Dynamics of the African Heat Low on climate scale R. Roehrig, F. Chauvin, J.-P. Lafore Météo-France, CNRM-GAME ENSEMBLES RT3 Working Meeting 10 February 2009

Motivation and Objectives The Saharan Heat Low (SHL) is considered as a key component of the West African Monsoon system. Few studies have addressed the SHL own variability and its impact on the WAM. The SHL can make a bridge between the midlatitudes and the West African region, and thus provide some potential predictability. Thus, the objectives here are to: Assess the SHL intraseasonal variability and its origin. Address the impacts of such modes onto the WAM system. 10 February 2009ENSEMBLES RT3 Working Meeting

Approach Use of the 850 hPa potential temperature (theta):  Highly-correlated to the SHL thickness (Lavaysse et al. 2009). 850 hPa is approximately the middle of the SHL boundary layer (~3000m deep)  Relatively less sensitive to surface and orography than the 2m temperature.  Easily diagnosed from reanalyses or GCM output. 10 February 2009ENSEMBLES RT3 Working Meeting

Data and Methodology NCEP/DOE AMIP-II reanalysis :  2.5° spatial resolution and 1 day temporal resolution  Time filtering (10-60 days) to investigate only the intraseasonal variability  Period from 1 June to 30 September (122 days per year)‏  Domain: 20°N-40°N, 20°W-40°E, to focus on the northern part of the SHL (i.e., mostly related to the midlatitudes)‏  Variable : theta at 850 hPa OLR-NOAA dataset as a proxy of the convective activity. Methodology:  Principal Component Analysis to extract the maximum of variance of the signal  As justified hereafter, Complex PCA to address propagative modes  Lagged composites 10 February 2009ENSEMBLES RT3 Working Meeting

850 hPa theta climatology 10 February 2009ENSEMBLES RT3 Working Meeting Colors: climatology (°C) Contours: day variance (°C)²

PCA Theta 850 hPa 10 February 2009ENSEMBLES RT3 Working Meeting 48% of the variance explained by the two first modes of the PCA. Most of the energy is in the day range. The eastern part of the two first EOFs are spatially in quadrature The first two PCs are correlated at lag 4-5 days for many years of the period (more than 0.6).  These two first modes partly represent the same propagative mode

Complex PCA Theta 850hPa 10 February 2009ENSEMBLES RT3 Working Meeting 51% of the variance explained by the two first modes of the CPCA. Most of the energy is also in the day range. Two poles in the first mode: One propagating southeastward. Another propagating southwestward.  In the following, we focus on the first mode Colors: module Vectors: local phase speed

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting HLW = warm anomaly in the West (Morocco and Mauritania HLE = warm anomaly in the East (Tunisia, Libya and Egypt) Vectors: 850 hPa horizontal wind

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

850 hPa Theta composites: HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

Composites MSLP and Wind at 925 hPa HLWHLE HLW - HLE 10 February 2009ENSEMBLES RT3 Working Meeting

Composite Geopotential Height at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting

Composite Geopotential Height at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting

Composite Geopotential Height at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting

Composite Geopotential Height at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting

Composite Geopotential Height at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting

Composite Geopotential Height at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting

Composite Geopotential Height at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting -2 days SHL mode associated with a upper tropospheric wave train Southeastward propagation The wave train precedes the SHL mode maximum by 2 days The SHL intraseasonal mode is related to the midlatitude intraseasonal variability during boreal summer

Composites Wind at 200 hPa 10 February 2009ENSEMBLES RT3 Working Meeting Shaded: 200 hPa windspeed Vectors: 200 hPa wind Contours: 400 hPa pressure vertical velocity

Composites OLR Propagation speed: 600 km/day. Triggering of AEW ? 10 February 2009ENSEMBLES RT3 Working Meeting

Conclusions An intraseasonal propagative mode of the SHL variability has been described. Its energy is mainly in the day range. It opposes Morocco/Mauritania to Central Libya in a West/East phase competition. Clearly related to the midlatitude intraseasonal variability (wave train along the northern hemisphere wave guide), and the Azores anticyclone, this mode is deep, almost barotropic above hPa. The link to the WAM is likely to appear through Eastern Africa (East of Lake Chad). A clear propagative signal in the OLR is initiated 2 days before the maximum of the mode, and is detected as far as the middle of the Atlantic. 10 February 2009ENSEMBLES RT3 Working Meeting

Perspectives 10 February 2009ENSEMBLES RT3 Working Meeting Futher investigations: Origin of the midlatitude intraseasonal wave train. Many studies exists for the winter season but very few for the summer season. They highlight barotropic instability of the jets (Simmons et al. 1983), or Rossby wave dispersion (Blackmon et al. 1984). The impact of this mode onto the WAM system: Precipitation and MCSs Monsoon dynamics (AEJ/AEW activity, monsoon flow). The understanding of the underlying mechanisms. Relationship with extratropical dry intrusions (Roca et al 2005). The impact of this mode onto the Atlantic tropical cyclogenesis. Relationship with the monsoon onset ? How these modes are simulated in climate models ?

Thanks ! 10 February 2009ENSEMBLES RT3 Working Meeting

Vertical structure 10 February 2009ENSEMBLES RT3 Working Meeting