The main modes are a 160–170 year trend and a 3-year periodicity; a secondary, 2.5-year peak is also highly significant (see Table 2 ).
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Intrinsic and climatic factors in North-American animal population dynamics.
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The sentences
They are 170 years and 2.5 years for the mean NH temperatures, and 4 years and 2 years for the Niño-3 SSTs, while the NAO has only spectral peaks that are both weaker and marginally significant.