Changes in Ocean Heat, Carbon Content, and Ventilation: A Review of the First Decade of GO-SHIP Global Repeat Hydrography

Abstract

Global ship-based programs, with highly accurate, full water column physical and biogeochemical observations repeated decadally since the 1970s, provide a crucial resource for documenting ocean change. The ocean, a central component of Earth's climate system, is taking up most of Earth's excess anthropogenic heat, with about 19% of this excess in the abyssal ocean beneath 2,000 m, dominated by Southern Ocean warming. The ocean also has taken up about 27% of anthropogenic carbon, resulting in acidification of the upper ocean. Increased stratification has resulted in a decline in oxygen and increase in nutrients in the Northern Hemisphere thermocline and an expansion of tropical oxygen minimum zones. Southern Hemisphere thermocline oxygen increased in the 2000s owing to stronger wind forcing and ventilation. The most recent decade of global hydrography has mapped dissolved organic carbon, a large, bioactive reservoir, for the first time and quantified its contribution to export production (∼20%) and deep-ocean oxygen utilization. Ship-based measurements also show that vertical diffusivity increases from a minimum in the thermocline to a maximum within the bottom 1,500 m, shifting our physical paradigm of the ocean's overturning circulation.

Confirmation of ENSO-Southern Ocean Teleconnections Using Satellite-Derived SST

“…Southern Ocean in situ observations were particularly limited prior to the Argo float program, with most of the high-quality collections derived from repeat hydrography programs [30,31], such as the World Ocean Circulation Experiment (WOCE). Compared to in situ observations, satellites have relatively high spatial resolution and for SST a long temporal record extending back to the 1980's.…”

The O2/N2 Ratio and CO2 Airborne Southern Ocean Study

“…The current trajectory of Southern Ocean airsea CO 2 fluxes is uncertain (Le Quéré et al 2007;Landschutzer et al 2015), the Southern Ocean is subject to strong decadal climate variability (e.g., Mayewski et al 2009), and the Southern Ocean appears to be uniquely sensitive to anthropogenic temperature forcing (Gille 2008;Purkey and Johnson 2010;Hogg et al 2015;Talley et al 2016). By midcentury, CMIP5 models disagree by up to 70 PgC on the cumulative ocean carbon uptake since 2006 (Lovenduski and Bonan 2017), which is a sizable amount in comparison to annual emissions of around 10 PgC yr −1 , and hinders evaluation of how much fossil fuel can be burned in coming decades to maintain a specific temperature target.…”

Observational Advances in Estimates of Oceanic Heating

“…For a few decades, global and regional ocean variability have been increasingly revealed by the synergy of several observing systems maintained and coordinated by strong international collaborations. The repeat of full-depth hydrography sections [66], the remote detection of sea-level changes [10], the systematic sampling of the upper ocean by profiling floats [59] and the maintenance of trans-basin moored arrays [43] became the heart of our current understanding of the ocean's role in climate change. They have, for instance, validated numerical models that provided complete explanations of the recent surface warming slowdown at global scale (e.g.…”

“…Our understanding of OHC changes in the deep and abyssal ocean comes from the synoptic shipboard occupations of repeat hydrographic sections [66]. While these sections represent the most accurate component of the observing system (accuracy of 0.002 • C), they have limited temporal resolution and spatial coverage.…”