Session OS1.3

[Programme]

OS1.3 | The ocean surface mixed layer: multi-scale dynamics and ecosystems in a changing climate

The ocean surface mixed layer: multi-scale dynamics and ecosystems in a changing climate

Co-organized by BG4

Convener: Anne Marie Treguier | Co-conveners: Baylor Fox-Kemper, Sinikka Lennartz, Francois MassonnetECSECS

Orals

| Thu, 27 Apr, 08:30–10:15 (CEST)

Room 1.14

Posters on site

| Attendance Thu, 27 Apr, 14:00–15:45 (CEST)

Hall X5

The ocean surface mixed layer mediates the transfer of heat, freshwater, momentum and trace gases between atmosphere, sea ice and ocean, thus playing a central role in the dynamics of our climate. This session will focus on the surface mixed layer globally, from the coasts to the pelagic ocean. We will review recent progress in understanding the key processes taking place in the mixed layer: dynamic processes such as surface waves, Langmuir circulations and turbulence, shear-induced mixing, internal waves, coherent structures, fronts, frontal instabilities, entrainment and detrainment at the mixed layer base, convection, restratification induce physical effects on this important layer. From a biogeochemical perspective, the physical dynamics of the euphotic layer, as well as biogeochemical processes affecting the cycling of carbon, micro- and macronutrients, production and degradation of trace gases as well as microbial dynamics from basic to higher trophic levels affect the layer’s role and sensitivity as part of the earth system. The improvement of the representation of surface mixed layer processes in numerical models is a complex and pressing issue: this session will bring together new advances in the representation of mixed layer processes in high resolution numerical models, as well as evaluation of mixed layer properties in climate models using most recent observational datasets. The coupling of the ocean and atmospheric boundary layers as well as the special processes occurring under sea ice and in the marginal sea ice zone will be given special consideration. This session welcomes all contributions related to the study of the oceanic mixed layer, independent of the time- and space scales considered. This includes small scale process studies, short-term forecasting of the mixed layer characteristics for operational needs, studies on the variability of the mixed layer’s physical and biogeochemical properties from sub-seasonal to multi annual time scales and mixed layer response to external forcing. The use of multiple approaches (coupled numerical modeling, reanalyses, observations, experiments) is encouraged.

Orals: Thu, 27 Apr | Room 1.14

Chairpersons: Anne Marie Treguier, Sinikka Lennartz

08:30–08:35

5-minute convener introduction

08:35–08:45

EGU23-726

OS1.3

ECS

On-site presentation

Ocean small-scale fronts in the Northwestern Tropical Atlantic: Assessment from the EUREC4A-OA/ATOMIC field experiment

Solange Coadou, Sabrina Speich, Sebastiaan Swart, Chelle Gentemann, Dongxiao Zhang, and Johannes Karstensen

Upper-ocean fronts are dynamically active features of the global ocean that have significant implications for air-sea interactions, vertical mass and heat transfers, stratification and phytoplankton production and export. They have a large range of spatial scales from submesoscale (1 – 10 km) to mesoscale (10 – 100s km) characterized by temporal variability from days to months. The small dimensions and short duration of these structures have limited our capacity of observing, modelling and understanding fully these processes and their impact.

The EUREC4A-OA/ATOMIC field experiment, that took place during January-February 2020 in the Northwest Tropical Atlantic, has tried to address this challenge. In particular, five Saildrones, which are uncrewed platforms instrumented to measure the air-sea interface, have been deployed. This region showed to be a well-suited laboratory to investigate horizontal density surface gradients over a wide range of scales. Strongly affected by the outflow of the Amazon River, the generation of fine-scale horizontal thermohaline gradients is favored by the stirring of this freshwater input by large anticyclonic eddies (a.k.a. North Brazil Current Rings). The distribution of these frontal structures highlights the presence of very intense gradients, including at the smaller spatial scales. The coherence of temperature and salinity fronts was estimated by a wavelet transform analysis. It reveals that large-scale density fronts are primarily controlled by horizontal variations in salinity but with increasing temperature-salinity coherence at the small scales range of the spectrum (O (0.1 km)) for strong gradients whereas they are poorly correlated for weaker fronts.

Our study shows that processes such as the mixed layer depth, the diurnal cycle, and air-sea exchanges are strongly affected by these small-scale frontal regimes. The parallel and quasi synchronous tracks of a 4-Saildrone formation provide a detailed picture of the upper ocean vorticity, divergence, and strain from their ADCP current measurements. Overall the methodology that has been developed could be extended on other datasets in order to assess the phenomenology of fine-scale structures in other dynamical regions.

How to cite: Coadou, S., Speich, S., Swart, S., Gentemann, C., Zhang, D., and Karstensen, J.: Ocean small-scale fronts in the Northwestern Tropical Atlantic: Assessment from the EUREC4A-OA/ATOMIC field experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-726, https://doi.org/10.5194/egusphere-egu23-726, 2023.

