Identification of Processes Driving Low-Level Westerlies in West Equatorial Africa

Wilfried M. Pokam Department of Physics, Higher Teacher Training College, University of Yaoundé 1, and Center for International Forestry Research, Central Africa Regional Office, Yaoundé, Cameroon

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Caroline L. Bain Met Office Hadley Centre, Exeter, United Kingdom

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Robin S. Chadwick Met Office Hadley Centre, Exeter, United Kingdom

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Richard Graham Met Office Hadley Centre, Exeter, United Kingdom

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Denis Jean Sonwa Center for International Forestry Research, Central Africa Regional Office, Yaoundé, Cameroon

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Francois Mkankam Kamga University of Mountain, Bangangté, Cameroon

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Abstract

This paper investigates and characterizes the control mechanisms of the low-level circulation over west equatorial Africa (WEA) using four reanalysis datasets. Emphasis is placed on the contribution of the divergent and rotational circulation to the total flow. Additional focus is made on analyzing the zonal wind component, in order to gain insight into the processes that control the variability of the low-level westerlies (LLW) in the region. The results suggest that the control mechanisms differ north and south of 6°N. In the north, the LLW are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the West African monsoon system. This northern branch of the LLW is well developed from June to August and disappears in December–February. South of 6°N, the seasonal variability of the LLW is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator, where ascent prevails. The heating contrasts lead to a Walker-type circulation with development of LLW as its lower branch. Thus, evidence is presented that the LLW are driven by differential heating. This contrasts with the traditional conceptual view that the Saint Helena high is the primary driver of low-level circulation off the Atlantic Ocean to WEA. Forest cover in WEA may modulate the latent heating that helps to drive the differential heating and maintain the LLW, and this interaction should be the focus of further study.

Denotes Open Access content.

Publisher’s Note: This article was revised on 7 August 2017 to correct the name of the first author, whose given name and surname was inadvertently reversed when originally published.

Corresponding author address: Wilfried M. Pokam, Department of Physics, Higher Teacher Training College, University of Yaoundé 1, P.O. Box 47, Yaoundé, Cameroon. E-mail: wpokam@yahoo.fr

Abstract

This paper investigates and characterizes the control mechanisms of the low-level circulation over west equatorial Africa (WEA) using four reanalysis datasets. Emphasis is placed on the contribution of the divergent and rotational circulation to the total flow. Additional focus is made on analyzing the zonal wind component, in order to gain insight into the processes that control the variability of the low-level westerlies (LLW) in the region. The results suggest that the control mechanisms differ north and south of 6°N. In the north, the LLW are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the West African monsoon system. This northern branch of the LLW is well developed from June to August and disappears in December–February. South of 6°N, the seasonal variability of the LLW is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator, where ascent prevails. The heating contrasts lead to a Walker-type circulation with development of LLW as its lower branch. Thus, evidence is presented that the LLW are driven by differential heating. This contrasts with the traditional conceptual view that the Saint Helena high is the primary driver of low-level circulation off the Atlantic Ocean to WEA. Forest cover in WEA may modulate the latent heating that helps to drive the differential heating and maintain the LLW, and this interaction should be the focus of further study.

Denotes Open Access content.

Publisher’s Note: This article was revised on 7 August 2017 to correct the name of the first author, whose given name and surname was inadvertently reversed when originally published.

Corresponding author address: Wilfried M. Pokam, Department of Physics, Higher Teacher Training College, University of Yaoundé 1, P.O. Box 47, Yaoundé, Cameroon. E-mail: wpokam@yahoo.fr
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