Oceanic Fronts Research Articles

Pacific oceanic front amplifies the impact of Atlantic oceanic front on North Atlantic blocking

Atmospheric blocking is a crucial driver of extreme weather events, but its climatological frequency is largely underestimated in state-of-the-art climate models, especially around the North Atlantic. While air-sea interaction along the North Atlantic oceanic frontal region is known to influence Atlantic blocking activity, remote effects from the Pacific have been less studied. Here we use semi-idealised experiments with an atmospheric general circulation model to demonstrate that the mid-latitude Pacific oceanic front is crucial for climatological Atlantic blocking activity. The front intensifies the Pacific eddy-driven jet that extends eastward towards the North Atlantic. The eastward-extended Pacific jet reinforces the North Atlantic circulation response to the Atlantic oceanic front, including the storm track activity and the eddy-driven jet. The strengthening of the eddy-driven jet reduces the Greenland blocking frequency. Moreover, the Pacific oceanic front greatly strengthens the stationary planetary-scale ridge in Europe. Together with a stronger northeastward extension of the Atlantic storm track, enhanced interaction between extratropical cyclones and the European ridge favours the occurrence of Euro-Atlantic blocking. Therefore, the North Atlantic circulation response amplified remotely by the Pacific oceanic front substantially increases Euro-Atlantic blocking frequency while decreasing Greenland blocking frequency.

An expanded database of Southern Hemisphere surface sediment dinoflagellate cyst assemblages and their oceanographic affinities

Abstract. Dinoflagellate cyst assemblages present a valuable proxy to infer paleoceanographic conditions, yet factors influencing geographic distributions of species remain largely unknown, especially in the Southern Ocean. Strong lateral transport, sea-ice dynamics, and a sparse and uneven geographic distribution of surface sediment samples have limited the use of dinocyst assemblages as a quantitative proxy for paleo-environmental conditions such as sea surface temperature (SST), nutrient concentrations, salinity, and sea ice (presence). In this study we present a new set of surface sediment samples (n=66) from around Antarctica, doubling the number of Antarctic-proximal samples to 100 (dataset wsi_100) and increasing the total number of Southern Hemisphere samples to 655 (dataset sh_655). Additionally, we use modelled ocean conditions and apply Lagrangian techniques to all Southern Hemisphere sample stations to quantify and evaluate the influence of lateral transport on the sinking trajectory of microplankton and, with that, to the inferred ocean conditions. k-means cluster analysis on the wsi_100 dataset demonstrates the strong affinity of Selenopemphix antarctica with sea-ice presence and of Islandinium spp. with low-salinity conditions. For the entire Southern Hemisphere, the k-means cluster analysis identifies nine clusters with a characteristic assemblage. In most clusters a single dinocyst species dominates the assemblage. These clusters correspond to well-defined oceanic conditions in specific Southern Ocean zones or along the ocean fronts. We find that, when lateral transport is predominantly zonal, the environmental parameters inferred from the sea floor assemblages mostly correspond to those of the overlying ocean surface. In this case, the transport factor can thus be neglected and will not represent a bias in the reconstructions. Yet, for some individual sites, e.g. deep-water sites or sites under strong-current regimes, lateral transport can play a large role. The results of our study further constrain environmental conditions represented by dinocyst assemblages and the location of Southern Ocean frontal systems.

Amplification of typhoon-generated near-inertial internal waves observed near the Tsushima oceanic front in the Sea of Japan

