Abstract. This study investigates three types of atmospheric forcing across the summertime subtropics that are shown to contribute in various ways to the occurrence of dry and wet climates in the subtropics. To explain the formation of desert over the western parts of continents and monsoon over the eastern parts, we propose a new mechanism of positive feedback between diabatic heating and vorticity generation that occurs via meridional advection of planetary vorticity and temperature. Monsoon and desert are demonstrated to coexist as twin features of multi-scale forcing, as follows.First, continent-scale heating over land and cooling over ocean induce the ascent of air over the eastern parts of continents and western parts of oceans, and descent over eastern parts of oceans and western parts of continents. Second, local-scale sea-breeze forcing along coastal regions enhances air descent over eastern parts of oceans and ascent over eastern parts of continents. This leads to the formation of the well-defined summertime subtropical LOSECOD quadruplet-heating pattern across each continent and adjacent oceans, with long-wave radiative cooling (LO) over eastern parts of oceans, sensible heating (SE) over western parts of continents, condensation heating (CO) over eastern parts of continents, and double dominant heating (D: LO+CO) over western parts of oceans. Such a quadruplet heating pattern corresponds to a dry climate over the western parts of continents and a wet climate over eastern parts. Third, regional-scale orographic-uplift-heating generates poleward ascending flow to the east of orography and equatorward descending flow to the west.The Tibetan Plateau (TP) is located over the eastern Eurasian continent. The TP-forced circulation pattern is in phase with that produced by continental-scale forcing, and the strongest monsoon and largest deserts are formed over Correspondence to: Y. Liu (lym@lasg.iap.ac.cn) the Afro-Eurasian Continent. In contrast, the Rockies and the Andes are located over the western parts of their respective continents, and orography-induced ascent is separated from ascent due to continental-scale forcing. Accordingly, the deserts and monsoon climate over these continents are not as strongly developed as those over the Eurasian Continent.A new mechanism of positive feedback between diabatic heating and vorticity generation, which occurs via meridional transfer of heat and planetary vorticity, is proposed as a means of explaining the formation of subtropical desert and monsoon. Strong low-level longwave radiative cooling over eastern parts of oceans and strong surface sensible heating on western parts of continents generate negative vorticity that is balanced by positive planetary vorticity advection from high latitudes. The equatorward flow generated over eastern parts of oceans produces cold sea-surface temperature and stable stratification, leading in turn to the formation of low stratus clouds and the maintenance of strong in situ longwave radiative cooling. The equatorward flow over wes...
The periodic wildfire is a natural process of Siberian forests. Wildfire is also an important natural factor for supporting biodiversity within the Siberian boreal ecosystems (Kharuk et al., 2021). However, wildfires can cause direct economic and life loss, and release air pollutants which can deeply threaten human health. Besides, wildfires create forest canopy gaps and major disturbances to wildlife habitats, biodiversity, and ecosystem services accompanied with soil erosion and changes in ecohydrology, altering long-term physical, chemical, and biological processes in the Arctic ecosystems and causing far-reaching environmental consequences (Mack et al., 2011). Furthermore, the boreal forests and tundra ecosystems store nearly 50% of global terrestrial carbon, accounting for about one third of global soil carbon (Higuera et al., 2008). The large quantities of carbon stored in Siberian boreal ecosystems and frozen soils can be released into the atmosphere through deepening of active layer and degradation of permafrost resulting from recent Arctic warming and wildfires (Kim et al., 2020). The released carbon may alter the global carbon budget and accelerate global and Arctic warming, and thereby forming positive feedback between the accumulation of atmospheric carbon and global warming (Mack et al., 2011;Turetsky et al., 2015).More than 70% of fires, and up to 90% of the total area burned in Russia occur in Siberia (Kharuk & Ponomarev, 2017). As the northern high-latitude ecosystems are experiencing climate warming at a rate twice of the global mean warming (Serreze & Barry, 2011), the Siberian wildfire has become more active in recent years (Kelly et al., 2013). Since the end of the twentieth century, the frequency and extent of forest fires has been
Wildfire is recognized as an increasing threat to southern boreal forest and permafrost beneath it while less occurring over the cold continuous permafrost before. However, we show continuous permafrost was a major contribution to the wildfire expansion in pan-Arctic over the last two decades. The expansion rate of burned area over continuous permafrost was 0.9Mha decade-1, in contrast to a decreasing trend (-0.5Mha decade-1) over the entire permafrost areas. Burned area has been rapidly growing in the north of Arctic Circle in particular, where the total burned area in the major fire seasons during 2011-2020 nearly doubled that during 2001-2010. The wildfire expansion is closely linked to increased soil moisture deficit, considering wildfires there combust more than 90% of belowground fuel. Continuous permafrost experiences more severe fire-induced degradation. Active layer thickening following wildfire over continuous permafrost lasts more than three decades to reach a maximum of more than triple the pre-fire thickness. These new findings highlight the massive expansion of wildfires over continuous permafrost, which can dramatically modify ecological processes, disturb organic carbon stock, and thus accelerate the positive feedback between permafrost degradation and climate warming.
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