Topographic influence on submesoscale dynamics in the Southern Ocean

Topography influences the circulation in the Southern Ocean, generating stationary meanders in the lee of topographic features, triggering hot spots of mesoscale eddy kinetic energy, and modifying jets and fronts. However, the relationship between topography and submesoscale flows (with length scales of order 1-10 km) has not yet been explored. The first submesoscale-resolving (1/80 resolution) ocean model, with realistic topography of the Indian sector of the Southern Ocean, is used to investigate this interaction. The results show spatial inhomogeneity in submesoscale activity that is correlated with topography. Topographic influence is primarily indirect: topography controls mesoscale flows, which in turn generate submesoscale activity. Mesoscale eddy kinetic energy and strain rate can be used, to first order, to infer submesoscale vertical velocity, implying a possible route to parameterise submesoscale activity in coarser resolution models

Quantifying the influence of sub-mesoscale dynamics on the supply of iron to Southern Ocean phytoplankton blooms

Southern Ocean phytoplankton growth is limited by iron. Episodes of natural iron fertilisation are pivotalto triggering phytoplankton blooms in this region, the Kerguelen Plateau bloom being one prominentexample. Numerous physical mechanisms that may supply iron to the euphotic zone in the KerguelenPlateau region, and hence trigger a phytoplankton bloom, have been identified. However, the impact ofsub-mesoscaleflows in delivering iron has been omitted. With a scale of order 10 km, sub-mesoscalefilaments and fronts can dramatically increase vertical velocities and iron transport.An innovative technique is developed to investigate the role of vertical advection associated with sub-mesoscale features on the supply of iron to the photic zone. First, Lagrangian trajectories are calculatedusing three dimensional velocityfields from high resolution numerical simulations; iron concentration isthen computed along these Lagrangian trajectories. The contribution of mesoscale- (1/20°resolution)and sub-mesoscale-resolving models (1/80°resolution) is compared, thereby revealing the sensitivity ofiron supply to horizontal resolution. Ironfluxes are clearly enhanced by a factor of 2 with the resolution,thus showing that the vertical motion induced by the sub-mesoscales represents a previously neglectedprocess to drive iron into the photic waters of the Kerguelen Plateau.A. Hogg was supported by Australian Research Council Future Fellowship FT120100842. We want to express our thanks to A. Bowie for constructive discussions

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

Abstract: The 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

Unsupervised classification identifies coherent thermohaline structures in the Weddell Gyre region

The Weddell Gyre is a major feature of the Southern Ocean and an important component of the planetary climate system; it regulates air–sea exchanges, controls the formation of deep and bottom waters, and hosts upwelling of relatively warm subsurface waters. It is characterised by low sea surface temperatures, ubiquitous sea ice formation, and widespread salt stratification that stabilises the water column. Observing the Weddell Gyre is challenging, as it is extremely remote and largely covered with sea ice. At present, it is one of the most poorly sampled regions of the global ocean, highlighting the need to extract as much value as possible from existing observations. Here, we apply a profile classification model (PCM), which is an unsupervised classification technique, to a Weddell Gyre profile dataset to identify coherent regimes in temperature and salinity. We find that, despite not being given any positional information, the PCM identifies four spatially coherent thermohaline domains that can be described as follows: (1) a circumpolar class, (2) a transition region between the circumpolar waters and the Weddell Gyre, (3) a gyre edge class with northern and southern branches, and (4) a gyre core class. PCM highlights, in an objective and interpretable way, both expected and underappreciated structures in the Weddell Gyre dataset. For instance, PCM identifies the inflow of Circumpolar Deep Water (CDW) across the eastern boundary, the presence of the Weddell–Scotia Confluence waters, and structured spatial variability in mixing between Winter Water and CDW. PCM offers a useful complement to existing expertise-driven approaches for characterising the physical configuration and variability of oceanographic regions, helping to identify coherent thermohaline structures and the boundaries between them

Variability in the meridional overturning circulation at 32°S in the Pacific Ocean diagnosed by inverse box models

