Hanna Link
HCOL 185
12/14/12
Discussion of Climactic Drivers of Drought in Amazonia
and
an Analysis of the Observed Effect
According to the United States Geological Survey, drought is “a period of below average water
content in streams, reservoirs, groundwater aquifers, lakes and soils” (2). I will discuss the climactic
factors which contribute to drought in the Amazon region of South America. Then I will talk in detail
about the Amazon droughts of 2005 and 2010. Finally, I will report on the latest findings about the
inaccuracy of analyses of Amazon forest greening and browning because of corrupted Moderate
Resolution Imaging Spectroradiometer enhanced vegetation index data.
Complex climactic factors contribute to droughts in the Amazon region of South America. One
factor is El Niño or Southern Oscillation events (8). El Niño and Southern Oscillation events, collectively
called ENSO events, are sea surface temperature changes in the tropical eastern Pacific Ocean which
cause air surface pressure changes. The two variations are “coupled”: El Niño events are the warm
oceanic phase in the eastern Pacific along with high air surface pressure in the western Pacific , while
Southern Oscillation events oppositely see a cold oceanic phase with low air surface pressure (4). When
sea surface temperatures and air pressure levels in the eastern Pacific are close to the long term mean, it is
considered to be neutral or normal conditions (Figure 1). ENSO episodes occur on an interannual time
scale (8). “Interannual” indicates an event that occurs between a number of years (5). According to the
National Oceanic and Atmospheric Administration’s Center for Weather and Climate Prediction, ENSO
episodes usually occur every three to five years. El Niño events can last between nine to twelve months
while South Oscillation events usually last one to three years (4).
El Niño and South Oscillation episodes are considered to be climactic factors of drought because
they force the relative precipitation in parts of the Amazon area (8). In El Niño years, eastern Pacific
Ocean sea surface temperatures rise which forces a decrease in precipitation over the north-central and
eastern Amazon regions. It forces a decrease in precipitation because warm eastern Pacific sea surface
temperatures “suppress convection in northern and eastern Amazonia” (9).
Another climactic driver of drought is the Intertropical Convergence Zone (ITCZ), which
influences dry season rainfall in Amazonia. The ITCZ is a band of clouds encircling the globe which is
located near the equator. In this zone, southeast and northeast trade winds meet, causing erratic weather
patterns (6). Its exact location varies with time, depending upon the intensity of the north-south Atlantic
gradient. This gradient is way to describe how far north the ITCZ is relative to the south. When sea
surface temperatures rise in the north Atlantic Ocean more than in the south Atlantic, the gradient shifts
northwards. As the gradient moves northwards, the ITCZ shifts northwards too (8). A northward shift of
the ITCZ significantly weakens the northeast trade winds which carry the moisture from the tropical
Atlantic to the Amazon region, which produces drought in the western and southern regions (9). Thus,
two major climactic factors govern drought in Amazonia: warm Pacific ocean temperatures and their
effect on El Nino events, and warm north Atlantic temperatures and their ultimate effect on local trade
winds.
Scientists have well established these two climactic events as drivers of drought in the Amazon
region. However, the Amazon Rainforest responds in varying ways to drought. The most recent research
has been produced from the major droughts in that region during 2005 and 2010, therefore I will discuss
them in detail.
First, the 2005 drought was linked with increased sea surface temperatures in the north Atlantic,
which pulled the Intertropical Convergence Zone northwards (9). This drought effected mostly the
southwestern Amazon (13). Figure 2 displays anomalies for the four 3-month periods of 2005 of rainfall
(OPI, mm d–1) and sea surface temperature (degree Celsius). It clearly displays the “evolution” of the
2005 drought (14).
In general, models predict that “interactions between changing global climate and terrestrial
vegetation” will produce “substantial carbon loss from tropical ecosystems” because drought amplifies
the forest’s physiological response to less water. (11). In a study published in Science by Saleska,
satellites were used to test the hypothesis that Amazon drought “reduced whole-canopy photosynthesis”.
