Abstract
Precipitation extremes have a strong influence on the exchange of energy and water between the land surface and the atmosphere. Although the Horn of Africa has faced recurrent drought and flood events in recent decades, it is still unclear how these events impact energy exchange and surface temperature across different ecosystems. Here, we analyzed the impact of precipitation extremes on spectral albedo (total shortwave, visible, and near-infrared (NIR) broadband albedos), energy balance, and surface temperature in four natural vegetation types: forest, savanna, grassland, and shrubland. We used remotely sensed observations of surface biophysical properties and climate from 2001 to 2016. Our results showed that, in forests and savannas, precipitation extremes led to divergent spectral changes in visible and NIR albedos, which cancelled each other limiting shortwave albedo changes. An exception to this pattern was observed in shrublands and grasslands, where both visible and NIR albedo increased during drought events. Given that shrublands and grasslands occupy a large fraction of the Horn of Africa (52%), our results unveil the importance of these ecosystems in driving the magnitude of shortwave radiative forcing in the region. The average regional shortwave radiative forcing during drought events (−0.64 W m−2, SD 0.11) was around twice that of the extreme wet events (0.33 W m−2, SD 0.09). Such shortwave forcing, however, was too small to influence the surface–atmosphere coupling. In contrast, the surface feedback through turbulent flux changes was strong across vegetation types and had a significant (P < 0.05) impact on the surface temperature and net radiation anomalies, except in forests. The strongest energy exchange and surface temperature anomalies were observed over grassland and the smallest over forest, which was shown to be resilient to precipitation extremes. These results suggest that land management activities that support forest preservation, afforestation, and reforestation can help to mitigate the impact of drought through their role in modulating energy fluxes and surface temperature anomalies in the region.
Highlights
Precipitation extremes across the Horn of Africa have become more frequent and intense in recent decades (Masih et al, 2014; Lyon and DeWitt, 2012)
VIS albedo consistently increased during droughts (SPI ≤ 1) and decreased in extreme wet events (SPI > 1) across all vegetation types
The VIS albedo changes during both extreme wet and drought events were stronger in magnitude than the NIR changes, resulting in a net increase in SW albedo during drought in all vegetation types and a decrease in SW albedo during extreme wet events with the exception of forests
Summary
Precipitation extremes across the Horn of Africa have become more frequent and intense in recent decades (Masih et al, 2014; Lyon and DeWitt, 2012). Precipitation volumes during the main rainfall season (March through June) have declined continuously in recent decades (Nicholson, 2017; Lyon and DeWitt, 2012). Extreme events affect soil moisture and vegetation, causing changes in the surface albedo and, the amount of available energy at the land surface. Drought-induced albedo changes have been studied since the 1970s (Charney et al, 1975). Earlier studies have hypothesized that an albedo increase triggers atmospheric subsidence, weakens convective activity, and leads to less precipitation, further amplifying droughts (Charney et al, 1975)
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