Dr Ryan Cloete, Dr Jean C. Loock, Van Horsten N., Dr Jan-Lukas Menzel Barraqueta, Fietz S., Mtshali T., Dr Hélène Planquette, Dr M. I. Garcia-Ibanez, Roychoudhury A. N.
Abstract

First winter measurements of dissolved zinc (dZn) and particulate zinc (pZn) are presented from seven stations, between 41 and 58°S, occupied in July 2017 along the 30°E longitude in the Indian Sector of the Southern Ocean. This unique spatial and seasonal dataset provided the opportunity to investigate Zn biogeochemical cycling in a region which is extremely data scarce and during a period when conditions are unfavourable for phytoplankton growth. Surface comparisons of our winter dZn and pZn to previous measurements during spring and summer revealed that Zn seasonality is most pronounced at the higher latitudes where higher dZn (and higher ratios of dZn to phosphate; dZn:PO4) and lower pZn in winter reflect decreased biological uptake and preferential dZn resupply (relative to PO4) to surface waters through deep winter mixing. The composition of pZn was majorly biogenic however localised lithogenic inputs were attributed to potential hydrothermal activity and transport of continental sediment via Agulhas waters. Calculated vertical attenuation factors (b values) for pZn (0.31) and phosphorus (P; 0.41) suggest that Zn has a longer remineralisation length scale than P, providing a mechanism as to why dZn appears to be remineralised deeper in the water column than PO4. Ratios of pZn to P (pZn:P) in surface waters increased with latitude from 1.12 to 8.28 mmol mol−1 due to increased dZn availability and the dominance of diatoms (with high cellular Zn quotas) in the high latitude Antarctic Zone (AAZ). Interestingly, the high surface pZn:P ratios in the AAZ did not change significantly with depth (in contrast to the northern stations where pZn:P increased with depth) suggesting the export of diatom cells below the winter mixed layer where remineralisation and rigorous mixing may resolve the linear dZn to silicic acid (dZn:Si(OH)4) correlation (dZn (nmol kg−1) = 0.064 Si(OH)4 (μmol kg−1) + 0.690; r2 = 0.93; n = 120) despite these elements being located in separate components of the diatom cell. Additionally, elevated concentrations of dZn and Si(OH)4 below 3000 m in the AAZ may reflect nutrient accumulation in bottom waters where northward flow is inhibited by the Indian mid-Ocean ridge.

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Surface water (~25 m) dZn, pZn, pZn:P, dFe and Si(OH)4 across the transect.

Dr Ryan Cloete, Dr Jean C. Loock, Van Horsten N., Fietz S., Mtshali T., Dr Hélène Planquette, Roychoudhury A. N.
Abstract

Winter distributions of dissolved cadmium (dCd) and particulate cadmium (pCd) were measured for the first time in the Indian sector of the Southern Ocean thereby contributing a unique spatial and seasonal dataset. Seven depth profiles, between 41°S and 58°S, were collected along the 30°E longitude during the 2017 austral winter to investigate the biogeochemical cycling of cadmium during a period characterized by contrasting upper water column dynamics compared to summer. Our results support an important role for biological uptake during winter months albeit weaker compared to summer. Distinct, biologically driven changes in cadmium cycling across the transect were observed. For example, surface ratios of pCd to phosphorus (P; pCd:P) increased from 0.37 to 1.07 mmol mol–1 between the subtropical zone (STZ) and the Antarctic zone (AAZ) reflecting increased Cd requirements for diatoms at higher latitudes which, in turn, was driven by a complex relationship between the availability of dCd and dissolved iron (dFe), zinc (dZn) and manganese (dMn). Vertical profiles of pCd:P displayed near-surface maxima consistent with (1) P occurring in two phases with different labilities and the lability of Cd being somewhere in-between and (2) increasing dCd to phosphate (PO4; dCd:PO4) ratios with depth at each station. North of the Antarctic Polar Front (APF), a secondary, deeper pCd:P maximum may reflect an advective signal associated with northward subducting Antarctic Intermediate Water (AAIW). The strong southward increase in surface dCd and dCd:PO4, from approximately 10–700 pmol kg–1 and 40–400 μmol mol–1, respectively, reflected the net effect of preferential uptake and regeneration of diatoms with high Cd content and the upwelling of Cd enriched water masses in the AAZ. Furthermore, distinct dCd versus PO4 relationships were observed in each of the intermediate and deep water masses suggesting that dCd and PO4 distributions at depth are largely the result of physical water mass mixing.