It is not fully understood how near-inertial kinetic energy (NIKE) is spatially distributed near Tsushima oceanic front (TOF) as a typhoon travels across the region. Underneath TOF, a year-round mooring covering a major part of water column was implemented in 2019. During summer, three massive typhoons (Krosa, Tapah, and Mitag) consecutively traversed the frontal area and delivered a substantial amount of NIKE into surface mixed layer. According to a mixed-layer slab model, NIKE was widely distributed near the cyclone’s track. The mooring observation exhibited the vertical distribution and pathways of surface-generated NIKE in response to the successive typhoon events. According to the modal decomposition, first three modes mostly explain the NIKE’s elevations following the typhoon events. According to ray-tracing experiments based on the internal-wave theory, large-scale near-inertial waves (NIWs) rapidly descend to a depth greater than 1000 m, while mesoscale NIWs slowly descend and rarely reached beyond the main pycnocline. Following the passage of Tapah, a profound energy mass was found nearly stationary at shallow depths coincident with vertical shear of geostrophic current. We infer that the descending rate of NIWs fell and then they were amplified through the energy conservation when the waves came from the north side of TOF.

External Dynamic Mechanisms Controlling the Periodic Offshore Blooms in Beibu Gulf

AbstractSince offshore waters are less affected by human activities and nutrient‐rich water masses, existing theories on periodic offshore blooms (POB) consider that the POB is proportional to the intensity of ocean fronts (nutrient supply from enhancing vertical mixing), ignoring external nutrient supply and external forcing (climatic oscillations). This study proposes an external dynamic mechanism of the POB on the basis of field observations and long‐term satellite remote sensing data (1981–2022) in Beibu Gulf, which is influenced by remarkable external forcing. A strong thermal front with an inverted‐V structure occurred in the central gulf every winter due to the strong wind stress. The intensity of the front on the east side is weaker due to the intrusion of the west‐Guangdong coastal current (WGCC). However, nutrient supply variation resulted in the POB's different intensity in different areas of the inverted‐V front. Chlorophyll a concentrations in the eastern (nutrients supplied by WGCC) and northern (nutrients supplied by vertical mixing) were obviously higher than that in the western front (limited nutrient supply) in winter. On an interannual scale, the intensity of POB in La Niña years is remarkably stronger than in El Niño years due to the stronger WGCC supplying more nutrients in La Niña. This study suggests that the intensity and range of POB are not proportional to the frontal intensity in the gulf, but are directly driven by the internal forcing (fronts and nutrient supply from WGCC), which is controlled by the external forcing.

An improved algorithm for detecting mesoscale ocean fronts from satellite observations: Detailed mapping of persistent fronts around the China Seas and their long-term trends

Mesoscale fronts are highly energetic ocean processes that dominate local biogeochemical processes and air-sea interactions and exert significant effects on ocean dynamics, ecology, and climate. An automatic front detection algorithm applied to satellite-based observations is a fundamental approach to acquiring frontal occurrence information. Histogram-based algorithms have been widely used in previous studies, and, despite being the most popular front detection algorithm, continuous improvements have been needed to address imperfect detection issues (undetected coastal fronts, discontinuities of fronts and repeated detection of fronts). In this study, a new algorithm is proposed to address the issues by coupling inverse distance weighting and mathematical morphology operators. Using 14,610 images acquired from 1982 to 2021, a series of comparisons between the new and existing algorithms have been implemented to validate the improvements. Statistical results suggest that the number of frontal pixels detected by our algorithm increased by 58.45% in the coastal domain when compared to previous methods. Statistical analysis also shows most of these increased fronts can be supported by a classic gradient-based method which is superior in detecting coastal fronts, which demonstrates the improvement of new algorithm in detecting coastal fronts. Furthermore, visual analyses suggest that our algorithm can better connect discontinuous fronts and reduce repeated detections, and statistical comparisons in the open seas present a 34.50% improvement in the average length of frontal segments. Moreover, based on an automated objective delineation method, an analysis of 40 years of seasonal frontal occurrences (as detected by our algorithm) produced a dataset that shows a detailed map of 59 persistent fronts around the China Seas. This map updates previously hand-delineated frontal positions and identifies some previously unreported persistent fronts. Further analyses show that the frontal occurrence or intensity of a majority of persistent fronts around the China Seas presents significant (P < 0.05) long-term variability over the last 40 years. The improved algorithm, together with its application in identifying persistent fronts around the China Seas, is expected to be extended to other waters to better investigate the response of marine ecosystems to front dynamics and predict the distribution of marine organisms within a changing climate.

