The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall

Abstract

This paper compares the characteristics of the Tropical Easterly Jet (TEJ) and upper-level winds in six reanalysis products, compares them with soundings at seven West African locations, examines the relationship between Sahel rainfall and the TEJ, and examines factors influencing the TEJ. The jet characteristics assessed by MERRA2, NCEP 1, JRA 55, and ERA 5 are similar. CFSR and 20th Century Reanalysis are outliers in nearly every analysis, overestimating wind speeds by as much as 25 to 40% compared to other reanalyses. Over the period 1948 to 2014, the correlation between rainfall and TEJ magnitude is .72. Arguments based on observations and modelling studies provide evidence that on interannual scales changes in the TEJ are not forced by rainfall, that largescale factors drive the TEJ. Potential mechanisms are discussed for a causal relationship such that a strong jet leads to high rainfall. However, further modelling efforts are needed to conclusively determine whether the TEJ/Sahel rainfall link is a result of common forcing factors. The factors that appear to control jet strength include sea-surface temperature (SST) contrast between the central equatorial Pacific and central equatorial Indian Ocean (correlation of −.64), SST contrast between the central equatorial and the southern subtropical Indian Ocean (correlation of −.39), the latitude of the shift between upper-tropospheric easterlies and westerlies in the Southern Hemisphere (correlation of −.84 at 150 hPa), and the intensity of the Southern Hemisphere westerlies (correlation of +.52 at 200 hPa). This suggests considerable control on the TEJ by extra-tropical circulation in the Southern Hemisphere.

GNSS Radio Occultation Infilling of the African Radiosonde Data Gaps Reveals Drivers of Tropopause Climate Variability

“…Figures 15d–15f reveal that IC4 of tropopause height variability and TEJ have a statistically significant correlation of 0.75 ( p < 0.05). The results are consistent with those of Nicholson and Klotter (2021), who showed that TEJ sustains a maximum strength at 150 hPa during August over the regions of 10°W–30°E and 5°N–15°N. Moreover, the temporal patterns indicate a relationship between tropopause height and TEJ (Figure 15f).…”

“…Grist and Nicholson (2001) showed that the tropospheric temperature gradient has linkage with the intensity of TEJ in the latitudinal region higher than 10°N. In addition, other factors appear to control the strength of TEJ, including the contrast of sea surface temperature over the southern subtropical Indian Ocean and central equatorial Pacific, the latitudinal shift of upper‐tropospheric easterlies and westerlies in the SH, and cooler tropical upper‐tropospheric temperature (Nicholson & Klotter, 2021). This suggests La Niña (i.e., 2010–2011 in Figure 15f) induces global upper‐tropospheric cooling (decrease in tropopause height) and is associated with a widespread and sustained TEJ.…”

“…Clearly, infilling of the African continent's UTLS will benefit the understanding of its climate, which is influenced by numerous global, regional, and local climate variability modes, for example, the Indian Ocean Dipole (IOD; Saji et al., 1999), El Niño‐Southern Oscillation (ENSO; Scherllin‐Pirscher et al., 2012), North Atlantic Oscillation (NAO; Hurrell et al., 2003), Southern Annular Mode (SAM; Fogt et al., 2011), Madden‐Julian Oscillation (MJO; Wheeler & Hendon, 2004), Quasi‐Biennial Oscillation (QBO; Baldwin et al., 2001), the Inter Tropical Convergence Zone (ITCZ; Basha et al., 2015), the Tropical Easterly Jet (TEJ; Nicholson & Klotter, 2021), West African Monsoon (WAM; Sultan & Janicot, 2003), growing ozone pollution in Southern Africa (Thompson et al., 2014), and changing climate in the mountainous region (Kubokawa et al., 2016). Even though these climate drivers are known to trigger multi‐effect severe weather and climate conditions (e.g., Awange, 2022; Awange et al., 2013), there are only a few studies that attempt to understand the African tropopause variability in relation to these climate drivers.…”

Contribution of Lakes in Sustaining Greening of the Sahara during the Mid-Holocene

“…Hence, the enhanced northward WAM forced by lakes can be explained by lake expansions that induce a cyclonic circulation in the lower atmosphere, accompanied by a weakened AEJ and stronger TEJ associated with weakened Sahara Highs and SHL. Similar mechanisms have been previously identified based on observations and simulations, although their physical mechanisms are still under discussion (Nicholson, 2009;Lavaysse et al, 2010;Klein et al, 2015;Nicholson & Klotter, 2020). Furthermore, we found that the lake-induced precipitation and SM increment were close to those induced by orbital forcing only, but restricted over ~10° N (Figure S4a and S4b).…”

The Eastern Mediterranean Sea as dominant driver of interannual rainfall variability in the Sahel

“…A last jet, at around 200 hPa, is the Tropical Easterly Jet. Originating over the western Indian Ocean, this jet passes south of the Sahel and is related to upper-level divergence of the air that rises in the rainstorms in the African rain belt (Nicholson and Klotter 2019). Together with zones of subsidence and uplift, these jets produce coherent circulation patterns north and south of the Sahel (Niang et al 2020, fig.…”