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Section plots of (A) dCd, (B) PO4 overlain by salinity, (C) pCd and (D) P. Each figure is separated into upper 1,000 m (upper panel) and below 1,000 m (bottom panel). Frontal positions, corresponding zones and water masses are shown.

Dr Charlotte M. Robinson, Dr Yannick Huot, Dr Nina Schuback, Ryan-Keogh T J, Thomalla S.J., Prof David Antoine
Abstract

Studying the biogeochemistry of the Southern Ocean using remote sensing relies on accurate interpretation of ocean colour through bio-optical and biogeochemical relationships between quantities and properties of interest. During the Antarctic Circumnavigation Expedition of the 2016/2017Austral Summer, we collected a spatially comprehensive dataset of phytoplankton pigment concentrations, particulate absorption and particle size distribution and compared simple bio-optical and particle property relationships as a function of chlorophyll a. Similar to previous studies we find that the chlorophyll-specific phytoplankton absorption coefficient is significantly lower than in other oceans at comparable chlorophyll concentrations. This appears to be driven in part by lower concentrations of accessory pigments per unit chlorophyll a as well as increased pigment packaging due to relatively larger sized phytoplankton at low chlorophyll a than is typically observed in other oceans. We find that the contribution of microphytoplankton (>20 µm size) to chlorophyll a estimates of phytoplankton biomass is significantly higher than expected for the given chlorophyll a concentration, especially in higher latitudes south of the Southern Antarctic Circumpolar Current Front. Phytoplankton pigments are more packaged in larger cells, which resulted in a flattening of phytoplankton spectra as measured in these samples when compared to other ocean regions with similar chlorophyll a concentration. Additionally, we find that at high latitude locations in the Southern Ocean, pheopigment concentrations can exceed mono-vinyl chlorophyll a concentrations. Finally, we observed very different relationships between particle volume and chlorophyll a concentrations in high and low latitude Southern Ocean waters, driven by differences in phytoplankton community composition and acclimation to environmental conditions and varying contribution of non-algal particles to the particulate matter. Our data confirm that, as previously suggested, the relationships between bio-optical properties and chlorophyll a in the Southern Ocean are different to other oceans. In addition, distinct bio-optical properties were evident between high and low latitude regions of the Southern Ocean basin. Here we provide a region-specific set of power law functions describing the phytoplankton absorption spectrum as a function of chlorophyll a.

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Map of underway sampling points during ACE legs 1-3, colourmapped with [Tchla]
(sum of monovinyl-chlorophyll a, divinyl-chlorophyll a, chlorophyll a epimers, chlorophyll
a isomers and chlorophyllide a concentrations). Grey points indicate location of samples
extracted from the NOMAD dataset. Red points indicate island locations. Locations of
the major fronts along the track identified in situ data are marked with ’X’ markers.
Dashed lines in background are the climatological positions of the major Southern Ocean fronts from Orsi et al. [33]. STF = Subtropical Front, SAF = Subantarctic Front, PF = Polar Front, SACCF = Southern Antarctic Circumpolar Current Front.

Dr Nina Schuback, Prof. Phillipe D. Tortell, Others, Ryan-Keogh T J, Thomalla S.J., et al.
Abstract

Phytoplankton photosynthetic physiology can be investigated through single-turnover variable chlorophyll fluorescence (ST-ChlF) approaches, which carry unique potential to autonomously collect data at high spatial and temporal resolution. Over the past decades, significant progress has been made in the development and application of ST-ChlF methods in aquatic ecosystems, and in the interpretation of the resulting observations. At the same time, however, an increasing number of sensor types, sampling protocols, and data processing algorithms have created confusion and uncertainty among potential users, with a growing divergence of practice among different research groups. In this review, we assist the existing and upcoming user community by providing an overview of current approaches and consensus recommendations for the use of ST-ChlF measurements to examine in- situ phytoplankton productivity and photo-physiology. We argue that a consistency of practice and adherence to basic operational and quality control standards is critical to ensuring data inter-comparability. Large datasets of inter-comparable and globally coherent ST-ChlF observations hold the potential to reveal large-scale patterns and trends in phytoplankton photo-physiology, photosynthetic rates and bottom-up controls on primary productivity. As such, they hold great potential to provide invaluable physiological observations on the scales relevant for the development and validation of ecosystem models and remote sensing algorithms.

