Profiling Floats Research Articles

Evolution of the Thermohaline Structure of One Agulhas Ring Reconstructed from Satellite Altimetry and Argo Floats

AbstractThe transfer of Indian Ocean thermocline and intermediate waters into the South Atlantic via the Agulhas leakage is generally believed to be primarily accomplished through mesoscale eddy processes, essentially anticyclones known as Agulhas Rings. Here we take advantage of a recent eddy tracking algorithm and Argo float profiles to study the evolution and the thermohaline structure of one of these eddies over the course of 1.5 years (May 2013–November 2014). We found that during this period the ring evolved according to two different phases: During the first one, taking place in winter, the mixing layer in the eddy deepened significantly. During the second phase, the eddy subsided below the upper warmer layer of the South Atlantic subtropical gyre while propagating west. The separation of this eddy from the sea surface could explain the decrease in its surface signature in satellite altimetry maps, suggesting that such changes are not due to eddy dissipation processes. It is a very large eddy (7.1×1013 m3 in volume), extending, after subduction, from a depth of 200–1,200 m and characterized by two mode water cores. The two mode water cores represent the largest eddy heat and salt anomalies when compared with the surrounding. In terms of its impact over 1 year, the north‐westward propagation of this long‐lived anticyclone induces a transport of 2.2 Sv of water, 0.008 PW of heat, and 2.2×105 kg s−1 of salt. These results confirm that Agulhas Rings play a very important role in the Indo‐Atlantic interocean exchange of heat and salt.

Spatiotemporal changes of ocean carbon species in the western North Pacific using parameterization technique

We constructed parameterizations for the estimation of dissolved inorganic carbon (DIC) and pH in the western North Pacific, including Japanese coastal regions. Parameterizations, determined as a function of potential temperature (θ) and dissolved oxygen (DO), provided strong correlations with direct measurements for DIC [the coefficient of determination (R2) = 0.99; the root mean square error (RMSE) = 8.49 µmol kg−1] and pH (R2 = 0.98, RMSE = 0.030). Predicted DIC and pH values were consistent with previous independent hydrographic observations, generally within 3 µmol kg−1 for DIC and 0.010 for pH. By applying these parameterizations to climatological seasonal θ and DO data and time-series θ and DO data from autonomous profiling floats (Argo) in the western North Pacific, large spatiotemporal variations in DIC and pH were demonstrated. Results indicated that the rate of increase in anthropogenic DIC above a depth of 400 m along 30° N in the western North Pacific was + 0.86 ± 0.11 µmol kg−1 year−1 between 1994 and 2004. This decreased by + 0.58 ± 0.05 µmol kg−1 year−1 after 2004 partly due to increasing anthropogenic nitrogen input, suggesting that the decrease has induced the slowdown of ocean acidification in the western North Pacific. The declining trend in anthropogenically induced ocean pH was found to be 0.0021 ± 0.0016 during 1994–2013.

Role of Iron in the Marquesas Island Mass Effect

AbstractA remarkable chlorophyll‐a concentration (Chl, a proxy of phytoplankton biomass) plume can be noticed on remotely sensed ocean color observations at the boundary separating the equatorial mesotrophic from the subtropical oligotrophic waters in the central South Pacific Ocean. This prominent biological feature is known as the island mass effect of the Marquesas archipelago. Waters surrounding these islands present high macronutrient concentrations but an iron depletion. In this study, the origin of Chl enhancement is investigated using a modeling approach. Four simulations based on identical physical and biogeochemical forcings but with different iron sources are conducted and analyzed. Only simulations considering an iron input from the island sediments present similar patterns (despite being too weak) of vertical and horizontal Chl distributions as compared to biogeochemical‐Argo profiling float and satellite observations. In addition, simulations with no other iron input than the boundary forcings reveal the relative importance of remote processes in modulating the seasonal pattern of Chl around the archipelago through horizontal advection of nutrient‐rich waters from the equator toward the archipelago and vertical mixing uplifting deep nutrient‐rich waters toward the upper lit layer.