MCSTNet: a memory-contextual spatiotemporal transfer network for prediction of SST sequences and fronts with remote sensing data

Ocean fronts are a response to the variabilities of marine hydrographic elements and are an important mesoscale ocean phenomenon, playing a significant role in fish farming and fishing, sea-air exchange, marine environmental protection, etc. The horizontal gradients of sea surface temperature (SST) are frequently applied to reveal ocean fronts. Up to now, existing spatiotemporal prediction approaches have suffered from low prediction precision and poor prediction quality for non-stationary data, particularly for long-term prediction. It is a challenging task for medium- and long-term fine-grained prediction for SST sequences and fronts in oceanographic research. In this study, SST sequences and fronts are predicted for future variation trends based on continuous mean daily remote sensing satellite of SST data. To enhance the precision of the predicted SST sequences and fronts, this paper proposes a novel memory-contextual spatiotemporal transfer network (MCSTNet) for SST sequence and front predictions. MCSTNet involves three components: the encoder-decoder structure, a time transfer module, and a memory-contextual module. The encoder-decoder structure is used to extract the rich contextual and semantic information in SST sequences and frontal structures from the SST data. The time transfer module is applied to transfer temporal information and fuse low-level, fine-grained temporal information with high-level semantic information to improve medium- and long-term prediction precision. And the memory-contextual module is employed to fuse low-level, spatiotemporal information with high-level semantic information to enhance short-term prediction precision. In the training process, mean squared error (MSE) loss and contextual loss are combined to jointly guide the training of MCSTNet. Extensive experiments demonstrate that MCSTNet predicts more authentic and reasonable SST sequences and fronts than the state-of-the-art (SOTA) models on the SST data.

Global High-Resolution Random Forest Regression Maps of Ocean Heat Content Anomalies Using In Situ and Satellite Data

Abstract The ocean, with its low albedo and vast thermal inertia, plays key roles in the climate system, including absorbing massive amounts of heat as atmospheric greenhouse gas concentrations rise. While the Argo array of profiling floats has vastly improved sampling of ocean temperature in the upper half of the global ocean volume since the mid-2000s, they are not sufficient in number to resolve eddy scales in the oceans. However, satellite sea surface temperature (SST) and sea surface height (SSH) measurements do resolve these scales. Here we use random forest regressions to map ocean heat content anomalies (OHCA) using in situ training data from Argo and other sources on a 7-day × 1/4° × 1/4° grid with latitude, longitude, time, SSH, and SST as predictors. The maps display substantial patterns on eddy scales, resolving variations of ocean currents and fronts. During the well-sampled Argo period, global integrals of these maps reduce noise relative to estimates based on objective mapping of in situ data alone by roughly a factor of 3 when compared to time series of CERES (satellite data) top-of-the-atmosphere energy flux measurements and improve correlations of anomalies with CERES on annual time scales. Prior to and early on in the Argo period, when in situ data were sparser, global integrals of these maps retain low variance, and do not relax back to a climatological mean, avoiding potential deficiencies of various methods for infilling data-sparse regions with objective maps by exploiting temporal and spatial patterns of OHCA and its correlations with SST and SSH. Significance Statement We use a simple machine learning technique to improve maps of subsurface ocean warming by exploiting the relationships between subsurface ocean temperature both sea surface temperature and sea level. Mapping ocean warming is important because it contributes to sea level rise through thermal expansion; impacts marine life through marine heatwaves and changes in mixing, oxygen, and carbon dioxide levels; increases energy available to tropical cyclones; and stores most of the energy building up in Earth’s climate system owing to the accumulation of anthropogenic greenhouse gases in the atmosphere. Our new estimates generally have lower noise energy and higher correlations than other products when compared with global energy fluxes at the top of the atmosphere measured by satellite.