The meridional circulation and transport at 32°S in the Pacific Ocean in 1992 and 2017 are compared with analogous data from 2003 and 2009 computed by Hernández-Guerra and Talley (2016). The hydrographic data come from the GO-SHIP database and an inverse box model has been applied with similar constraints as in Hernández-Guerra and Talley (2016). In 1992, 2003 and 2017 the pattern of the overturning streamfunction and circulation are similar, but in 2009 the pattern of the circulation changes in the whole water column. The horizontal distribution of mass transports at all depths in 1992 and 2017 resembles the familiar shape of the “classical gyre” also observed in 2003 and is notably different to the “bowed gyre” found in 2009. The hydrographic data have been compared with data obtained from the numerical modelling outputs of ECCO, SOSE, GLORYS, and MOM. Results show that none of these models properly represents the “bowed gyre” circulation in 2009, and this change in circulation pattern was not observed during the entire length of model simulations. Additionally, the East Australian Current in the western boundary presents higher mass transport in the hydrographic data than in any numerical modelling output. Its poleward mass transport ranges from −35.1 ± 2.0 Sv in 1992 to −54.3 ± 2.6 Sv in 2003. Conversely, the Peru-Chile Current is well represented in models and presents an equatorward mass transport from 2.3 ± 0.8 Sv in 2009 to 4.4 ± 1.0 Sv in 1992. Furthermore, the Peru-Chile Undercurrent presents a more intense poleward mass transport in 2009 (−3.8 ± 1.2 Sv). In addition, the temperature and freshwater transports in 1992 (0.42 ± 0.12 PW and 0.26 ± 0.08 Sv), 2003 (0.38 ± 0.12 PW and 0.25 ± 0.02 Sv), and 2017 (0.42 ± 0.12 PW and 0.34 ± 0.08 Sv) are similar, but significantly different from those in 2009 (0.16 ± 0.12 PW and 0.50 ± 0.03 Sv, respectively). To clarify the causes of these different circulation schemes, a linear Rossby wave model is adopted, which includes the wind-stress curl variability as remote forcing and the response to sea surface height changes along 30°S.This study was supported by the SAGA project (RTI2018-100844-B-C31) funded by the Ministerio de Ciencia, Innovación y Universidades of the Spanish Government. This article is a publication of the Unidad Océano y Clima from Universidad de Las Palmas de Gran Canaria, an R&D&I CSIC-associate unit. The wind data were collected from NCEP Reanalysis Derived data (http://www.eslr.noaa.gov/psd/). Hydrographic data were collected from the CCHDO website in the frame of International WOCE and GO-SHIP projects (https://cchdo.ucsd.edu/). We gratefully acknowledge the major efforts of the WOCE/GO-SHIP program’s chief scientists that collected these transect data: H. L. Bryden, M. McCartney, J. Toole M. Fukasawa, S. Watanabe, Y. Yoshikawa, A. Macdonald, R. Curry, S. Mecking, and K. Speer. ECCO data are available for download at https://ecco.jpl.nasa.gov/. MOM data are available at https://www.gfdl.noaa.gov/mom-ocean-model/. SOSE data are available at http://sose.ucsd.edu. GLORYS data are available for download at https://resources.marine.copernicus.eu/. The SSHA data were collected from the Aviso database (http://las.aviso.oceanobs.com). The authors declare no competing interests. This work has been completed as part of C. Arumí-Planas work at IOCAG, in the doctoral program in Oceanography and Global Change. C. Arumí-Planas acknowledges the Agencia Canaria de Investigación, Innovación y Sociedad de la Información (ACIISI) grant program of “Apoyo al personal investigador en formación” TESIS2021010028.Peer reviewe

Global disparities in surgeons’ workloads, academic engagement and rest periods: the on-calL shIft fOr geNEral SurgeonS (LIONESS) study

