Contribution of topographically-generated submesoscale turbulence to Southern Ocean overturning

The ocean’s global overturning circulation regulates the transport and storage of heat, carbon and nutrients. Upwelling across the Southern Ocean’s Antarctic Circumpolar Current and into the mixed layer, coupled to water mass modification by surface buoyancy forcing, has been highlighted as a key process in the closure of the overturning circulation. Here, using twelve high-resolution hydrographic sections in southern Drake Passage, collected with autonomous ocean gliders, we show that Circumpolar Deep Water originating from the North Atlantic, known as Lower Circumpolar Deep Water, intersects sloping topography in narrow and strong boundary currents. Observations of strong lateral buoyancy gradients, enhanced bottom turbulence, thick bottom mixed layers and modified water masses are consistent with growing evidence that topographically generated submesoscale flows over continental slopes enhance near-bottom mixing, and that cross-density upwelling occurs preferentially over sloping topography. Interactions between narrow frontal currents and topography occur elsewhere along the path of the Antarctic Circumpolar Current, which leads us to propose that such interactions contribute significantly to the closure of the overturning in the Southern Ocean

Sources and pathways of glacial meltwater in the Bellingshausen Sea, Antarctica

Meltwater content and pathways determine the impact of Antarctica's melting ice shelves on ocean circulation and climate. Using ocean glider observations, we quantify meltwater distribution and transport within the Bellingshausen Sea's Belgica Trough. Meltwater is present at different densities and with different turbidities: both are indicative of a layer's ice shelf of origin. To investigate how ice-shelf origin separates meltwater into different export pathways, we compare these observations with high-resolution tracer-release model simulations. Meltwater filaments branch off the Antarctic Coastal Current into the southwestern trough. Meltwater also enters the Belgica Trough in the northwest via an extended western pathway, hence the greater observed southward (0.50 mSv) than northward (0.17 mSv) meltwater transport. Together, the observations and simulations reveal meltwater retention within a cyclonic in-trough gyre, which has the potential to promote climactically important feedbacks on circulation and future melting

2014-2021, 8 years without bottom-reaching deep water formation in the Western Mediterranean. Probably, the longest known period

Deep Water Formation (DWF) appeared almost regularly every year, during central winter months, in an area located offshore the Gulf of Lions in the NW Mediterranean Sea. Since the early 1960s, the processes involved in the DWF have been monitored, more or less intensively by regular hydrographic surveys or by moored instruments. It is worth noting the international efforts carried out in late 60s-early 70s by the so-called MEDOC Group to obtain a quite precise description of the whole process. Although the intensity of the DWF, as well as the amount of the newly formed Western Mediterranean Deep Water (WMDW), have shown high interanual variability, those years when the DWF was absent were exceptional, e.g. 1990, and those not reaching the bottom were scarce, e.g. 1997. Typically, they were years with almost no cold northerly winds during winter. By contrast, in some years the amount of newly formed WMDW was exceptional, e.g. 1987, and in some cases, an extra amount of this water came from dense shelf cascading, e.g. 1999. Moreover, in some years, the so-called variable Bottom Water, a slightly warm and salty layer, appeared near the bottom. It was a layer not thicker than 300 m, attributed to a large area affected by DWF which caused an extra amount of Levantine Intermediate Water (LIW) involved in the process, e.g. 1973. Other concomitant conditions that contributed to the DWF variability across the years was the presence of a blocking anticyclone in the Balearic Sea, that would play a role in intensifying the exposure of surface water to the northerlies, e.g. 1999. In winter 2005, all the factors contributing to an intense DWF process acted simultaneously, resulting in a new structure within the WMDW. The amount of newly formed WMDW, with higher density, T and S, was so extraordinary that affected the entire western Mediterranean basin, and it was identified as the Western Mediterranean Transition (WMT). The remnants of the WMDW previous to the WMT have been uplifted as to being available for a relevant contribution to the Mediterranean Outlfow Water (MOW) through the Gibraltar sill. After the WMT, the MOW showed both lower T and S than previously recorded up to around 2015, indicating that the old WMDW has been almost completely lost by leakage and diffusion. After the 2005 episode, the WMDW has evolved, changing its TS shape and increasing both T and S at the bottom, but still maintaining a deep layer with higher stratification than before 2005. In a previous work, we attributed the long period (2014-2018) without DWF to a combination of mild winters, the absence of the old WMDW, and the deep stratification. Such a process would be similar to the recovery of the Eastern Mediterranean Transient. In the present communication we incorporate 3 new years of data to the series, discuss the current situation and try to identify the requirements for a successful bottom-reaching DWF

Autonomous sampling of ocean submesoscale fronts with ocean gliders and numerical model forecasting

Submesoscale fronts arising from mesoscale stirring are ubiquitous in the ocean and have a strong impact on upper-ocean dynamics. This work presents a method for optimizing the sampling of ocean fronts with autonomous vehicles at meso- and submesoscales, based on a combination of numerical forecast and autonomous planning. This method uses a 48-h forecast from a real-time high-resolution data-assimilative primitive equation ocean model, feature detection techniques, and a planner that controls the observing platform. The method is tested in Monterey Bay, off the coast of California, during a 9-day experiment focused on sampling subsurface thermohaline-compensated structures using a Seaglider as the ocean observing platform. Based on model estimations, the sampling “gain,” defined as the magnitude of isopycnal tracer variability sampled, is 50% larger in the feature-chasing case with respect to a non-feature-tracking scenario. The ability of the model to reproduce, in space and time, thermohaline submesoscale features is evaluated by quantitatively comparing the model and glider results. The model reproduces the vertical (~50–200 m thick) and lateral (~5–20 km) scales of subsurface subducting fronts and near-bottom features observed in the glider data. The differences between model and glider data are, in part, attributed to the selected glider optimal interpolation parameters and to uncertainties in the forecasting of the location of the structures. This method can be exported to any place in the ocean where high-resolution data-assimilative model output is available, and it allows for the incorporation of multiple observing platforms