The Long-term variability of Global Typical Oceanic Fronts

Using high-resolution NOAA Optimum Interpolation sea surface temperature (OISST) data, the temporal and spatial characteristics of typical oceanic fronts in the past 39 years, including Oyashio front, Gulf Stream front, Peru current front and southern Indian Ocean polar front, are analyzed. The results show that the intensities of the four typical fronts have all strengthened, among which the Peru current is the most significant and the South Indian Ocean polar front is the weakest. The movement of the fronts are quite different: the Oyashio front has a significant poleward movement, and the Gulf Stream front extending into the open sea part has a marginal poleward movement; while the south Indian Ocean polar front and Peru current front have no obvious movements.

Improvements of ocean fronts forecasting in the South China Sea by using ensemble optimal interpolation assimilation

Based on the operational ocean forecasting system in the South China Sea (SCS) runned by National Marine Environmental Forecasting Center (NMEFC), current Ensemble Optimal Assimilation (EnOI) system are updated by developing the dynamic coordination of multi-source data assimilation system, and has carried on the systematic evaluation. The diagnostic analysis of ocean fronts in the SCS is carried out using the daily average hindcast results in 2018, and the improvement effect of data assimilation on ocean mesoscale processes was evaluated. The statistical analysis results show that the occurrence frequency and diagnostic parameters of the ocean front after assimilation are closer to the observation. The comparison of the scores of the ocean front prediction skills before and after assimilation shows that the assimilated model has higher front prediction skills.

SQNet: Simple and Fast Model for Ocean Front Identification

The ocean front has a non-negligible role in global ocean–atmosphere interactions, marine fishery production, and the marine military. Hence, obtaining the positions of the ocean front is crucial in oceanic research. At present, the positioning method of recognizing an ocean front has achieved a breakthrough in the mean dice similarity coefficient (mDSC) of above 90%, but it is difficult to use to achieve rapid extraction in emergency scenarios, including marine fisheries and search and rescue. To reduce the its dependence on machines and apply it to more requirements, according to the characteristics of an ocean front, a multi-scale model SQNet (Simple and Quick Net) dedicated to ocean front position recognition is designed, and its perception domain is expanded while obtaining current scale data. In experiments along the coast of China and the waters of the Gulf of Mexico, it was not difficult to find that SQNet exceedingly reduced running time while ensuring high-precision results (mDSC of higher than 90%). Then, after conducting intra-model self-comparison, it was determined that expanding the perceptual domain and changing the weight ratio of the loss function could improve the accuracy and operational efficiency of the model, which could be better applied in ocean front recognition.

Role of physical processes in regulating the biological responses in the Eastern Arabian Sea during the late southwest monsoon

Understanding physical properties and associated biological responses in the marine ecosystem is critical for fisheries and assessing the ocean’s role in determining the fate of anthropogenic carbon dioxide emissions. This study reports the different roles of southwest monsoon (SWM) upwelling, oceanic fronts, and mesoscale eddies in supplying nutrients and fueling phytoplankton production in the eastern Arabian Sea (EAS). In-situ measurements of physical parameters along with the distribution of biogeochemical parameters in the 100 m water column over a wide latitudinal transect from 8°N to 21°N (Station II-1 to II-14 at 1° interval) along the 68°E were carried out during the late SWM (September 11–19) of 2017. We found distinct features in the study area from the in-situ measurements and satellite data. Primarily, oceanic fronts along with higher instabilities caused vertical mixing at stations II-6, and II-10 (13°N and 17°N). Secondly, eddy-induced lateral advection of high productive water from the Western Arabian Sea (WAS) resulted in high phytoplankton biomass at stations II-7, and II-8 (14°N and 15°N), which led to high concentrations of pigments (chlorophyll-a (Chl-a) and fucoxanthin). SWM upwelling induced higher phytoplankton biomass at northern stations II-13 and II-14 and is evident from the higher total organic carbon (TOC), high total bacterial count (TBC) and Chl-a concentration (up to 0.371μg/L at II-14). The difference in time lag for vertical mixing at II-6 and II-10 are responsible for the contrasting biological responses in these stations. The present study showed that along with SWM upwelling, eddy-induced lateral advection from WAS and oceanic fronts are also increasing phytoplankton biomass in the EAS. The present study suggests that the high phytoplankton biomass prompted by upwelling, oceanic fronts, and mesoscale eddies can influence CO2 sequestration and the prevailing subsurface oxygen minimum zone in the EAS.

