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Malay Archipelago Forest Loss to Cash Crops and Urban Expansion Contributes to Weaken the Asian Summer Monsoon: An Atmospheric Modelling Study

Forest is lost to large scale cash crops plantations (oil palm, rubber and acacia, including fallow lands) and urban expansion in the Malay Archipelago (Indonesia and Malaysia). Land surface properties and fluxes are changed by deforestation, which thereby modify wind and rainfall. The impact that results on the Asian summer monsoon has not been studied, in spite of land-cover change over a climatologically sensitive region of the tropics. This study investigated the atmospheric response that is caused by the island surface change resulting from deforestation into cash crop plantation and urban expansion. It is shown by this study that deforestation warms the Malay Archipelago, which is caused by an increase in soil warming that is due to a decrease in evapotrasnspirative cooling. Island warming agrees well with in situ and satellite observations; it causes moisture to converge from the surrounding seas into Sumatra and Malaya, and updrafts, rainfall, and cyclonic circulations to spread northeastwards into southern India and the Arabian Sea as well as a drying anticyclonic circulation over the Indo-Gangetic plains, Indochina, and the South China Sea, which weakens the Asian summer monsoon. The modelled monsoon weakening agrees well with, and tends to enhance, the long-term trend that is observed which suggests the potential for continued weakening with protracted cash-crop plantation and urban expansion.

Biodiversity is threatened and irreversible anthropogenic changes to the biosphere are caused by logging and conversion of tropical rainforest (e.g., Indonesia) into cash crops such as oil palm, rubber and acacia, as well as urban expansion (Bradshaw & Muller, 1998; Wilcove & Koh, 2010; Seto et al., 2012; National Geographic, 2017). Land surface characteristics are altered by cash-crop plantation and urban expansion (CPU), as well as affecting the regional climate by regulating energy and moisture fluxes at the land-atmosphere interface (Bradshaw & Muller, 1998; Rosenfeld, 2000; Shepherd et al., 2002; Arnfield, 2003; Dallmeyer & Claussen, 2011; Kusaka et al., 2012; Mawalagedara & Oglesby, 2012; Li et al., 2013; Devaraju et al., 2015). The highest rate of forest loss, globally, occurs in the Malay Archipelago of the Maritime Continent (Economist, 2011; Hansen et al., 2013; Crowther et al., 2015); for accuracy Wallace’s term “Malay Archipelago” is used in the paper (Wallace, 1863) to include only Indonesia and Malaysia of the present. The loss of forest from 2000 to 2014 is approximately 263,900 km2, an area that is larger than the UK, mostly over Sumatra, Malaya         (i.e., the Malay Peninsula), and Borneo. At the same time areas of CPU continue to expand (Grimm et al., 2008; Ellis, 2009; Seto et al., 2011; Mertes et al., 2015; Drescher et al., 2016; Ratnasari et al., 2016; United Nations, 2018). CPU area has grown to about 50,000 to 100,000 km2 since the 1990s in the western Malay Archipelago (west of 120oE). As the vegetation of the island (such as the leaf area, canopy heights and root depths) and surface properties (such as Albedo, emissivity and surface roughness) changes, the fluxes such as albedo, emissivity and evapotranspiration also change (an introductory summary is provided in the supplemental material). An idealised regional climate model experiment of “Southeast Asia”        defined as (15oS-17oN and 92o-140oE, which included Indochina and the Philippians as well as parts of northern Australia was conducted (Tölle et al., 2017), by changing all grids in that region that had 80% or more vegetation into grassland, and found that the surface would warm and local precipitation would generally increase. The issues of deforestation that resulted from cash crop plantations and urbanisation in the Malay Archipelago, in particular its western portion, and how there may be changes in large scale wind and precipitation were not addressed. There are 2 climate models (with horizontal resolution of 1.9o x 2.5o and 1.9o x 3.8o, respectively) that were used (Devaraju et al., 2018) to assess the effects of deforestation into grassland, finding that the increase that resulted in aerodynamic resistance reduced turbulent transfer of heat from the surface. The surface would warm over the tropics, though the degree of warming would differ by 10 times between the 2 models, from ~11 to 1.11 K. Cash-crops surface properties differ markedly from those of grasslands and further analysis that targets specifically the land type changes of the Malay Archipelago are necessary to identify the effects. That the atmospheric circulation, including the transport of water vapour and precipitation can be particularly sensitive to changes in the surface flux near the equator; have been shown by observations, theory and numerical experiment (Ramage, 1968; Gill, 1980; Neale & Slingo, 2003; Schiemann et al., 2014). Yet there have been no studies of the potential impact on the atmospheric circulation and rainfall by CPU expansion in the Malay Archipelago.

