The Saharan heat low is a major climate feature of the West African monsoon. Lying poleward of the main rainband throughout the year it is associated with a distinctive shallow meridional circulation (SMC) south of it, with low-level poleward flow and a significant return flow around 700hPa. The heat low and associated SMC have a marked annual cycle with significant changes in the intensity and location of both the heat low and the SMC. They are both most intense in Spring suggesting their potentially crucial role in the rapid Spring-to-Summer transition of several environmental factors including the monsoon onset for locations over land.

The low-level transport of water vapour from the Gulf of Guinea to the continent is a crucial component of the "atmospheric bridge" between the rapidly developing cold tongue and the land, and likely aids the poleward migration of the main rainband between Spring and Summer. The mid-tropospheric return flow around 700hPa advects dry air towards the main rainband and likely hinders its poleward migration.

We will explore the annual variations of the heat low, the SMC and associated water vapour transport using three reanalyses (ERA40, NCEP1 and NCEP2) together with available observations from the AMMA observing campaign in 2006.