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The three energy pathway concept and ChlF transients from typical single turnover (ST) protocols.

Dr Hanna M. Kauko, Dr Tore Hattermann, Ryan-Keogh T J, Singh A., Dr Laura de Steur, Dr Agneta Frannson, Dr Melissa Chierici, Dr Tone Falkenburg, Dr Elvar H. Hallfredsson, Dr Gunnar Bratbak, Dr Tatiana Tsagaraki, Dr Terje Berge
Abstract

Knowing the magnitude and timing of pelagic primary production is important for ecosystem and carbon sequestration studies, in addition to providing basic understanding of phytoplankton functioning. In this study we use data from an ecosystem cruise to Kong Håkon VII Hav, in the Atlantic sector of the Southern Ocean, in March 2019 and more than two decades of satellite-derived ocean color to study phytoplankton bloom phenology. During the cruise we observed phytoplankton blooms in different bloom phases. By correlating bloom phenology indices (i.e., bloom initiation and end) based on satellite remote sensing to the timing of changes in environmental conditions (i.e., sea ice, light, and mixed layer depth) we studied the environmental factors that seemingly drive phytoplankton blooms in the area. Our results show that blooms mainly take place in January and February, consistent with previous studies that include the area. Sea ice retreat controls the bloom initiation in particular along the coast and the western part of the study area, whereas bloom end is not primarily connected to sea ice advance. Light availability in general is not appearing to control the bloom termination, neither is nutrient availability based on the autumn cruise where we observed non-depleted macronutrient reservoirs in the surface. Instead, we surmise that zooplankton grazing plays a potentially large role to end the bloom, and thus controls its duration. The spatial correlation of the highest bloom magnitude with marked topographic features indicate that the interaction of ocean currents with sea floor topography enhances primary productivity in this area, probably by natural fertilization. Based on the bloom timing and magnitude patterns, we identified five different bloom regimes in the area. A more detailed understanding of the region will help to highlight areas with the highest relevance for the carbon cycle, the marine ecosystem and spatial management. With this gained understanding of bloom phenology, it will also be possible to study potential shifts in bloom timing and associated trophic mismatch caused by environmental changes.

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(A) Chl a concentration during the cruise, as obtained from calibrated in situ fluorescence profiles and underway fluorescence measurements. St. 53, station 53; 6◦E tr., 6◦E transect. (B) Mean Chl a profiles of Astrid Ridge and Maud Rise stations and the profile from station 53. The magenta star shows the MLD (mean for Astrid Ridge and Maud Rise).

Ryan-Keogh T J, Dr Charlotte M. Robinson
Abstract

The uptake and application of single turnover chlorophyll fluorometers to the study of phytoplankton ecosystem status and microbial functions has grown considerably in the last two decades. However, standardization of measurement protocols, processing of fluorescence transients and quality control of derived photosynthetic parameters is still lacking and makes community goals of large global databases of high-quality data unrealistic. We introduce the Python package Phytoplankton Photophysiology Utilities (PPU), an adaptable and open-source interface between Fast Repetition Rate and Fluorescence Induction and Relaxation instruments and python. The PPU package includes a variety of functions for the loading, processing and quality control of single turnover fluorescence transients from many commercially available instruments. PPU provides the user with greater flexibility in the application of the Kolber-Prasil- Falkowski model; tools for plotting, quality control, correcting instrument biases and high-throughput processing with ease; and a greater appreciation for the uncertainties in derived photosynthetic parameters. Using data from three research cruises across different biogeochemical regimes, we provide example applications of PPU to fit raw active chlorophyll-a fluorescence data from three commercial instruments and demonstrate tools which help to reduce uncertainties in the final fitted parameters.
Keywords:

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Schematic overview of the functions available within Phytoplankton Photophysiology Utilities.