Ocean Mesoscale and Frontal-Scale Ocean–Atmosphere Interactions and Influence on Large-Scale Climate: A Review

Abstract Two decades of high-resolution satellite observations and climate modeling studies have indicated strong ocean–atmosphere coupled feedback mediated by ocean mesoscale processes, including semipermanent and meandrous SST fronts, mesoscale eddies, and filaments. The air–sea exchanges in latent heat, sensible heat, momentum, and carbon dioxide associated with this so-called mesoscale air–sea interaction are robust near the major western boundary currents, Southern Ocean fronts, and equatorial and coastal upwelling zones, but they are also ubiquitous over the global oceans wherever ocean mesoscale processes are active. Current theories, informed by rapidly advancing observational and modeling capabilities, have established the importance of mesoscale and frontal-scale air–sea interaction processes for understanding large-scale ocean circulation, biogeochemistry, and weather and climate variability. However, numerous challenges remain to accurately diagnose, observe, and simulate mesoscale air–sea interaction to quantify its impacts on large-scale processes. This article provides a comprehensive review of key aspects pertinent to mesoscale air–sea interaction, synthesizes current understanding with remaining gaps and uncertainties, and provides recommendations on theoretical, observational, and modeling strategies for future air–sea interaction research. Significance Statement Recent high-resolution satellite observations and climate models have shown a significant impact of coupled ocean–atmosphere interactions mediated by small-scale (mesoscale) ocean processes, including ocean eddies and fronts, on Earth’s climate. Ocean mesoscale-induced spatial temperature and current variability modulate the air–sea exchanges in heat, momentum, and mass (e.g., gases such as water vapor and carbon dioxide), altering coupled boundary layer processes. Studies suggest that skillful simulations and predictions of ocean circulation, biogeochemistry, and weather events and climate variability depend on accurate representation of the eddy-mediated air–sea interaction. However, numerous challenges remain in accurately diagnosing, observing, and simulating mesoscale air–sea interaction to quantify its large-scale impacts. This article synthesizes the latest understanding of mesoscale air–sea interaction, identifies remaining gaps and uncertainties, and provides recommendations on strategies for future ocean–weather–climate research.

Improving the combined use of reflectance and thermal channels for ocean and coastal cloud detection for the Sea and Land Surface Temperature Radiometer (SLSTR)

Reflectance imagery is used to aid daytime cloud detection in thermal remote sensing. This paper presents new approaches to utilization of reflectance for sea surface temperature (SST) remote sensing for the Sea and Land Surface Temperature Radiometer (SLSTR). SLSTR is an along-track scanning sensor with a complex instrumental viewing geometry, and no co-registration of the fields-of-view of reflectance and thermal pixels. Reflectance channels have twice the spatial resolution of thermal channels, and observations are placed on compatible “image grids” for the convenience of users of Level-1 data. We highlight limitations of simple methods, based on these image grids, of using reflectance imagery to inform cloud detection at the thermal resolution. We present improvements from averaging the N-nearest reflectance observations directly to the infrared instrument geometry, where N = 10 is chosen in this study when using the A and B stripes together. We show that the standard deviation of the N-nearest reflectance observations is another calculable quantity of use to improve the discrimination of clouds in the infrared image, beneficially reducing the weight placed on coarser-scale thermal spatial variability. The developments are illustrated by case studies in coastal zones over optically bright waters and around strong ocean fronts, and the benefit for SST products is quantified by the impacts on coverage and validation statistics. In a case study over optically bright waters, the clear-sky fraction increases from 46.2% to 93.1%. Coastal zone validation shows a 23.8-31.6% reduction in the false alarm rate and a corresponding 27.1-33.3% increase in the cloud detection true skill score. Globally the robust standard deviation for clear-sky matches between SLSTR-A and drifting buoys reduces from 0.3 to 0.29 with a 6% reduction in data due to improved screening of scattered cloud.

