Effects of the subtropical anticyclones over North Africa and Arabian Peninsula on the African easterly jet

Abstract

North African climate is analysed between 1979 and 2010 with an emphasis on August using the European Center for Medium‐Range Weather Forecast (ECMWF) global dataset to investigate the effects of the subtropical anticyclones over North Africa and the Arabian Peninsula on the African easterly jet (AEJ). It was found that the AEJ encloses a core with a local wind maximum (LWM) in both West and East Africa, in which the west LWM core has a higher zonal wind speed. The strength of both cores is distinctly different by way of thermal wind balance. As found in previous studies, the AEJ is formed through baroclinicity with influence from Saharan lower level heating along and to the north of the Intertropical Front (ITF). As there are two separate anticyclonic centres, the AEJ is maintained by the anticyclonic systems in West Africa, East Africa as well as the Arabian Peninsula. The presence of these two anticyclonic centres provides the AEJ with two separate core maxima with the western LWM located at 15°W, 17°N and the eastern LWM located at 35°E, 15°N for August.

Evaluation of the ECMWF Sub-seasonal to Seasonal Precipitation Forecasts during the Peak of West Africa Monsoon in Nigeria

“…Climatologically, the model reproduces the spatial and behavioral patterns of the three cores of the AEJ (the eastern, central and western cores Figure 6c); though, with slight deviation in the positions and strength. These findings are similar to those of Grist and Nicholson (2001), Afiesimama (2007), and Spinks et al (2015). These imply that, irrespective of the ENSO phase considered and apart from wind directions, the ECMWF-S2S model poorly reproduce the wind strength that could depict AEJ at the 700 hPa level.…”

Variability of the subtropical highs, African easterly jet and easterly wave intensities over North Africa and Arabian Peninsula in late summer

“…It was demonstrated in Spinks et al (2014) that the LWMs were reproduced in other data sets such as ECMWF 40-year reanalysis (ERA-40) at 2.5 ∘ (Uppala et al, 2005), the National Centre of Environmental Prediction (NCEP) reanalysis at 2.5 ∘ (Kalnay et al, 1996), and the Climate Forecasting System Reanalysis version 1 (CFSR) at 0.3 ∘ (Saha et al, 2006). Over the years, climatological analyses of the AEJ were carried out using global model data with relatively low spatial resolutions.…”

“…It was shown that the divergent centre was formed from east-west differential heating by thermal low heating (hot) and downward (cold) branch of the Indian Monsoon. Similar procedure was performed by Spinks et al (2014) for the Arabian High. With the usage of a higher spatial resolution data set, two divergent centres were found respectively for the Saharan and Arabian Highs.…”

“…In this research, we plan to extend the study of Spinks et al (2014) to investigate the yearly variability of AEJ, LWMs, easterly waves, and AEWs and their intensities over North Africa and the Arabian Peninsula during the same period of 1979-2010 for the month of August. The sensitivities of these features to El Niño Southern Oscillation (ENSO) will also be studied.…”

“…In addition, North Africa experiences its most active month for precipitation in August. Spinks et al (2014) investigated the AEJ and local wind maxima in the west (LWM W ) and east (LWM E ) Africa, respectively, to the Saharan and Arabian Highs. In their study, an analysis of the European Centre for Medium-Range Weather Forecasts Intermediate Reanalysis (ERA-Interim or ERA-I) at 0.75 for the month of August over 32 years was carried out to compare the differences of the AEJ's maintenance and formation associated with LWM W and LWM E .…”