Amazon basin rainfall variability

In the study “Spatio-temporal rainfall variability in the Amazon basin countries” by Villar et al., 756 pluviometric stations throughout Brazil, Peru, Bolivia, Colombia, and Ecuador were used to analyze rainfall variability in the period ranging from 1964 to 2003. What distinguishes this study from previous ones is that it includes significant data from areas other than the Amazon forest within Brazil, which tends to have the most data since most of the forest lies within its borders.

The study confirmed the fact that there are not only great differences in rainfall between the Amazon basin and other surrounding ecosystems, but also within the basin itself. As mentioned in previous posts, the differences lie between north and south, and east and west, with the wetter areas (generally) towards the north and west. However, the more interesting findings are in relation to rainfall changes over the second half of the 20^th century. The study is careful to mention that there is much disagreement with regard to whether the forest is getting drier or wetter, and cites papers that attempt to solve this question. But in favor of the paper at hand, the authors use a larger amount of pluviometric stations over a larger area, and also seek to analyze changes over a longer period.

Figure 1. Mean 1975–2003 annual rain (mm/year). The pluviometric stations mentioned in the text are indicated. Andean regions above 500 m are limited by a black and white line.

They found that there is an average decrease in rainfall at a rate of -0.32% from 1975 to 2003, and that “the decrease has been particularly important since 1982”. So, in essence, the forest as a whole is getting drier, or at least receiving less rainfall. But, in the same way there are great differences in how much water the forest receives according to different regions, the changes in rainfall trends are also region-specific. In the 1970’s and 1990’s, in the wet season there was more rainfall in the northwest of the forest, and less in the southeast. In the 1980’s, the opposite was true. Also, up until 1992, rainfall was decreasing more significantly in the dry season, and also decreasing but not as much in the wet season. Since 1992, the wet season has seen an increase in rainfall, and the dry season continues to become drier.

Figure 2. (a) 1975–2003 evolution of the average annual rainfall (mm) in the Amazon basin at the delta and trend line (significant at the 99% level). (b) 1975–2003 evolution of the average quarterly rainfall (mm) in the Amazon basin at the delta and trend lines DJF, JJA, and SON have significant trends at the 95, 90, and 99% levels, respectively. In MAM there is no significant trend.

In context with my other posts about extreme droughts and flooding, this paper shows data that agrees with the notion that there is an increase in seasonal variability, but also brings in the argument that the forest as a whole is becoming drier. Thus, it seems that the dryness of the dry season outweighs the wet season. This is sure to bring some kind of change to the ecosystem in the years to come.
Sources: Espinoza Villar, J. C., Ronchail, J., Guyot, J. L., Cochonneau, G., Naziano, F., Lavado, W., De Oliveira, E., Pombosa, R. and Vauchel, P. (2009), Spatio-temporal rainfall variability in the Amazon basin countries (Brazil, Peru, Bolivia, Colombia, and Ecuador). International Journal of Climatology, 29: 1574–1594. doi: 10.1002/joc.1791