West African Monsoon influence on the summer Euro-Atlantic circulation

Gaetani et al 2011, GRL 38, L09705

M. Gaetani (1), B. Pohl (2),H. Douville (3), and B. Fontaine (2)

(1) IBIMET, CNR, Roma, Italy(2) CRC, CNRS / Université de Bourgogne, Dijon, France(3) CNRM-GAME, Meteo-France / CNRS, Toulouse, France

m.gaetani@ibimet.cnr.it11th EMS Annual Meeting / 10th ECAM

CL2 - Climate modelling, long-range forecasting, climate predictionand scenarios from seasons to century

12-16 September 2011Berlin, Germany

Gaetani 2011.09.13 - EMS2011 2/14

Outline1.Motivations & objectives2.The “grid-point nudging” technique3.WAM/Euro-Atlantic interannual and intraseasonal relationship4.Conclusions

Gaetani 2011.09.13 - EMS2011 3/14

Rodwell & Hoskins (2001 J Climate)> role of the monsoons in maintaining the summer subtropical circulation;> Rossby-wave response to the west of the monsoon heating + midlatitude westerlies > region of adiabatic descent;> Asian monsoon > descent over east Med & subtropical North Atlantic.

Euro-Atlantic tropical connections

column mean heating

1 Motivations & Objectives

streamfunction900hPa

omega700hPa

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Fontaine et al (2010 Int J Climatol)> recovery of monsoonal precipitation + ITCZ convection > increased subsidence over the Mediterranean.

Cassou et al (2005 J Climate)> anomalous tropical Atlantic heating + wetter-than-average conditions the Caribbean basin and the Sahel > excitation of the “blocking” and “Atlantic low” regimes > record warm summer of 2003.

WAM > Euro-Atlantic circulation1 Motivations & Objectives

Gaetani 2011.09.13 - EMS2011 5/14

Objectives> investigate the WAM influence on the interannual and intraseasonal variability of the atmospheric circulation over North Atlantic and Europe;> highlight the dynamical features underlying the relationship.

1 Motivations & Objectives

Gaetani 2011.09.13 - EMS2011 6/14

IRCAAM project: http://www.cnrm.meteo.fr/ircaam/

> Main idea: Prescribing monsoon climate in global simulations, with prescribed or interactive SST.> Methodology: Regional grid-point nudging (U,V,T,Q,SLP) towards ERA40 reanalyses; ensembles of simulations with different initial conditions.

> IRCAAM protocol:> U, V, every 5h (λ = 0.1); T, q, SLP every 12h (λ = 0.04);> nudging weaker for 3 (5) lower (upper) levels to let the model adjust to the nudging and/or prescribed SST fields;

> model geometry, ARPEGE v4.6 tl63l31r.

Grid-point nudgingδX/δt = D(X) + P(X) – λ(X-Xref) >>> ERA40

λ = tstep/etime (where tstep = model timestep, etime relaxation time)

2 Grid-point nudging technique

Gaetani 2011.09.13 - EMS2011 7/14

Experimental set-up> Investigation period: JAS 1971-2000 and 1994 (strong WAM).> Observations: NOAA OLR, GPCP and ERA40.> ARPEGE runs (*nudging, **SST):

> Ct IV**, AfNQ IV* IV**, AfNQ IV* Cl**(30 years, 1 member);

> Ct 94**, AfNQ 94* 94**, AfNQ 94* Cl**(1 year, 30 members).

2 Grid-point nudging technique

Gaetani 2011.09.13 - EMS2011 8/14

SVD: JAS ERA40 Z500 vs NOAA OLR 8-year high-pass filter

Wet Sahel <-> H over east Med + positive NAO

3 WAM/Euro-Atlantic relationship

Gaetani 2011.09.13 - EMS2011 9/14

SVD: JAS Arpege Z500 vs OLR 8-year high-pass filter3 WAM/Euro-Atlantic relationship

Gaetani 2011.09.13 - EMS2011 10/14

Diedhiou et al. (1999 Clim Dyn): African Easterly Waves

1994 JAS NAO intraseasonal variability: reproducibility3 WAM/Euro-Atlantic relationship

Gaetani 2011.09.13 - EMS2011 11/14

Rep=var[ensemble]/var[members] African nudging> improvement of the reproducibility of the 1994 JAS NAO intraseasonal variability;> strong signal over the subtropical North Atlantic & in the AEWs periodicities.

3 WAM/Euro-Atlantic relationship

Gaetani 2011.09.13 - EMS2011 12/14

Height

Latitude

Azores index Max-min differences (AEWs periods)travelling AEWs > convection anomalies in the tropical Atlantic > overturning circulation anomalies > vertical velocity anomalies in the subtropical North Atlantic.

1994 JAS NAO intraseasonal variability: mechanism3 WAM/Euro-Atlantic relationship

Gaetani 2011.09.13 - EMS2011 13/14

> Observations confirm that a significant part of the JAS Euro‐Atlantic circulation variability is related to the WAM.

> Specific relationships have been emphasized with the NAO, the subtropical Atlantic and the Mediterranean.

> Reinforced convection in the Sahelian region is associated with positive NAO phases and subsidence over eastern Mediterranean.

> The role of the WAM in driving the mid latitude circulation is ‐confirmed in the model simulations: an improvement in the description of the mid latitude interannual variability is ‐observed when the Arpege Climat model is nudged towards ERA40, a ‐strong effect is observed in the subtropical North Atlantic.

> The atmospheric nudging is effective in replacing the NAO centers of action in a more realistic configuration.

4 Conclusions

Gaetani 2011.09.13 - EMS2011 14/14

> The observed Sahelian convection is positively correlated to the intensification of the meridional overturning circulation and to increased subsidence over the Azores.> In Arpege Climat, the intensification of the Hadley cell is ‐reproduced, but the western sahel convection is underestimated.

> The possibility of an alternative or additive dynamical linkage (a westward propagating Rossby wave pattern?) should be taken into account and investigated in future analysis

4 Conclusions