08:45–08:55

EGU23-2874

OS1.3

ECS

On-site presentation

Local variability of internal wave driven upwelling at four remote Pacific Reefs

Megan O'Hara, Peter Robins, Gareth Williams, and Mattias Green

Internal wave driven upwelling has been shown to deliver cool, nutrient-rich water (‘cold pulses’) to the shallow shores of remote reefs, providing potential thermal refugia and nutrient enrichment. However, the spatial variability within nearby islands is yet to be explored. Two methods were used to quantify the distribution of cold pulse events between two sets of remote tropical Pacific Islands: Kingman Reef and Palmyra Atoll (66 km apart), and Howland and Baker Islands (70 km apart); the two groups of islands are ~1700 km apart.

Using data from subsurface temperature loggers (STR’s) from 2008 to 2018, moored on the forereefs of our four Islands, we show that there were clear differences in upwelling behaviour. Around Palmyra Atoll, the northwest and west logger sites are <1 km apart and have a difference in degree cooling hours (DCH – hours in a day during which cold pulses were present) of up to 0.6 per day. The temperature drop at these sites differs by up to 1°C per cold pulse. Kingman Reef showed up to 0.5 DCH difference between sites (~4 km apart) per day, with temperature drop differences of up to 2°C per pulse. In contrast, Howland and Baker Islands showed up to 3 DCH difference per day between islands, whereas the temperature drop around Baker Island differed by up to 2°C per pulse. During the very strong 2015/2016 El Niño, Palmyra showed an increase of up to 1.3 DCH in a day, whereas Kingman reached <0.2 DCH per day. Howland and Baker Islands showed a similar response during this El Niño event but differed during normal ENSO phases. For example, during 2013/2014, Baker Island showed a maximum of up to 2.75 DCH per day, whereas Howland Island did not reach past 0.5 DCH per day.

We conclude that cold pulse behaviour varies between geographically close reefs and so one reef’s data cannot be used as a proxy for other islands and reefs. Subsequently, we hypothesise that the slope angle of the reef may be correlated to the presence of cold pulse activity, and that increased cold pulses may be able to mitigate the effects of a global warming on reefs with specific characteristics.

How to cite: O'Hara, M., Robins, P., Williams, G., and Green, M.: Local variability of internal wave driven upwelling at four remote Pacific Reefs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2874, https://doi.org/10.5194/egusphere-egu23-2874, 2023.

08:55–09:05

EGU23-11167

OS1.3

ECS

On-site presentation

Unveiling the Recent Changes in Barrier Layer Dynamics over the Arabian Sea Mini Warm Pool

Sankar Prasad Lahiri and Vimlesh Pant

The reversal of monsoon wind and restriction of further northward oceanic heat transport makes the Indian Ocean unique compared to the other two tropical oceans. The circulation over the north Indian Ocean (NIO) also reverses following this change in the wind pattern. Two basins of NIO, i.e., the Bay of Bengal and the Arabian Sea, witness distinct physical and dynamical properties in response to this wind pattern and freshwater influx, although they lie within the same latitudinal band. The focus of this study is over the south-eastern Arabian Sea (SEAS) (7.5 - 12.5°N, 72.5 - 76.5°E), where sea surface temperature (SST) of more than 29.5°C is observed during late April and early May. This warm temperature over the SEAS is associated with the formation of a monsoon onset vortex that influences the onset of the Indian Summer Monsoon. Previous studies have suggested that high SST over the SEAS is independent of the tropical Indian Ocean warm pool. This high SST region is referred to as the Arabian Sea Mini Warm Pool (ASMWP). The development of ASMWP starts in November when the coastal Kelvin wave packets initiate the formation of an equatorward flowing boundary current along the east coast of India, East Indian Coastal Current (EICC). EICC transports the low saline Bay of Bengal water to the SEAS, resulting in a strong haline stratification which leads to the formation of a barrier layer. Once this layer forms, it restricts the vertical mixing of water in the mixed layer with the thermocline water. The objective of this study is to observe the recent change in the dynamics of this barrier layer thickness (BLT) over SEAS. Using reanalysis data from Copernicus Marine Services, the seasonal and yearly evolution of BLT is analyzed from 1993 to 2018. This study calculates the isothermal layer depth (ILD) based on the 1°C temperature criteria. The density change is computed following this temperature change which is used to calculate mixed layer depth (MLD). The monthly climatology suggests the presence of thick BLT (i.e., ILD - MLD) over SEAS from December to February, although some remnant is present in March. A seasonal average (December - February) of BLT suggests a significant increasing trend from 1993 to 2018. Although the MLD is not showing any significant changes, the ILD is witnessing a substantial increase over these years. The effect of the ILD increase is also reflected in the stratification and heat content. Using geostrophic eddy kinetic energy, the energetics of the EICC in October-November are noticed in three different regions along the southeast coast of India and south of Sri Lanka. The influence of local forcings on the dynamics of the BLT is investigated to understand the mechanism behind this evolution of BLT and its role in ASMWP variability.

How to cite: Lahiri, S. P. and Pant, V.: Unveiling the Recent Changes in Barrier Layer Dynamics over the Arabian Sea Mini Warm Pool, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11167, https://doi.org/10.5194/egusphere-egu23-11167, 2023.

