Acceleration of ocean warming, salinification, deoxygenation and acidification in the surface subtropical North Atlantic Ocean

Nicholas Robert Bates,Rodney J Johnson

doi:10.1038/s43247-020-00030-5

Abstract

Ocean chemical and physical conditions are changing. Here we show decadal variability and recent acceleration of surface warming, salinification, deoxygenation, carbon dioxide (CO2) and acidification in the subtropical North Atlantic Ocean (Bermuda Atlantic Time-series Study site; 1980s to present). Surface temperatures and salinity exhibited interdecadal variability, increased by ~0.85 °C (with recent warming of 1.2 °C) and 0.12, respectively, while dissolved oxygen levels decreased by ~8% (~2% per decade). Concurrently, seawater DIC, fCO2 (fugacity of CO2) and anthropogenic CO2 increased by ~8%, 22%, and 72% respectively. The winter versus summer fCO2 difference increased by 4 to 8 µatm decade−1 due to seasonally divergent thermal and alkalinity changes. Ocean pH declined by 0.07 (~17% increase in acidity) and other acidification indicators by ~10%. Over the past nearly forty years, the highest increase in ocean CO2 and ocean acidification occurred during decades of weakest atmospheric CO2 growth and vice versa.

Highlights

Ocean chemical and physical conditions are changing
The Bermuda Atlantic Time-series Study (BATS) site is located ~80 km southeast of Bermuda (31o40′N, 64o10′W)[3]. It consists of a comprehensive monthly sampling of the physics, chemistry and biology of the entire water column commencing in October 19884–7, and one of only a handful of ship-based biogeochemical time-series sites remaining in the global ocean[7]
BATS–Hydrostation S time-series data provide partial support for the findings of a recent study indicating that the highest increases in ocean CO2 and ocean acidification (OA) occur during decades of weakest atmospheric CO2 growth and vice versa[8]

Summary

Introduction

We show decadal variability and recent acceleration of surface warming, salinification, deoxygenation, carbon dioxide (CO2) and acidification in the subtropical North Atlantic Ocean (Bermuda Atlantic Time-series Study site; 1980s to present). Recent acceleration of changes in ocean physics and chemistry since the 1980s is demonstrated at two sustained open-ocean hydrographic stations in the North Atlantic Ocean near Bermuda Of these two time-series sites, Hydrostation S (formerly known as the Panulirus site) is the oldest, located ~25 km southeast of Bermuda at 32°10′N, 64°30′W, and consists of repeat biweekly hydrographic observations of temperature, salinity and dissolved oxygen (DO) conducted through the water column since 19541,2. DIC and TA data are normalised to a salinity of 36.6 (mean value in the Sargasso Sea) to remove the influence of evaporation and precipitation, and denoted as nDIC and nTA, respectively

Methods

Results

Conclusion