Ryan-Keogh T J, Walker O. Smith Jr.
Abstract

The Ross Sea is one of the most productive regions in the Southern Ocean, with a significant role in carbon cycling as well as the massive abundance of higher trophic levels. The seasonal cycle is well established with an early summer Phaeocystis antarctica bloom that declines followed by a diatom bloom in late summer. This seasonal progression of the phytoplankton has been linked to the availability of iron in the mixed layer. Investigating the temporal progression of iron limitation is often limited by both the decreased sampling resolution from traditional platforms, such as ships, and the lack of regular deployments of specific sensors that can measure phytoplankton physiology. Through the use of a novel technique that uses the degree of quenching (NPQGlider), determined from a standard fluorometer deployed on a buoyancy glider, a proxy for iron limitation, αNPQ, was calculated for a glider time series in the Ross Sea from December 2011 to February 2012. Surface chlorophyll concentrations indicated that there were four stages: the first being a pre-bloom phase, the second in which phytoplankton growth was rapid, resulting in the accumulation of biomass; the third in which biomass in the surface layer decreased, and the fourth in which chlorophyll concentrations remained low but the POC:Chl ratio increased. The levels of NPQGlider in this region were much higher compared to other Southern Ocean regions, with the highest levels in the third phase. Similarly, αNPQ remained low throughout most of the time series except for the transition from the second to third phase when the surface biomass decreases. The increase in POC:Chl ratios in the final phase combined with the low values of αNPQ suggest the switch from a Phaeocystis antarctica bloom to a potentially non-iron limited diatom bloom. These results confirm that the application of novel methodologies to proven and reliable sensors will provide a greater understanding of biogeochemical cycles and their controls throughout the ocean.

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Temporal progression of the αNPQ with (a) chlorophyll and (b) POC:Chl. synthesis

Emannuel Boss, Others, Thomalla S.J., et al
Abstract

Tracking how ocean life is responding to increased human use and climate change will empower the global community to predict, mitigate, and manage our ocean. In this document we demonstrate the existence of mature technologies to measure ‘biology’ as a combination of biomass and diversity indicators across the plankton size spectrum. These are now ready to deploy within the GO-SHIP constraints.

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Abstract

Chlorophyll fluorescence, primarily used to derive phytoplankton biomass, has long been an underutilized source of information on phytoplankton physiology. Diel fluctuations in chlorophyll fluorescence are affected by both photosynthetic efficiency and non-photochemical quenching (NPQ), where NPQ is a decrease in fluorescence through the dissipation of excess energy as heat. NPQ variability is linked to iron and light availability, and has the potential to provide important diagnostic information on phytoplankton physiology. Here we establish a relationship between NPQsv (Stern-Volmer NPQ) and indices of iron limitation from nutrient addition experiments in the sub-Antarctic zone (SAZ) of the Atlantic Southern Ocean, through the derivation of NPQmax (the maximum NPQsv value) and αNPQ (the light limited slope of NPQsv). Significant differences were found for both Fv/Fm and αNPQ for iron versus control treatments, with no significant differences for NPQmax. Similar results from CTDs indicated that changes in NPQ were driven by increasing light availability from late July to December, but by both iron and light from January to February. We propose here that variability in αNPQ, which has removed the effect of light availability, can potentially be used as a proxy for iron limitation (as shown here for the Atlantic SAZ), with higher values being associated with greater iron stress. This approach was transferred to data from a buoyancy glider deployment at the same location by utilizing the degree of fluorescence quenching as a proxy for NPQGlider, which was plotted against in situ light to determine αNPQ. Seasonal increases in αNPQ are consistent with increased light availability, shoaling of the mixed layer depth (MLD) and anticipated seasonal iron limitation. The transition from winter to summer, when positive net heat flux dominates stratification, was coincident with a 24% increase in αNPQ variability and a switch in the dominant driver from incident PAR to MLD. The dominant scales of αNPQ variability are consistent with fine scale variability in MLD and a significant positive relationship was observed between these two at a ∼10 day window. The results emphasize the important role of fine scale dynamics in driving iron supply, particularly in summer when this micronutrient is limiting.

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(A) A time-series of a 5-day rolling mean of αNPQ and MLD (m). (B) Correlation coefficients of rolling averages of αNPQ and MLD using different time windows (days).

