Seasonal types in homogeneous rainfall regions of the Amazon basin

Due to its size and geographical features, different average annual rainfall regimes co-exist in the Amazon basin, with distinct year-to-year variability dependent on regions within the basin. In this study, we define and explain the seasonal regional types of annual regimes, that is, years with similar seasonal anomalies. Our work is based on a 205 rain gauge network distributed over five Amazonian countries, spanning a period over 30 years. Using a spectral clustering method, we identified seven sub-regions within the basin in which annual rainfall regimes are spatially homogenous. For each sub-domain, we estimated specific parameters that characterize the rainy season (onset and demise dates, sign and duration of rainfall anomalies). Finally, using spectral analysis we identified between two and four ‘seasonal type’ of precipitation in these seven sub-domains. Most of these seasonal types are in phase with the large-scale atmospheric circulation, which explains the temporal link with rainfall anomalies. The seasonal types result of the superposition of inter-annual and intra-seasonal variability whose factors are then difficult to identify and attribute. Part of the rainfall anomalies characterizing seasonal types is related to the inter-annual variability of the sea surface temperature in the Atlantic or the Pacific oceans, especially in the northeast and southeast part of the Amazon basin, whereas in other parts, strong intra-seasonal and local factors have a larger impact. The same sign and duration of anomalies do not concomitantly affect the various regions of the Amazon basin, confirming that one mode of variability does not homogeneously affect precipitation in different parts of the basin.