: The workload of general surgeons is multifaceted, encompassing not only surgical procedures but also a myriad of other responsibilities. From April to May 2023, we conducted a CHERRIES-compliant internet-based survey analyzing clinical practice, academic engagement, and post-on-call rest. The questionnaire featured six sections with 35 questions. Statistical analysis used Chi-square tests, ANOVA, and logistic regression (SPSS® v. 28). The survey received a total of 1.046 responses (65.4%). Over 78.0% of responders came from Europe, 65.1% came from a general surgery unit; 92.8% of European and 87.5% of North American respondents were involved in research, compared to 71.7% in Africa. Europe led in publishing research studies (6.6 ± 8.6 yearly). Teaching involvement was high in North America (100%) and Africa (91.7%). Surgeons reported an average of 6.7 ± 4.9 on-call shifts per month, with European and North American surgeons experiencing 6.5 ± 4.9 and 7.8 ± 4.1 on-calls monthly, respectively. African surgeons had the highest on-call frequency (8.7 ± 6.1). Post-on-call, only 35.1% of respondents received a day off. Europeans were most likely (40%) to have a day off, while African surgeons were least likely (6.7%). On the adjusted multivariable analysis HDI (Human Development Index) (aOR 1.993) hospital capacity > 400 beds (aOR 2.423), working in a specialty surgery unit (aOR 2.087), and making the on-call in-house (aOR 5.446), significantly predicted the likelihood of having a day off after an on-call shift. Our study revealed critical insights into the disparities in workload, access to research, and professional opportunities for surgeons across different continents, underscored by the HDI

Sub-mesoscale dynamics in The Southern Ocean

The Southern Ocean circulation is dominated by the Antarctic Circumpolar Current (ACC), a quasi-zonal current that encircles Antarctica. Typical features of the ACC are an energetic eddy field and jets that influence both the large scale flow and heat and carbon fluxes and, consequently, impact the climate system. Due to the strong zonal flow and weak stratification of the Southern Ocean, topography steers and influences the ACC. For example, Rossby waves or stationary meanders can be found in the lee of topographic features and the structure of jets and fronts can be modified by topography. ACC dynamics are very complex and understanding these dynamics is crucial, given the Southern Ocean role in the global climate system. The Southern Ocean is an environment where, despite a large nutrient availability, the biological productivity is very low. This biological activity is limited by light irradiance and iron availability. However, there exist several locations in the Southern Ocean where, due to a natural iron fertilisation, phytoplankton blooms can be observed. One such location is the Kerguelen Plateau (KP) region in the south Indian Ocean. Numerous physical mechanisms that drive iron into the euphotic zone of KP waters have been identified. However, in these studies sub-mesoscale dynamics, occurring at horizontal scales of several kilometers, have never been included and their contribution to the iron supply never estimated. These structures have been seen to dramatically trigger an ecosystem response in other parts of the ocean, suggesting that they might represent a significant contribution to Southern Ocean blooms. This thesis is focused on the development and analysis of the first sub-mesoscale-resolving (1/80 resolution) ocean model of the KP area. Resolving sub-mesoscale structures results in an enhancement of vertical velocities and transport, compared to mesoscale-resolving simulations (1/20). Results show that sub-mesoscale fields, such as eddy kinetic energy or vertical velocities, are spatially inhomogeneous. Evidence is presented that this inhomogeneity is strongly related to the topographic features of this region. In particular, it is in part due to internal waves excited by the interaction of the large-scale flow with topography and largely due to an indirect generation by the topography: topography controls mesoscale flows, which in turn generate sub-mesoscale activity. The correlation between mesoscale eddy kinetic energy and strain rate fields with sub-mesoscale vertical velocities suggests a possible new route to parameterise sub-mesoscales in coarser resolution models. The modelled velocity field is used to advect Lagrangian particles. The 1/80 resolution experiments are compared to the 1/20 case, finding that waters reach greater depths at the highest resolution. Built on these Lagrangian experiments is the development of an innovative technique for the study of iron supply, used to contrast the contribution of mesoscales and sub-mesoscales. This technique highlights the sensitivity of iron supply to the horizontal resolution, showing a clear enhancement of iron fluxes (by a factor of 2) at higher resolution. Thus, the vertical motion induced by the sub-mesoscales represents a new process to drive iron into the euphotic waters of the KP region