2005 Amazon drought due to warming

Ocean warming - not el Nino - drove severe Amazon drought in 2005
Original article from Mongabay.com

Download Journal article Hydro-climatic and ecological behaviour of

the drought of Amazonia in 2005February 21, 2008

One of the worst droughts on record in the Amazon was caused by high
temperatures in the Atlantic rather than el Niño, according to analyses
of climatic and hydrological records. The research, published in
Philosophical Transactions of the Royal Society B, suggests that
human-driven warming is already affecting the climate of Earth's largest
rainforest.

In 2005 the Amazon experienced the worst in memory. As rivers dried up,
remote communities were isolated while commerce slowed to a standstill.
Thousands of square kilometers of land burned for months on end,
releasing more than 100 million metric tons of carbon into the
atmosphere.

At the time of the drought scientists observed an apparent correlation
between sea surface temperatures in the Atlantic Ocean and precipitation
in the Amazon. Now research by Dr. Jose Marengo and colleagues from
Brazil's Instituto Nacional de Pesquisas Espaciais (INPE) and the
Instituto de Aeronáutica e Espaço (IEA), confirms that "the 2005 drought
was linked not to El Niño as with most previous droughts in the Amazon,
but to warming sea surface temperatures in the tropical North Atlantic
Ocean."

According to the WHRC, the above map is "a product of our ongoing
drought monitoring effort from Oct. 2005, the worst month we have in our
record going back to 1995. It shows moisture stored in the soil which is
available for use by plants, what we call 'Plant-Available Water' or
'PAW', expressed as a percentage of the total water-holding capacity of
the top 10m of soil at any given point; %PAW is one of the strongest
indicators we have of severity of drought and of forest susceptibility
to fire.. Courtesy of WHRC.
"Very simply, during 2005 there was no El Niño episode occurring in the
Tropical Pacific Ocean," INPE's Dr. Carlos Nobre told mongabay.com.
"Secondly, when El Niños occur, they rarely affect the southwestern
Amazon, where the drought hit more intensely."

"The Tropical North Atlantic Ocean was much warmer during all of 2005
and the meteorological analysis described in the paper showed that
anomalous warm sea surface temperature over the Tropical North Atlantic
created a relatively low pressure system over that oceanic region. That
low pressure changed the wind system over the Tropics. The overall
result was descending motion over SW Amazonia, which reduced cloudiness
and rainfall."

Lead author Marengo told mongabay.com that while previous droughts have
been linked to El Niño events, 2005 was an anomaly.

Scene from the 2005 drought in the Amazon. Picture courtesy of
Greenpeace.
"The idea of drought episodes during El Niño years is not always
correct," he explained. "During El Niño the drought is more on the
central and eastern side of Amazonia (between 5N-10S). That was the case
of the droughts of 1926, 1983, 1998 while the drought of 2005 was not
during El Niño, and was on the SW side of the basin, similar to another
non El Niño-related drought in 1964."

"The warming in the tropical North Atlantic was responsible for this
drought, since the tropical Pacific was almost neutral (absence of El
Niño)."

The conclusion is significant as climate models forecast continue
warming in the North Atlantic Ocean due to rising emissions of
greenhouse gases. Warm conditions in the North Atlantic Ocean have also
been correlated with the formation of hurricanes in the Gulf of Mexico
and along the Eastern seaboard of the United States. Hurricane Katrina
struck New Orleans in August 2005 [note].

Several climate models for the Amazon project significant warming and
drying by 2100 due to rising concentrations of carbon dioxide in the
atmosphere. These changes, combined with forecast deforestation and
continuing use of fire, could result in the conversion of much of the
Amazon rainforest to savanna and scrub.

Marengo, J.A. et al (2008). Hydro-climatic and ecological behaviour of
the drought of Amazonia in 2005 [FREE OPEN ACCESS]. Phil. Trans. R. Soc.
B, DOI: 10.1098/rstb.2007.0026