Thorncroft and Blackburn (1999): Maintenance of the African Easterly Jet

In a recent discussion with some colleagues someone asked the simple question, “Do you know why the African easterly jet occurs at 600mb?” and I realized didn’t actually know the whole story. The usual explanation is related thermal wind balance and the meridional temperature gradient at the surface. However, this explanation misses some subtle aspects of the vertical structure of wind over Africa. This paper appears to be the first to provide a thorough explanation of the processes which maintain the African easterly jet (AEJ).

Thorncroft, C. D., and M. Blackburn, 1999: Maintenance of the African Easterly Jet. Quart. J. Roy. Met. Soc., 125, 763–786.

For the uninitiated, the AEJ occurs over West Africa year-round and leads to the generation of African easterly waves (AEW), which play a big role in the weather over the African Sahel region (see satellite image below). These waves are also known to often be associated with tropical cyclone formation over the Atlantic.

Several studies have looked into the AEJ and AEWs starting back in the 1970’s. The plot below of zonal mean zonal wind is from Reed et al. (1977) and was made using data from a field experiment in 1974 known by a the nested acronym: GATE. The AEJ is the easterly wind (marked “E”) at ~5N and 600mb.

The mean zonal wind during the GATE (Global Atmospheric Research Programme Atlantic Tropical Experiment) Phase III period between 23 August and 19 September 1974 from Reed et al. (1977), contour interval 2.5 m s-1.This is averaged between 10E and 31W. the ‘zero’ latitude corresponds to the average latitude of a disturbance path which was 11N over land and 12N over the ocean.

The existence of the AEJ is a consequence of thermal wind balance, related to the fact that the meridional surface temperature gradient (i.e. dT/dy) is positive south of the Sahara desert.

Boreal summer mean skin temperature from era interim reanalysis years 2000-2009.

Thorncroft and Blackburn point out that this explanation is insufficient to explain why the shear becomes westerly above the jet core. They demonstrate why the jet sits at its particular altitude by showing that typical temperature profiles from regions south and north of the jet “cross” at the height of the jet.

“Making the simple assumption that the mean temperature soundings in these two regions can be approximated by pseudo-adiabatic parcel ascent curves from the surface, we can see in Fig. 2(a) that the temperature soundings cross at about 700 mb. This implies that the positive meridional temperature gradient decreases with height in association with contrasting moist and dry adiabatic profiles in the equatorward and poleward regions respectively.”

This seems like an overly complicated way to say that the meridional temperature gradient is negative below the jet, and becomes positive above the jet. From thermal wind balance this would predict maximum easterlies at the altitude where the meridional temperature gradients is zero. I think it’s much nicer to see this by just plotting the zonal mean of the meridional temperature gradient, as I’ve done below using ERA interim data.

Zonal and boreal summer mean meridional temperature gradient (colors) and zonal wind (contours) over West Africa (0-20E) from ERA interim reanalysis data (2000-2009).

That was kind of a long tangent about the AEJ, but It’s relevant to the results of this current paper. The main goal of the paper is to use a zonally symmetric dry model (See Hoskins and Simmons 1975) to isolate the effect of two mechanisms that work to maintain the AEJ against the dissipating effects of AEWs.

“The aim of this paper is to show that the AEJ is associated with two separate diabatically forced meridional circulations, one associated with dry convection in the Sahara and the other with deep moist convection in the intertropical convergence zone (ITCZ) equatorward of the Sahara. Although it was not the main purpose of their paper, Schubert et al. (1991; henceforward S91) emphasized the role of the ITCZ convection in maintaining the AEJ.”

In other words, Schubert et al. was correct in saying the ITCZ convection plays a role in maintaining the AEJ, but without the dry convective boundary layer of the Sahara, the jet would look very different. The results of this paper can be summarized by just showing the results of the 3 main experiments:

  1. prescribed heat low north of 15N
    • surface temperature gradient is maximized at 15N
    • outside of gradient region surface fluxes are suppressed
  2. prescribed ITCZ heating south of 15N
    • Idealized heating profiles based on sine and sine2
    • They also test the effect of a boundary layer scheme
  3. Combined forcings

Below is the result of the heat low forcing. A strong easterly jet develops at 15N as expected.

Zonal wind response from heat low experiment.

The next figure is the response from the imposed ITCZ heating. The easterly wind at 17N is analogous to the AEJ. The jet is weaker than observed.

Zonal wind response from ITCZ experiment.

The result of the third experiment below combines the two forcing to yield the most realistic picture of the AEJ. The jet sits lower than observed, but I don’t think this is an issue given the simplicity of the model. The authors discuss how the idealized jet here is missing several key processes, most notably dissipation by AEWs.

Zonal wind response from combined forcings.

“The results from the present study … lead us to suggest that, although ITCZ convection alone can result in an easterly jet, the dry convection in the heat-low plays a crucial role in AEJ maintenance. In fact the heat-low heating alone leads to a more realistic AEJ structure and magnitude than the ITCZ heating alone. Also, the ITCZ heating produces a jet pair, with a stronger westerly jet equatorward of the easterly, which is not observed.”

One particularly interesting point about the effect of the two forcings is that the negative PV gradient is due to the near zero PV over the Sahara. S91 suggested that it result from diabatic heating and PV generation in the ITCZ, but the experiments in this paper suggest that the low PV north of the equator might be the more factor determining the negative meridional gradient.

Another interesting tidbit from this paper was how the meridional circulation that develops in response to the heat low reinforces a negative meridional temperature gradient.

“The meridional circulation through maintaining a state close to thermal wind balance, does not only provide easterly acceleration in the midtroposphere. Through adiabatic cooling in the rising branch and adiabatic warming in the sinking branch above the level of the diabatic heating, it also provides the negative meridional temperature gradient consistent with a reversal in the zonal wind shear. The situation is shown schematically in Fig. 8, which indicates rising motion in the region of heating.”

Keep in mind that this paper used a highly idealized model. Another more recent study on this topic that goes into much more interesting detail on the role of various surface processes can be found here:

Wu, M.-L. C., O. Reale, S. D. Schubert, M. J. Suarez, R. D. Koster, and P. J. Pegion, 2009: African Easterly Jet: Structure and Maintenance. J. Climate, 22, 4459–4480.

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