Footprint of sustained poleward warm water flow within East Antarctic submarine canyons.

The intrusion of relatively warm water onto the continental shelf is widely recognized as a threat to Antarctic ice shelves and glaciers grounded below sea level, as enhanced ocean heat increases their basal melt. While the circulation of warm water has been documented on the East Antarctic continental shelf, the modes of warm water transport from the deep ocean onto the shelf are still uncertain. This makes predicting the future responses of major East Antarctic marine-grounded glaciers, such as Totten and Ninnis glaciers, particularly challenging. Here, we outline the key role of submarine canyons to convey southward flowing currents that transport warm Circumpolar Deep Water toward the East Antarctic shelf break, thus facilitating warm water intrusion on the continental shelf. Sediment drifts on the eastern flank of the canyons provide evidence for sustained southward-directed flows. These morpho-sedimentary features thus highlight areas potentially prone to enhanced ocean heat transport toward the continental shelf, with repercussions for past, present, and future glacial melting and consequent sea level rise.

Seawater physics and chemistry along the Med-SHIP transects in the Mediterranean Sea in 2016.

The Mediterranean Sea has been sampled irregularly by research vessels in the past, mostly by national expeditions in regional waters. To monitor the hydrographic, biogeochemical and circulation changes in the Mediterranean Sea, a systematic repeat oceanographic survey programme called Med-SHIP was recommended by the Mediterranean Science Commission (CIESM) in 2011, as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Med-SHIP consists of zonal and meridional surveys with different frequencies, where comprehensive physical and biogeochemical properties are measured with the highest international standards. The first zonal survey was done in 2011 and repeated in 2018. In addition, a network of meridional (and other key) hydrographic sections were designed: the first cycle of these sections was completed in 2016, with three cruises funded by the EU project EUROFLEETS2. This paper presents the physical and chemical data of the meridional and key transects in the Western and Eastern Mediterranean Sea collected during those cruises.

Experimental evidence of long-term oceanic circulation reversals without wind influence in the North Ionian Sea.

Under the emerging features of interannual-to-decadal ocean variability, the periodical reversals of the North Ionian Gyre (NIG), driven mostly by the mechanism named Adriatic-Ionian Bimodal Oscillating System (BiOS), are known as impacting on marine physics and biogeochemistry and potentially influencing short-term regional climate predictability in the Eastern Mediterranean. Whilst it has been suggested that local wind forcing cannot explain such variability, aspects of the alternative hypothesis indicating that NIG reversals mainly arises from an internal ocean feedback mechanism alone remain largely debated. Here we demonstrate, using the results of physical experiments, performed in the world's largest rotating tank and numerical simulations, that the main observed feature of BiOS, i.e., the switch of polarity of the near-surface circulation in the NIG, can be induced by a mere injection of dense water on a sloping bottom. Hence, BiOS is a truly oceanic mode of variability and abrupt polarity changes in circulation can arise solely from extreme dense water formation events.

Inorganic and Organic Carbon Uptake Processes and Their Connection to Microbial Diversity in Meso- and Bathypelagic Arctic Waters (Eastern Fram Strait).

The deep Arctic Ocean is increasingly vulnerable to climate change effects, yet our understanding of its microbial processes is limited. We collected samples from shelf waters, mesopelagic Atlantic Waters (AW) and bathypelagic Norwegian Sea Deep Waters (NSDW) in the eastern Fram Strait, along coast-to-offshore transects off Svalbard during boreal summer. We measured community respiration, heterotrophic carbon production (HCP), and dissolved inorganic carbon utilization (DICu) together with prokaryotic abundance, diversity, and metagenomic predictions. In deep samples, HCP was significantly faster in AW than in NSDW, while we observed no differences in DICu rates. Organic carbon uptake was higher than its inorganic counterpart, suggesting a major reliance of deep microbial Arctic communities on heterotrophic metabolism. Community structure and spatial distribution followed the hydrography of water masses. Distinct from other oceans, the most abundant OTU in our deep samples was represented by the archaeal MG-II. To address the potential biogeochemical role of each water mass-specific microbial community, as well as their link with the measured rates, PICRUSt-based predicted metagenomes were built. The results showed that pathways of auto- and heterotrophic carbon utilization differed between the deep water masses, although this was not reflected in measured DICu rates. Our findings provide new insights to understand microbial processes and diversity in the dark Arctic Ocean and to progress toward a better comprehension of the biogeochemical cycles and their trends in light of climate changes.