09:05–09:15

EGU23-9840

OS1.3

ECS

Virtual presentation

Observing and Modeling the variability of DWLs during the summer Monsoon in the Northern Indian Ocean

Siddhant Kerhalkar, Amit Tandon, Tamara Schlosser, J.Thomas Farrar, Andrew Lucas, Leah Johnson, Verena Hormann, and Luca Centurioni

Diurnal Warm Layers (DWLs) play an important role in coupling the atmosphere and the ocean, but their observations in the freshwater dominated Northern Indian Ocean in summer Monsoons are rare. This study focuses on the following aspects of DWLs observed during a 5-day suppressed atmospheric convection phase of the southwest monsoon season in 2019: (i) DWL observations using innovative drifting flux profilers to simultaneously measure high resolution shear and stratification as well as the surface meteorological forcing variables to compute air-sea fluxes (ii) Observed spatial gradients of SST over 1-100 km scales and (iii) Modeling using the popular one-dimensional models increasing in complexity. These observations show regions of marginal shear instability at the DWL base in agreement with previous studies in the tropical Pacific. The commonly used constant stratification assumption within the DWL (e.g. Fairall et al. 1996) breaks down in scenarios with weaker winds and salinity-driven stratification. The vertical structure of DWLs is therefore explored using k-e turbulence closure scheme in General Ocean Turbulence Model (GOTM) framework. Insights from model-observation comparisons show that for days with similar wind speeds, the DWL response can vary based on whether warm water or freshwater advection plays a role. Notably, warm water advection leads to deeper DWLs, whereas the freshwater advection traps the DWL to shallower depths. Further, spatial differences of O(1 C) in diurnal cycles of SST are observed over O (1-100 km), showing remarkable lateral inhomogeneity in the evolution of DWLs.

How to cite: Kerhalkar, S., Tandon, A., Schlosser, T., Farrar, J. T., Lucas, A., Johnson, L., Hormann, V., and Centurioni, L.: Observing and Modeling the variability of DWLs during the summer Monsoon in the Northern Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9840, https://doi.org/10.5194/egusphere-egu23-9840, 2023.

09:15–09:25

EGU23-15118

OS1.3

On-site presentation

Dissipation due to ageostrophic turbulence in the upper-ocean mixed layer

Nils Brüggemann, Leonidas Linardakis, and Peter Korn

09:25–09:35

EGU23-3283

OS1.3

ECS

Highlight

On-site presentation

Sub-frontal niches of marine plankton driven by transport and trophic interactions at ocean fronts

Inès Mangolte, Marina Lévy, and Mark Ohman

Observations and theory have suggested that ocean fronts are ecological hotspots, associated with higher diversity and biomass across many trophic levels. The hypothesis that these hotspots are driven by frontal nutrient injections is seemingly supported by the frequent observation of opportunistic diatoms at fronts, but the behavior of the rest of the plankton community is largely unknown.
Here we investigate the organization of planktonic communities across fronts by analyzing 8 high resolution transects in the California Current Ecosystem containing extensive data for 24 groups of bacteria, phytoplankton and zooplankton.
We find that a distinct frontal plankton community characterized by enhanced biomass of not only diatoms and copepods but many other groups of plankton such as chaetognaths, rhizarians and appendicularians emerges over most fronts. Importantly, we find spatial variability at a finer scale (typically 1-5 km) than the width of the front itself (typically 10-30 km) with peaks of different plankton taxa at different locations across the width of a front. Our results suggest that multiple processes, including both horizontal stirring and biotic interactions, are responsible for creating this fine-scale patchiness.

How to cite: Mangolte, I., Lévy, M., and Ohman, M.: Sub-frontal niches of marine plankton driven by transport and trophic interactions at ocean fronts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3283, https://doi.org/10.5194/egusphere-egu23-3283, 2023.

09:35–09:45

EGU23-3723

OS1.3

ECS

On-site presentation

Changes and mechanisms of long-lived warm blobs in the Northeast Pacific in low-warming climates

Jian Shi, Cong Tang, and Yu Zhang

In the last decade, three persistent warm blob events (2013–2014, 2015, and 2019–2020) in the Northeast Pacific (NEP) have been hotly debated given their substantial effects on climate, ecosystem, and socioeconomy. This study investigates the changes of such long-lived NEP warm blobs in terms of their intensity, duration, structure, and occurrence frequency under Shared Socioeconomic Pathway (SSP) 119 and 126 low-warming scenarios of the Coupled Model Intercomparison Project Phase 6. Results show that the peak timing of the warm blobs shifts from cold season to boreal summer. For the summer-peak warm blobs, their maximum intensity increases by 6.7% (10.0%) under SSP119 (SSP126) scenario, but their duration reduces by 31.0% (20.4%) under SSP119 (SSP126) scenario. In terms of their vertical structure, the most pronounced temperature signal is located at the surface, and their vertical penetration is mostly confined to the mixed layer, which becomes shallower in warming climates. Based on a mixed-layer heat budget analysis, we reveal that shoaling mixed layer depth plays a dominant role in driving stronger intensity of the warm blobs under low-warming scenarios, while stronger magnitude of ocean heat loss after their peaks explains the faster decay and thus shorter duration. Regarding occurrence frequency, the total number of the warm blobs does not change robustly in the low-warming climates. Following the summer peak of the warm blobs, extreme El Niño events may occur more frequently under the low-warming scenarios, possibly through stronger air-sea coupling induced by tropical Pacific southwesterly anomalies.

