Winter Mixed Layer Research Articles

The Relative Importance of Ocean Advection and Surface Heat Fluxes During the Madden–Julian Oscillation in a Coupled Ocean–Atmosphere Model

AbstractIntraseasonal variability of ocean surface mixed layer temperature (MLT) in the tropics can be linked to the Madden–Julian Oscillation (MJO), the main source of intraseasonal atmospheric variability in the tropics. Here, we conduct a boreal winter mixed layer heat budget using 10‐day forecast composites of a coupled ocean–atmosphere Numerical Weather Prediction model of the UK Met Office to reveal that ocean advection plays a major role in modulating intraseasonal anomalies of MLT over the tropical Indian Ocean and the Maritime Continent. Large scale ( 10) intraseasonal anomalies of MLT ( 0.1 C) are driven by net surface heat flux. Ocean advection dominates at smaller horizontal scales ( 1), contributing up to 0.5 C to the intraseasonal MLT anomaly. Prior to the development of the enhanced MJO convection in the western Indian Ocean (phases 8 and 1), ocean advection reinforces the net heat flux warming in this region. However, ocean advection opposes the net heat flux warming in the Maritime Continent prior to the development of suppressed MJO convection in this region. When the MJO convection develops over the central Indian Ocean (phase 3), ocean advection (net surface heat flux) drives the intraseasonal MLT anomalies in the western Indian Ocean (central Indian Ocean and the Maritime Continent). Our results demonstrate the importance of ocean dynamics during the initiation and growth of the MJO, and raise implications for models that do not resolve these processes.

Modeling of the Black Sea circulation using equations of heat and salt advection–diffusion having discrete nonlinear invariants

In this work the accuracy of reconstructing the Black Sea circulation by using new approximations of nonlinear terms in the transport equations, ensuring the conservation of temperature and salinity to a power greater than two, is analyzed based on the results of forecast calculations. Three numerical experiments with differences in the schemes for calculating temperature and salinity are carried out. In the first experiment – traditional schemes are used to conserve of temperature and salinity in the first and second degrees; in the second one – the temperature is conserved in the first and fifth degrees, salinity in the first and third; in the third one – the temperature in the first and third, salinity in the first and fifth degrees. Calculations are performed on the basis of the MHI model with a resolution of 1.6 km and taking into account realistic atmospheric forcing for 2016. Validation of the results is carried out based on comparison of model fields with in-situ and satellite measurements of temperature and salinity in 2016. Analysis of mean and root mean square errors showed that new schemes for the advection-diffusion equations of heat and salt, ensuring the conservation of predictive parameters to a power greater than two, improve the accuracy of reconstructing the salinity in the Black Sea upper 100m layer throughout the year compared with traditional approximation. The root mean square errors in the salinity field are reduced by 15–20%, the thickness of the upper mixed layer in winter and the depth of the upper boundary of the thermocline layer in summer in the central part of the sea are modeled approximately 10% more accurately. Based on the results of three experiments, the smallest deviations from observational data are obtained when using approximations that ensure the conservation of temperature to the third power and salinity to the fifth power.

Spatiotemporal variability of satellite-derived abundance of Karenia spp. during 2021 in shelf waters along the Pacific coast of Hokkaido, Japan

