openAccessArticle: FalsePage Range: 13-13doi: 10.1016/j.yqres.2012.03.012Harvest Date: 2016-01-12 15:09:40issueName:cover date: 2012-07-01pubType
This paper examines the mechanisms controlling the year-to-year variability of rainfall over western equatorial Africa during the rainy season of October–December. Five regions with distinct behavior are analyzed separately. Only two show strong associations with the ocean and atmospheric features in the global tropics. These two regions, in the east (the eastern Zaire basin) and west (Angolan coast) of the study area, respectively, demonstrate strikingly opposite relationships with the anomalies of sea surface temperatures (SSTs), sea level pressure (SLP), and east–west atmospheric circulation. The wet (dry) conditions in the eastern Zaire basin are associated with El Niño(La Niña)–like phases. The inverse pattern is apparent for the Angolan coast. The other three regions, lying between these two poles of variability, represent a transition zone with a weak linear relationship to the circulation features. The vital impact of the east–west circulation cells on rainfall variability results in a stronger association with zonal wind than with SSTs or SLP. In addition to the zonal shift, changes in intensity of the zonal cells also play a crucial role. Variability in both magnitude and location of the circulation cells appear to be modulated by the remote forcing from the Pacific via an atmospheric bridge. However, the eastern sector is impacted mainly when synchronous changes occur in the Indian Ocean, and the western sector is impacted mainly when synchronous changes occur in the Atlantic Ocean.
a semiquantitative precipitation dataset for the nineteenth century to add to the more modern gauge data.
This paper examines the factors governing rainfall variability in western equatorial Africa (WEA) during the April–June rainy season. In three of the five regions examined some degree of large-scale forcing is indicated, particularly in the region along the Atlantic coast. Interannual variability in this coastal sector also demonstrates a strong link to changes in local sea surface temperatures (SSTs) and the South Atlantic subtropical high. To examine potential causal mechanisms, various atmospheric parameters are evaluated for wet and dry composites. The results suggest that the intensity of the zonal circulation in the global tropics is a crucial control on rainfall variability over WEA. A La Niña (El Niño)–like signal in both SSTs and zonal circulation over the Pacific is apparent in association with the wet (dry) conditions in the western sector. However, remote forcing from the Pacific modulates the circulation over Africa indirectly by way of synchronous changes in the entire Indian or Atlantic Ocean. Anomalies in the local zonal winds are similar in all three regions: the wet (dry) composite is associated with an intensification (weakening) of the upper-tropospheric easterlies and low-level westerlies, but a weakening (intensification) of the midlevel easterlies. This work also suggests that, in most cases, the relationship between local SSTs and rainfall reflects a common remote forcing by the large-scale atmosphere–ocean system. This forcing is manifested via changes in the zonal circulation. Thus, the statistical associations between rainfall and SSTs do not indicate direct forcing by local SSTs. One point of evidence for this conclusion is the stronger association with atmospheric parameters than with SSTs.
Understanding of hydroclimatic processes in Africa has been hindered by the lack of in situ precipitation measurements. Satellite-based observations, in particular, the TRMM Multisatellite Precipitation Analysis (TMPA) have been pivotal to filling this void. The recently released Integrated Multisatellite Retrievals for GPM (IMERG) project aims to continue the legacy of its predecessor, TMPA, and provide higher-resolution data. Here, IMERG-V04A precipitation data are validated using in situ observations from the Trans-African Hydro-Meteorological Observatory (TAHMO) project. Various evaluation measures are examined over a select number of stations in West and East Africa. In addition, continent-wide comparisons are made between IMERG and TMPA. The results show that the performance of the satellite-based products varies by season, region, and the evaluation statistics. The precipitation diurnal cycle is relatively better captured by IMERG than TMPA. Both products exhibit a better agreement with gauge data in East Africa and humid West Africa than in the southern Sahel. However, a clear advantage for IMERG is not apparent in detecting the annual cycle. Although all gridded products used here reasonably capture the annual cycle, some differences are evident during the short rains in East Africa. Direct comparison between IMERG and TMPA over the entire continent reveals that the similarity between the two products is also regionally heterogeneous. Except for Zimbabwe and Madagascar, where both satellite-based observations present a good agreement, the two products generally have their largest differences over mountainous regions. IMERG seems to have achieved a reduction in the positive bias evident in TMPA over Lake Victoria.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.