How to cite: Shi, J., Tang, C., and Zhang, Y.: Changes and mechanisms of long-lived warm blobs in the Northeast Pacific in low-warming climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3723, https://doi.org/10.5194/egusphere-egu23-3723, 2023.

09:45–09:55

EGU23-14823

OS1.3

ECS

On-site presentation

How subsurface and double-core anticyclones intensify the winter mixed layer deepening in the Mediterranean sea

Alexandre Barboni, Solange Coadou-Chaventon, Alexandre Stegner, Briac Le Vu, and Franck Dumas

The mixed layer is the uppermost layer of the ocean, connecting the atmosphere to the subsurface ocean through atmospheric fluxes. It is subject to pronounced seasonal variations: it deepens in winter due to buoyancy loss and shallows in spring while heat flux increase and restratify the water column. A mixed layer depth (MLD) modulation over this seasonal cycle has been observed within mesoscale eddies.

Taking advantage of the numerous Argo floats deployed and trapped within large Mediterranean anticyclones over the last decades, we reveal for the first time this modulation at a 10-day temporal scale and free of the smoothing effect of composite approaches. The analysis of 16 continuous MLD time series inside 13 long-lived anticyclones at a fine temporal scale brings to light the importance of the eddy preexisting vertical structure in setting the MLD modulation by mesoscale eddies. Extreme MLD anomalies of up to 330m are observed when the winter mixed layer connects with a preexisting subsurface anticyclonic core, greatly accelerating mixed layer deepening. The winter MLD sometimes does not achieve such connection but homogenizes another subsurface layer, then forming a multi-core anticyclone with spring restratification. A MLD restratification delay is always observed, reaching more than 2 months in 3 out the 16 MLD timeseries. The water column starts to restratify outside anticyclones while mixed layer keeps deepening and cooling at the eddy core for a longer time.

These new elements provide direct observation of double-core anticyclone formation, which dominant formation mechanism was previously considred to be vertical alignement, and provides new keys for understanding anticyclone vertical structure evolution.

How to cite: Barboni, A., Coadou-Chaventon, S., Stegner, A., Le Vu, B., and Dumas, F.: How subsurface and double-core anticyclones intensify the winter mixed layer deepening in the Mediterranean sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14823, https://doi.org/10.5194/egusphere-egu23-14823, 2023.

09:55–10:05

EGU23-187

OS1.3

ECS

On-site presentation

A parameter space for evaluating oceanic convection regimes

Alexandre Legay, Bruno Deremble, Thierry Penduff, and Pierre Brasseur

Oceanic convection, parameterized through vertical mixing schemes, is still not well captured by ocean general circulation models. A preliminary step necessary to improve these schemes is to evaluate and compare how the models behave for different forcing regimes. Literature often proposes single-case comparison (either on a specific location or a specific time or with a specific metrics). The goal of our work is to propose a more systematic framework allowing evaluations and comparisons over a larger range of forcing regimes. For doing so, we define a parameter space which has been derived thanks to a theoretical 1D model of the mixed-layer depth (MLD) evolution. This parameter space is formed by two dimensionless numbers : λ_s which describes the relative contribution of the buoyancy flux and the wind in the surface layer, and the Richardson number R_h which characterizes the stability of the water column at the mixed layer base. In this presentation, I will highlight the key features of this parameter space and I will illustrate its physical robustness with an ensemble of 1D simulations. These simulations were conducted by applying a 10 years JRA55-do 1.4.0 atmospheric forcing within a 1D standalone code making use of the NEMO Turbulent Kinetic Energy + Enhanced Vertical Diffusivity (TKE + EVD) scheme. Then, I will present a test case to study the impact of the horizontal resolution on the convection regimes for a TKE + EVD scheme in 1D, 1°, 1/12° and 1/60° realistic NEMO simulations. I will define convective regimes by sorting the values according to the normalized evolution of the mixed layer depth d_t MLD / MLD and I will show that these regimes are almost kept in the parameter space between 1D and 1° but become generally less convective / more restratifiying when increasing the resolution, highlighting the restratification processes by lateral fluxes. Moreover, I will show that the dynamics in the Mediterranean is much more affected by the increase of resolution than the Labrador sea, suggesting that it involves more intense lateral restratification processes.

How to cite: Legay, A., Deremble, B., Penduff, T., and Brasseur, P.: A parameter space for evaluating oceanic convection regimes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-187, https://doi.org/10.5194/egusphere-egu23-187, 2023.

10:05–10:15

EGU23-2545

OS1.3

ECS

On-site presentation

Surface cooling as an internal wave generator in high latitudes.

Matheus Azevedo and Yujiro Kitade

Heat-forced convection is a phenomenon observed frequently in high-latitude oceans. An inherent part of the Global Ocean Conveyor, it affects the global climate state over a wide range of spatial and temporal scales while being fundamentally tied to the diurnal cycle. Despite the importance of convective phenomena, most ocean general circulation models do not fully resolve it, instead parametrizing convection with adjustment schemes that remove static instability in the water by mixing vertically adjacent grid cells. However, the mixed layer response to daily-averaged fluxes is not necessarily the same as the average response to the diurnal cycle. Neglecting the diurnal cycle replaces periodic nightly convective pulses with chronic mixing that does not reach as deep. (Soloviev and Klinger, 2008).