Unprecedented catastrophic damage to coastal fisheries attributable to harmful Karenia outbreaks were reported in Pacific coastal shelf waters off the southern coast of Hokkaido from late summer to autumn in 2021. To understand the spatiotemporal variability of the Karenia blooms, we analyzed Sentinel 3-derived abundances of Karenia spp. together with marine environmental variables. Karenia spp. were very widely distributed over a maximum of more than 400 km along the shelf from the easternmost Pacific coast of Hokkaido to Cape Erimo, where there was a nearly stable water-mass front, to the west, where pure subtropical water inhibited the westward expansion of Karenia spp. blooms. The duration of the appearance of Karenia spp. at a fixed point was very long—about 45 days—in the middle part of the shelf. East of the Tokachi River, the time-averaged abundances of Karenia spp. were robustly correlated with the time-averaged alongshore velocity and stability of the Coastal Oyashio, a coastal boundary current; more intense and stable alongshore currents were associated with less developed Karenia spp. blooms. Time-averaged abundances of Karenia spp. were the highest in the middle part of the shelf, west of the Tokachi River, where low-salinity water from the river suppressed the development of the surface winter mixed layer and might have fostered favorable growth conditions and supplied nutrients of land origin. During the period of Karenia spp. blooms, abundances changed rapidly on a small scale (typically, ≤2 days and ≤50 km) in association with physical-biochemical coupled submesoscale variations. Subsampling of these variations of Karenia spp. abundances at 1-day intervals showed that the maxima and center of gravity of Karenia spp. abundances moved slowly westward along the coast at a typical velocity of 4 cm s−1. This velocity was one-third that of the time-averaged alongshore velocity of the Coastal Oyashio. Particle-tracking experiments implied that horizontal advection by the Coastal Oyashio, which supplied Karenia spp. eliminated from the upstream shelf to the downstream shelf, contributed to the long duration of Karenia spp. blooms on the middle part of the shelf.

Planktonic foraminifera response to the azores high and industrial-era global warming in the central-western Mediterranean Sea

The Mediterranean Sea is warming about 20 % more rapidly than global ocean and this phenomenon is impacting ecosystems and biodiversity. Planktonic foraminifera are an important component of surface and subsurface water ecosystems and food chains. Their species communities have been altering across the oceans since the Industrial Era, in response to the ongoing climate change, especially in the western Mediterranean Sea, where a significant productivity decrease has been recently reported.Here we show planktonic foraminifera and multispecies stable isotopes from three short sediment cores, recovered on the eastern flank of the Sicily Channel, central Mediterranean Sea. Results fully confirm the planktonic foraminifera productivity decrease in the Industrial Era, which is especially relevant for the second half of the 20th century. The planktonic foraminifera productivity decrease matches with a higher number of Large Azores High events, i.e., the establishment of an exceptional and persistent winter atmospheric high-pressure ridge over the western-central Mediterranean Sea. This is an unprecedented atmospheric phenomenon for the last millennia Mediterranean Sea history, as a direct response of the global warming. Surface productivity and DCM species are especially declining since ∼1960 CE, at expenses of winter mixed layer taxa, suggesting that the Azores High activity prevents a sustained water column vertical mixing and surface water nutrient fuelling. Our results document and confirm that the climate change has already been affecting Mediterranean marine ecosystems and the basic level of the trophic chain, by extending the surface water stratification period.

Deep Winter Mixed Layer Anchored by the Meandering Antarctic Circumpolar Current: Cross-Basin Variations

Abstract A deep winter mixed layer forms north of the Antarctic Circumpolar Current (ACC) in the Indo-Pacific sectors, while the mixed layer depth (MLD) is shallow in the Atlantic. Using observations and a global atmospheric model, this study investigates the contribution of surface buoyancy flux and background stratification to interbasin MLD variations. The surface heat flux is decomposed into broad-scale and frontal-scale variations. At the broad scale, the meandering ACC path is accompanied by a zonal wavenumber-1 structure of sea surface temperature (SST) with a warmer Pacific than the Atlantic; under the prevailing westerly winds, this temperature contrast results in larger surface heat loss facilitating deeper MLD in the Indo-Pacific sectors than in the Atlantic. In the Indian sector, the intense ACC fronts strengthen surface heat loss compared to the Pacific. The surface freshwater flux pattern largely follows that of evaporation and reinforces the heat flux pattern, especially in the southeast Pacific. A diagnostic relationship is introduced to highlight the role of ACC’s sloping isopycnals in setting a weak submixed layer stratification north of ACC. This weak stratification varies in magnitude across basins. In the Atlantic and western Indian Oceans where the ACC is at a low latitude (∼45°S), solar heating, intrusions of subtropical gyres, and energetic mesoscale eddies together maintain relatively strong stratification. In the southeast Pacific, in comparison, the ACC reaches the southernmost latitude (56°S), far away from the subtropical front. This creates weaker stratification, allowing deep mixed layers to form, aided by surface buoyancy loss.