Interannual variation in winter thermal front to the east of the Shandong Peninsula in the Yellow Sea

Oceanic fronts play an important role in ocean dynamics and ecology systems. The ear-shape front (ESF), first detected in the1980s, is a winter thermal front to the east of the Shandong Peninsula in the Yellow Sea. Using high-resolution satellite observation and ocean reanalysis data, we investigated the interannual variation in ESF and underlying dynamics during 2006–2020. The results indicate that ESF shows a strengthening trend of approximately 0.1–0.5 °C/100 km/year, accompanying the temperature on its eastern side. This was caused by the westward shift and enhancement of the Yellow Sea Warm Current (YSWC) through transporting more warm water to the eastern side of the ESF. Further analyses indicate that changes in the YSWC resulted from the decrease in wind stress divergence. Decreasing wind stress divergence induced increasing sea surface height (SSH) anomalies in the central Yellow Sea. The resultant positive SSH anomalies generated an anti-cyclonic gyre anomaly via geostrophic effects, which led to the westward shift and enhancement of the YSWC on the eastern side of the ESF.

Feasibility of ice sheet conservation using seabed anchored curtains.

Sea level rise is expected to be rapid and extremely damaging to coastal communities and infrastructure, with unavoidable losses and coastal protection costs in the tens of billions per year. Retreat of the Thwaites and Pine Island Glaciers is likely already in an unstable regime as their oceanic fronts are ablated by deep intruding layers of relatively warm seawater. Warm water can be blocked from reaching the grounding line by thin flexible buoyant curtains anchored to the seabed. The consequent reduction in ice shelf melting could result in increased ice sheet buttressing as the shelf makes contact with seabed highs. Flexible curtains are less costly than solid artificial barriers, more robust against iceberg collisions, and easier to repair or remove in the event of unforeseen side effects. We illustrate the technical viability of this approach by considering curtain design concepts that should withstand oceanographic forces, and feasible methods of installation. Suitable materials are commonly available. Installation of a seabed curtain in temperate ocean waters would be entirely within the capabilities of existing offshore and deep ocean construction techniques. Installing in polar waters presents severe challenges from icebergs, harsh weather, and brief working seasons, which can however, be overcome with present-day technology. An 80 km long curtain installed in 600 m deep waters on alluvial sediments could help stabilize Pine Island and Thwaites glaciers over the next few centuries at much lower cost ($40-80 billion + $1-2 billion/yr maintenance) than the global coastline protection (∼$40 billion/yr) needed due to their collapse.

Phytoplankton Patches at Oceanic Fronts Are Linked to Coastal Upwelling Pulses: Observations and Implications in the California Current System

AbstractLocally enhanced biological production and increased carbon export are persistent features at oceanic density fronts. Studies often assume biological properties are uniform along fronts or hypothesize that along‐ and across‐front gradients reflect physical‐biological processes occurring in the front. However, the residence times of waters in fronts are often shorter than biological response times. Thus, an alternate—often untested—hypothesis is that observed biological patchiness originates upstream of a front. To test these two hypotheses, we explore an eddy‐associated front in the California Current System sampled during two surveys, separated by 3 weeks. Patches of high phytoplankton biomass were found at the northern ends of both surveys, and phytoplankton biomass decreased along the front. While these patches occurred in similar locations, it was unclear whether the same patch was sampled twice, or whether the two patches were different. Using an advection‐reaction framework combined with field and satellite data, we found that variations in along‐front gradients in dissolved oxygen, particle biovolume, and salinity support the conclusion that the two phytoplankton patches were different. They were only coincidentally sampled in similar locations. Backward‐ and forward‐in‐time tracking of water parcels showed that these phytoplankton patches had distinct origins, associated with specific, strong coastal upwelling pulses upstream of the front. Phytoplankton grew in these recently upwelled waters as they advected into and along the frontal system. By considering both local and upstream physical‐biological forcings, this approach enables better characterizations of critical physical and biogeochemical processes that occur at fronts across spatial and temporal scales.

