Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation UH seminar, 29 August 2001 Masahiro Watanabe Department of Meteorology, SOEST, University of Hawaii * On leave from Center for Climate System Research (CCSR), University of Tokyo refs: Watanabe, Kimoto, and Jin (2001, submitted to JC) Watanabe, Jin, and Kimoto (2001, to be submitted to JC)

introduction motivation of the study –What is the leading mode of the atmospheric circulation? e.g. barotropic & baroclinic instabilities, teleconnection patterns, MJO,…. –Attempt to specify and understand a principal mode in the global circulation fields ( teleconnection patterns in hemispheric fields) outline –observational data analysis –AGCM simulation –linear model diagnoses UH seminar, 29 August 2001

previous studies near zonally uniform pattern (superrotational flow) –(e.g. Kang & Lau 1994) coherence with atmospheric angular momentum –(e.g. Anderson & Rosen 1983; Rosen & Salstein 1983) found in intraseasonal time scale –(e.g. Weickmann et al. 1997) UH seminar, 29 August 2001

EOF1(23%) for monthly  300, UH seminar, 29 August 2001 Tropical Axisymmetric Mode (TAM) = ‘global mode’ (Higgins et al. 2000; Bell & Halpert 2000) = ‘tropical mode’ (vonStorch 1999) principal mode in 

UH seminar, 29 August 2001 time series  PC1 M  LOD Niño3  PC M  LOD.41 Niño3 (defined as the TAM index)

UH seminar, 29 August 2001 Regression of monthly NCEP anomalies on the  300 PC1 structure of TAM

UH seminar, 29 August 2001 ResidualTAM  R EOF1 explains 19% of total variance, significantly correlated with M (0.62) and  LOD (0.31) TAM in the zonal-mean winds

UH seminar, 29 August 2001 spectral characteristics NCEP AGCM

UH seminar, 29 August 2001 (=TAM index) persistence of the TAM

UH seminar, 29 August 2001 question (1) The features of the TAM –high correlation with Niño3 SST index –low-level divergence (convergence) over the maritime continent (eastern eq. Pacific) –spectral peak around 4 yr –persistence up to 5 mo Do they imply the TAM nothing more than the atmospheric response to El Niño? But ENSO residual fields do reveal the same variability dominating In essence, is TAM independent of ENSO? –How can we explore it? use AGCM !

UH seminar, 29 August 2001 T42L20 CCSR/NIES AGCM, 50yr run with climatological SST TAM simulated by an AGCM CCSR/NIES AGCM

UH seminar, 29 August 2001 simulated TAM zonal wind meridional wind CCSR/NIES AGCM

UH seminar, 29 August 2001 question (2) AGCM reproduced an overall feature of the observed TAM –e.g. horizontal circulation patterns, even the equatorial surface wind spectrum of the coefficient is much whiter than observations in addition to the absence of a peak around 4yr TAM may essentially be an internal atmospheric mode –What is the dynamics responsible for such a mode? –We need to diagnose it using a simpler dynamical framework

Linearized multi-level PE model –derived from a dynamical core of the CCSR/NIES AGCM –spectral truncation of T21 –vertical 20 levels steady version –zonally symmetric basic state obtained from the NCEP or AGCM climatology (monthly or seasonal mean) –zonal wavenumber truncated at m=5 time integration –3D basic state obtained from the NCEP or AGCM climatology (monthly or seasonal mean) –refs: Watanabe & Kimoto (1999, GRL) – Watanabe & Kimoto (2000, QJRMS) UH seminar, 29 August 2001 linear baroclinic model (LBM)

zonal-wave coupling term stationary wave feedback UH seminar, 29 August 2001 linear operators

＝ , D, T, ... Xa Xa* m=0 m=1 m=2 m=M ,D,T,,D,T, ZRM PWM N (N ～ 30,000 for T21L20) F*(Xa,Xc*) F(Xa*,Xc*) UH seminar, 29 August 2001 linear operator matrices

calculate singular vectors of L associated stationary wave anomalies readily obtained as u-vectors singular values v-vectors ∴ singular mode with the smallest  will have the longest persistence UH seminar, 29 August 2001 neutral mode detection

UH seminar, 29 August 2001 neutral mode zonal wind meridional wind

UH seminar, 29 August 2001 Leading singular mode + associated stationary waves, v 1 +L *-1 F * (X c *,v 1 ) ・ much prevailing zonal structure in  300 ・ low-level features less similar to obs./AGCM TAM ・ decay time ～ dissipation timescale of the free troposphere (< month) anomalous circulation associated with the neutral mode

UH seminar, 29 August 2001 zonal asymmetry observed TAM neutral mode Ua ・ neutral mode seems consistent with the observed TAM in a considerable part except for the Pacific

UH seminar, 29 August 2001 Zonal-mean zonal momentum budget close to neutrality on the neutrality of the mode

UH seminar, 29 August 2001 NCEP zonal-mean wind regressed on the PC1  300 Coincidence between Ua and  c further suggests the momentum feedback actively working for the neutrality role of the basic state vorticity