Furthermore, the current understanding of upper-layer processes does not elucidate the consequences of the oscillatory behavior of the diurnal convection at the boundaries of the mixed layer. To address this issue, we devised a numerical experiment to investigate whether an upward heat flux is enough to generate internal waves capable of propagation despite their original forcing having a non-propagating period (~24 hours).

To reproduce the surface cooling-induced convection and the consequent internal wave generation, we formulated a 2-D model incorporating non-hydrostatic dynamics. Although pressure is the most computationally intensive term to calculate in such models, we could exclude it from our calculation by employing the Navier-Stokes equation with a rigid-lid, incompressible, and Boussinesq approximation, and cross-differentiating the equation system to reach a single equation defined in terms of vorticity and stream function. The model was set with a 60s time step, implemented using a leap-frog scheme, constant step Δx=200m for the horizontal and Δz=5m for the vertical axis, over a 40000 x 2000 m domain. The bottom and lateral boundaries were respectively set to a reflective non-slip and a cyclic boundary. The inertial period for the domain was set at 13.81h, simulating the 60°S latitude. The experiment started from a stratified condition and was forced using a sinusoidal heat-flux function at the middle of the domain with a diurnal period and varying amplitudes.

Our experiment indicates that internal waves are generated at the boundary of the mixed layer by nonlinear wave-wave interactions of the diurnal and inertial periods. The enhancement of the near-inertial period was observed as well as the generation of higher frequency waves of 8, 6 and 4 hours. These waves travel far beyond their generation site and propagate down to 2000 m deep, as deep as the vertical domain allows.

The internal waves observed in the numerical experiment might play an important role in enhancing mixing in the ocean interior at high latitudes, especially during the winter. This mechanism could also help to explain deep and bottom ocean variability and establish a pathway for the upper layer and deep ocean interaction.

How to cite: Azevedo, M. and Kitade, Y.: Surface cooling as an internal wave generator in high latitudes., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2545, https://doi.org/10.5194/egusphere-egu23-2545, 2023.

Posters on site: Thu, 27 Apr, 14:00–15:45 | Hall X5

Chairpersons: Anne Marie Treguier, Sinikka Lennartz

X5.283

EGU23-4833

OS1.3

ECS

Negative surface chlorophyll concentration anomalies in the southeastern Arabian Sea during 2015 and 2019 summers

Huaming Huang

Two extremely low surface chlorophyll concentration events in the southeast Arabian Sea (SEAS, 6^oN-15^oN, 72^oE-77^oE) during summers of 2015 and 2019 have been found since 1998. Although warm sea surface temperature (SST) and low nutrients are the direct cause for the anomalously low surface chlorophyll concentration, the physical processes leading to the warm SST anomalies during 2015 and 2019 summer are different. Satellite observations, model outputs and reanalysis data are used to explore the related mechanisms. In 2019, the combined effects of northward local wind anomaly due to extreme positive IOD and westward-propagating downwelling Kelvin wave driven by the easterly anomaly in eastern Sri Lanka weaken the upwelling in the SEAS, leading to warm SST anomaly and suppressing the upward transport of the subsurface nutrients to the surface. A weaker positive IOD occurred in 2015, leading to stronger upwelling in the SEAS than during 2019. Yet, seawater in the SEAS experienced extreme warming (lowest SST exceeded 28.5^oC) due to the development of super El Niño in 2015. The significant seawater warming can shoal mixed layer and prevent the nutrients in the subsurface from reaching surface, which is unfavorable for the chlorophyll growth. The thermal balance analysis suggests that the extreme warming in the SEAS was mainly related to more downward shortwave radiation.

How to cite: Huang, H.: Negative surface chlorophyll concentration anomalies in the southeastern Arabian Sea during 2015 and 2019 summers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4833, https://doi.org/10.5194/egusphere-egu23-4833, 2023.

X5.284

EGU23-7758

OS1.3

ECS

Highlight

Seasonal variation in the barrier layer of the Banda Sea

Mochamad Furqon Azis Ismail and Johannes Karstensen

The Banda Sea is crucial to the circulation of the world's oceans and atmosphere due to its location within the equatorial regions of the Indonesian Maritime Continent. It links the Pacific and Indian Oceans' circulation via the Indonesian Throughflow and contributes to driving atmospheric conditions via heat and moisture fluxes. Strong salinity-stratified barrier layers have the potential to play a significant role in air-sea interaction by separating the base of the mixed layer from the top of the thermocline and reducing the exchange of surface heat and momentum with the ocean's subsurface. In this study, we present the seasonal variability of barrier layer thickness (BLT) and its formation mechanism in the Banda Sea using the eddy-resolving ocean reanalysis Bluelink version 2020 (BRAN2020) for 1993 to 2021 and air-sea flux data. The findings show that the BLT is a persistent feature in the Banda Sea with a strong seasonal cycle. The BLT maxima appear in the southeast monsoon season period from May to July and the minima in the pre-northwest monsoon season from October-November. The spatial distribution of BLT is zonally oriented along the sea surface salinity (SSS) front from the west to the east of the Banda Sea. We suggest that the horizontal advection of low salinity water from the Java Sea and precipitation contributes to the formation of BLT formation and variability in the Banda Sea.

