Southern Ocean Acidification Revealed by Biogeochemical‐Argo Floats.

Ocean acidification has potentially large impacts on calcifying organisms and ecosystems. Argo floats equipped with biogeochemical (BGC) sensors have been continuously measuring Southern Ocean pH since 2014. These BGC‐Argo floats were deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project. Here we present a SOCCOM‐era Objectively Mapped pH (SOM‐pH) 2014–2019 climatology and explain the method for constructing this product. We show example SOM‐pH fields demonstrating the spatial and temporal structure of Southern Ocean pH. Comparison with previous ship‐based measurements reveals decreases in pH of up to 0.02 per decade, with a structure decaying with depth. An assessment of the trend structure reveals a pattern indicative of the meridional overturning circulation. Upwelling waters that have not been in recent contact with the atmosphere show negligible or small trends, while surface and downwelling waters that have had more exposure to the atmosphere show the strongest trends. Thus comparison of this new BGC‐Argo mapped pH estimate to historic observations allows quantifying the structure of Southern Ocean acidification. Plain Language Summary: Robotic instruments that measure ocean acidity levels (pH) have been deployed in the Southern Ocean since 2014. Here we compile these observations into a map that is an estimate of the current Southern Ocean pH. This paper discusses how the map is made and gives a brief description of its features. We then compare it to pH measurements made from ships prior to 2014. Analyzing the structure of these differences in space and time reveals how the Southern Ocean is becoming more acidic. We find that there is significant structure to the changes. In general the changes are largest at the surface and become smaller at greater depth. However, there are horizontal differences. Ocean locations where winds pull up deep waters have smaller trends than regions where the winds are pushing waters down. Thus the ocean changes show a signature associated with the large‐scale circulation. Key Points: We present a novel 12‐month Southern Ocean pH mapped product, made possible by the Biogeochemical‐Argo array initiated in 2014Comparing to ship‐based measurements above 1,500 m reveals a decrease in pH of up to 0.02 per decadepH changes are widespread with varying magnitudes reflecting the pattern of the meridional overturning circulation [ABSTRACT FROM AUTHOR]