The African Easterly Jet - PowerPoint Presentation

The African Easterly Jet SO 442 – Tropical Meteorology

Formation of the African Jet We’ve discussed previously how intense heating over Northern Hemisphere land masses (particularly deserts) is reflected in global average sensible heating Peak ~30°N (approx. latitude of N.H. deserts) The intense heating over the Sahara Desert, in particular, has additional consequences in the atmosphere a bit more localized to the African continent and adjacent Atlantic Ocean Note that the average temperatures over the Sahara during summer are exceptionally warmer than the corresponding temperatures over the Gulf of Guinea immediately to the south What is the consequence of this to upper-level winds? Gulf of Guinea Sahara

Formation of the African Easterly Jet (AEJ) We’ve discussed previously how intense heating over Northern Hemisphere land masses (particularly deserts) is reflected in global average sensible heating Peak ~30°N (approx. latitude of N.H. deserts) The intense heating over the Sahara Desert, in particular, has additional consequences in the atmosphere a bit more localized to the African continent and adjacent Atlantic Ocean Note that the average temperatures over the Sahara during summer are exceptionally warmer than the corresponding temperatures over the Gulf of Guinea immediately to the south What is the consequence of this to upper-level winds? Gulf of Guinea Sahara

Hot Gulf of Guinea Ghana Sahara Desert 0° 10°N 20°N Formation of the African Easterly Jet (AEJ) The view in the sketch below is looking westward The intense heating over the desert in the summer compared with the cooler temperatures over the Gulf of Guinea results in potential temperature surfaces (isentropic surfaces) that slope downward with latitude up to about mid-troposphere         To maintain thermal wind balance this requires that the sign of is what?     Negative means that the -wind (positive from the west) is becoming more negative with height (i.e., the wind is becoming stronger from the east with height)   changes sign above about mid-troposphere which means what?           e asterly jet

Hot Gulf of Guinea Ghana Sahara Desert 0° 10°N 20°N 800 hPa 6 00 hPa 400 hPa 800 hPa 6 00 hPa 400 hPa Formation of the African Easterly Jet (AEJ) There is another way to describe how the AEJ forms: Consider the heights of three pressure surfaces, 800, 600, and 400 hPa over the Gulf of Guinea How do the heights of the same pressure surfaces compare over the Sahara? Why? Higher due to warmer layer average temps (greater thickness)       The difference in the heights of the pressure surfaces in the two locations implies that the isobaric surfaces slope upward with latitude This results in horizontal pressure gradient forces oriented from north to south Coriolis turns the air to the west (from the east) and brings it into geostrophic balance       PGF Coriolis

Formation of the African Easterly Jet (AEJ) Reanalysis data taken from 1971-2000 indeed reveals the presence of an easterly wind maximum of about 20-25 kts located along about 11°N latitude and at a height of ~4 km (~600 hPa) jet The jet exhibits both barotropic instability from vorticity gradients, and baroclinic instability from temperature gradients, both of which are important initial energy sources for the disturbances in the jet known as African easterly waves (AEWs)…which we’ll discuss soon. jet AEWs

Cyclonic shear vorticity From a vorticity standpoint, which side of the jet would be more favorable for cyclogenesis? Southern side due to presence of cyclonic shear vorticity AEWs propagating westward induce the development of thunderstorms which occasionally organize into mesoscale convective systems (MCSs) of several hundred km diameter The combination of these relatively large disturbances with pre-existing environmental helps to prolong their lifespan (by lowering Rossby radius) increasing the likelihood of TC genesis  

Lastly for today, we’ll take a closer look at vertical wind shear in the vicinity of the AEJ (and a more quantitative look at vertical wind shear in general) Vertical Wind Shear in the Vicinity of the AEJ In the sketch below, consider that the 700 hPa winds (magenta vectors) at point A are 4 m/s from the east and at point B are 10 m/s from the east B A Consider also that the 200 hPa winds (green vectors) at point A are 15 m/s from the west and at point B are 5 m/s from the east What is the 700-200 hPa vertical wind shear at A and B? at A: 19 m/s of westerly shear at B: 5 m/s of westerly shear N E z s hear vector Vertical wind shear is the vector difference between the lower- level wind and the upper-level wind

Vertical Wind Shear in the Vicinity of the AEJ In general, lighter shear is found near the axis of the AEJ Smaller vertical shear Cyclonic shear vorticity The overlap region characterized by both small vertical wind shear and lower-level cyclonic shear vorticity is an especially favorable zone for possible TC genesis