Quantifying Drivers of Seasonal and Interannual Variability of Dissolved Oxygen in the Canada Basin Mixed Layer

AbstractAnalysis of dissolved oxygen (O2) in the Arctic's surface ocean provides insights into gas transfer between the atmosphere‐ice‐ocean system, water mass dynamics, and biogeochemical processes. In the Arctic Ocean's Canada Basin mixed layer, higher O2 concentrations are generally observed under sea ice compared to open water regions. Annual cycles of O2 and O2 saturation, increasing from summer through spring and then sharply declining to late summer, are tightly linked to sea ice cover. The primary fluxes that influence seasonal variability of O2 are modeled and compared to Ice‐Tethered Profiler O2 observations to understand the relative role of each flux in the annual cycle. Findings suggest that sea ice melt/growth dominates seasonal variations in mixed layer O2, with minor contributions from vertical entrainment and atmospheric exchange. While the influence of biological activity on O2 variability cannot be directly assessed, indirect evidence suggests relatively minor contributions, although with significant uncertainty. Past studies show that O2 molecules are expelled from sea ice during brine rejection; sea ice cover can then inhibit air‐sea gas exchange resulting in winter mixed layers that are super‐saturated. Decreasing mixed layer O2 concentrations and saturation levels are observed during winter months between 2007 and 2019 in the Canada Basin. Only a minor portion of the decreasing trend in wintertime O2 can be attributed to decreased solubility. This suggests the O2 decline may be linked to more efficient air‐sea exchange associated with increased open water areas in the winter sea ice pack that are not necessarily detectable via satellite observations.

Sea Ice Interannual Variability and Sensitivity to Fall Oceanic Conditions and Winter Air Temperature in the Gulf of St. Lawrence, Canada

AbstractThe Gulf of St. Lawrence has been nearly free of sea ice in the winter six times in its recorded history, four of which have occurred since 2010. This study examines the inter‐annual variability of sea ice cover characteristics (1969–2024) and winter mixed layer heat content (1996–2024), their sensitivity to fall oceanic conditions (since fall of 1995) and to winter air temperatures. The study finds no relationship between fall oceanic conditions with either the first occurrence of sea ice, maximum seasonal estimated volume or winter mixed layer heat content. However, it shows that the first occurrence of sea ice in the northwestern Gulf is related to the timing of sea surface temperature crossing the 1°C threshold with a lag time of 30–37 days, and with air temperature dropping below −2°C with a lag of 37–44 days; longer lags have weak correlations. The seasonal maximum conditions in area or estimated volume can be estimated by the preceding measurements of the same metrics with a lead time of only 29 days for volume and 36 days for area. The average air temperature over the Gulf between December and February or March is highly correlated to seasonal maximum sea ice area and estimated volume, as well as ice season duration. The six nearly ice‐free winters correspond to the warmest December to February (or December to March) average air temperatures over the Gulf. A warming of >1.9°C–2.4°C (DJFM) or >2.2°C–2.9°C (DJF) above the 1991–2020 climatology leads to nearly ice‐free conditions.

Effects of Freezing Temperature Parameterization on Simulated Sea‐Ice Thickness Validated by MOSAiC Observations

AbstractFreezing temperature parameterization significantly impacts the heat balance at sea‐ice bottom and, consequently, the simulated sea‐ice thickness. Here, the single‐column model ICEPACK was used to investigate the impact of the freezing temperature parameterization on the simulated sea‐ice thermodynamic growth during the MOSAiC expedition from October 2019 to September 2020. It is shown that large model errors exist with the standard parameterization and that different formulations for calculating the freezing temperature impact the simulated sea‐ice thickness significantly. Considering the winter mixed layer temperature, a modified parameterization of the freezing point temperature based on Mushy scheme was developed. The mean absolute error (ratio) of simulating sea‐ice thickness for all buoys reduces from 7.4 cm (4.9%) with the “Millero” scheme, which performs the best among the existing schemes in the ICEPACK model, to 4.2 cm (2.9%) with the new developed scheme.

