Date of Award

1-1-2022

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Atmospheric and Environmental Sciences

Content Description

1 online resource (xix, 168 pages) : color illustrations, color maps.

Dissertation/Thesis Chair

Liming Zhou

Committee Members

Aiguo Dai, Brian E. J. Rose, Robert Fovell

Keywords

Precipitation (Meteorology), Climatic changes, Wildfires, Rain forests

Subject Categories

Atmospheric Sciences

Abstract

Tropical rainforests are global epicenters of biodiversity and modulators of climate. A long-term drying trend has occurred in the Congo Basin since the 1980s and this raises great concerns for sustaining the Congolese rainforest and local rain-fed agriculture. Motivated by documented rainfall changes and the need to improve understanding of main factors influencing precipitation variability and changes in Central Equatorial Africa (CEA), this dissertation conducts synthesized studies on 1) quantifying variations in precipitation seasonality by focusing on dry season duration, 2) estimating cascading wildfire risks associated with changes in precipitation patterns, and estimating impacts of 3) the tropical Indian Ocean and 4) rainforest evapotranspiration (ET) on precipitation.First, combining multiple datasets of precipitation, vegetation greenness, canopy water content and photosynthesis, a trend analysis is conducted to assess variations in dry season length (DSL). The boreal summer dry season (mainly during June-August) has become longer in the Congo Basin over the period 1979-2015. The onset of the dry season has become earlier based on precipitation data, which was associated with the strong drying trend during April-June. A delayed end of the dry season was indicated by vegetation products and was attributed to a prolonged deficit in root-zone soil moisture during the dry season. Second, using the latest satellite-derived burned area products and an advanced labeling method, changes in total burned area, frequency, and size of different fire categories are quantified over Central Africa. The random forest model is applied to identify leading factors regulating wildfires and their changes. Wildfires commonly occur during dry seasons. Total burned area has declined by ~1.3% yr-1 from 2003-2017, particularly in northern Central Africa. This decline was due to significant decreases in both fire frequency and size, particularly for large fires (>100 ha). Declined burned area mainly occurred in savannas and grasslands and was attributed to decreased fuel load, which was associated with decreased rainfall and increased DSL. Nevertheless, burned area has increased in the southern edge of the Congolese rainforest, which might result from reduced rainfall and warmer temperature. Third, the association between the Indian Ocean Dipole (IOD) and CEA rainfall is estimated. Combining partial correlation and composite analyses, IOD’s independent impact and its combined impact with the Madden-Julian-Oscillation (MJO) on precipitation are identified. There is a significant positive correlation between CEA rainfall and the IOD during September-December (SOND) over the period 1981-2019. During a positive IOD events, the warmer western Indian Ocean enables stronger convection and a wetter troposphere to enhance rainfall amount and frequency over CEA. An increase in CEA rainfall has been observed since the 1990s and is concurrent with the increasing correlation between IOD and rainfall. The increasing IOD index might contribute to much of the rainfall trends over the western, southern, and eastern Congo basin. Last, the impact of rainforest ET on precipitation over the Congo Basin is estimated by using satellite isotopic data and running the International Centre for Theoretical Physics Regional Climate Model (RegCM4.9.3). Sensitivity experiments with ET perturbations via prescribed changes in leaf area index (LAI) and stem area index (SAI) are conducted to quantify precipitation sensitivity to ET changes. Estimates by satellite data indicate that ET contributes to atmospheric moisture for precipitation and sustains moisture convergence over the Congo Basin during March-May and June-August. Simulated precipitation increases (decreases) linearly with enhanced (reduced) ET over the Congo Basin. The sensitivity of precipitation to ET changes is mainly attributed to the sensitivity of precipitation intensity to ET. Model simulations suggest that ET affects precipitation by modulating atmospheric moisture distribution in the lower troposphere. The results imply a positive feedback loop for declined ET to exacerbate the aridity.

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