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Decadal Changes in Properties of El Niño-Southern Oscillation (ENSO) Rong-Hua Zhang Earth System Science Interdisciplinary Center (ESSIC), University of Maryland, College Park Observations => El Nino Property shift in the late 1970s Pre-shift period: 2-3 yr oscillation; westward propagation of SSTA; weak SSTA in far east. Equ. Pacific Post-shift period: 4-5 yr oscillation; eastward propagation of SSTA; strong SSTA in far east. Equ. Pacific What caused the decadal changes ? Mechanisms for Decadal ENSO Changes • Random factors Stochastic forcing, Nonlinearity, Residual effect, … • Deterministic factors Changes in mean climate states: Atmosphere: wind stress Ocean : the thermocline depth, stratification, SST, … Changes in the spatial structure of subsurface ocean thermal fields The role of atmos stochastic forcing (SF) A null hypothesis for causing ENSO irregularity Significant effects on ENSO & its changes ENSO characteristics, Predictability, … Limitations to previous studies: Random factor systematic & coherent way in Obs? Focusing on amplitude & period; propagation ? Decadal subsurface temperature changes? ICM : Zebiak-Cane model: Te specified HCM: Statistical + OGCM What are the respective roles relative to Te ? Observed decadal changes in ocean temperature in the Pacific in the late 1970s Data: World Ocean Database at NODC/NOAA (Levitus et al.) Decadal variability in the Pacific basin: 1. The Midlatitude/extratropics 2. The tropics 3. Oceanic connections from the midlatitude/extratropics to the tropics Temp anomaly at 250 m depth 157W-22N 172W-2N 157W-2N Decadal change in entrainment temperature (Te) & its role in ENSO Hypothesis: Subsurface thermal structure change => ENSO changes Testing using coupled models The Temperature of subsurface water entrained into the mixed layer (Te) Decadal changes in the structure of Te Te : The Temperature of subsurface water entrained into the mixed layer Wind stress (τ) SST mixed layer Te We Te SST anomaly model T ' T T ' T T ' u ' (u u ' ) v' (v v ' ) t x x y y ( w w ) M ( w w ) wM ( w) ' ' (Te T ) H (Te T ' ) ' ( w w ) M ( w w ) ' ' T ' H h 2 v h ( H hT ) (Te T ' ) ' ' H H (H H 2 ) Te: The Temperature of subsurface water entrained into the mixed layer Outline • Introduction • Intermediate ocean model (IOM) with an empirical parameterization for Te • The Role of Subsurface Entrainment Temperature (Te) (1) Intermediate coupled model ( ICM; Zhang and Busalacchi, 2005) Ocean: Intermediate ocean model (IOM) + an SST anomaly model + Te Atmospheric wind stress (τ) model: SVD-based statistical (2) Hybrid coupled model (HCMAGCM ) Ocean: The IOM as in the ICM Atmosphere: AGCM (ECHAM4.5) • The respective roles of atmospheric stochastic forcing (SF) & Te • Summary Intermediate ocean model (IOM): Ocean dynamical model + a SST anomaly model Linear part: u t fv p x v h 2 u (v v u z ) z h vt fu py vh2v (vvuz ) z h p z g 0 u 0 w 2 t N h 2 ( ) zz h g Non-linear part utnl u (u ) vh 2 (u nl ) (vv u z ) z h nl u nl 0 Vertical modal decomposition u n 1 un n v n 1 vn n p n1 pn n 10 10 10 w n 1 wn n dz n1 n nz 10 z 10 H SST anomaly model T ' T T ' T T ' u ' (u u ' ) v' (v v ' ) t x x y y ( w w ) M ( w w ) wM ( w) ' ' (Te T ) H (Te T ' ) ' ( w w ) M ( w w ) ' ' T ' H h 2 v h ( H hT ) (Te T ' ) ' ' H H (H H 2 ) Te: The Temperature of subsurface water entrained into the mixed layer Te Parameterization scheme 1. Cane-Zebiak scheme Te Tsub (1 )To Td 1 tanh b1 (h h) tanh b1 h , h0 Tsub Td 2 tanh b2 (h h) tanh b2 h , h0 2. Empirical parameterization scheme SST anomaly model T ' T T ' T T ' u ' (u u ' ) v' (v v ' ) t x x y y ( w w ) M ( w w ) wM ( w) ' ' (Te T ) H (Te T ' ) ' ( w w ) M ( w w ) ' ' T ' H h 2 v h ( H hT ) (Te T ' ) ' ' H H (H H 2 ) Te: The Temperature of subsurface water entrained into the mixed layer An empirical model for