Before this drought, we had not had the technological capacity to compare actual “forest drought response
to expectation at large scales”. Using the satellite technology of Moderate Resolution Imaging
Spectroradiometer (MODIS) enhanced vegetation index (EVI), they found that the rainforest “greenedup” during the 2005 drought. The canopy was significantly skewed towards greenness (P < 0.001)
(11).The study proposes that during drought, a decrease in water is not extremely problematic for the
Amazon because the trees have deep roots and there is more sunshine. However, is areas heavily
deforested by human activity, the Amazon is more susceptible to forest fires during drought, which leads
to tree mortality. This was clearly observable during the 2005 drought as well (11) (14).
Secondly, the 2010 drought impacted almost all of “South America’s tropical region south of the
equator” (13). The 2010 was linked to high Atlantic sea surface temperatures and weak trade winds and
water vapor transport during “the boreal spring and summer”(7)(3). On top of that, there was an El Nino
episode in the “austral summer”.
Comparatively, the drought of 2010 was more severe than the 2005 drought. The drought of 2010
was 1.65 times larger than the 2005 drought. Using the satellite technology of MODIS EVI, the Amazon
was found to decline in greenness during 2010 “spanning an area four times greater (2.4 million km2) than
in 2005”(13). According to one study, 14% of the drought affected forest showed browning in 2005,
whereas 51% of drought affected forests showed browning in 2010 (13).
As mentioned above, scientists measured the large scale greening of the 2005 and the 2010
drought using NASA Moderate Resolution Imaging Spectroradiometer (MODIS) enhanced vegetation
index (EVI) data (12). A study published in February 2012 in Environmental Research Letters analyzes
the accuracy of the data collected using this technology and exposes that “corruption of optical remote
sensing data with clouds and aersols”, “spatial sampling constraints” and “reduced record lengths”
introduce “large biases in estimates of greenness anomalies”. This is their official statement:
“There were no changes in the greenness of these forests, or if there were changes, the EVI Data failed to
capture these either because the constituent reflectances were saturated or the moderate resolution
precluded viewing small-scale variations”
Therefore, the analysis of scientists using satellite imaging to observe the Amazon’s response to drought
have been inaccurate up until this point (12). Although this makes null much of the hard work of
scientists over the course of this decade, it proves to be an arrow pointing towards the right direction,
bringing science closer and closer to the truth and the nature of complex systems.
To conclude, two major climactic factors force drought in the Amazon region: El Nino and Southern
Oscillation episodes and the Intertropical Convergence Zone. Furthermore, both the major droughts of
2005 and 2010 were linked with high sea surface temperatures in the north Atlantic ocean. While
scientists have attempted to conclude about the relative greenness/brownness of the two droughts using
satellites, their analyses have now been proved void due to data corruption.
FIGURES (1-3)
FIGURE 1 (4)
Top chart (Normal
Conditions): Shows sea
surface temperature
anomalies (in standard
deviation) vs. latitudes.
Most temperatures do not
deviate far from the long
term mean.
Bottom Chart (El Niño
Conditions): Sea surface
temperatures in the eastern
Pacific deviate far from
the long term average,
specifically near the
northeastern South
American’s coast.
FIGURE 2 (14)
“Rainfall (OPI, mm d−1) and SST (degree Celsius) anomalies for the four 3month periods of 2005, showing the evolution of the
Amazon drought. The drought was most severe during the dry season Jul–Sep
when everywhere in the Amazon rainfall was reduced.
Note the small La Nina cold event during Oct.-Dec.”
Work Cited
1. "Climate Prediction Center - ENSO FAQ." Climate Prediction Center - ENSO FAQ. NOAA/ National
Weather Service, 26 Apr. 2012. Web. 09 Dec. 2012.
2. "Definitions of Drought." Water Information. United States Geological Survey, 6 Aug. 2012. Web. 28
Oct. 2012. <http://md.water.usgs.gov/drought/define.html>.
3. Espinoza, J. C., J. Ronchail, J. L. Guyot, C. Junquas, P. Vauchel, W. Lavado, G. Drapeau, and R.
Pombosa (2011), Climate variability and extreme drought in the upper Solimões River (western Amazon
Basin): Understanding the exceptional 2010 drought, Geophys. Res. Lett., 38, L13406,
doi:10.1029/2011GL047862.
