Temporal series – Synoptic forecast

The time series available here fall into two categories:

• Raw fields such as precipitable water (parameter=PW) derived from observations/analyses and forecasts, available over the West African capitals and the main cities of Senegal and Burkina Faso. The grey dashed line represents the smoothed annual climatological cycle of PW
• Indices of activity for intraseasonal modes.
  • Sahelian Heat Low (SHL):Evolution of the intraseasonal SHL index (normalised) (Chauvin et al. 2010). This index is calculated as the spatial projection (regression) of potential temperature anomalies at 850 hPa on the spatial structure of the intraseasonal SHL mode. The SHL index is shown for the last 20 days using ECMWF analyses and for the next 9 days using ECMWF forecasts. SHL index > 0: Eastern phase with warm anomaly over the eastern Mediterranean and cold anomaly near the Moroccan coast. When the index is greater than 1 (standard deviation), the SHL is in a marked easterly event. SHL index < 0: Western phase with cold anomaly over the eastern Mediterranean and warm anomaly near the Moroccan coast. When the index is below -1 (standard deviation), the SHL is in a markedly warm westerly event. A cold wave (Vizy and Cook 2009) forms over north-east Africa. The dotted line shows the same intraseasonal SHL index, but using band-pass filtered anomalies from 10 to 90 days.
  • African Easterly Waves (AEW): An AEW activity index is calculated from the kinetic energy (EKE) of the wind disturbances (u',v') associated with the AEW. The EKE (for Eddy Kinetic Energy) is calculated from the wind anomalies (u' and v') at 700 hPa filtered in frequency-wavenumber in the AEW domain (see the time-space filtering part of the documentation). The EKE is defined as EKE=0.5 x (u'2 +v'2) and calculated for the current period and for each year over the climatological period. We then define the intraseasonal EKE anomaly, i.e. the abnormally high or low activity of African easterly waves. A positive (negative) anomaly indicates increased (reduced) AEW activity. The ECMWF analysis is used before the current date and the ECMWF deterministic forecasts after the forecast initialization date. The graphs show the EKE of the African Easterly Waves (left) and its anomaly (right) over different boxes of interest in the Sahel.