Primary Production and Respiration in the Norwegian Sea Estimated From Biogeochemical Argo Floats

AbstractBiogeochemical (BGC) Argo floats were used in this study to investigate phytoplankton blooms. We assessed the seasonal and annual rates of net primary and community production, along with respiration in the Norwegian Sea. The years 2020 and 2021 were contrasted to illuminate similarities and differences. In both years the onset of the bloom occurred at the beginning of February, coinciding with a deep winter mixed layer. However, during spring and summer the biological production appeared to develop differently. In 2020 the mixed layer depth shoaled quickly in April due to surface heating, triggering a strong spring bloom event. In contrast, a significant surface cooling in April 2021 triggered a substantial vertical mixing, that delayed the mixed layer's shoaling. This delay initiated cascading effects impacting nitrate consumption and bloom development that resulted in increased respiration in 2021 compared to 2020. In both years, vertical nitrate mixing from deeper layers to the surface emerged as a pivotal factor determining primary production. Using a mixed layer model in combination with Argo observations, we found that the upward nutrient fluxes due to mixing doubled net community production. These findings underscore the capability of BGC‐Argo floats, operating at a 5‐day resolution, to unveil the intricate interplay between hydrography, physical drivers, and biogeochemical processes in shaping phytoplankton dynamics and overall ecosystem productivity in the Norwegian Sea.

A Scale-Aware Parameterization of Restratification Effect of Turbulent Thermal Wind Balance

Abstract The vertical buoyancy flux Bf under the turbulent thermal wind (TTW) balance plays an important role in restratifying the surface mixed layer in winter. So far, most of the global ocean models are too coarse to resolve this process. In this paper, a scale-aware parameterization is proposed for and implemented in a hierarchy of regional ocean simulations over the winter Kuroshio Extension with horizontal resolutions ranging from 27 to 1 km. The parameterization depends on the Coriolis parameter, model-simulated turbulent vertical viscosity, horizontal density gradient, and a scaling relationship to adjust for the effects of model horizontal resolution on the simulated horizontal density gradient. It shows good skills in reconciling the difference between in the coarse-resolution simulations (27, 9, and 3 km) and in the 1-km simulation where is well resolved. Furthermore, implementation of the parameterization improves the simulated stratification in the surface mixed layer in coarse-resolution simulations.

Interannual Variability of Isopycnal Ocean Heat Content in the Subtropical Northeast Pacific

AbstractThe subsurface thermal anomaly originating from the mid‐latitude Pacific significantly influences climate variability in the tropical Pacific. Traditionally, the spiciness anomaly, defined on an isopycnal surface, is used to estimate this thermal anomaly. However, the thermal anomaly is confined to an isopycnal layer with a certain thickness, not spread on a specific surface. Its impact on the tropical Pacific depends on the amount of heat reaching the equator. Therefore, the thickness‐weighted isopycnal ocean heat content (IPOHC), defined as the ocean heat content in an isopycnal layer of a given potential density range, can more accurately estimate the subsurface thermal anomaly. This study focuses on the interannual variability of the IPOHC in the subtropical northeast Pacific, a region crucial for extratropical‐tropical oceanic exchange. The IPOHC in the 24.8–25.6 kg/m3 potential density layer exhibits a significant interannual variability with a 5–6‐year period. Subduction and salt fingering are identified as key processes driving this variability. Specifically, increased subduction volume and enhanced salt fingering both can induce positive IPOHC anomalies, while the opposite results in negative anomalies. Late winter variations in subduction volume and salt fingering strength, which originate from the winter mixed layer depth variability and halocline intensity variability, work together to cause the observed interannual IPOHC anomalies. We emphasize that the subduction‐induced IPOHC anomalies are determined by anomalous subduction volume rather than mixed layer temperature. Furthermore, salt fingering plays an essential role in driving substantial IPOHC anomalies in the deep, poorly‐ventilated halocline/thermocline.

