mode CM2.1 The bridge between ENSO and the Asian Monsoon is a C-mode Stuecker, Jin, Timmermann, McGregor, 2015, JCL in press

18 Impacts: El Nino Taimasa events Sea level low stands in SW tropical Pacific are a result of the atmospheric Combination tone.

19 Outlook: orbital modulation of ENSO Annual cycle strength AMOC down AMOC affects ACY and in turn ENSO Orbital forcing affects Annual cycle and BJ index, Possible compensation AMOC up Full forcing To understand paleo- ENSO we need to understand annual cycle ENSO intractions Orbital forcing Liu et al. (2014), Nature

20 Summary Our results show: Seasonal weakening of winds south of equator during DJF/MAM leads to southward shift of El Nino-related wind anomalies (McGregor et al. 2012) This effect is described by wind EOF2 Wind EOF2 (includes Philippine anticyclone) is a combination of El Nino and western annual cycle Wind EOF2 accelerates Termination of El Nino in DJF/MAM via upwelling Kelvin wave and northward discharging of heat (Kug et al. 2003) Operates only for strong El Nino events, not for La Nina or weak El Nino and Modoki Explains why El Nino events are shorter than La Nina events, skewness This effect can be crudely parameterized as a seasonal modulation of the coupling strength (Thompson and Battisti (2000), Stein et al (2010), An and Jin (2011)) -- still symmetric for El Nino and La Nina Seasonal recharge model explains combination tone, seasonal variance modulation and annual cycle