UH seminar, 29 August 2001 eigenmodes of the zonal-mean shallow-water eqs. ・ basic state  is not crucial for the presence of the mode ・ scattering on  i =0, due to viscosity? origin of the neutral mode

conclusions (1) Tropical Axisymmetric Mode (TAM): –tightly related to the angular momentum variability and  LOD –contains a signature of El Niño (may suggest ENSO forces TAM) dynamics of the TAM –AGCM with climatological SST does reproduce the observed TAM –A near-neutral mode found in the singular mode computation of the linear model is considerably similar to the observed/AGCM TAM –The essence of the TAM can be interpreted as an internal atmospheric mode which is easily excited by forcing –The neutrality partially arises from a positive momentum feedback in the zonal mean state (i.e. coupling between Ua and Hadley circulation), although the process may not be crucial for the origin of mode UH seminar, 29 August 2001

question(3) UH seminar, 29 August 2001 Neutral mode failed to reproduce the lower-tropospheric feature in the observed TAM –Why? –Interaction between convection and the dynamics?

UH seminar, 29 August 2001 Composite OLR anomaly based on the TAM index AGCM NOAA convection associated with TAM

implication UH seminar, 29 August 2001 Can we interpret the change in zonal mean state during ENSO in terms of an excitation of the neutral mode? Role of the zonal mean flow (Ua) in: –ENSO upstream teleconnection –ENSO-monsoon coupling

UH seminar, 29 August 2001 Regression of Z 500 /  300 on monthly Nino3 SSTA, global ENSO teleconnection

UH seminar, 29 August 2001 DJF idealized heating Q ENSO-forced zonal-mean flow The Ua response is independent of the Rossby wave train over the Pacific! NCEP compositeLBM response

calculate singular vectors of L (zonal-mean dynamical operator) The phase and amplitude of each mode depend not only on the singular value, but on the projection of u-vector onto forcing u-vectors singular values v-vectors UH seminar, 29 August 2001 zonal mean response represented by singular modes ‘projection coefficient’

UH seminar, 29 August 2001 ・ a large part of the forced zonal wind is reproducible with two singular modes ・ different optimal heating profiles for the neutral mode ( ～ TAM) & a second (baroclinic) mode optimal thermal forcing reconstruction by singular modes

zonal-wave coupling El Niño heating UH seminar, 29 August 2001 stationary wave response to ENSO-forced Ua T 850 in winter NCEP composite for El Niño LBM response to Q 95% significance idealized heating LBM response to Q+ZW

UH seminar, 29 August 2001 ‘tropical-belt’ teleconnection upslope cooling (downslope warming) due to orographic forcing (cf. Hoskins & Karoly 1981)

UH seminar, 29 August 2001 time series of : TAM index (JJA avg.), all-India monsoon rainfall (IMR), Webster & Yang ‘s dynamical monsoon index r(TAM,IMR) = -0.50r(TAM,DMI) = Relationship between TAM and summer monsoon

question (4) UH seminar, 29 August 2001 There is an argument that change in the subtropical jet associated with El Niño is involved in the coupled ENSO-monsoon system. –(e.g. Nigam 1994; Ju & Slingo 1995) The TAM index indeed shows a significant correlation with indices of the Asian summer monsoon variability Does the anomalous zonal-mean flow forced by El Niños (whatever the mechanism) plays any role in the ENSO-monsoon coupling?

UH seminar, 29 August 2001 Composite OLR anomaly in summer, following Kawamura (1998) convection associated with ENSO/monsoon weak monsoon/warm event: 1979, 1983, 1987, 1991, 1992, 1993 strong monsoon/cold event : 1981, 1984, 1985, 1988, 1989, 1990

UH seminar, 29 August 2001 precursors for weak summer monsoon observed composite in May OLR anomaly T 300 & V 850 anomalies

UH seminar, 29 August 2001 simulated circulation anomalies in May 10-member ensemble difference for El Niño run Vertically averaged Q T 300 & V 850 response CCSR/NIES AGCM

UH seminar, 29 August 2001 simulated monsoon precursor in May CCSR/NIES AGCM

UH seminar, 29 August 2001 role of Ua in forcing the continental cooling T 300 & V 850 response to the AGCM heating LBM at day 25 Indian Ocean heating removed & zonal mean response damped

UH seminar, 29 August 2001 NCEP composite cooling over the Himalayan upslope CCSR AGCM LBM (day 25) temperature longitude-pressure section along 30N in May vertical phase tilt upslope cooling for long wave (K<Ks)

UH seminar, 29 August 2001 cooling over the Himalayan upslope C p1p1 p0p0 C  <0

conclusions (2) zonal flow anomaly (Ua) during ENSO –subtropical westerlies and equatorial easterly anomaly –the anomalous zonal-mean flow is plausibly independent of the Rossby wave train over the Pacific –may be an indication of neutral modes excited by the El Niño heating role of zonal flow anomaly: tropical-belt teleconnection –Ua-induced teleconnection seems to explain how and why the anomalous circulation occurs in the upstream region of El Niño –tropical-belt teleconnection may further play an active role in the ENSO-monsoon coupling such that it helps to precondition the weak monsoon during El Niño-like condition further question: how ENSO forces the zonal mean flow anomaly? –what is the role of stationary wave feedback on to the zonal mean? UH seminar, 29 August 2001