Characteristics of a Seasonal Front in the Southern Bay of Bengal: Dynamics, Mixing, and Water Mass Transport

Abstract The formation of a sharp oceanic front located south-southeast of Sri Lanka during the southwest monsoon is examined through in situ and remote observations and high-resolution model output. Remote sensing and model output reveal that the front extends approximately 200 km eastward from the southeast coast of Sri Lanka toward the southern Bay of Bengal (BoB). This annually occurring front is associated with the boundary between the southwest monsoon current with high-salinity water to the south, and a weak flow field comprised of relatively fresh BoB water to the north. The front contains a line of high chlorophyll extending from the coastal upwelling zone, often for several hundred kilometers. Elevated turbulent diffusivities ∼10−2 m2 s−1 along with large diapycnal fluxes of heat and salt were found within the front. The formation of the front and vertical transports are linked to local wind stress curl. Large vertical velocities (∼50 m day−1) indicate the importance of ageostrophic, submesoscale processes. To examine these processes, the Ertel potential vorticity (PV) was computed using the observations and numerical model output. The model output shows a ribbon of negative PV along the front between the coastal upwelling zone and two eddies (Sri Lanka Dome and an anticyclonic eddy) typically found in the southern BoB. PV estimates support the view that the flow is susceptible to submesoscale instabilities, which in turn generate high vertical velocities within the front. Frontal upwelling and heightened mixing show that the seasonal front is regionally important to linking the fresh surface water of the BoB with the Arabian Sea. Significance Statement Within the ocean, motions span extraordinarily wide ranges of sizes and time scales. In this study we focus on a narrow, intensified feature called a front. This front occurs in the southern Bay of Bengal during the summer monsoon and forms a boundary between fresher water to the north and saltier water to the south. Features such as this are difficult to study, however, by combining observations made from ships and satellites with output from numerical models of the ocean, we are able to better understand the front. This is important because fronts like the one studied here play a role in determining the pathways of heat within the ocean, which, in turn, may feedback into the atmosphere and weather patterns.

Biophysical Dynamics at Ocean Fronts Revealed by Bio‐Argo Floats

AbstractThe straining regions of the ocean in between mesoscale eddies contain large vertical velocities that may be important in regulating phytoplankton accumulation rates. We analyze time series of variables measured by ocean Bio‐Argo floats (mixed layer depths [MLDs], chlorophyll, and carbon concentrations) in conjunction with variables derived from satellite altimetry (strain rates, Lyapunov exponents, vertical velocities) to determine the evolution of mixed layer phytoplankton biomass in response to straining by the mesoscale geostrophic flow. A Lagrangian (water parcel following) framework is justified by restricting the analysis to profiles whose value of a Quasi‐Planktonic Index—an index quantifying averaged distance between a float trajectory and a surface geostrophic trajectory over three consecutive time steps—is less than 5 km. Bin‐averaged Lagrangian derivatives of phytoplankton biomass and chlorophyll concentration are positive for elevated strain rate and upwelling quasigeostrophic vertical velocities. Lagrangian derivatives of MLD and phytoplankton carbon averaged at straining fronts (in rotated along‐ and across‐front coordinates) have features in common with submesoscale dynamics, including increasing phytoplankton carbon (and chlorophyll) and a shoaling mixed layer over the front. To elucidate a mechanism, we average time derivatives of modeled cell division rates, finding the pattern approximately matches the pattern of phytoplankton accumulation rates and is controlled primarily by the term modulating light stress, suggesting that frontal dynamics cause accelerations of division rates by increasing available light. Regions of increasing chlorophyll are also approximately co‐located with upwelling quasigeostrophic velocity, suggesting non‐Lagrangian behavior of floats causes some imprint of larger scale, more persistent mesoscale signals.