the Temperature of water Entrained (Te) into the mixed layer Historical data: simulated & observed Forced ocean model run => currents, pressure fields ( Sea level (SL) ) • Inverse modeling of Te Obs. SST fields etc. => SST anomaly model => Te • Statistical relationships based on history: SLTe SVD-based analysis of covariance between SL and Te Given SL from ocean model => Te => SST calculation Nothing could be better than this procedure in simulating SSTA !! Data Sets and Construction of Te models • SST (Reynolds et al.): 1950-1999 • Dynamical ocean model run forced by NCEP: 1960-1999 => mean fields; anomaly fields: sea level (SL) etc Inverse modeling of SSTA equation => Te ________________________________________________________________________ • Te model construction: SL Te Seasonally invariant ( 1 model for all months); 1963-1996 1963-1979 1980-1996 Intermediate coupled model (ICM): IOM + statistical anomaly model Atmos: A SVD-based wind stress model: Observed SSTAs Wind stress anomalies (τ) from ECHAM4.5 24 member ensemble mean Ocean: IOM with Te Hybrid coupled model (HCM): IOM + an AGCM Atmosphere: The European Center + The Max Planck Institute for Meteorology (MPI) Atmospheric GCM (ECHAM4.5) T42; 19 layers Ocean: IOM with Te ICM HCM AGCM (Intermediate coupled model) (Hybrid coupled model) AGCM (ECHAM4.5) SST anomaly model s ,w ,v s us SL Te model Intermediate ocean model Realizations of atmos stochastic forcing (SF) in the coupled models Wind stress response: Signal + Noise (SF) SST HCM: AGCM + IOM with Te SF: implicitly included ICM: statistical + IOM with Te (1) Signal part in wind stress anomalies (No SF) (2) Both parts: SF explicitly taken into account τ τ Construction of signal & SF Wind stress ( τ ) : Signal + noise (stochastic forcing) obs. SSTAs - AGCM (ECHAM4.5) 24 ensemble mean HCM (ECHAM4.5 + IOM) Residual A first order autocorrelation model (AR1) τ τ Signal + SF => Used in ICM The ICM experiments τSignal τSignal + τSF Te model : 63-96; 63-79; 80-96 τ model : 63-96; 63-79; 80-96 Te63-79, Te80-96 The HCMAGCM experiments AGCM (ECHAM4.5) SST anomaly model ,ws vs u s, SL Te model Intermediate ocean model Te63-79 , Te80-96 Coupled modeling experiments on the effects of Te & SF on ENSO properties ICM without HCM ICM with SF SF Te(63-79) Amplitude: Period: Propagation ? ? ? : Te(80-96) Amplitude: Period: Propagation ? ? ? : Nino3 SSTA power sprectra AGCM (ECHAM4.5) + an IOM with Te Effects of SF & Te on ENSO properties ICM without HCM ICM with SF SF Amplitude: west of 140W west of 14 0W west of 14 0W Te(63-79) Period: regular (2yr) irregular (3-5yr) irregular (3-5yr) Propagation: Westward Westward Westward Te(80-96) Amplitude: east of 140W east of 140W east of 140W Period: regular (5yr) irregular (3-7yr) irregular (3-7yr) Propagation: Eastward Eastward Eastward Oceanic processes involved T ' T T ' T T ' u ' (u u ' ) v' (v v ' ) t x x y y ( w w ' ) M ( w w ' ) wM ( w) (T H T ) e (Te T ' ) ' ( w w ) M ( w w ) ' ' T ' H h 2 v h ( H hT ) (Te T ' ) ' ' H H (H H 2 ) Off-equatorial Te SST Hori adv. Vert adv. mixed layer Te thermolcine Summary • Demonstrate a new factor determining the El Nino properties • Different modulating effects of SF & Te: SF Irregularity of ENSO amplitude & oscillation periods Te Phase propagation Implication for observations Explain El Nino Property shift in the late 1970s Pre-shift period: 2-3 yr oscillation; westward propagation of SSTA; weak SSTA in far east. Equ. Pacific Post-shift period: 4-5 yr oscillation; eastward propagation of SSTA; strong SSTA in far east. Equ. Pacific Implications for coupled modeling Common biases with OGCM-based Coupled Models: Quas-biennial (~2 yr) oscillation; westward propagation of SSTA on equ.; weak SSTA in far eastern equatorial Pacific Improvements: better Te simulations Thank you !!!
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