PHILIP M. FEARNSIDE
The future of water vapor transport to São Paulo depends not only on the source of water vapor in the Amazon, but also on the strength of the SALLJ (South American Low Altitude Jet Wind) mechanism. Interdecadal cycles affect these water sources [1]. Changes in SALLJ may offset the effect of decreased water vapor supply by deforestation.
The future strength of SALLJ depends on whether the frequency of the episodes is causally related to El Niño. This connection is weak, the sea surface temperature in the Pacific explains only 20% of the frequency of SALLJ episodes during the period 1950 to 1998 [2].
However, two observations are suggestive of a connection: the higher frequency of SALLJ episodes during the El Niño event in the austral summer of 1997 – 1998 compared to the La Niña event a year later [3] and a general trend toward greater precipitation in southern Amazonia and south-central Brazil since the mid-1970s that is consistent with both increases in El Niño events and SALLJ episodes. [2].
If a “permanent El Niño” were to develop after 2050, caused by global warming according to the Hadley Center model (eg, [4, 5]), the result would be an increase in precipitation in south-central Brazil.
A more recent version of the same model indicates that the Amazon rainforest is more resistant to the effects of drought than previously thought, but confirms the trend of a substantial decrease in precipitation in the Amazon with an increase in the frequency of El Niño. [6-8].
The drought in the Cantareira System in November 2014 (Photo: Mídia NINJA/Conta da Água)
The fall in precipitation in São Paulo in 2014-2015 is very out of proportion with the increase in deforested area from 2013 to 2014. Therefore, although the gradual increase in deforested area in the Amazon may have some contribution, it cannot explain a drop in deforestation. such a drastic fallout for this effect alone.
Some sort of break in the low-altitude jet winds could explain it, but we don’t have the data to say this happened. In fact, the year 2014 had an El Niño beginning, and this phenomenon leads to more, not less, transport of water vapor from the Amazon to the southeast by the SALLJ [9].
What seems most likely as an explanation is a combination of factors that reduced the condensation of water vapor present in the air over São Paulo. A mass of hot air stationed over the State of São Paulo inhibited the entry of cold fronts that normally cause the condensation of water vapor to generate precipitation.
There was also a shift of the South Atlantic Convergence Zone (SACZ) to the north, moving to the border between Minas Gerais and Bahia in place of its normal position over São Paulo, which took from São Paulo an important mechanism to cause condensation. of rain precisely in 2014-2015.
The SACZ is a line that crosses Brazil diagonally, normally extending from São Paulo to Acre, where the cold air coming from Antarctica causes the formation of clouds and rain when it encounters warm air loaded with water vapour. That’s why Acre has its famous “friagens”.
Despite the uncertainties about the causes of drought, it is important to learn the lessons that this experience teaches us. First, if the current course of “development” in the Amazon continues, with grand plans to build roads, dams and other works that lead to deforestation, and with subsidies to forest destruction through a wide range of perverse policies, then water will be lacking. , yes, in São Paulo. This shortage would be in a more permanent form, not just a year-to-year variation.
The second lesson is that the so-called “natural” climate variability is increasing due to global warming. This leads to more severe and more frequent extreme events of droughts and floods compared to historical patterns. The increase in drought and flood extremes in the Amazon is already clear [10-12].
GRADES
[1] Marengo, JA 2004. Interdecadal and long term rainfall variability in the Amazon basin. Theoretical and Applied Climatology 78: 79-96.
[2] Marengo, JA; Liebmann, B.; Vera, CS; Nogués-Paegel, J.; Báez, J. 2004b. Lowfrequency variability of the SALLJ. Cleave Exchange 9(1): 26-27.
[3] Marengo, JA; Soares, WR; Saulo, C.; Nicolini, M. 2004a. Climatology of the lowlevel jet East of the Andes derived from NCEP-NCAR reanalyses: Characteristics and temporal variability. Journal of Climate 17(12): 2261-2280.
[4] Cox, PM; Betts, RA; Jones, CD; Spall, SA; Totterdell, IJ 2000. Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature 408: 184-187.
[5] Cox, PM; Betts, RA; Collins, M.; Harris, P.; Huntingford, C.; Jones, CD 2004. Amazonian dieback under climate-carbon cycle projections for the 21st century. Theoretical and Applied Climatology 78: 137-156.
[6] Cox, PM; Pearson, D.; Booth, BB; Friedlingstein, P.; Huntingford, C.; Jones, CD; Luke, CM 2013. Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability. Nature 494: 341-344.
[7] Good, P.; Jones, C.; Lowe, J.; Betts, R.; Gedney, N. 2013. Comparing tropical forest projections from two generations of Hadley Center Earth System models, HadGEM2-ES and HadCM3LC. Journal of Climate 26(2): 495-511.
[8] Huntingford, C. & 25 others. 2013. Simulated resilience of tropical rainforests to CO2-induced climate change. Nature Geoscience 6: 268-273. doi:10.1038/ngeo1741
[9] Drumond, A.; Marengo, J.; Ambrizzi, T.; Nieto, R.; Moreira, L.; Gimeno, L. 2014. The role of the Amazon Basin moisture in the atmospheric branch of the hydrological cycle: a Lagrangian analysis. Hydrology and Earth System Sciences 18: 2577-2598. doi:10.5194/hess-18-2577-2014
[10] Marengo, JA; Tomasella, J.; Soares, WR; Alves, LM; Nobre, CA 2011. Extreme climatic events in the Amazon basin. Theoretical and Applied Climatology 107: 73-85.3
[11] Marengo, JA; Borma, LS; Rodrigues, DA 2013. Recent extremes of drought and flooding in Amazonia: Vulnerabilities and human adaptation. American Journal of Climate Change 2: 87-96.
[12] Updated and expanded from Fearnside, PM 2015. Drought and deforestation. Ciência Hoje 55(322): 52. The author’s research is funded by the National Council for Scientific and Technological Development (CNPq) (proc. 304020/2010-9; 573810/2008-7), by the Fundação de Amparo à Pesquisa do Estado do Amazonas (FAPEAM) (proc. 708565) and by the National Institute for Research in the Amazon (INPA) (PRJ1).
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