Intra-annual and interannual non-stationary cycles of chlorophyll concentration in the Northeast Atlantic

Intra-annual and interannual variability of satellite derived chlorophyll-a concentration (Chl) is studied using wavelet analysis during 1997–2010 in tropical, subtropical and temperate zones of the Eastern North Atlantic (20–50° N and 5–40° W). It was found that during the period of observations the Chl dynamics was dominated by the four major periodicities: 0.5-year, 1-year, 2-year and 3–5-year cycles. The 0.5-year wavelet cycle predominantly is a function of the intensity of the secondary bloom (autumn or summer); it attains maximum of 80–100% of the fundamental seasonal (1-year) cycle in the subtropics and 30–60% in the southern tropics. The 2-year and 3–5-year wavelet cycles characterise the variations in the annual mean Chl concentrations and their amplitudes change in-parallel with the change of the amplitude of the 1-year cycle. Expressed as the percentage of the 1-year cycle, the interannual cycles play the most significant role in Chl variations in the southern tropics (40–50%). The 2-year and 3–5 year cycles are also present in the North Atlantic Oscillation Index (NAOI), but only the 3–5-year Chl cycle is closely coupled with the corresponding cycle in the NAOI. Weak correlations with the 2-year cycle are suggested to be a result of the NAOI variations that does not always directly translate into a biological response.