So what’s going on?

    The Amazon Rainforest has a dry and a wet season, so the fact that there are large variations in rainfall throughout the year is a natural part of the region’s climate. However, what has been happening in recent years is that, while there are still the two seasons, they are becoming more intense. In other words, the dry season can turn into a full-on drought, and the wet season can have floods that become destructive to both the rainforest itself and the areas surrounding the forest.
    According to a paper by Chen et al. in 2009, the northern and central parts of the Amazon rainforest experienced the worst floods in over half a century, and left nearly 400,000 people homeless. The paper details the results of a study carried out over 7 years using the GRACE (Gravity Recovery and Climate Experiment) satellite. The GRACE satellite measures levels of gravity change, and in the case of this experiment the data collected allowed the researchers to calculate the terrestrial water storage of the Amazon Rainforest. They found that from 2002-2009 there was an increase in how much water was stored in the forest, culminating with the 2009 floods.

Figure 5. The evolution of yearly TWS anomalies (in cm of equivalent water thickness change) in the Amazon basin and surrounding regions during the 7 year period from August 2002 to June 2009. Yearly averages are mean TWS changes from July of the previous year through June of the current year; for example, the 2004 TWS anomalies are the mean during July 2003 through June of 2004. Seasonal (annual and semiannual) signals and the 161 day S2 alias error have been removed through unweighted least squares fit as well as the mean field for the 7 year period.

    One shocking comparison is that the surplus water stored in 2009 was “roughly equal to U.S. water consumption for a year”. Besides the effect this had on the people who were left without a home, excessive flooding can put the ecosystem at risk simply because it is not meant to withstand this kind of weather.
    This study also points to the link between extreme climate variability in the Amazonian Rainforest and El Niño/Southern Oscillation (ENSO). An exceptionally dry season in 2002-2003 is linked to El Niño, and the 2009 flood with La Niña. In fact, the paper has a great comparison between two graphs, one showing precipitation anomalies and the other showing sea surface temperature anomalies i.e. presence of El Niño/La Niña.

Figure 8. (a) Monthly precipitation anomalies for 1997–2009 in the lower Amazon basin (the area encircled by the magenta contour in Figure 3). Each monthly precipitation anomaly is computed by removing the mean monthly precipitation (for that month), estimated for 1997–2008. (b) The NINO3.4 index over the period 1997–2009. NINO3.4 is the average sea surface temperature (SST) anomaly in the region bounded by 5°N–5°S, 170°W–120°W. This region has large variability on El Niño time scales and is close to the region where changes in local sea surface temperature are important for shifting the large region of rainfall typically located in the far western Pacific. The NINO3.4 index time series is provided by the Royal Netherlands Meteorological Institute (http://www.knmi.nl).

    I think that the most important thing to take from this paper is that, while the Amazon Rainforest is by definition subject to intense weather shifts, the climactic extremes are becoming too much, even by Rainforest standards.

Sources: Myneni R.B, et al. 2007 Large seasonal swings in leaf area of Amazon rainforests. Proc. Natl Acad. Sci. USA. 104, 4820–4823. doi:10.1073/pnas.0611338104.

Chen, J. L., C. R. Wilson, and B. D. Tapley (2010), The 2009 exceptional Amazon flood and interannual terrestrial water storage change observed by GRACE, Water Resour. Res., 46, W12526, doi:10.1029/2010WR009383.