Bridging ocean color observations of the 1980s and 2000s in search of long‐term trends

TLDR

The study aims to create a global, multiyear, multisensor ocean color record from past, present, and future missions to assess past biogeochemical evolution and detect near‑future trends. The authors revised CZCS data‑processing algorithms and applied them to both the 1979–1986 CZCS archive and the 1998–2002 SeaWiFS data, validating the approach to produce an internally consistent time series for detecting long‑term ocean color trends. Decadal analysis reveals a ~22% rise in global mean chlorophyll, largely from intertropical regions with altered seasonal cycles, modest increases at higher latitudes, and declines in oligotrophic gyres.

Abstract

A comprehensive revision of the Coastal Zone Color Scanner (CZCS) data‐processing algorithms has been undertaken to generate a revised level 2 data set from the near‐8‐year archive (1979–1986) collected during this “proof‐of‐concept” mission. The final goal of this work is to establish a baseline for a global, multiyear, multisensor ocean color record, to be built from observations of past (i.e., CZCS), present, and future missions. To produce an internally consistent time series, the same revised algorithms also have been applied to the first 5 years of the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color observations (1998–2002). Such a database is necessary in order to determine whether or not the ocean biogeochemistry has evolved in the past years and, if so, to be able to detect near future trends. Algorithmic and calibration aspects, along with validation results presented in this paper, are tailored toward the identification of long‐term trends, which mandated this reprocessing effort. The analysis of decadal changes from the CZCS to the SeaWiFS era shows an overall increase of the world ocean average chlorophyll concentration by about 22%, mainly due to large increases in the intertropical areas, where the seasonal cycles also substantially changed over the past 2 decades. Increases in higher latitudes, where seasonal cycles did not change, contribute to a lesser extent to the general trend. In contrast, oligotrophic gyres display declining concentrations.