Mdutyana M, Thomalla S.J., Raissa Philibert, Bess B. Ward, Dr Sarah Fawcett
Abstract

Net primary production (NPP) fueled by nitrate is often equated with carbon export, providing a metric for CO2 removal to the deep ocean. This “new production paradigm” assumes that nitrification, the oxidation of regenerated ammonium to nitrate, is negligible in the sunlit upper ocean. While surface layer nitrification has been measured in other oceanic regions, very few data exist for the Southern Ocean. We measured NPP, nitrogen (N) uptake, and nitrification in the upper 200 m across the Atlantic Southern Ocean in winter and summer. Rates of winter mixed-layer nitrate uptake were low, while ammonium uptake was surprisingly high. NPP was also low, such that NPP and total N (nitrate+ammonium) uptake were decoupled; we attribute this to ammonium consumption by heterotrophic bacteria. By contrast, NPP and total N uptake were strongly coupled in summer except at two stations where an additional regenerated N source, likely dissolved organic N, apparently supported 30–45% of NPP. Summertime nitrate uptake rates were fairly high and nitrate fueled >50% of NPP, indicating the potential for significant carbon export. Nitrification supplied <10% of the nitrate consumed in summertime surface waters, while in winter, mixed-layer nitrification was on average 16 times higher than nitrate uptake. Despite the near-zero nitrification rates measured in the summer mixed layer, the classically defined f ratio does not well-represent Southern Ocean carbon export potential annually. This is because some fraction of the nitrate regenerated in the winter mixed layer is likely supplied to phytoplankton in summer; its consumption cannot, therefore, be equated with export.

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Specific uptake rates of carbon (VC) versus nitrogen (VN_total = VNO3 + VNH4) for (a) summer and (b) winter. Colored symbols show the rates at the individual incubation depths, and the corresponding black symbols represent the 0–200 m‐weighted average rates at each station. Integrating to the base of the mixed layer or euphotic zone rather than 200 m makes little difference to the averages because the specific rates at 200 m are very low. The solid
black line represents VC:VN_total = 1:1, which is expected for balanced phytoplankton growth assuming that the only N
forms being assimilated are NO3− and NH4+.

Dr Sarah Lou Carolin Giering, Dr Emma Louise Cavan, Others, Thomalla S.J., et al
Abstract

Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide (CO2) in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the mesopelagic zone of the ocean than traditional techniques, such as sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results. Here we provide a practical overview of the challenges and potential of using these instruments, as a step toward improvement and expansion of their applications.

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Working steps to derive small and large particles from single photodetectors. A median filter is fitted and assumed to be representative of small particles. Spikes are caused by large particles passing through the sampling frame.

Moutier W, Thomalla S.J., Dr Stewart Bernard, Galina Wind, Ryan-Keogh T J, Marie Smith
Abstract

The Southern Ocean (SO) is highly sensitive to climate change. Therefore, an accurate estimate of phytoplankton biomass is key to being able to predict the climate trajectory of the 21st century. In this study, MODerate resolution Imaging Spectroradiometer (MODIS), on board EOS Aqua spacecraft, Level 2 (nominal 1 km 1 km resolution) chlorophyll-a (CSat) and Particulate Organic Carbon (POCsat) products are evaluated by comparison with an in situ dataset from 11 research cruises (2008–2017) to the SO, across multiple seasons, which includes measurements of POC and chlorophyll-a (Cin situ) from both High Performance Liquid Chromatography (CHPLC) and fluorometry (CFluo). Contrary to a number of previous studies, results highlighted good performance of the algorithm in the SO when comparing estimations with HPLC measurements. Using a time window of 12 h and a mean satellite chlorophyll from a 5 5 pixel box centered on the in situ location, the median CSat:Cin situ ratios were 0.89 (N = 46) and 0.49 (N = 73) for CHPLC and CFluo respectively. Differences between CHPLC and CFluo were associated with the presence of diatoms containing chlorophyll-c pigments, which induced an overestimation of chlorophyll-a when measured fluorometrically due to a potential overlap of the chlorophyll-a and chlorophyll-c emission spectra. An underestimation of 0.13 mg m-3 was observed for the global POC algorithm. This error was likely due to an overestimate of in situ POCin situ measurements from the impact of dissolved organic carbon not accounted for in the blank correction. These results highlight the important implications of different in situ methodologies when validating ocean colour products.

