Global spatiotemporal variability of chlorophyll a and environmental drivers revealed by empirical orthogonal function analysis

Abstract

Chlorophyll a (Chl a ) is a key indicator of phytoplankton biomass and marine primary productivity, linking ocean ecosystem change to biogeochemical cycling and climate. Understanding the environmental controls on long‐term Chl a variability is therefore essential. Here, we examine spatiotemporal variability in surface Chl a across the non‐polar global ocean (60°S–60°N) from 1998 to 2022 using Copernicus biogeochemical hindcast and ocean reanalysis datasets. Empirical orthogonal function (EOF) analysis is applied to Chl a and associated physical and biogeochemical variables, including sea surface temperature, surface sensible heat flux, mixed‐layer depth, wind speed, and nutrient concentrations. The results reveal strong regional contrasts in Chl a variability across the major ocean basins. Declining regions are associated mainly with enhanced upper‐ocean stratification and reduced nutrient resupply, whereas increasing regions are more strongly linked to enhanced nutrient availability. These findings highlight the importance of coupled physical–biogeochemical controls on global phytoplankton variability under climate forcing.