The Impact of ENSO on NAO Variability Zhaohua
The Impact of ENSO on NAO Variability Zhaohua Wu, B. P. Kirtman, E. K. Schneider Center for Ocean-Land-Atmosphere Studies and N. E. Huang Goddard Space Flight Center, NASA
OUTLINE n A question n A natural filter based on EMD n ENSO and NAO in observation n Mechanism (incomplete) n Summary
AN OLD STORY “Editorial Note: In the following suite of formal comments and responses we are following unusual procedure; we are providing a venue for a valuable but unusual scientific exchange that involves multiple articles. ” BAMS, 1999, p 2721 1. 2. 3. 4. 5. Wunsch, BAMS, 1999, p 245 -255. Comments on “ 1” by Trenberth and Hurrell Reply to “ 2” by Wunsch Comments on “ 2” by Rajagoplan, Lall, and Cane Reply to “ 4” by Trenberth and Hurrell
A QUESTION NAO Index X(t) = S(t) + N(t) Does white noise like NAO index contains the signals related to the interannual ENSO variability? (The lag one autocorrelation for the observed NAO index is 0. 08. )
BAND PASS FILTERING n Traditional Filters • Subjective parameters • “local” -> “global” n Results • Tuned • fuzzy
EMPIRICAL MODE DECOMPOSITION signal source 1 signal source 2 signal source 3 signal source 4 receiver signal source 5
SIFTING PROCESS
IMFs OF NAO INDEX
STATISTICAL SIGNIFICANCE
BAND PASS FILTER n EMD Filter • Adaptive without prescribed parameter • “local” ->”local” • Physical n Results • Clear • Dyadic
“LOCAL” -> “LOCAL”
AN IDEALIZED MODEL n NAO = Internal Variability + ENSO forced signal • For simplicity, assuming • When the standard deviation of the noise is one, A is 0. 4, T is about 4. 2 yr.
DATA
SIGNIFICANCE TEST
PHYSICAL DECOMPOSITION
ENSO/NAO IN OBSERVATION
POSSIBLE ROUTES
APPROACHES n GCM Studies: Globally coupled vs. regionally uncoupled • In the regional uncoupled cases, the atmospheric model is forced by climatological SST while the ocean model is driven by the interactive fluxes.
WAVE CHAIN ROUTE • NAO and SST variability in the North Atlantic will still have a significant simultaneous (or slightly delayed) correlation with ENSO cycle if the causal effect is through PNA type of wave chains in the North Atlantic uncoupled case.
LOW LATITUDE ROUTE • If the ENSO effect is through the low latitude route by affecting the wind stress that drives the subtropical gyre, the correlation between ENSO and NAO will be missing.
GCMS & EXPERIMENTS COLA Anomaly Coupled GCM: n • • • ATMOSPHERE: COLA AGCM version 2, T 42, 18 layers OCEAN: GFDL MOM 2, 1. 5˚*0. 5˚ in the tropics, 1. 5˚*1. 5˚ in the high latitude, 25 layers (15 layers in upper 250 m) ICE: observed climatology EXPERIMENTS: n • • CONTROL: coupled globally PAC_UNCOUPLED: Tropical Pacific (between 30˚N and 30˚ S) uncoupled ATL_UNCOUPLED: North Atlantic (north of 15˚N) uncoupled Additional
ENSO/NAO IN MODEL (I)
ENSO/NAO IN MODEL (II)
ENSO/NAO IN MODEL (III)
STORY TO BE CONTINUE n n … …
CONCLUSIONS n n The new natural filtering method provides a unique opportunity to examine the impact of ENSO on NAO variability on interannual timescale The observational evidence showed that NAO interannual variability is well correlated to ENSO variability in the tropical Pacific The modeling studies demonstrate that the relation of ENSO and NAO is hidden in the seemingly white noise like NAO variability More analysis need to be carried out to figure out causal relationships between ENSO and NAO
- Slides: 26