Abyssal ocean overturning shaped by seafloor distribution

Deep ocean hydrographic variability estimated from distributed geodetic sensor arrays off northern Chile

Observations of spatio-temporal variability of the deep ocean are rare and little is known about occurrence of deep ocean mesoscale dynamics. Here, we make use of 2.5 years of time series data from three distributed sensor arrays, which acquired high-resolution temperature, pressure and sound speed data of the bottom layer offshore northern Chile. Estimating salinity and density from the direct observations enable access to the full spectrum of hydrographic variability from a multi-hourly to annual time scale and with average inter-station distances of less than 1 km. Analyses revealed interannual warming over the continental slope of 0.002 °C yr-1-0.003 °C yr-1, and could trace periodic hydrographic anomalies, likely related to coastal-trapped waves, as far as to the lower continental slope. A concurrent change in the shape of the warm anomalies and the rate of deep-sea warming that occurs with the crossing of the deep-sea trench suggests that the abyssal part of the eastern boundary current system off Chile does not extend past the deep sea trench. Furthermore, the comparison of anomaly timing and shape in between stations implies southwards flow over the mid to lower continental slope, centred closer to the trench.

Deep water pathways in the North Pacific Ocean revealed by Lagrangian particle tracking

Lagrangian particle tracking experiments are conducted to investigate the pathways of deep water in the North Pacific Ocean. The flow field is taken from a state-of-the-art deep circulation simulation. An unprecedented number of particles are tracked to quantify the volume transport and residence time. Half of the North Pacific deep water returns to the Southern Ocean, and its principal pathway is along the western boundary current in the Southwest Pacific Basin in the deep layer. About 30% is exported to the Indian Ocean after upwelling to the shallow layer in the western North Pacific Ocean. The rest is transported to the Arctic Ocean through the Bering Strait or evaporates within the Pacific Ocean. Upwelling of deep water is confined in the western North Pacific Ocean owing to the strong vertical mixing. The mean residence time of deep water in the North Pacific Ocean is estimated to be several hundred years, which is consistent with the observed radiocarbon distribution.

Diffusion controls the ventilation of a Pacific Shadow Zone above abyssal overturning

Mid-depth North Pacific waters are rich in nutrients and respired carbon accumulated over centuries. The rates and pathways with which these waters exchange with the surface ocean are uncertain, with divergent paradigms of the Pacific overturning: one envisions bottom waters upwelling to 1.5 km depth; the other confines overturning beneath a mid-depth Pacific shadow zone (PSZ) shielded from mean advection. Here global inverse modelling reveals a PSZ where mean ages exceed 1400 years with overturning beneath. The PSZ is supplied primarily by Antarctic and North-Atlantic ventilated waters diffusing from below and from the south. Half of PSZ waters re-surface in the Southern Ocean, a quarter in the subarctic Pacific. The abyssal North Pacific, despite strong overturning, has mean re-surfacing times also exceeding 1400 years because of diffusion into the overlying PSZ. These results imply that diffusive transports – distinct from overturning transports – are a leading control on Pacific nutrient and carbon storage.

The deep North Pacific is the end of the road for global ocean circulation, but the circulation patterns and ventilation are poorly understood. Here the authors show that diffusive transports both along and across density layers play a leading role in returning 1,400 year old water to the surface.

Viral Infections Boost Prokaryotic Biomass Production and Organic C Cycling in Hadal Trench Sediments

Hadal trenches are among the most remote and least explored ecosystems on Earth and can support high benthic microbial standing stocks and activities. However, information on the role of viruses in such ecosystems and their interactions with prokaryotic hosts is very limited. Here, we investigated activities of benthic viruses and prokaryotes and their interactions in three hadal trenches (Japan, Izu-Ogasawara and Mariana trenches) and in their nearby abyssal sites. Our findings reveal that these hadal trenches, compared with the surrounding abyssal sites, support higher abundances and biomasses of prokaryotes. In addition, the high prokaryotic biomasses of hadal trenches could favor high rates of viral infection and cell lysis, especially in the Japan Trench. Hadal viruses can release large amounts of highly labile and promptly available organic material by inducing cell lysis, which could contribute to sustain benthic prokaryotes and decrease their dependency on the enzymatic digestion of the more refractory fraction of sediment organic matter. Our results suggest that this process can contribute to explain the discrepancy between high prokaryote biomass and apparent low efficiency in the utilization of the sedimentary organic matter in the hadal ecosystems. Concluding, hadal trenches may be characterized by a highly dynamic viral component, which can boost prokaryotic biomass production, thereby profoundly influencing the functioning of these remote and extreme ecosystems.

Deep-ocean mixing driven by small-scale internal tides

Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth’s climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate—the relative importance of their local versus remote breaking into turbulence—remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models.

The geography of deep-ocean mixing driven by internal tides is poorly constrained in ocean models. Here the authors unveil the global variability of energetic small-scale internal tides, combining an analytical model with satellite and in situ observations, paving the way to future parameterisations.

Impact of deep ocean mixing on the climatic mean state in the Southern Ocean

The Southern Ocean is of great importance for the global stratification and biological carbon storage because it is connected to the global ocean conveyor by which atmospheric information absorbed in the Southern Ocean is redistributed globally and buffered over centuries. Therefore, understanding what controls the Southern Ocean climate, the global ocean conveyor, and links between them is a key to quantifying uncertainties in future climate projections. Based on a set of climate model experiments, here we show that the tide-induced micro-scale mixing in the Pacific deep ocean has significant impacts on the wintertime Southern Ocean climate through basin-scale reorganization of ocean stratification and resultant response of the global ocean conveyor. Specifically, Pacific deep water, which is modified by the deep ocean mixing while travelling south, reinforces the subsurface stratification and suppresses deep convection in the Southern Ocean. Resultant increase of the Ross Sea sea-ice leads to decrease of incoming shortwave radiation and strengthening of the westerly and storms. Because the Southern Ocean could regulate the global warming progress through its role as heat and carbon sink, our study implies that better representation of deep ocean mixing in climate models contributes to reliability improvement in regional-to-global climate projections.