4. "Hurricane Season 2012." Weather Spectrum. N.p., n.d. Web. 09 Dec. 2012.
5. "Inter-About Our Definitions: All Forms of a Word (noun, Verb, Etc.) Are Now Displayed on One
Page." Merriam-Webster. Merriam-Webster, n.d. Web. 09 Dec. 2012.
6. "Inter-Tropical Convergence Zone." NWS JetStream -. N.p., n.d. Web. 10 Dec. 2012
7. Lewis, S., Brando P. M., G.M.F. Van Der Heijden, and Daniel Nepstad. "The 2010 Amazon Drought."
Science 331 (2011): 554.
8. Malhi, Y., J. T. Roberts, R. A. Betts, T. J. Killeen, W. Li, and C. A. Nobre. "Climate Change,
Deforestation, and the Fate of the Amazon." Science 319.5860 (2008): 169-72.
9. Marengo, José A., Carlos A. Nobre, Javier Tomasella, Marcos D. Oyama, Gilvan Sampaio De Oliveira,
Rafael De Oliveira, Helio Camargo, Lincoln M. Alves, and I. Foster Brown. "The Drought of Amazonia
in 2005." Journal of Climate 21.3 (2008): 495-516.
10. Phillips, O. L., L. E. O. C. Aragao, S. L. Lewis, J. B. Fisher, J. Lloyd, G. Lopez-Gonzalez, Y. Malhi,
A. Monteagudo, J. Peacock, C. A. Quesada, G. Van Der Heijden, S. Almeida, I. Amaral, L. Arroyo, G.
Aymard, T. R. Baker, O. Banki, L. Blanc, D. Bonal, P. Brando, J. Chave, A. C. A. De Oliveira, N. D.
Cardozo, C. I. Czimczik, T. R. Feldpausch, M. A. Freitas, E. Gloor, N. Higuchi, E. Jimenez, G. Lloyd, P.
Meir, C. Mendoza, A. Morel, D. A. Neill, D. Nepstad, S. Patino, M. C. Penuela, A. Prieto, F. Ramirez, M.
Schwarz, J. Silva, M. Silveira, A. S. Thomas, H. T. Steege, J. Stropp, R. Vasquez, P. Zelazowski, E. A.
Davila, S. Andelman, A. Andrade, K.-J. Chao, T. Erwin, A. Di Fiore, E. H. C., H. Keeling, T. J. Killeen,
W. F. Laurance, A. P. Cruz, N. C. A. Pitman, P. N. Vargas, H. Ramirez-Angulo, A. Rudas, R. Salamao, N.
Silva, J. Terborgh, and A. Torres-Lezama. "Drought Sensitivity of the Amazon Rainforest." Science
323.5919 (2009): 1344-347.
11. Saleska, S. R., K. Didan, A. R. Huete, and H. R. Da Rocha. "Amazon Forests Green-Up During 2005
Drought." Science 318.5850 (2007): 612.
12. Samanta, Arindam, Sangram Ganguly, Eric Vemote, Ramakrishna R. Nemani, and Ranga B. Myneni.
"Interpretation of Variations in MODIS-measured Greenness Levels of Amazon Forests during 2000 to
2009." Environmental Research Letters 7.024018 (2012): n. pag. Print.
13. Xu, Liangu, Arindam Samanta, Marcos H. Costa, Sangram Ganguly, Ramakrishna R. Nemani, and
Ranga B. Myneni. "Widespread Decline in Greenness of Amazonian Vegetation Due to the 2010
Drought." Geophysical Research Letters 38 (2011): n. pag.
14. Zeng, Ning, Jin-Ho Yoon, Jose A. Marengo, Ajit Subramaniam, Carlos A. Nobre, Annarita Mariotti,
and J. David Neelin. "Causes and Impacts of the 2005 Amazon Drought." Environmental Research
Letters 3.1 (2008): 014002.