Observations of oceanic fronts and water-mass properties in the central Japan Sea: Repeated surveys from an underwater glider

Abstract Repeated glider surveys were conducted in the Japan Sea during April–June 2016. Observations revealed mesoscale hydrographic structures of the subpolar front, the Tsushima Warm Current, and eddies formed around flows in the central Japan Sea off Sado Island, in addition to fine-scale water properties associated with mesoscale variation. Data collected from satellite altimeters and thermometers, profiling floats, and ship conductivity–temperature–depth profiles were also analyzed to elucidate spatiotemporal characteristics in multiscale phenomena: mesoscale to fine scale. We present new quantitative estimates of the geostrophic volume transports of the Tsushima Warm Current and the subpolar front current in the central Japan Sea, based on hydrographic data from the glider's transects: they are, respectively, 2.5–2.7 Sv and 1.1–1.4 Sv. The mesoscale frontal/eddy structures varied greatly on a timescale of less than one month. We classified water masses, which revealed that the upper-layer, the low-salinity water within an anticyclonic eddy, was characterized in terms of salinity 100 m in this region, for which there are few hydrographic measurement data.

Evaluating satellite estimates of particulate backscatter in the global open ocean using autonomous profiling floats.

Satellite retrievals of particulate backscattering (bbp) are widely used in studies of ocean ecology and biogeochemistry, but have been historically difficult to validate due to the paucity of available ship-based comparative field measurements. Here we present a comparison of satellite and in situ bbp using observations from autonomous floats (n = 2,486 total matchups across three satellites), which provide bbp at 700 nm. With these data, we quantify how well the three inversion products currently distributed by NASA ocean color retrieve bbp. We find that the median ratio of satellite derived bbp to float bbp ranges from 0.77 to 1.60 and Spearman's rank correlations vary from r = 0.06 to r = 0.79, depending on which algorithm and sensor is used. Model skill degrades with increased spatial variability in remote sensing reflectance, which suggests that more rigorous matchup criteria and factors contributing to sensor noisiness may be useful to address in future work, and/or that we have built in biases in the current widely distributed inversion algorithms.

Observational Requirements for Long-Term Monitoring of the Global Mean Sea Level and Its Components Over the Altimetry Era

Present-day global mean sea level rise is caused by ocean thermal expansion, ice mass loss from glaciers and ice sheets, as well as changes in terrestrial water storage. For that reason, sea level is one of the best indicators of climate change as it integrates the response of several components of the climate system to internal and external forcing factors. Monitoring the global mean sea level allows detecting changes (e.g., in trend or acceleration) in one or more components. Besides, assessing closure of the sea level budget allows us to check whether observed sea level change is indeed explained by the sum of changes affecting each component. If not, this would reflect errors in some of the components or missing contributions not accounted for in the budget. Since the launch of TOPEX/Poseidon in 1992, a precise 27-year continuous record of sea level change is available. It has allowed major advances in our understanding of how the Earth is responding to climate change. The last two decades are also marked by the launch of the GRACE satellite gravity mission and the development of the Argo network of profiling floats. GRACE space gravimetry allows the monitoring of mass redistributions inside the Earth system, in particular land ice mass variations as well as changes in terrestrial water storage and in ocean mass, while Argo floats allow monitoring sea water thermal expansion due to the warming of the oceans. Together, satellite altimetry, space gravity, and Argo measurements provide unprecedented insight into the magnitude, spatial variability, and causes of present-day sea level change. With this observational network, we are now in a position to address many outstanding questions that are important to planning for future sea level rise. Here, we detail the network for observing sea level and its components, underscore the importance of these observations, and emphasize the need to maintain current systems, improve their sensors, and supplement the observational network where gaps in our knowledge remain.

Polar Ocean Observations: A Critical Gap in the Observing System and Its Effect on Environmental Predictions From Hours to a Season.

There is a growing need for operational oceanographic predictions in both the Arctic and Antarctic polar regions. In the former, this is driven by a declining ice cover accompanied by an increase in maritime traffic and exploitation of marine resources. Oceanographic predictions in the Antarctic are also important, both to support Antarctic operations and also to help elucidate processes governing sea ice and ice shelf stability. However, a significant gap exists in the ocean observing system in polar regions, compared to most areas of the global ocean, hindering the reliability of ocean and sea ice forecasts. This gap can also be seen from the spread in ocean and sea ice reanalyses for polar regions which provide an estimate of their uncertainty. The reduced reliability of polar predictions may affect the quality of various applications including search and rescue, coupling with numerical weather and seasonal predictions, historical reconstructions (reanalysis), aquaculture and environmental management including environmental emergency response. Here, we outline the status of existing near-real time ocean observational efforts in polar regions, discuss gaps, and explore perspectives for the future. Specific recommendations include a renewed call for open access to data, especially real-time data, as a critical capability for improved sea ice and weather forecasting and other environmental prediction needs. Dedicated efforts are also needed to make use of additional observations made as part of the Year of Polar Prediction (YOPP; 2017–2019) to inform optimal observing system design. To provide a polar extension to the Argo network, it is recommended that a network of ice-borne sea ice and upper-ocean observing buoys be deployed and supported operationally in ice-covered areas together with autonomous profiling floats and gliders (potentially with ice detection capability) in seasonally ice covered seas. Finally, additional efforts to better measure and parameterize surface exchanges in polar regions are much needed to improve coupled environmental prediction.

