Interdecadal and long-term variability of precipitation in the Brazilian Amazon basin

Abstract

Rainfall and river indices were constructed for both northern and southern Amazonia, and together with river data were used to identify possible climate trends in Amazonia. No systematic unidirectional long-term trends towards drier or wetter conditions were identified since 1920's. This study has focused on long-term rainfall anomalies, where cycles rather than unidirectional trends have been identified, for both northern and southern Amazonia. On interannual scales, the spectral analysis shows peaks between 2-4 years that reflected a time variability that is also typical of ENSO. Even though, the spectrum is dominated by low-frequency variability. Both sections of the basin exhibit low-frequency variability, with a 25-30 years cycle, even though they are not in phase. Periods of time like 1975-77 have been linked to a shift on climate regime, identified as part of the interdecadal variability of the Pacific sector and that has also been detected in Amazonia as a whole, even though northern and southern Amazonian variability do not vary at unison. This decadal and interdecadal variability seems to be linked to the variability of the South Atlantic, where changes in the strength of the subtropical high and the associated surface wind are dynamically linked to the distribution of SST anomalies, suggesting the important of the atmospheric forcing in the decadal time scale. Superimposed to the interdecadal variation of rain are considerably year- to-year fluctuations of precipitation that affect especially Northern Amazonia, but not the southern basin.