Ground thermal regimes and implications for permafrost distribution on Kilimanjaro, Tanzania

Abstract

Tropical mountain permafrost has a unique thermal regime due to ground surface exposure to strong solar radiation. The intensity of the surface offset resulting from snow cover also strongly affects the absence or presence of permafrost. Latent heat transfer and reflected solar radiation (higher albedo) that occur during the snow-covered season contribute to a positive feedback that cools the ground. Eleven ground temperature monitoring sites were established on the mountain at 2,780 to 5,820 m.a.s.l. The geothermal heat flow is locally high in the caldera of this volcano, as shown by borehole temperature data. Permafrost is located near the only glacier entirely within the caldera (Furtwängler). These threeyear continuous records of ground temperature data encompass years of high and low snow cover. Our results show that the current lower boundary of permafrost is slightly above summit altitude and relict permafrost is present due to the influence of saturated sand on latent heat transfer. Permafrost tends to be lost more rapidly during drought years. The remaining permafrost seems likely to disappear in the future. The presence of permafrost and its thermal resistance depends on the ice content of caldera sand and the duration of snow cover.

A multiannual ground temperature dataset covering sixteen high elevation sites (3493–4377 m a.s.l.) in the Bale Mountains, Ethiopia

“…The offset can be explained by the strong insolation and relatively low air density of the highest tropical mountains. A similar offset between the mean annual air temperature and ground temperature has also been observed on Kilimanjaro (Yoshikawa et al, 2021). Moreover, the comparison on the Sanetti Plateau reveals that temporal ground temperature variations are predominantly controlled by fluctuations in the net radiation as well as changes in the ground water content, which regulates the thermal balance through heat absorption and evaporative cooling (Fig.…”

“…4) speaks against contemporary permafrost anywhere in the southern Ethiopian Highlands. Recent studies indicate that contemporary permafrost in Africa is restricted to the highest mountain Kilimanjaro (Yoshikawa et al, 2021) and outside the tropics to the upper reaches of the High Atlas (Vieira et al, 2017). Similar to the thermal conditions on Mount Kenya (Grab et al, 2004), nocturnal superficial ground frost is common in the Bale Mountains during the dry season (Fig.…”

“…The presented ground temperature dataset from the tropical Bale Mountains in the southern Ethiopian Highlands comprises hourly measurements from multiple depths and sites along an elevation gradient from 3493 up to 4377 m. Besides the measurements that have been obtained in the course of the permafrost monitoring programme on Kilimanjaro (Yoshikawa et al, 2021), the multiannual dataset from the Bale Mountains represents the most comprehensive ground temperature observation from an afro-alpine study site. Many of the installed data loggers were collected in January 2020 after three years of operation, but the hourly ground temperature monitoring will be continued at five sites between 3877 and 4377 m on the Sanetti Plateau and on Mount Wasama (see Fig.…”

An hourly ground temperature dataset for 16 high-elevation sites (3493–4377 m a.s.l.) in the Bale Mountains, Ethiopia (2017–2020)

“…The presented ground temperature dataset from the tropical Bale Mountains in the southern Ethiopian Highlands comprises hourly measurements from multiple depths and sites along an elevation gradient from 3493 up to 4377 m. Besides the measurements that have been obtained in the course of the permafrost monitoring programme on Kilimanjaro (Yoshikawa et al, 2021), the multiannual dataset from the Bale Mountains represents the most comprehensive ground temperature observation from an afro-alpine study site. Many of the installed data loggers were collected in January 2020 after 3 years of operation, but the hourly ground temperature monitoring will be continued at five sites between 3877 and 4377 m on the Sanetti Plateau and on Mount Wasama (see Fig.…”

“…The tropical alpine environment in Africa is mainly confined to the eastern part of the continent and comprises the Ethiopian Highlands in the Horn of Africa as well as the Rwenzori Mountains and numerous isolated volcanic peaks in eastern Africa (Groos et al, 2021a). With regards to the modern afro-alpine climate and ecosystem, Mount Kilimanjaro is by far the best-studied and best-monitored tropical high-elevation site on the continent, with a maximum elevation of 5985 m. Biodiversity (Peters et al, 2019), ground temperature (Yoshikawa et al, 2021), and various meteorological variables (see Appelhans et al, 2016;Pepin et al, 2016;and Mölg et al, 2020) have been monitored temporarily or continuously along elevation gradients of up to 5000 m in the course of the last 2 decades. Meteorological and ground temperature data are also available for limited periods from Mount Kenya (Grab et al, 2004;Nicholson et al, 2013) and outside the tropics from the High Atlas (Vieira et al, 2017).…”