On the Future of Argo: A Global, Full-Depth, Multi-Disciplinary Array

The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo’s global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al., 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities.

Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea

AbstractThe Scotia Sea is the site of one of the largest spring phytoplankton blooms in the Southern Ocean. Past studies suggest that shelf‐iron inputs are responsible for the high productivity in this region, but the physical mechanisms that initiate and sustain the bloom are not well understood. Analysis of profiling float data from 2002 to 2017 shows that the Scotia Sea has an unusually shallow mixed‐layer depth during the transition from winter to spring, allowing the region to support a bloom earlier in the season than elsewhere in the Antarctic Circumpolar Current. We compare these results to the mixed‐layer depth in the 1/6° data‐assimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixed‐layer variability in the region. Results indicate the importance of lateral advection of Weddell Sea surface waters in setting the stratification. A Lagrangian particle release experiment run backward in time suggests that Weddell outflow constitutes 10% of the waters in the upper 200 m of the water column in the bloom region. This dense Weddell water subducts below the surface waters in the Scotia Sea, establishing a sharp subsurface density contrast that cannot be overcome by wintertime convection. Profiling float trajectories are consistent with the formation of Taylor columns over the region's complex bathymetry, which may also contribute to the unique stratification. Furthermore, biogeochemical measurements from 2016 and 2017 bloom events suggest that vertical exchange associated with this Taylor column enhances productivity by delivering nutrients to the euphotic zone.

Towards quantifying uncertainty in ocean heat content changes using synthetic profiles

Observational estimates of global ocean heat content (OHC) change are used to assess Earth’s energy imbalance over the 20th Century. However, intercomparison studies show that the mapping methods used to interpolate sparse ocean temperature profile data are a key source of uncertainty. We present a new approach to assessing OHC mapping methods using ‘synthetic profiles’ generated from a state-of-the-art global climate model simulation. Synthetic profiles have the same sampling characteristics as the historical ocean temperature profile data but are based on model simulation data. Mapping methods ingest these data in the same way as they would real observations, but the resultant mapped fields can be compared to a model simulation ‘truth’. We use this approach to assess two mapping methods that are used routinely for climate monitoring and initialisation of decadal forecasts. The introduction of the Argo network of autonomous profiling floats during the 2000s drives clear improvements in the ability of these methods to reconstruct the variability and spatial structure of OHC changes. At depths below 2000 m, both methods underestimate the magnitude of the simulated ocean warming signal. Temporal variability and trends in OHC are better captured in the better-observed northern hemisphere than in the southern hemisphere. At all depths, the sampling characteristics of the historical data introduces some spurious variability in the estimates of global OHC on sub-annual to multi-annual timescales. However, many of the large scale spatial anomalies, especially in the upper ocean, are successfully reconstructed even with sparse observations from the 1960s, demonstrating the potential to construct historical ocean analyses for assessing decadal predictions. The value of using accurate global covariances for data-poor periods is clearly seen. The results of this ‘proof-of-concept’ study are encouraging for gaining further insights into the capabilities and limitations of different mapping methods and for quantifying uncertainty in global OHC estimates.