Surface Heat Fluxes Drive a Two‐Phase Response in Southern Ocean Mode Water Stratification

AbstractSubantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual‐average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely‐signed advective haline contribution dominates. This two‐phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current.

Intrathermocline Eddy with Lens-Shaped Low Potential Vorticity and Diabatic Forcing Mechanism in the South China Sea

Abstract Intrathermocline eddies (ITEs), characterized by subsurface lens-shaped low potential vorticity (PV), are pervasive in the ocean. However, the abundance and generation mechanisms of these low-PV lenses are poorly understood owing to their weak surface signals and awkward sizes, which present an observational barrier. Using in situ observations of the northern South China Sea (NSCS), a typical ITE with a lens-shaped low PV at a core depth of 30–150 m and a horizontal size of ∼150 km was captured in May 2021. Combined with PV budget analysis, we investigate the underlying generation mechanism of low PVs within these ITEs using high-resolution reanalysis products. The results suggest that wintertime surface buoyancy loss driven by atmospheric diabatic forcing rather than frictional forcing is a crucial favorable condition for the ITE formation. These enhanced surface buoyancy losses produce a net upward PV flux and decrease PV in the weakly stratified and deep winter mixed layer, which are preconditioned by anticyclonic eddies (AEs). While surface heating in the following spring tends to weaken the surface buoyancy loss and gradually causes a downward PV flux, the surface-injected high PV subsequently caps the low-PV water within the surface-intensified AEs and transforms them into ITEs. Approximately 22% of the 58 AEs detected by satellite altimetry in the NSCS are ITEs. More importantly, the lens-shaped low PVs within them are produced primarily by the enhanced surface buoyancy loss during wintertime. These findings provide a new dynamic explanation for the low-PV generation in ITEs, highlighting the crucial role of atmospheric diabatic forcing. Significance Statement Intrathermocline eddies (ITEs), characterized by a lens-like isopycnal structure that bounds low potential vorticity (PV), are active in the oceanic interior. Although a few previous studies revealed the existence of ITEs in the South China Sea, the source and dynamic generation mechanisms of the lens-shaped low PV still remain elusive. We find that the enhanced surface buoyancy loss due to atmospheric diabatic forcing drives an upward surface PV flux and is identified to produce the low PV. The preexisting anticyclonic eddy, combined with seasonal surface heating in spring, can be easily transformed into the ITE. This study provides a new dynamic understanding for the generation mechanism of ITEs’ low PVs and highlights the contribution of atmospheric diabatic forcing.

Anthropogenic carbon pathways towards the North Atlantic interior revealed by Argo-O2, neural networks and back-calculations

The subpolar North Atlantic (SPNA) is a region of high anthropogenic CO2 (Cant) storage per unit area. Although the average Cant distribution is well documented in this region, the Cant pathways towards the ocean interior remain largely unresolved. We used observations from three Argo-O2 floats spanning 2013-2018 within the SPNA, combined with existing neural networks and back-calculations, to determine the Cant evolution along the float pathways from a quasi-lagrangian perspective. Our results show that Cant follows a stepwise deepening along its way through the SPNA. The upper subtropical waters have a stratified Cant distribution that homogenizes within the winter mixed layer by Subpolar Mode Water formation in the Iceland Basin. In the Irminger and Labrador Basins, the high-Cant footprint (> 55 μmol kg−1) is mixed down to 1400 and 1800 dbar, respectively, by deep winter convection. As a result, the maximum Cant concentration is diluted (<45 μmol kg−1). Our study highlights the role of water mass transformation as a first-order mechanism for Cant penetration into the ocean. It also demonstrates the potential of Argo-O2 observations, combined with existing methods, to obtain reliable Cant estimates, opening ways to study the oceanic Cant content at high spatio-temporal resolution.

