African Easterly Jet: Barotropic Instability, Waves, and Cyclogenesis

Abstract

This study investigates the structure of the African easterly jet, focusing on instability processes on a seasonal and subseasonal scale, with the goal of identifying features that could provide increased predictability of Atlantic tropical cyclogenesis. The Modern-Era Retrospective Analysis for Research and Applications (MERRA) is used as the main investigating tool. MERRA is compared with other reanalyses datasets from major operational centers around the world and was found to describe very effectively the circulation over the African monsoon region. In particular, a comparison with precipitation datasets from the Global Precipitation Climatology Project shows that MERRA realistically reproduces seasonal precipitation over that region. The verification of the generalized Kuo barotropic instability condition computed from seasonal means is found to have the interesting property of defining well the location where observed tropical storms are detected. This property does not appear to be an artifact of MERRA and is present also in the other adopted reanalysis datasets. Therefore, the fact that the areas where the mean flow is unstable seems to provide a more favorable environment for wave intensification, could be another factor to include—in addition to sea surface temperature, vertical shear, precipitation, the role of Saharan air, and others—among large-scale forcings affecting development and tropical cyclone frequency. In addition, two prominent modes of variability are found based on a spectral analysis that uses the Hilbert–Huang transform: a 2.5–6-day mode that corresponds well to the African easterly waves and also a 6–9-day mode that seems to be associated with tropical–extratropical interaction.

A meteorological and chemical overview of the DACCIWA field campaign in West Africa in June–July 2016

“…The AEJ is maintained by the anticyclonic circulation associated with the monsoonal subsidence, which characterises the mid-upper troposphere over the Sahara (Chen, 2005;Thorncroft and Blackburn, 1999) and above the shallow dry convection in the SHL (GarciaCarreras et al, 2015;Ryder et al, 2015). Barotropic and baroclinic instabilities associated with the jet create an environment favourable to the generation of African easterly waves (AEWs) (Thorncroft and Hoskins, 1994a, b;Wu et al, 2012), synoptic-scale disturbances characterised by a 2-6-day period in the Sahel. Diedhiou et al (1999) present evidence for a more intermittent, slower (6-9-day period) wave regime with cyclonic and anticyclonic centres straddling the AEJ, longer wavelengths and an activity maximum over the continent in June and July.…”

Tropical Cyclones in the 7-km NASA Global Nature Run for Use in Observing System Simulation Experiments

“…Modern-Era Reanalysis for Research and Applications, version 2 (MERRA-2), described and documented by Bosilovich et al (2015a, b) and by Wargan and Coy (2016), is used for comparison. MERRA-2 is the new generation of the well-known MERRA (Rienecker et al 2011) which has been successfully used, among many others, in studies concerning the meteorology of the African monsoon and tropical Atlantic region (e.g., Wu et al 2012,Wu et al 2013. Figure 1 shows a meridional vertical cross-section of zonal wind at 0°, comparing the NR in the July, August, and September (JAS) months of two different years with the corresponding MERRA-2 years.…”

“…It can only be noted that the NR represents the basic features of the African Monsoon circulation, namely: a) the Tropical Easterly Jet (TEJ), an uppertropospheric jet located close to the Equator at about 100-200 hPa, b) the African Easterly Jet (AEJ) at about 600 hPa and peaking at about 12°N −16°N, c) the low-level westerly monsoonal flow confined below 800 hPa, and, d) the low-level easterly flow (also known as Harmatthan flow) at about 27°N. The overall depiction of the AEJ in the NR is about 15% weaker than in MERRA-2, but it should be remembered that the AEJ depiction is affected by very large uncertainties, with differences of 20% in speed even among state-of-the-art reanalyses such as the ECMWF Reanalysis-40 (ERA-40), the National Centers for Environmental Predictions, Reanalysis 2, (NCEP-R2), the Japanese 25-year Reanalysis (JRA-25) and MERRA, as discussed in detail in Wu et al (2009) and in Wu et al (2012). On the contrary, the representation of the Harmattan flow is stronger in the NR than in MERRA-2.…”

“…African Easterly Waves (AEWs) arise out of a combination of mechanisms: the presence of localized triggers, which can be convective in nature and may alter the vorticity and thermal profile of the atmosphere, and the favorable large-scale environment in which barotropic-baroclinic instability of the Charney-Stern type can occur (e.g., Kiladis et al 2006;Hall et al 2006;Thorncroft et al 2008;Wu et al 2012). Moreover, the presence of the TEJ, which is responsible for strong easterly shear and is generally unfavorable for development of vertically aligned structures, is an important forcing that confines the potential development of TCs to a narrow latitude range (just a few degrees south of the AEJ and north of the TEJ).…”

AIRS impact on analysis and forecast of an extreme rainfall event (Indus River Valley, Pakistan, 2010) with a global data assimilation and forecast system

“…In addition, because it is aided by the analysis tendencies, its products are very smooth and are minimally affected by spin‐down problems. MERRA precipitation has been compared to other analysis and observational data sets and found to be very realistic globally, regionally and on various timescales [e.g., Bosilovich et al , 2011; Wu et al , 2012].…”