Century-scale variability of Chlorophyll a concentration in the North Atlantic Subtropical Gyre

Abstract Globally, continuous measurements of Chlorophyll- a (Chl a ) have been available for over 40 years. However, these measurements do not provide a clear insight into its variability during periods of significant changes in ocean circulation or water properties in the last century. To address this, high-resolution Chl a hindcasts enhance our understanding of Chl a variability over multi-decadal timescales. In this study, we reconstructed time-series of Chl a and nutrients in the upper 300 m at key observatories within the North Atlantic Subtropical Gyre (NASTG), specifically Bermuda Atlantic Time-series Study (BATS) on the western side and Kiel276 on the eastern side, between 1871 and 2010. This method was further applied to the NASTG region. The method proved effective at BATS, accurately capturing both the onset and end of spring blooms, and the depth of Chl a maximum. We showed that, on interannual timescales, integrated Chl a correlated with the phases of the North Atlantic Oscillation (Kiel276) and the Atlantic Multidecadal Oscillation (BATS). However, no statistically significant trends were estimated at BATS or Kiel276 for Chl a . Furthermore, inter-decadal variability in Chl a was observed at all sites, with changes in downward solar radiation flux linked to periods of increasing or decreasing Chl a . In the NASTG region, we estimated decreases in integrated nitrate (−0.15 µ mol l −1 decade −1 ) and Chl a (−0.05±0.01 mg m −2 decade −1 ) in the upper 300 m. This reduction led to decreased carbon biomass and export within the NASTG, estimated to account for 4 to 10% of global carbon export estimates.