The oceanic mixed layer changes along with the state transition of the Beaufort Gyre

Recent state transition of the Beaufort Gyre has drawn great interest in the Arctic research community, but how the upper ocean hydrographic structure varies with this transition remains poorly understood. The upper ocean mixed layer plays an important role in climatic and ecological processes. Therefore, we analyze the Ice-Tethered Profiler (ITP) observations over the last two decades (2004–2022) to investigate the long-term trend of the mixed layer in the Arctic Ocean’s Beaufort Gyre (BG) from an observational perspective. Results show that the linear trend of the BG surface mixed layer depth (MLD) before and after 2015 has changed significantly, characterized by the vanishing or even reversal of the significant deepening trend. This transition is most pronounced in winter. The BG winter mixed layer is significantly cooler, saltier and denser in the mid-transition period (2013–2017) compared to the pre-transition period (2004–2012), but becomes significantly warmer, fresher and lighter in the post-transition period (2018–2022). The transition feature of the depth of maximum buoyancy frequency in the upper BG is similar to that of MLD, while this maximum decreases significantly in both the mid- and post-transition period when compared to their previous period. The deepening signal of MLD is propagated eastward, which coincides with the recent transition of BG position and freshwater distribution. Mechanism analysis further reveals that the reversal of winter MLD trend before and after 2015 may be due to changes in surface wind stirring and Ekman pumping. This study extends the investigation of the recent state transition of BG considering the upper hydrographic structure.

Near-Inertial Energy Variability in a Strong Mesoscale Eddy Field in the Iceland Basin

An 18-month deployment of moored sensors in Iceland Basin allows characterization of near-inertial (frequencies near the Coriolis frequency f with periods of ~14 h) internal gravity wave generation and propagation in a region with an active mesoscale eddy field and strong seasonal wind and heat forcing. The seasonal cycle in surface forcing deepens the mixed layer in winter and controls excitation of near-​inertial energy. The mesoscale eddy field modulates near-inertial wave temporal, horizontal, and vertical scales, as well as propagation out of the surface layer into the deep permanent pycnocline. Wind-forced near-inertial energy has the most active downward propagation within anticyclonic eddies. As oceanic surface and bottom boundaries act to naturally confine the propagation of internal waves, the vertical distribution of these waves can be decomposed into a set of “standing” vertical modes that each propagate horizontally at different speeds. The lowest modes, which propagate quickly away from their generation sites, are most enhanced when the mixed layer is deep and are generally directed southward.

Blocked Drainpipes and Smoking Chimneys: Discovery of New Near-Inertial Wave Phenomena in Anticyclones

Time-varying winds blowing over an eddying ocean generate near-​inertial waves (NIWs) that tend to be trapped in anticyclones. Such anticyclones have been termed inertial chimneys in the past but have recently been renamed inertial drainpipes, given their propensity to funnel NIW energy downward to the deep ocean. Here, we present evidence of a semi-blocked inertial drainpipe where downward-​propagating NIWs trapped in an anticyclone are partially reflected off the permanent pycnocline, returned toward the surface, and dissipated at the top of the seasonal pycnocline in a submesoscale filament of anticyclonic vorticity. Observations made on the northern rim of an anticyclone in the Iceland Basin include a high-​resolution survey of velocity, hydrography, and microstructure. Upward-propagating NIWs were observed in a salty, submesoscale filament of anticyclonic vorticity near the edge of the eddy, potentially trapped there. Above the filament and at the top of the seasonal pycnocline, turbulence was enhanced over what could be attributed to local winds and surface cooling. Ray tracing suggests the filament could have channeled and focused trapped upward-propagating NIWs, acting as an inertial chimney in a truer sense of the term, possibly intensifying the wave energy sufficiently to sustain the observed turbulence. Numerical simulations of NIWs in anticyclonic vorticity and stratification representative of the observations suggest that the upward-propagating NIWs could have been generated by a wind event 12 days prior and reflected off a sharp jump in stratification at the base of the anticyclone. Here, the transition between the weakly stratified winter mixed layer and the permanent pycnocline partially reflects downward-​propagating NIWs, limiting the inertial drainpipe effect.

