Abstract
AbstractIt was suggested in a recent article that sunspots drive decadal variation in Amazon River flow. This conclusion was based on a novel time series decomposition method used to extract a decadal signal from the Amazon River record. We have extended this analysis back in time, using a new hydrological proxy record of tree ring oxygen isotopes (δ18OTR). Consistent with the findings of Antico and Torres, we find a positive correlation between sunspots and the decadal δ18OTR cycle from 1903 to 2012 (r = 0.60, p < 0.001). However, the relationship does not persist into the preceding century and even becomes weakly negative (r = −0.30, p = 0.11, 1799–1902). This result casts considerable doubt over the mechanism by which sunspots are purported to influence Amazon hydrology.
Highlights
Frequency and mode analysis of time series are often used in climate studies to explore covariation between variables and identify potential causal relationships
It was suggested in a recent article that sunspots drive decadal variation in Amazon River flow. This conclusion was based on a novel time series decomposition method used to extract a decadal signal from the Amazon River record. We have extended this analysis back in time, using a new hydrological proxy record of tree ring oxygen isotopes (δ18OTR)
We show that the relationship between sunspots and Amazon River flow is not constant over time and suggest it might even arise by chance, just as the hands of a broken clock point to the right time twice a day
Summary
Frequency and mode analysis of time series are often used in climate studies to explore covariation between variables and identify potential causal relationships. EEMD is a noise-assisted data analysis approach, which involves decomposing a noise-added time series into its constituent oscillations, and calculating the average of these over a large number of trials Using this technique, the authors found a good correlation between the decadal mode of Amazon River flow and variation in sunspot number. Their findings led them to suggest that solar activity influences the Amazon hydrological cycle at decadal timescales by modulating Atlantic sea surface temperatures (SSTs) They suggest that maxima and minima in the sunspot cycle respectively increase or decrease the difference between SSTs in the tropical North and tropical South Atlantic, weakening or strengthening the trade winds that transport moisture into the Amazon basin. We use a new, annually resolved δ18OTR chronology to test the relationship between sunspots and the Amazon hydrological cycle over the past two centuries
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