How to cite: Ismail, M. F. A. and Karstensen, J.: Seasonal variation in the barrier layer of the Banda Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7758, https://doi.org/10.5194/egusphere-egu23-7758, 2023.

X5.285

EGU23-8346

OS1.3

ECS

Variability of Warming at Mixed Layer Base in the Subtropical South Indian Ocean Salinity Maxima Region

Madhu Kaundal, Jithendra Raju Nadimpalli, and Mihir Kumar Dash

Subtropical South Indian Ocean salinity maxima region plays an important role in transporting temperature anomalies towards north along the isopycnals following geostrophic pathways. In this study, interannual and decadal changes in temperature and salinity at the base of mixed layer during austral winters are investigated for the Argo era. Winter time deep mixed layer allows these Temperature/Salinity (T/S) changes to penetrate to the permanent pycnocline. Interannual changes in the mixed layer depth (MLD) are mostly driven by convective buoyancy and wind forcing. Contribution of different atmospheric and oceanic forcing to the changes in mixed layer temperature and salinity are shown using mixed layer budget calculation. It is observed that net heat flux term dominates the temperature changes whereas meridional advection plays a important role in driving salinity changes in the mixed layer. Mixed layer T/S changes are subducted to the permanent pycnocline mainly by lateral induction process because of large meridional MLD gradient. Density compensated anomalies also contribute to the T/S changes at the bottom of the mixed layer. Interannual temperature anomalies due to spiciness and heaving are further explored.

How to cite: Kaundal, M., Nadimpalli, J. R., and Dash, M. K.: Variability of Warming at Mixed Layer Base in the Subtropical South Indian Ocean Salinity Maxima Region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8346, https://doi.org/10.5194/egusphere-egu23-8346, 2023.

X5.286

EGU23-17246

OS1.3

ECS

Dynamics and impact of diurnal warm layers in the ocean

Mira Schmitt, Sutanu Sarkar, Hieu T. Pham, and Lars Umlauf

Thin Diurnal Warm Layers (DWLs) form near the surface of the ocean on days of large solar radiation,
weak to moderate winds, and small surface waves. DWLs are characterized by complex dynamics,
and are relevant to the ocean especially by modifying surface-layer mixing and atmosphere-ocean
fluxes. Here, we use idealized Large Eddy Simulations (LES) and second-moment turbulence
modelling, both including the effects of Langmuir turbulence, to identify the key non-dimensional
parameters of the problem, and explore DWL properties and dynamics across a wide parameter
space. Comparison of LES and the second-moment turbulence models shows that the latter provide
an accurate representation of the DWL structure and dynamics. We find that, for equilibrium wave
conditions, Langmuir effects are significant only in the Stokes layer very close to the surface. While
we see pulses in the turbulent stresses and shear in the LES, there are no relevant effects of
Langmuir turbulence on DWL bulk properties and total entrainment. Results of the parameter space
analysis agree with the midday scaling by Pollard et al. (1986), however, with modified model
coefficients and deviations of up to 30% especially at high-latitudes. We develop non-dimensional
expressions for the strength and timing of the DWL temperature peak in the afternoon, and discuss
the mixing efficiency and energetics of DWLs in the presence of Langmuir turbulence.

How to cite: Schmitt, M., Sarkar, S., Pham, H. T., and Umlauf, L.: Dynamics and impact of diurnal warm layers in the ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17246, https://doi.org/10.5194/egusphere-egu23-17246, 2023.

X5.287

EGU23-17247

OS1.3

Highlight

Subducting filaments in frontal zones in the Western Mediterranean Sea: Physical, turbulent and biological evidences

Francesco Marcello Falcieri, Lorenzo Pasculli, and Giovanni Testa

Oceanic frontal areas are well known as sites prone to the generation of submesoscale instabilities that can lead to surface waters subduction along isopycnal surfaces well below the mixed layer. Those processes can play an important role in the vertical exchange of physical properties and biogeochemical tracers.
In the framework of the CALYPSO DRI research initiative (Coherent Lagrangian Pathways from the Surface Ocean to Interior), turbulent dissipation rates characterizing subducting filaments originated form frontal areas were studied with a free falling microstructure profiler. Microstructure profiles, along ancillary data, were collected on several transects along and across frontal areas and mesoscale eddies in the Western Mediterranean Sea during two cruises: one in the Alboran Sea (March/April 2019) and one in the Balearic Sea (February/March 2022).
The presence of subducting filaments moving along isopycnal surfaces was identified at depths between 100 and 250 m by the combined analysis of physical (i.e. temperature and salinity), chemical (i.e. dissolved oxygen) and biological properties (i.e. high chlorophyll concentration well below the mixed layer and the deep chlorophyll maximum). The majority of the subducting filaments were characterized by turbulent kinetic energy dissipation rates (TKE, values of 10^-7 W·m^-2) much higher than rates generally observed at such depths. The TKE values were found in conjunction with an increase in Brunt Vaisala frequency and low Thorpe scale values. The same conclusion can be drawn from Turner angle values.

How to cite: Falcieri, F. M., Pasculli, L., and Testa, G.: Subducting filaments in frontal zones in the Western Mediterranean Sea: Physical, turbulent and biological evidences, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17247, https://doi.org/10.5194/egusphere-egu23-17247, 2023.