The study was conducted using a suite of moderately large (36 member) ensembles of atmospheric general circulation model experiments with different land type changes in order to identify and understand changes in wind and rainfall that were due to expansion of CPU. This study demonstrates that the simulated wind and rainfall are sensitive to GPU changes near the equator, especially in the western Malay Archipelago; northern Sumatra, Malaya and Borneo. It is shown by the model that expansion of CPU results in significant island warming, which agrees with observations, which causes moisture to converge from the surrounding seas; the impact on wind and rainfall reaches as far west and north as the Indian subcontinent and southern China, contributing to weakening of the South Asian summer monsoon has a long-term weakening trend (Naidu et al., 2009; Turner and Annamalai, 2012). The weakening has been attributed to various factors such as:

·       increased cropland in India (Devaraju et al., 2015; Krishnan et al., 2016; Paul et al., 2016),

·       increased aerosols (Krishnan et al., 2016; Bollasina et al., 2011; Ganguly et al., 2012; Sanap et al.,2015),

·       Indian Ocean warming (Roxy et al., 2015; Saha et al., 2014; Liang et al., 2017) and/or

·       Tropospheric cooling over China (Menon et al., 2002; Yu et al., 2004; Wu et al., 2005).

According to Huang & Oey a single common thread to these different attributions is that the various factors combine to contribute to weakening the summertime land-sea temperature contrast between continental Asia in the Indian Ocean, either because the land has cooled or the sea has warmed, or both the cooled land and warmed sea occur simultaneously. CPU expansion in the Malay Archipelago is indicated by the model of Huang & Oey to also contribute to the weakening of the monsoon, though it does so from the equator side of the north Indian Ocean via the faster-than-ocean warming of the Malay Archipelago surfaces caused by reduced evapotranspiration of reduced forested trees. While the island areas of the Malay Archipelago are much smaller than the Indo-Asian continent, the strategic location of the islands on the equator and plentiful availability of moisture from the surrounding seas mean the warming can have a significant impact on the circulation of the monsoon. The result of the warming is increased rainfall under the northwestwards-tilted cyclone that emanates from the Malay Archipelago, and decreased rainfall under the accompanying cyclone to the north. It increases the sea-island contrast between the Indian Ocean and the Malay Archipelago, which forces a northeasterly wind anomaly that weakens the summer monsoon. This study emphasises the need for high resolution observations and modelling that resolves the warming of the Malay Archipelago. According to the findings of this study, e.g., the reanalysis data of the NCEP, do not indicate the warming trend of the islands that has been shown in this paper for high resolution APHRODITE and CRU datasets. Of interest is that the increased precipitation and updraft over the western Malay Archipelago may potentially contribute to the well-recognised dry bias over the same region in climate models (Neale and Slingo, 2003; Schiemann et al., 2014), which often also have a similarly coarse resolution as NCEP. Potentially, the weakened monsoon wind may also decrease the frequency and/or weaken the strength, of the north/northeastwards propagating Madden-Julian oscillation events that originate over the central-eastern Indian Ocean directly west of Sumatra during summer (the so-called monsoon intraseasonal oscillation [MISO]; Goswami, 2012). Then, the reduced MISO events may lead to drier conditions over northern Bay of Bengal, Indochina, and the South China Sea. Further research is needed.

While the experiments using the atmosphere-land   surface model that were conducted in this study could isolate the atmospheric response to expansion of CPU in the Malay Archipelago, the lack of an air-sea coupling in the model may modify the result. In a fully coupled model of the atmosphere, land, and ocean, a weakened summer monsoon would reduce evaporative cooling over the Indian Ocean and lead to a warmer SST (du et al., 2009). The temperature between the Malay Archipelago and the Indian Ocean, in the near surface, and therefore the convergence of wind onto the islands may be reduced sufficiently to potentially moderate the weakened effects of the monsoon on CPU expansion.

On the other hand, as marine layer moisture increases as a result of the warming of the sea, the moisture flux convergence and associated CAPE that fuel the updraft over the western Malay Archipelago may actually be enhanced which would then boost the CPU-induced response. Again, further research is needed.

It is expected that cash-crop will continue into the foreseeable future, particularly in Indonesia. It is dictated by population growth and economic growth that urban areas will also continue to expand (Seto et al., 2012; Seto et al., 2011; Gamba & Herold, 2009). It is projected by the results of this study that a protracted weakening of the Asian summer monsoon. It is suggested by the research in this study that there is urgency in implementing policies that can soften or mitigate the resulting irreversible and possibly unanticipated consequence of the altered climate.  


Huang, S. and L. Oey (2019). "Malay Archipelago Forest Loss to Cash Crops and Urban Expansion Contributes to Weaken the Asian Summer Monsoon: An Atmospheric Modeling Study." Journal of Climate 32(11): 3189-3205.



Author: M. H. Monroe
Last updated: 13/12/2020
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