An observing system simulation experiment for the calibration and validation of the surface water ocean topography sea surface height measurement using in situ platforms

Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 35 (2018): 281-297, doi:10.1175/JTECH-D-17-0076.1.The wavenumber spectrum of sea surface height (SSH) is an important indicator of the dynamics of the ocean interior. While the SSH wavenumber spectrum has been well studied at mesoscale wavelengths and longer, using both in situ oceanographic measurements and satellite altimetry, it remains largely unknown for wavelengths less than ~70 km. The Surface Water Ocean Topography (SWOT) satellite mission aims to resolve the SSH wavenumber spectrum at 15–150-km wavelengths, which is specified as one of the mission requirements. The mission calibration and validation (CalVal) requires the ground truth of a synoptic SSH field to resolve the targeted wavelengths, but no existing observational network is able to fulfill the task. A high-resolution global ocean simulation is used to conduct an observing system simulation experiment (OSSE) to identify the suitable oceanographic in situ measurements for SWOT SSH CalVal. After fixing 20 measuring locations (the minimum number for resolving 15–150-km wavelengths) along the SWOT swath, four instrument platforms were tested: pressure-sensor-equipped inverted echo sounders (PIES), underway conductivity–temperature–depth (UCTD) sensors, instrumented moorings, and underwater gliders. In the context of the OSSE, PIES was found to be an unsuitable tool for the target region and for SSH scales 15–70 km; the slowness of a single UCTD leads to significant aliasing by high-frequency motions at short wavelengths below ~30 km; an array of station-keeping gliders may meet the requirement; and an array of moorings is the most effective system among the four tested instruments for meeting the mission’s requirement. The results shown here warrant a prelaunch field campaign to further test the performance of station-keeping gliders.The authors would like to acknowledge the funding sources: the SWOT mission (JW, LF, DM); NASA Projects NNX13AE32G, NNX16AH76G, and NNX17AH54G (TF); and NNX16AH66G and NNX17AH33G (BQ). AF and MF were funded by the Keck Institute for Space Studies (which is generously supported by the W. M. Keck Foundation) through the project Science-driven Autonomous and Heterogeneous Robotic Networks: A Vision for Future Ocean Observations (http://kiss.caltech.edu/?techdev/seafloor/seafloor.html).2018-08-0

Autonomous sampling of ocean submesoscale fronts with ocean gliders and numerical model forecasting

Submesoscale fronts arising from mesoscale stirring are ubiquitous in the ocean and have a strong impact on upper-ocean dynamics. This work presents a method for optimizing the sampling of ocean fronts with autonomous vehicles at meso- and submesoscales, based on a combination of numerical forecast and autonomous planning. This method uses a 48-h forecast from a real-time high-resolution data-assimilative primitive equation ocean model, feature detection techniques, and a planner that controls the observing platform. The method is tested in Monterey Bay, off the coast of California, during a 9-day experiment focused on sampling subsurface thermohaline-compensated structures using a Seaglider as the ocean observing platform. Based on model estimations, the sampling “gain,” defined as the magnitude of isopycnal tracer variability sampled, is 50% larger in the feature-chasing case with respect to a non-feature-tracking scenario. The ability of the model to reproduce, in space and time, thermohaline submesoscale features is evaluated by quantitatively comparing the model and glider results. The model reproduces the vertical (~50–200 m thick) and lateral (~5–20 km) scales of subsurface subducting fronts and near-bottom features observed in the glider data. The differences between model and glider data are, in part, attributed to the selected glider optimal interpolation parameters and to uncertainties in the forecasting of the location of the structures. This method can be exported to any place in the ocean where high-resolution data-assimilative model output is available, and it allows for the incorporation of multiple observing platforms

Satellites to seafloor : toward fully autonomous ocean sampling

Author Posting. © The Oceanography Society, 2017. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 30, no. 2 (2017): 160–168, doi:10.5670/oceanog.2017.238.Future ocean observing systems will rely heavily on autonomous vehicles to achieve the persistent and heterogeneous measurements needed to understand the ocean’s impact on the climate system. The day-to-day maintenance of these arrays will become increasingly challenging if significant human resources, such as manual piloting, are required. For this reason, techniques need to be developed that permit autonomous determination of sampling directives based on science goals and responses to in situ, remote-sensing, and model-derived information. Techniques that can accommodate large arrays of assets and permit sustained observations of rapidly evolving ocean properties are especially needed for capturing interactions between physical circulation and biogeochemical cycling. Here we document the first field program of the Satellites to Seafloor project, designed to enable a closed loop of numerical model prediction, vehicle path-planning, in situ path implementation, data collection, and data assimilation for future model predictions. We present results from the first of two field programs carried out in Monterey Bay, California, over a period of three months in 2016. While relatively modest in scope, this approach provides a step toward an observing array that makes use of multiple information streams to update and improve sampling strategies without human intervention.This work is funded by the Keck Institute for Space Studies (generously supported by the W.M. Keck Foundation) through the project “Science-driven Autonomous and Heterogeneous Robotic Networks: A Vision for Future Ocean Observation