Spatial and Temporal Structures and Mechanisms of the TBO Tim Li, Ping Liu, Bin Wang, X. Fu, Jerry Meehl Outline 1.Observational analysis --An season-sequence EOF analysis 2. Physical mechanisms -- results from a hybrid coupled GCM and an intermediate coupled model

Motivation: Compared to clear temporal and spatial structure of ENSO (Rasmussen and Carpenter 1982), it is not clear what is the spatial structure of the TBO and how does it evolve with time. Nicholls 1982 Meehl 1987,1993,1997 Yasunari 1990 Objectives: To reveal the temporal and spatial structures of the TBO using EEOF To understand the physical mechanisms for the TBO

Li et al 2001 GRL

Scientific Questions: What is the coherent 3-d structure of the observed TBO in the tropical Pacific and Indian Oceans? How does it evolve with time? What mechanisms are responsible for the observed TBO structure and evolution? How does the QB component of ENSO related to the biennial variation of the monsoon?

Data: NCEP-NCAR reanalysis for Seasonal mean (DJF,MAM,JJA,SON) field of Precipitation 925mb U and V 200mb U and V SLP 850mb geopotential height SST Goal  Reveal seasonal evolution of the TBO structure Basic feature of TBO: Seasonal dependent anomalies (e.g., JJA rainfall pattern differs from DJF rainfall pattern) Traditional EOF  dominant modes with spatial structure (50x4=200 samples) DJF,MAM,SON,DJF,MAM,JJA,SON,…DJF,MAM,JJA,SON Extended EOF  50 sequences (each sequence has 4 seasons) DJF,MAM,JJA,SON,DJF,MAM,JJA,SON,…DJF,MAM,JJA,SON 1 250

TBO Issues: Where are maximum convective activity centers associated with the TBO? What is the coherent spatial structure and seasonal evolution of the TBO? What is the origin of the TBO? Does it result from remote ENSO forcing or air-sea interaction in the warm oceans ? How is the quasi-biennial component of El Nino related to the TBO?

TBO activity centers Time filter

A seasonal-sequence EOF analysis using NCAR/NCEP reanalysis data ( ) Shading: rainfall Vector: 925mb wind 200mb wind: first baroclinic mode structure Data: seasonal mean (DJF, MAM, JJA, SON) fields of  Precipitation, SLP, SST  925mb U and V  200mb U and V  850mb geopotential height

TAS feedback in western North Pacific (Wang et al. 2000)

EQ Sumatra Anticyclonic Circulation Warm Cold A Season-dependent Air-Sea Feedback in the Southeast Indian Ocean  A primary mechanism that leads to the phase locking of the Indian Ocean Dipole (Li et al. 2003, JAS) Mean southeasterly in boreal summer

What is the origin of the TBO ? Hypothesis 1: TBO is forced by remote forcing from the eastern equatorial Pacific. Hypothesis 2: TBO is an air-sea coupled mode in the monsoon region.  This implies that the QB component of ENSO may result from the inter-basin teleconnection between the monsoon/warm ocean and eastern Pacific.

A Monsoon-Warm Ocean Interaction Scenario for the Tropospheric Biennial Oscillation

QB Monsoon LF Monsoon QB ENSO LF ENSO ? Delayed oscillator ? TBO mechanism

IPRC Hybrid coupled atmosphere-ocean GCM (Fu et al. 2003, 2004): ECHAM4 AGCM UH 2.5-layer intermediate ocean model (Wang, Li and Chang 1995; Fu and Wang 1999) 50-yr integration

Hybrid coupled GCM simulations: Summer vs. Winter

Coupled ECHAM4-ocean model 50-yr simulation

The hybrid coupled GCM experiments suggest that the TBO originates from the monsoon-warm ocean interaction.  Analogy to the PNA pattern (internal atmospheric dynamics vs. external forcing) The TBO is an inherent monsoon mode, while the El Nino forcing may magnify the signal. Question: Through what processes does the air-sea interaction in the warm ocean lead to a TBO?

Top: TRMM precipitation (mm/day) (annual mean removed) Bottom: ECHAM4 simulated precipitation (mm/day)

A: Total SST tendency B: Net heat flux term C: Sum of ocean dynamic (3D advection) terms Box average (10-15N, E)

An intermediate Coupled Atmosphere-Ocean Model Dynamic Oceanic Model --Cane-Zebiak reduced gravity model Empirical + Dynamic Atmospheric Model Four types of wind patterns that are either regressed from observational data (Type I, II, III) or derived based on the Gill model given atmospheric heating (Type IV) A possible scenario for QB ENSO without involvement of delayed oscillation dynamics.

Connection of the TBO to ENSO A CZ ocean model coupled to an empirical atmospheric model The coupled model excludes the delayed oscillator dynamics by suppressing the effect of thermocline variations.

Simulations from an intermediate coupled model that suppresses the delayed oscillator mode (i.e., no El Nino dynamics)

A monsoon-warm ocean interaction scenario for the TBO

Conclusions TBO structure and evolution: The major convective activity centers associated with the TBO appear in SEIO and WNP, accompanied with anticyclonic (or cyclonic) circulation patterns with distinctive life cycles. TBO originates from air-sea interactions in the warm ocean. Key processes involved include the variability of the WNP monsoon and associated cross-equatorial flows, convective activity over Southeast Asia/maritime continent and anomalous Walker circulation, and ocean dynamic responses to anomalous wind stress curl in the western Pacific.

TBO A two step process Northern Summer  Northern Winter (A strong Indian monsoon precedes a strong Australian monsoon) Southeastward anomalous convection progression hypothesis (Meehl 1987) South Asian monsoon-east west circulation-western Pacific SST teleconnection hypothesis (Chang and Li 2000) Remote SST forcing from Eastern Pacific (Yanai & Liu 2001) Northern Winter  Next Summer (A strong Australian monsoon precedes a strong Indian monsoon) Remote Eastern Pacific SSTA forcing hypothesis (Meehl 1987) Western Pacific SSTA-Walker cell-Indian Ocean SSTA linkage hypothesis (Chang and Li 2000) Tropical-midlatitude teleconnection hypothesis (e.g., Meehl 1997)

Indian and Australian Monsoon in-Phase Relationship Why does a strong (weak) Australian monsoon often follow a strong (weak) Indian monsoon? Hypothesis: El Nino control (Yanai and Liu 2001)

Velocity potential difference fields

The result suggests that Indian Ocean SSTA may play a active role in bridging the Indian and Australian summer monsoons.

How does the El Nino impact South Asian monsoon? Why is the atmosphere response equatorially asymmetric to a symmetric forcing? How the SSTA off Sumatra is initiated? Why is the Indian monsoon anomaly opposite to the WNP monsoon anomaly? 12-El Nino composite JJA(0)

Anomaly dry AGCM with specified 3D summer mean flow and diabatic heating How does the El Nino remotely impact the Asian monsoon? Large-scale east-west overturning ? Equatorial asymmetric response to a symmetric El Nino forcing, why?

Conclusions Asian-Australian monsoon in-phase relationship: in addition to remote ENSO forcing, eastward propagation/expansion of Indian Ocean SSTA may contribute to the in- phase relationship. Asymmetric monsoon response to symmetric El Nino forcing is attributed to the asymmetric mean state: Rossby wave response modulated by the easterly shear of the mean flow.