Observing the Oceans Acoustically

Acoustics play a central role in mankind’s interactions with the ocean and the life within. Passive listening to ocean “soundscapes” informs us about the physical and bio-acoustic environment from earthquakes to communication between fish. Active acoustic probing of the environment informs us about ocean topography, currents and temperature, and abundance and type of marine life vital to fisheries and biodiversity related interests. The two together in a multi-purpose network can lead to discovery and improve understanding of ocean ecosystem health and biodiversity, climate variability and change, and marine hazards and maritime safety. Passive acoustic monitoring (PAM) of sound generated and utilized by marine life as well as other natural (wind, rain, ice, seismics) and anthropogenic (shipping, surveys) sources, has dramatically increased worldwide to enhance understanding of ecological processes. Characterizing ocean soundscapes (the levels and frequency of sound over time and space, and the sources contributing to the sound field), temporal trends in ocean sound at different frequencies, distribution and abundance of marine species that vocalize, and distribution and amount of human activities that generate sound in the sea, all require passive acoustic systems. Acoustic receivers are now routinely acquiring data on a global scale, e.g., Comprehensive Nuclear-Test-Ban Treaty Organization International Monitoring System hydroacoustic arrays, various regional integrated ocean observing systems, and some profiling floats. Judiciously-placed low-frequency acoustic sources transmitting to globally distributed PAM and other systems provide: 1) high temporal resolution measurements of large-scale ocean temperature/heat content variability, taking advantage of the inherent integrating nature of acoustic travel-time data using tomography; and 2) acoustic positioning (“underwater GPS”) and communication services enabling basin-scale undersea navigation and management of floats, gliders, and AUVs. This will be especially valuable in polar regions with ice cover. Routine deployment of sources during repeat global-scale hydrographic ship surveys would provide high spatial coverage snapshots of ocean temperatures. To fully exploit the PAM systems, precise timing and positioning need to be broadly implemented. Ocean sound is now a mature Global Ocean Observing System (GOOS) “essential ocean variable”, which is one crucial step toward providing a fully integrated global multi-purpose ocean acoustic observing system.

The Scientific and Societal Uses of Global Measurements of Subsurface Velocity

Ocean velocity defines ocean circulation, yet the available observations of subsurface velocity are under-utilized by society. The first step to address these concerns is to improve visibility of and access to existing measurements, which include acoustic sampling from ships, subsurface float drifts, and measurements from autonomous vehicles. While multiple programs provide data publicly, the present difficulty in finding, understanding, and using these data hinder broader use by managers, the public, and other scientists. Creating links from centralized national archives to project specific websites is an easy but important way to improve data discoverability and access. A further step is to archive data in centralized databases, which increases usage by providing a common framework for disparate measurements. This requires consistent data standards and processing protocols for all types of velocity measurements. Central dissemination will also simplify the creation of derived products tailored to end user goals. Eventually, this common framework will aid managers and scientists in identifying regions that need more sampling and in identifying methods to fulfill those demands. Existing technologies are capable of improving spatial and temporal sampling, such as using ships of opportunity or from autonomous platforms like gliders, profiling floats, or Lagrangian floats. Future technological advances are needed to fill sampling gaps and increase data coverage.

Multi-objective optimization of the shell in autonomous intelligent argo profiling float

To improve the performance of the first-generation profiling float and ensure that the float smoothly completes the profile motion, a shell optimization method based on response surface approximation model and multi-objective genetic algorithm is proposed. The second-order nonlinear response surface models of resistance, mass and envelope volume of the shell are established based on the data from Design of Experiments (DOE). Taking the minimum resistance, the minimum mass and the maximum envelope volume of the shell as optimization objectives, the Pareto Front is obtained based on the fast elitist Non-dominated Sorting Genetic Algorithm II (NSGA-II). The influence rule and degree of each design variable on the optimization objective are gained through the main effect analysis. A set of solutions are selected as the design scheme of the shell for second-generation profiling float. The hydrodynamic performance, endurance and carrying capacity of the profiling float have been greatly improved compared with the first-generation. Finally, the feasibility and effectiveness of the optimization results are verified through simulation analysis and high pressure test. The proposed optimization process can greatly improve the performance of Argo profiling float, shorten the optimization period of shell design and improve the optimization efficiency. Therefore, it has a good reference value for the design optimization of other similar submarines. • To improve the hydrodynamic performance, endurance and carrying capacity of the first-generation Argo profiling float, a shell optimization method based on response surface approximation model and (NSGA-II) is proposed. The Pareto Front is obtained based on the NSGA-II. A set of solutions are selected as the design scheme of the shell for second-generation profiling float. • The feasibility and effectiveness of the optimization results are verified through hydrodynamic analysis by FLUENT and buckling analysis by ABAQUS. In order to prove the feasibility of the final optimization scheme and the reliability of the simulation analysis, 48 M P a high pressure test is carried out by the high pressure experimental device. • Through the analysis of the main effect plots, we can get the influence law and degree of each design variable on the optimization objective. In the design of the shell, the range of design variables can be determined according to the requirements of design indicators. • The proposed optimization process can greatly improve the performance of the second-generation Argo profiling float, shorten the optimization period of shell design and improve the optimization efficiency. Therefore, it has a good reference value for the design optimization of other similar submarines.

