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Sources of Sahelian Sudan moisture: insights from a moisture-tracing atmospheric model
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Physical Geography.
Stockholm University, Faculty of Science, Department of Meteorology .
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The summer rainfall across Sahelian Sudan is one of the main sources of water for agriculture, human and animal needs. However, the rainfall is characterized by large interannual variability, which has attracted extensive scientific effort to understand it. This study attempts to identify the source regions that contribute to Sahelian Sudan moisture budget during the monsoon months – July through September. We have used an atmospheric general circulation model with embedded water-tracing module (CAM3), forced by observed (1979-2013) sea-surface temperatures (SST). The result suggests that about 38.6% of the moisture comes with the atmospheric flow associated with the seasonal migration of the Intertropical convergence zone (ITCZ), which originates from Guinea Coast, Central Africa and the Western Sahel. The Mediterranean, Arabian Peninsula, and South Indian Ocean regions accounts for 10.2%, 8.1%, and 6.4%, respectively. Local evaporation and the rest of the globe supply the region with 20.3% and 13.2%, respectively. We also compared the result from this study to previous analysis that used the Lagrangian model FLEXPART. The two approaches differ when comparing individual regions, but are in better agreement when neighboring regions of similar atmospheric flow features are grouped together. Interannual variability of the rainfall over the region is highly correlated with contribution from regions that are associated with the ITCZ movement, which is linked to the Atlantic Multidecadal Oscillation (AMO). A dry regime during the time period 1979-1994 seems to be associated with the cold phase of AMO, and relatively wet period 1995-2013 is associated with the warm mode phase of AMO. 

Keyword [en]
Sahel, Sudan, rainfall, moisture sources, moisture tagging, moisture transport
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-122712OAI: oai:DiVA.org:su-122712DiVA: diva2:868279
Available from: 2015-11-10 Created: 2015-11-10 Last updated: 2015-11-13Bibliographically approved
In thesis
1. On Sahelian-Sudan rainfall and its moisture sources
Open this publication in new window or tab >>On Sahelian-Sudan rainfall and its moisture sources
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The African Sahel is one of the most vulnerable regions to climate variability at different time scales. It is an arid to semi-arid region with limited water resources. The summer rainfall is one of these sources, but it exhibits pronounced interannual variability. This thesis presents several aspects of Sahelian Sudan rainfall. Sudan is located at the eastern fringe of the Sahel and its least studied part. We have examined the impact of tropical deforestation on the rainfall, the moisture sources of the region and the temporal characteristics of the observed and modeled rainfall. In a sensitivity study we performed three simulations, one control simulation and then setting the surface condition of South Sudan to either grass or desert conditions. The rainfall was reduced by 0.1 − 0.9 in the grass scenario and by 0.1 − 2.1 mm day−1 (hereafter mm d−1) in the desert scenario. These changes also propagated northward into Sahelian Sudan, indicating a remote impact. The total moisture convergence into Sahelian Sudan was reduced by 11.5% and 21.9% for grass and desert conditions, respectively. The change in moisture convergence into the region motivated a comprehensive analysis of the moisture sources for the region. Two different modeling approaches, Lagrangian and Eulerian, were applied to identify the moisture sources and quantify their contributions to the total annual rainfall budget. The analysis shows that atmospheric flows associated with the Inter-Tropical Convergence Zone (ITCZ), e.g. from Guinea Coast, Central African and Western Sahel, brings about 40% − 50% of the annual moisture supply, while local evaporation adds about 20%. The rest of the moisture comes from the Mediterranean, Arabian Peninsula and the Southern part of the Indian Ocean. While there were differences in the details between the results from the two modeling approaches, they agree on the larger scale results. In an attempt to characterize the temporal character of the rainfall, observed and modeled daily rainfall from different regional climate models was classified into five categories: weak (0.1 −1.0), moderate (>1.0 − 10.0), moderately strong (>10.0 − 20.0), strong (>20.0 − 30.0), and very strong (>30.0) mm d−1. We found that most rain-days were in the weak to moderate rainfall categories, accounting for 60% − 75%. Days that have strong rainfall represent about 6% of the total rain-days, yet they represent about 28% − 48% of the total amount of the annual rainfall. Regional climate models fail to produce the strong rainfall, instead most of the modeled rain-days are in the moderate category and consequently the models overestimated the number of rain-days per year.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2015. 36 p.
Keyword
Sudan, Sahel, rainfall, land use, deforestation, moisture sources, moisture transport, Lagrangian, moisture tagging, regional model, climate modeling
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-122731 (URN)978-91-7649-281-9 (ISBN)
Public defence
2015-12-11, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2015-11-19 Created: 2015-11-10 Last updated: 2015-11-25Bibliographically approved

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