Interannual variability of chlorophyll and the influence of low‐frequency climate modes in the North Atlantic subtropical gyre

Abstract

The impact of low‐frequency climate modes on the large‐scale variability of chlorophyll‐a (a proxy for phytoplankton biomass) concentration in the subtropics is little known. Here we examined the concurrent monthly chlorophyll‐a and hydrographical records obtained at two subtropical time series stations (Bermuda Atlantic Time series Study, BATS and European Station for Time series in the Ocean Canary Islands, ESTOC) from the beginning of the time series (1989 at BATS and 1994 at ESTOC) to 2003, in order to analyze its seasonal and interannual variability and investigate the potential correlation with large‐scale atmospheric oscillations. At BATS, Chl‐a variations are mainly caused by differences in the convective mixing and mesoscale phenomena. Variability in winter mixing is a significant factor at ESTOC as exemplified by years with anomalously deep mixed layer depths. An additional nutrient source causing Chl‐a variability at this station likely occurs due to nutrient advection driven by the baroclinic flow caused by the Trade Winds during summertime. We found that interannual variability in mean integrated total Chl‐a (TChl‐a) is significantly correlated with temperature and salinity anomalies at BATS. Chl‐a also covaried with changes in temperature although the correlation was not significant at ESTOC. We could not find any direct correlation between TChl‐a and NAO at BATS; the correlation improved between TChl‐a and NAO +1 year but was still insignificant. However, significant correlations were found for ESTOC between TChl‐a anomalies and ENSO (El Niño Southern Oscillation) +3 years and NAO +1 year time lag.