Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP5 models

Transcription

1 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models F. Lott{1}, S. Denvil{2}, N. Butchart{3}, C. Cagnazzo{4}, M. A. Giorgetta{}, S. C. Hardiman{3}, E. Manzini{}, T. Krismer{}, J-P Duvel{1}, P. Maury{1}, J. F. Scinocca{6}, S. Watanabe{7}, and S. Yukimoto{9} {1} Laboratoire de Météorologie Dynamique, Paris, France {2} IPSL, Paris France, {3} Met Office, Exeter, United Kingdom {4} ISAC-CNR, Roma, Italy {} MPI Meteorologie, Hamburg, Germany {6} CCCMA, University of Victoria, Canada {7} JAMSTEC, Yokohama, Japan {8} CRD-Met. Res. Institute, Tsukuba, Japan. List of models (and data) used: CMIP with stratosphere, the 4 that has a QBO and 4+1 without to compare with.

2 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Motivation: An important step in CMIP is that many models represent the stratosphere : We need to diagnose how well they do, and in the stratosphere the day to day variability is dominated by Kelvin and Rossbygravity wave packets How well are they represented? What make the differences between models? CMIP3 models present a large spread in their representation of convection variability (ENSO, MJO, Convectively Coupled Waves (Straub Haertel and Kiladis 21). If the same is true for CMIP: Does it impact the largest scales stratospheric waves or does the dynamical filtering? Does other sources of waves balance tropospheric defects in large scale organization of equatorial precipitations?

3 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. The method only used the fact that the gravest equatorial waves are characterized by one dynamical field being of uniform sign over the Equatorial Band and at a given longitude: u, T, and Z for the Kelvin waves (n=-1) and v for the Rossby Gravity waves (n=). Rossby-Gravity 1 N-1 S 1 N-1 S Kelvin In ERA4 and NCEP see the climatology in Lott et al.~(29)

4 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. In the low equatorial stratosphere, the day to day variability is dominated by Kelvin and Rossby Gravity wave packets : We need to have them correct in the physical space (not only in the Spectral space), since they impact advection, de-hydratation, i.e. not only the QBO. Zonal wind at Eq. In February in Feb 1999 (ERA4, CI=2m/s)) Meridional wind at Eq. In September 199 (ERA4, CI=2m/s)) From Lott et al. (JAS 29)

5 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Spectral analysis of hpa fields averaged over the Eq. Band and filter used Right: zonal wind spectra in the eastward propagating direction and Kelvin wave band-pass filter (in spectral space) Frequency (ω, Cy/day) Left : meridional wind spectra in the westward propagating direction and Rossby-Gravity wave band-pass filter (in spectral space) ½ power point of F ½ power point of F Filtered zonal wind (u) build from the zonal wind double Fourier transform (û): s =nlon / 2 n=nday / 2 u (λ,ϕ, z, d)= s =1 n =1 F(s,ω n) u (s, ϕ, z, ω n)e The same filter is applied to all models i ( s λ ω n d )

6 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Zonal mean zonal wind u Kelvin wave index: K = Max < λ<36 [ 1 1 u d φ 1 2 ] Larger when u< : westward QBO Rossby gravity wave index: [ 1 1 RG= Max 2 1 v d φ <λ <36 ] Larger when u> : eastward QBO An exampl of dynamical filtering: waves propagate better when u and the wave phase speed have opposite signs KW = 1 1 u d ϕ 2 1

7 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. No CCEWs at all The PREC variability varies a lot from one model to the other (as in CMIP3, see Straub Hertel Kiladis 213) Frequency (Cy/day) Frequency (Cy/day) CCEWs but not strong enough to impact the PREC spectra (only visible in the coherency) Frequency (Cy/day) CCEWs strong enough to affect the precipitation Variablity ERAI and GPCP Symmetric precipitations : spectra (lines).1.1 And Increasing precipitation variability Some models have : Frequency (Cy/day) Convectively coupled waves Frequency (Cy/day) Kelvin waves.1.1 IPSLA.1.1 IPSLB Wavenumber With QBO Wavenumber 8 1 Without QBO coherency with U8 (color) A large variability in precipitations spectra

8 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Kelvin waves Zonal wind at the equator spectra at hpa All models have Kelvin waves (see eastward panel) More substantial signals are for the models with QBO, which is confirmed when we compare the 2 MPI runs (Take care of the CIx4 factor in The models with QBO only!!!). Some sensitivity to the convection, confirmed when we compare the two IPSL runs. Between models, the effect of the convection is not as pronounced as in Horinouchi et al.~23. ERAI IPSLA IPSLB

9 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. With QBO The amplitude are the strongest for the models with a QBO and largest precipitation variability. Hovmoller of u at the Equator Kelvin wave composites at hpa Kelvin waves Without QBO ERAI IPLA (High precip variability) IPLB (Low precip variability) Longitude ( is arbitrary) Longitude ( is arbitrary) 18

10 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. With QBO The eastward phase speeds are quite correct The amplitude are the strongest for the models with a QBO and largest precipitation variability. Hovmoller of u at the Equator Kelvin wave composites at hpa Kelvin waves Without QBO ERAI IPLA (High precip variability) IPLB (Low precip variability) Longitude ( is arbitrary) Longitude ( is arbitrary) 18

