The last 50 years have enabled significant progress in our ability to describe and model both the oceanic and atmospheric environments of the Indian Ocean and their interactions with the sea floor and coastal regimes. However, our understanding of the major processes of the Indian Ocean is still far from complete and is rudimentary in many respects. This SI compiles various synthesis articles about the recent results of the 2nd International Indian Ocean Expedition Program (IIOE-2: https://iioe-2.incois.gov.in/). The synthesis articles will review and highlight results of physical, biogeochemical, ecological, geological, coastal and atmospheric studies from all parts of the Indian Ocean. Moreover, the SI will serve to illustrate some of the key questions that are relevant for future research on the grand challenges in the Indian Ocean system.

In the vast low-nutrient—low-chlorophyll (LNLC) ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be 1 order of magnitude greater over short timescales. Recently, significant experimental, field and modeling work allowed us to better link atmospheric deposition with nutrient availability, ocean productivity, carbon cycling and marine emissions. By improving our understanding on how atmospheric inputs are impacting biological activity and the carbon balance in oligotrophic environments, their representation in biogeochemical models at different timescales and space scales will be better assessed. In particular, processes may be affected by ongoing environmental changes in the ocean such as a decrease in pH and an increase in temperature.

This special issue aims at gathering contributions from both experimental and modeling approaches showing how natural or anthropogenic inputs from the atmosphere can impact biogeochemical and ecological processes in the present and future oligotrophic ocean. It will present the results obtained during the PEACETIME (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea) cruise conducted in 2017 in the Mediterranean Sea, but the special issue is open to any submissions provided that the subject is consistent with the objectives defined above.

The distribution of oxygen in the ocean is controlled by physical, biogeochemical and biological processes. Both the supply and consumption of oxygen are sensitive to climate change in ways that are not fully understood. Recent observations suggest that the oxygen content of the ocean is declining (ocean deoxygenation) and the oxygen minimum zones and costal hypoxia sites are expanding with tremendous effects on the ocean’s ecosystems and living organisms. Following the EUR-OCEANS conference on “Ocean Deoxygenation” in Toulouse, France, in 2011, the 46th International Liège Colloquium on “Low Oxygen Environments in Marine, Fresh and Estuarine Waters” in 2014 and the discussion meeting of the Royal Society London on “Ocean Ventilation and Deoxygenation in a Warming World” in 2016, the purpose of the international conference on “Ocean Deoxygenation: Drivers & Consequences – Past|Present|Future” that took place in Kiel, 3–7 Sept 2018 organised by the Collaborative Research Centre 754 “Climate-Biogeochemical Interactions in the Tropical Ocean” was to − focus on the past, present and future state of oxygen in the ocean on global, regional and local scales − analyse mechanisms and feedbacks critical to identify natural and anthropogenic causes of oxygen variability − determine impacts on biogeochemical cycles and ecosystems.

This special issue is open to all submissions on ocean deoxygenation and is not limited to conference participants.

Purpose of special issue: The Collaborative Research Centre 754 “Climate-Biogeochemical Interactions in the Tropical Ocean” has been investigating the newly recognised threat of ocean deoxygenation since 2008. Before the project's end in 2019, the International Conference on Ocean Deoxygenation was organised in Kiel, Germany in September 2018 with over 300 participants (https://conference.sfb754.de/event/1/). Following the conference the proposed inter-journal special issue aims at presenting the recent research advances and highlights in the field of ocean deoxygenation. The special issue shall not be limited to conference participants but be open to all submissions.

Paleoclimate data provide unique insights into climate dynamics across a range of timescales. Importantly, paleoclimate data are the only means of evaluating and constraining climate models under boundary conditions different from today and the recent past. However, most paleoclimate data are presently archived in a fragmented and non-standardized way, necessitating synthesis efforts in order to allow meaningful analysis of spatio-temporal climate dynamics and data-model comparison. Moreover, paleoclimate data are inherently uncertain since they are based on indirect evidence (proxies) and associated with chronological error, requiring rigorous uncertainty analysis to separate signal from noise and to make full use of paleoclimate data syntheses.

This special issue provides a platform to present paleoclimate synthesis products, to review the current state of proxy uncertainty analysis, as well as to present new developments. The issue is organized within the paleoclimate data synthesis working group of the PALMOD (http://www.palmod.de/ ) project, which focuses on the past 130,000 years. However, this is an open submission issue and we explicitly invite contributions from across the paleoclimate community describing synthesis methods and results from any kind of archive and/or parameter. We welcome contributions presenting paleoclimate synthesis products and their analysis across timescales, with regional or global focus and both time slice and transient approaches as well as conceptual contributions to proxy data uncertainty analysis (theoretical, empirical, Bayesian).