X5.288

EGU23-10563

OS1.3

ECS

Buoy Observations of Turbulent Mixing in the northwestern subtropical Pacific Ocean

Hsin-I Lin and Yiing-Jang Yang

Strong winds from typhoons decrease sea surface temperatures, and this cooling area is called the cold wake. It is well known that the primary mechanisms causing this phenomenon include turbulent mixing and Ekman pumping that result in the upwelling of cold water in the lower layer. The amplitude of surface cooling is influenced by the typhoon’s moving speed, strength, the radius of the storm, and the pre-typhoon conditions of the upper ocean. The cooling phenomenon affects air-sea interactions, but observing the upper ocean in such an extreme environment is challenging. To understand the physical process of mixing, and to improve the predictions of numerical models, more observations of turbulent mixing are needed. In addition, the passage of eddy also affects the occurrence of background conditions and turbulent mixing. The northwestern subtropical Pacific Ocean is an area where typhoons are prevalent, and eddies often pass through it. Therefore, this is a suitable area to study turbulent mixing when typhoons and eddies pass by.

These data were obtained from the surface buoy and the ADCP subsurface moorings located in the northwestern subtropical Pacific Ocean. In September 2022, category 5 typhoon Hinnamnor passed the buoy site during observation period. The upper ocean profiles of temperature and current were obtained, in order to estimate the Richardson numberwhen the typhoon and the cold eddy passed by. The observation results show that the peak value of the probability distribution of the Richardson number was about 3 to 4, and the probability of being less than 0.25 was about 12% at a depth of 20 m before the passage of Typhoon Hinnamnor. When the buoy system was within the 34-knot wind radius (R34) of a typhoon, the peak value of the probability distribution of the Richardson number decreased to slightly smaller than 0.25, and the probability of being less than 0.25 is about 62% at a depth of 20 m. At a depth of 75 m, the probability distribution of the Richardson number did not significantly change within the 34-knot wind radius (R34) of a typhoon,and it was not even close to 0.25. It shows that typhoon-induced turbulent mixing has no effect at this depth. In addition, during the normal period without a cold eddy, the mixed layer was deeper than the depth of 20 m. Marginal instability was evident within the mixed layer, in which the probability distribution of the Richardson number oscillated around 0.25. During the passage of the cold eddy, the upwelling of cold water made the surface mixed layer thinner, and stratification was more stable. Therefore, the cold eddy would prohibit turbulent mixing. The probability distribution of the Richardson number shifted to a larger value. However, the probability distribution of the Richardson number at a depth of 75 m did not change significantly. As a result, the observed cold eddy had no effect on the turbulent mixing at this depth. These results will be presented herein in detail.

How to cite: Lin, H.-I. and Yang, Y.-J.: Buoy Observations of Turbulent Mixing in the northwestern subtropical Pacific Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10563, https://doi.org/10.5194/egusphere-egu23-10563, 2023.

X5.289

EGU23-8431

OS1.3

Imaging small-scale ocean dynamics at interfaces of the Earth System with the SeaSTAR Earth Explorer 11 mission candidate

Christine Gommenginger, Adrien C. H. Martin, David L. McCann, Alejandro Egido, Kevin Hall, Petronilo Martin-Iglesias, and Tânia Casal

SeaSTAR is a satellite mission candidate for ESA Earth Explorer 11 that proposes to measure small-scale ocean dynamics below 10 km at ocean/atmosphere/land/ice interfaces of the Earth System. SeaSTAR products consist of high-resolution images of total surface current vectors and wind vectors of unprecedented resolution (1 km) and accuracy over a wide swath. A key objective of SeaSTAR is to characterize, for the first time, the magnitude, spatial structure, regional distribution and temporal variability of upper ocean dynamics on daily, seasonal and multi-annual time scales, with particular focus on coastal seas, shelf seas and Marginal Ice Zone boundaries. The mission addresses an urgent need for new measurements of small-scale ocean processes to help understand and model their impacts on air-sea interactions, horizontal water pathways, vertical mixing and marine productivity. High-resolution imaging of total currents with collocated wind and waves data would bring new means of validating and developing models to improve operational forecasts and climate projections. The presentation will outline the key elements of the mission and the latest status of the mission concept evolution, with the technical solutions and trade-offs that are being considered. We will also present the latest results of the SEASTARex airborne campaign in Iroise Sea using the OSCAR (Ocean Surface Current Airborne Radar) demonstrator.

How to cite: Gommenginger, C., Martin, A. C. H., McCann, D. L., Egido, A., Hall, K., Martin-Iglesias, P., and Casal, T.: Imaging small-scale ocean dynamics at interfaces of the Earth System with the SeaSTAR Earth Explorer 11 mission candidate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8431, https://doi.org/10.5194/egusphere-egu23-8431, 2023.