11 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. With QBO Kelvin wave composites at hpa Latitude Kelvin waves But models with QBO have stronger Kelvin waves, clearly due to their increased vertical resolution since in them the Kelvin waves are filtered out half of the time 1S Latitude Latitude N S N IPLA (High precip variability) 1S 1N Latitude in the models without QBO, the zonal mean Zonal winds are westward all the time (U< not shown), which is the favourable Situation for Kelvin waves vertical propagation. Hence it is not surprise that all the models simulate well the Kelvin waves IPLB (Low precip variability) 1S -9 Latitude All models have realistic Kelvin waves, when compared to ERAI (which probably underestimates them, Ern et al. 28 and after) ERAI 1N -9 u, v, and T at -day lag Without QBO N S Longitude ( is arbitrary) Longitude ( is arbitrary)

12 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Rossby Gravity waves Meridional wind spectra at the equator and at hpa Models with a QBO have a much Larger Rossby-GW signature (remember that model without QBO have westward Winds, a situation not favorable for waves with westward phase Speeds) (Take care of the CIx4 factor in The models with QBO except HadGEM2!!!). ERAI IPSLA f IPSLB

13 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Rossby Gravity wave composites at hpa Hovmoller of V at equator Without QBO With QBO Rossby Gravity waves v -12 ERAI IPLA (High precip variability) IPLB (Low precip variability) Longitude ( is arbitrary) Longitude ( is arbitrary) 12

14 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. This long distance travel is particularly pronounced in the UKMO model! In models without QBO, the RGWs packets stay at the same place, the eastward intrinsic group speed balancing the westward advection. This is not the case in Models with a QBO, and where the RG waves packets travel over very large distances. Eastward phase speed but Westward group velocity as expected for RGWs. Rossby Gravity wave composites at hpa Hovmoller of V at equator Without QBO With QBO Rossby Gravity waves v -12 ERAI IPLA (High precip variability) IPLB (Low precip variability) Longitude ( is arbitrary) Longitude ( is arbitrary) 12

15 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. This long distance travel is particularly pronounced in the UKMO model! In models without QBO, the RGWs packets stay at the same place, the eastward intrinsic group speed balancing the westward advection. This is not the case in Models with a QBO, and where the RG waves packets travel over very large distances. Eastward phase speed but Westward group velocity as expected for RGWs. Rossby Gravity wave composites at hpa Hovmoller of V at equator Without QBO With QBO Rossby Gravity waves v -12 ERAI IPLA (High precip variability) IPLB (Low precip variability) Longitude ( is arbitrary) Longitude ( is arbitrary) 12

16 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. This long distance travel is particularly pronounced in the UKMO model! In models without QBO, the RGWs packets stay at the same place, the eastward intrinsic group speed balancing the westward advection. This is not the case in Models with a QBO, and where the RG waves packets travel over very large distances. Eastward phase speed but Westward group velocity as expected for RGWs. Rossby Gravity wave composites at hpa Hovmoller of V at equator Without QBO With QBO Rossby Gravity waves v -12 ERAI IPLA (High precip variability) IPLB (Low precip variability) Longitude ( is arbitrary) Longitude ( is arbitrary) 12

17 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. With QBO As situation with U> never occurs in models without a QBO, it is natural that these models do not reproduce the RGWs as well as the models with QBO do. Latitude Latitude 1N 1S N 1S N 1S N 1S IPLA (High precip variability) 1N 1S -9 Latitude All models have Rossby gravity waves, but the more realistic and large signals are in models with QBO. In them, the composite technique tends to pick dates when the QBO is eastward (U>) which is a favourable situation for RGWs vertical propagation. Without QBO ERAI 1N -9 Latitude Rossby-gravity waves composites at hpa: Latitude Rossby Gravity waves 1N S 9-9 1N IPLB (Low precip variability) 1S N 1S-9 9 1N S Longitude ( is arbitrary)c 1S Longitude ( is arbitrary)c

18 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. The High top CMIP models simulate realistic aspects of the stratospheric largescale Kelvin and RG waves. They represent them better than the tropospheric convectively coupled waves. There is nevertheless a large spread among the models, and those with a QuasiBiennial Oscillation (QBO) produce larger amplitude waves than the models without For the RG waves this follows that models without a QBO never have u>, a situation that is favorable to the propagation of westward propagating waves.. For the Kelvin waves, larger amplitudes in the presence of a QBO is counter intuitive because Kelvin waves are expected to be larger when u< and this is always satisfied in models without QBO. We attribute the larger amplitude to the fact that models tuned to have a QBO require finer vertical resolution in the stratosphere. Models with large precipitation variability tend to produce larger amplitude waves. The effect is not as pronounced as found in previous studies. In fact, even models with weak precipitation variability still have quite realistic stratospheric waves : (i) other sources can be significant or (ii) the dynamical filtering mitigates the differences in the sources between models. Lott et al.~(jgr 214)

19 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Other sources in models : Exemple of the Composite of EP fluxes between a model and in re-analysis (Maury and Lott ACP 214)

20 Kelvin and Rossby gravity wave packets in the lower stratosphere of CMIP models. Other sources in reality : Exemple of the Composite of RGWs in re-analysis, when QBO winds underneath are negative (Stratospheric reloading in Maury and Lott ACP 214)