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(a) Comparison between measured (Cin situ) and retrieved (CSat) chlorophyll-a for box with a time window of 12 h. Blue and orange dots indicate samples measured from HPLC (CHPLC) and fluorometry (CFluo) respectively, while the dashed line shows the 1:1 relationship. (b) Probability density function of the logarithm base 10 of the ratio between CSat and Cin situ from fluorometry (blue line) and HPLC (orange line).

Dean Roemmich, Others, Thomalla S.J., et al
Abstract

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.

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The three main models of BGC-Argo floats presently in use include (A) Navis, (B) APEX, and (C) PROVOR.

Abstract

Underwater gliders have become widely used in the last decade. This has led to a proliferation of data and the concomitant development of tools to process the data. These tools are focused primarily on converting the data from its raw form to more accessible formats and often rely on proprietary programming languages. This has left a gap in the processing of glider data for academics, who often need to perform secondary quality control (QC), calibrate, correct, interpolate and visualize data. Here, we present GliderTools, an open-source Python package that addresses these needs of the glider user community. The tool is designed to change the focus from the processing to the data. GliderTools does not aim to replace existing software that converts raw data and performs automatic first-order QC. In this paper, we present a set of tools, that includes secondary cleaning and calibration, calibration procedures for bottle samples, fluorescence quenching correction, photosynthetically available radiation (PAR) corrections and data interpolation in the vertical and horizontal dimensions. Many of these processes have been described in several other studies, but do not exist in a collated package designed for underwater glider data. Importantly, we provide potential users with guidelines on how these tools are used so that they can be easily and rapidly accessible to a wide range of users that span the student to the experienced researcher. We recognize that this package may not be all-encompassing for every user and we thus welcome community contributions and promote GliderTools as a community-driven project for scientists.

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The GliderTools “cheat sheet,” a quick reference guide to using and implementing the tool. The boxes represent the various modules within the package.

Dr Daniel B. Whitt, Nicholson S., Dr Magdalena Carranza
Abstract

Subseasonal surface wind variability strongly impacts the annual mean and subseasonal turbulent atmospheric surface fluxes. However, the impacts of subseasonal wind variability on the ocean are not fully understood. Here, we quantify the sensitivity of the ocean surface stress (τ), buoyancy flux (B) and mixed‐layer depth (MLD) to subseasonal wind variability in both a one‐dimensional (1D) vertical column model and a three‐dimensional (3D) global mesoscale‐resolving ocean/sea‐ice model. The winds are smoothed by time‐filtering the pseudo‐stresses, so the mean stress is approximately unchanged and some important surface flux feedbacks are retained. The 1D results quantify the sensitivities to wind variability at different timescales from 120 days to 1 day at a few sites. The 3D results quantify the sensitivities to wind variability shorter than 60 days at all locations, and comparisons between 1D and 3D results highlight the importance of 3D ocean dynamics. Globally, subseasonal winds explain virtually all of subseasonal τ variance, about half of subseasonal B variance, but only a quarter of subseasonal MLD variance. Subseasonal winds also explain about a fifth of the annual mean MLD and a similar and spatially‐correlated fraction of the mean friction velocity, urn:x-wiley:21699275:media:jgrc23678:jgrc23678-math-0001 where ρsw is the density of seawater. Hence, the subseasonal MLD variance is relatively insensitive to subseasonal winds despite their strong impact on local B and τ variability, but the mean MLD is relatively sensitive to subseasonal winds to the extent that they modify the mean u*, and both of these sensitivities are modified by 3D ocean dynamics.

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Annual mean (top) and seasonal cycle amplitude (bottom) of the magnitude of the ocean mixed-layer depth from the control run (CTL, left) and the fractional difference between CTL and a low-pass run with smoothed winds (CTL-LP, right; see section 2.5 for more on the metrics). In (B) and (D), points are blanked if zero is included in the 95% confidence interval, which is derived non-parametrically using 1000 bootstrap samples at each point. In (B) and (D), red means the metric is greater in CTL, whereas blue means the metric is greater in LP.