Sea Ice‐Driven Variability in the Pacific Subantarctic Mode Water Formation Regions

AbstractSubantarctic Mode Water (SAMW) forms north of the Subantarctic Front, in regions of deep winter mixed layers, and is important to the absorption and storage of anthropogenic CO2 and heat. Two SAMW pools exist in the Pacific, a lighter Central mode (CPSAMW), and a denser Southeast mode (SEPSAMW). Both have experienced significant interannual variability in thickness and properties in recent years. We compute mixed layer temperature and salinity budgets for the two SAMW formation regions, to determine the relative contribution of processes driving variability in the properties of mixed layers that subduct to form SAMW. The dominant drivers of temperature and salinity variability are shown to be surface fluxes, horizontal advection, and entrainment of deeper water. Salt advection into each SAMW formation region is found to be strongly correlated with changes in sea ice area in the northern Ross Sea, with lags of up to 2 years. Further correlation is found between meridional salt advection in the southeast Pacific formation regions, and sea ice area in the northern Amundsen/Bellingshausen seas, suggesting that freshwater derived from sea ice melt reaches the SEPSAMW formation region within 6 months. In 2016, strong advective freshening of the SEPSAMW formation region, linked to increased winter sea ice in the Amundsen/Bellingshausen seas, led to anomalously fresh mixed layers. However, a regime change in Antarctic sea ice in 2016 resulted in a subsequent lack of the usual advective freshening in the SEPSAMW formation region, driving increased salinity of the mixed layer the following year.

Sea‐Ice Impacts Inter‐Annual Variability of Phytoplankton Bloom Characteristics and Carbon Export in the Weddell Sea

AbstractThe Antarctic Marginal Ice Zone (MIZ) accounts for 15% of the Southern Ocean's primary production (PP), but limited data has hindered understanding of its variability and connection to carbon export. Using a combination of gliders, biogeochemical Argo floats and satellite observations in the northeast Weddell Sea, we show that years with more sea‐ice formation over winter are followed by more intense phytoplankton blooms (∼15% greater daily PP) and export to 100 m (∼50% higher daily carbon export) the following summer. However, the carbon export beyond the deepest winter mixed layer did not vary in proportion to PP, suggesting different drivers of carbon export at depth compared to surface waters. Furthermore, across the entire MIZ, the response of blooms to sea‐ice volume was spatially variable, indicating the need to consider spatial heterogeneity in the response of the biological carbon pump to future sea‐ice changes.

Analysis of ecologically relevant sea ice and ocean variables for the Southern Ocean using a high-resolution model to inform ecosystem studies

Southern Ocean organisms are uniquely adapted to the extreme environmental conditions that characterise this region, making them especially vulnerable to climate change. Alterations to the physical environment have already been linked to alterations in the structure and functioning of entire ecosystems, and ecological disruptions are expected to continue to occur. Although our understanding of the physical processes driving ecological change in the Southern Ocean has improved in recent years, significant knowledge gaps remain largely as a result of insufficient observational data being available. High resolution ocean models are an important tool that can help us overcome data scarcity. However, models generally contain biases that may affect their ability to accurately represent environmental conditions in the region of interest. Thus, their outputs must be evaluated to understand if and how model outputs can be used to answer questions about ecological impacts. Here, we examined the suitability of ACCESS-OM2-01, a high-resolution coupled ocean–sea ice model, for ecological applications. Our results highlight the heterogeneous nature of the mean state of the environmental variables examined and their trends across the Southern Ocean. Our assessment shows that the ACCESS-OM2-01 model simulated well the observed seasonal cycle and broad baseline climatological conditions of the mixed layer depth and sea ice variables for the Southern Ocean over the past 50 years. However, the model performance varies across regions and at seasonal time scales: the model simulated much deeper winter mixed layer depth in the Weddell Sea and within the Antarctic Circumpolar Current, and the climatological sea-ice concentration for summer was lower in the model compared to available observations. Finally, we provide detailed Python-based scripts with all the code and steps for this analysis, which can be used as reference material to reapply elsewhere and evaluate the suitability of other model outputs for ecological applications.