Long-term stability in the circumpolar foraging range of a Southern Ocean predator between the eras of whaling and rapid climate change

Assessing environmental changes in Southern Ocean ecosystems is difficult due to its remoteness and data sparsity. Monitoring marine predators that respond rapidly to environmental variation may enable us to track anthropogenic effects on ecosystems. Yet, many long-term datasets of marine predators are incomplete because they are spatially constrained and/or track ecosystems already modified by industrial fishing and whaling in the latter half of the 20th century. Here, we assess the contemporary offshore distribution of a wide-ranging marine predator, the southern right whale (SRW, Eubalaena australis), that forages on copepods and krill from ~30°S to the Antarctic ice edge (>60°S). We analyzed carbon and nitrogen isotope values of 1,002 skin samples from six genetically distinct SRW populations using a customized assignment approach that accounts for temporal and spatial variation in the Southern Ocean phytoplankton isoscape. Over the past three decades, SRWs increased their use of mid-latitude foraging grounds in the south Atlantic and southwest (SW) Indian oceans in the late austral summer and autumn and slightly increased their use of high-latitude (>60°S) foraging grounds in the SW Pacific, coincident with observed changes in prey distribution and abundance on a circumpolar scale. Comparing foraging assignments with whaling records since the 18th century showed remarkable stability in use of mid-latitude foraging areas. We attribute this consistency across four centuries to the physical stability of ocean fronts and resulting productivity in mid-latitude ecosystems of the Southern Ocean compared with polar regions that may be more influenced by recent climate change.

Thermodynamic characteristics of marine atmospheric boundary layer across frontal regions of the Indian Ocean Sector of the Southern Ocean based on three field campaigns

The Southern Ocean (SO) marine atmospheric boundary layer (MABL), regulated by continuous air-sea exchanges, plays a key role in transporting exchanges between tropics and poles. Among SO sectors, the Indian Ocean sector of SO (ISSO) remained least explored in terms of MABL characterization and is examined in this study. In ISSO, occurrence of sharp oceanic thermohaline fronts regulates the vertical thermodynamic structure of MABL, clouds, and inversions. In this study, these properties of ISSO MABL are investigated over three oceanic domains (Sub-Tropical Indian Ocean (STIO), ISSO, and High-Latitude SO (HLSO)). To achieve this, near-sea surface air-sea exchanges along with profiles of meteorological parameters generated between 25°S and 68°S and 57°E to78°E during three field campaigns conducted in the austral summers of 2017, 2018, and 2020 were utilized. Results showed strong SST-Tair variability across the study region. In STIO, positive SST-Tair indicated Low-Level Cold Air Advection (LLCAA)-induced destabilized and coupled MABLs capped by multiple inversions (>3INV's of strength∼0.35 K m−1 above∼1200 m) and thin mid/high-altitudes clouds (cloud-base∼830 m, cloud-top∼2309 m, and cloud-thickness∼758 m). Over ISSO and northern HLSO, weak and negative SST-Tair indicated Low-Level Warm Air Advection (LLWAA)-induced stratified and decoupled MABLs. Aided by advective mixing of multiple air-masses, low-level thick multilayered clouds (>2 layers, average cloud-base∼604 m, cloud-top∼2288 m, and cloud-thickness∼1314 m) and multiple strong high-level inversions (>2INV's, strength>0.4 K m−1,∼1836 m) was observed. In HLSO, weakly positive SST-Tair indicated LLCAA-induced weakly destabilized MABLs capped by mid-altitude inversions (strength∼0.22 K m−1,∼1632 m) and mid-altitude clouds (cloud-base∼979 m, cloud-top∼2465 m, and cloud-thickness∼1263 m) supported by sublimation leading to virga conditions.