Evolution of the Seasonal Surface Mixed Layer of the Ross Sea, Antarctica, Observed With Autonomous Profiling Floats

AbstractOceanographic conditions on the continental shelf of the Ross Sea, Antarctica, affect sea ice production, Antarctic Bottom Water formation, mass loss from the Ross Ice Shelf, and ecosystems. Since ship access to the Ross Sea is restricted by sea ice in winter, most upper ocean measurements have been acquired in summer. We report the first multiyear time series of temperature and salinity throughout the water column, obtained with autonomous profiling floats. Seven Apex floats were deployed in 2013 on the midcontinental shelf, and six Air‐Launched Autonomous Micro Observer floats were deployed in late 2016, mostly near the ice shelf front. Between profiles, most floats were parked on the seabed to minimize lateral motion. Surface mixed layer temperatures, salinities, and depths, in winter were −1.8 °C, 34.34, and 250–500 m, respectively. Freshwater from sea ice melt in early December formed a shallow (20 m) surface mixed layer, which deepened to 50–80 m and usually warmed to above −0.5 °C by late January. Upper‐ocean freshening continued throughout the summer, especially in the eastern Ross Sea and along the ice shelf front. This freshening requires substantial lateral advection that is dominated by inflow from melting of sea ice and ice shelves in the Amundsen Sea and by inputs from the Ross Ice Shelf. Changes in upper‐ocean freshwater and heat content along the ice shelf front in summer affect cross‐ice front advection, ice shelf melting, and calving processes that determine the rate of mass loss from the grounded Antarctic Ice Sheet in this sector.

New Insights of the Sicily Channel and Southern Tyrrhenian Sea Variability

The dynamics of the Sicily Channel and the southern Tyrrhenian Sea are highly influenced by the seasonal variability of the Mediterranean basin-wide circulation, by the interannual variability of the numerous mesoscale structures present in the Channel, and by the decadal variability of the adjacent Ionian Sea. In the present study, all these aspects are investigated using in-situ (Lagrangian drifter trajectories and Argo float profiles) and satellite data (Absolute Dynamic Topography, Sea Level Anomaly, Sea Surface Temperature, wind products) over the period from 1993 to 2018. The availability of long time series of data and high-resolution multi-sensor surface currents allow us to add new details on the circulation features and on their driving mechanisms and to detect new permanent eddies not yet described in literature. The structures prevailing in winter are mainly driven by wind, whereas those prevailing in summer are regulated by topographical forcing on surface currents. The strength of the surface structures located at the western entrance of the Ionian Sea and of the mesoscale activity along the northern Sicily coast is modulated by the large-scale internal variability. The vertical hydrological characteristics of these mesoscale eddies are delineated using the Argo float profiles inside these structures.

Atlantic Meridional Overturning Circulation: Observed Transport and Variability

The Atlantic Meridional Overturning Circulation (AMOC) extends from the Southern Ocean to the northern North Atlantic, transporting heat northwards throughout the South and North Atlantic, and sinking carbon and nutrients into the deep ocean. Climate models indicate that changes to the AMOC both herald and drive climate shifts. Intensive trans-basin AMOC observational systems have been put in place to continuously monitor meridional volume transport variability, and in some cases, heat, freshwater and carbon transport. These observational programs have been used to diagnose the magnitude and origins of transport variability, and to investigate impacts of variability on essential climate variables such as sea surface temperature, ocean heat content and coastal sea level. AMOC observing approaches vary between the different systems, ranging from trans-basin arrays (OSNAP, RAPID 26°N, 11°S, SAMBA 34.5°S) to arrays concentrating on western boundaries (e.g., RAPID WAVE, MOVE 16°N). In this paper, we outline the different approaches (aims, strengths and limitations) and summarize the key results to date. We also discuss alternate approaches for capturing AMOC variability including direct estimates (e.g., using sea level, bottom pressure, and hydrography from autonomous profiling floats), indirect estimates applying budgetary approaches, state estimates or ocean reanalyses, and proxies. Based on the existing observations and their results, and the potential of new observational and formal synthesis approaches, we make suggestions as to how to evaluate a comprehensive, future-proof observational network of the AMOC to deepen our understanding of the AMOC and its role in global climate.

Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean

Hydrothermal activity is significant in regulating the dynamics of trace elements in the ocean. Biogeochemical models suggest that hydrothermal iron might play an important role in the iron-depleted Southern Ocean by enhancing the biological pump. However, the ability of this mechanism to affect large-scale biogeochemistry and the pathways by which hydrothermal iron reach the surface layer have not been observationally constrained. Here we present the first observational evidence of upwelled hydrothermally influenced deep waters stimulating massive phytoplankton blooms in the Southern Ocean. Captured by profiling floats, two blooms were observed in the vicinity of the Antarctic Circumpolar Current, downstream of active hydrothermal vents along the Southwest Indian Ridge. These hotspots of biological activity are supported by mixing of hydrothermally sourced iron stimulated by flow-topography interactions. Such findings reveal the important role of hydrothermal vents on surface biogeochemistry, potentially fueling local hotspot sinks for atmospheric CO2 by enhancing the biological pump.

Horizontal velocity structure of mesoscale eddies in the South China Sea

The horizontal velocity structure of mesoscale eddies is investigated using data from 18-year altimetry data in the South China Sea (SCS). A total of 2,433 cyclonic eddies (CEs) and 3,132 anticyclonic eddies (AEs) are detected in this data set. This analysis shows that the eddy velocity structure is strongly linked to the eddy amplitude, with the band of maximum velocity moving closer to the eddy core as the eddy amplitude increases. By combining the records of Argo float profiles and satellite altimetry data, the eddy feature found in sea level anomaly (SLA) data can also be seen at the subsurface. This feature is related to sub-mesoscale processes accompanying with water exchange and energy cascade at the outer edge of mesoscale eddy. A modified Taylor vortex model is built including this new finding to describe the eddy velocity structure. This vortex model is found to be useful as it better describes the eddy’s life history, the knowledge of which is important for better understanding physical, biological and chemical processes in the ocean.

Winter Upper-Ocean Stability and Ice–Ocean Feedbacks in the Sea Ice–Covered Southern Ocean

AbstractIn this study, under-ice ocean data from profiling floats, instrumented seals, and shipboard casts are used to assess wintertime upper-ocean stability and heat availability in the sea ice–covered Southern Ocean. This analysis reveals that the southern Weddell Sea, which features a weak upper-ocean stratification and relatively strong thermocline, is preconditioned for exceptionally high rates of winter ventilation. This preconditioning also facilitates a strong negative feedback to winter ice growth. Idealized experiments with a 1D ice–ocean model show that the entrainment of heat into the mixed layer of this region can maintain a near-constant ice thickness over much of winter. However, this quasi-equilibrium is attained when the pycnocline is thin and supports a large temperature gradient. We find that the surface stress imparted by a powerful storm may upset this balance and lead to substantial ice melt. This response can be greatly amplified when coincident with anomalous thermocline shoaling. In more strongly stratified regions, such as near the sea ice edge of the major gyres, winter ice growth is weakly limited by the entrainment of heat into the mixed layer. Thus, the thermodynamic coupling between winter sea ice growth and ocean ventilation has significant regional variability. This regionality will influence the response of the Southern Ocean ice–ocean system to future changes in ocean stratification and surface forcing.

The Dominant Global Modes of Recent Internal Sea Level Variability

AbstractThe advances in the modern sea level observing system have allowed for a new level of knowledge of regional and global sea level in recent years. The combination of data from satellite altimeters, Gravity Recovery and Climate Experiment (GRACE) satellites, and Argo profiling floats has provided a clearer picture of the different contributors to sea level change, leading to an improved understanding of how sea level has changed in the present and, by extension, may change in the future. As the overlap between these records has recently extended past a decade in length, it is worth examining the extent to which internal variability on timescales from intraseasonal to decadal can be separated from long‐term trends that may be expected to continue into the future. To do so, a combined modal decomposition based on cyclostationary empirical orthogonal functions is performed simultaneously on the three data sets, and the dominant shared modes of variability are analyzed. Modes associated with the trend, seasonal signal, El Niño–Southern Oscillation, and Pacific decadal oscillation are extracted and discussed, and the relationship between regional patterns of sea level change and their associated global signature is highlighted.