X5.290

EGU23-10370

OS1.3

ECS

Atmospheric weather patterns and their contributions to the fall stratification breakdown on the Southern New England shelf

Lukas Lobert, Glen Gawarkiewicz, and Al Plueddemann

High-wind events predominantly cause the rapid breakdown of seasonal stratification on mid-latitude continental shelfs. It is well established that downwelling-favorable wind forcing, i.e., wind vectors with the coastline to their right (on the northern hemisphere), leads to enhanced coastal destratification. A categorization scheme for high-wind events has identified the two atmospheric weather patterns that locally cause such favorable wind conditions on the Southern New England shelf and have the largest contribution to the annual breakdown of stratification in the region. These patterns are i) cyclonic storms that propagate south of the continental shelf and cause strong anticyclonically rotating winds, and ii) persistent large-scale high-pressure systems over eastern Canada causing steady north-easterly winds. Despite both patterns generally producing downwelling-favorable winds on the shelf, the two patterns differ in their wind direction steadiness and tend to produce opposite temperature and salinity contributions to destratification, implying differences in the dominant processes driving ocean mixing. We hypothesize that local mechanical mixing and surface cooling dominate for cyclonic storms due to their strong wind energy input and shear production. In contrast, the weaker but steady downwelling-favorable winds from high-pressure systems can lead to an enhanced cross-shelf Ekman cell that advects salty and less buoyant Slope Water onto the continental shelf. To assess which process dominates for the different impactful high-wind event patterns, we apply a simplified two-dimensional mixed-layer model framework that incorporates horizontal buoyancy gradients across the shelfbreak front. The model allows to determine the stratification change caused by one-dimensional surface forcing (wind stress and surface buoyancy flux) and Ekman-driven advection individually. Observations from moorings and glider transects across the shelfbreak, provided by the Ocean Observatories Initiative Coastal Pioneer Array (2015-2022) at the Southern New England shelfbreak, allow a comparison to investigate the importance of along-shelf processes for predicting shelf stratification changes on synoptic to intra-seasonal timescales.

How to cite: Lobert, L., Gawarkiewicz, G., and Plueddemann, A.: Atmospheric weather patterns and their contributions to the fall stratification breakdown on the Southern New England shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10370, https://doi.org/10.5194/egusphere-egu23-10370, 2023.

X5.291

EGU23-8162

OS1.3

The mixed layer depth in Ocean Model Intercomparison Project (OMIP) high resolution models

Anne Marie Tréguier, Clement de Boyer Montégut, Eric Chassignet, Baylor Fox-Kemper, Andy Hogg, Doroteaciro Iovino, Andrew Kiss, Julien le Sommer, Camille Lique, Pengfei Lin, Hailong Liu, Guillaume Serazin, Dmitry Sidorenko, Steve Yeager, and Qiang Wang

The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice, and plays a key role in climate variability. Numerical models used in climate studies should therefore have a good representation of the mixed layer, especially its depth (MLD). Here we use simulations from the Ocean Model Intercomparison Project (OMIP), which have been forced by a common atmospheric state, to assess the realism of the simulated MLDs. For model validation, an updated MLD dataset has been computed from observations using the fixed density threshold recommended by the OMIP protocol. We evaluate the influence of horizontal resolution by using six pairs of simulations, non-eddying (typically 1° resolution) and eddy-rich (1/10° to 1/16° resolution). In winter, low resolution models exhibit large biases in the deep water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode water formation regions of the northern hemisphere, where the eddy-rich models produce a more robust MLD and deep biases are reduced. The Southern Ocean offers a more contrasted view, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability of MLD in winter. Contrary to an hypothesis that the deepening of the MLD in anticyclones would make the MLD deeper globally, eddy-rich models tend to have a shallower MLD in the zonal mean. In summer, a deep MLD bias is found in all the non-eddying models north of the equator; this bias is greatly reduced at high resolution. In addition, our study highlights the sensitivity of the MLD computation to choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects.

How to cite: Tréguier, A. M., de Boyer Montégut, C., Chassignet, E., Fox-Kemper, B., Hogg, A., Iovino, D., Kiss, A., le Sommer, J., Lique, C., Lin, P., Liu, H., Serazin, G., Sidorenko, D., Yeager, S., and Wang, Q.: The mixed layer depth in Ocean Model Intercomparison Project (OMIP) high resolution models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8162, https://doi.org/10.5194/egusphere-egu23-8162, 2023.

X5.292

EGU23-8577

OS1.3

ECS

On the ability of OMIP models to simulate the seasonal cycle of the ocean mixed layer depth in pan-Arctic Seas

Sofía Allende, Thierry Fichefet, Hugues Goosse, and Anne-Marie Tréguier

In this study, we assess the ability of the ocean-sea ice general circulation models that participated in the Ocean Model Intercomparison Project (OMIP) to simulate the seasonal cycle of the ocean mixed layer depth in pan-Arctic seas. We focus on the central Arctic Ocean, Beaufort, Chukchi, East Siberian, Laptev, Kara, and Barents Seas. All models underestimate the mixed layer depth by about 15m on average during summertime compared to the MIMOC (Monthly Isopycnal/Mixed layer Ocean Climatology) observational data. In fall and winter, differences of several tens of meters are noticed between the models themselves, and between the models and the observational data. Some models generate too deep mixed layers, while others produce too shallow mixed layers. The magnitude of these inter-model variations differs depending on the sea under consideration.

In almost all the seas, OMIP models with similar ocean stratification compared to MIMOC observational data display the best mixed layer depth at the end of the winter. Furthermore, all models simulate more or less the same sea ice mass balance and thus salt flux into the ocean during sea ice freezing. We argue that the discrepancies between models are not so much linked to the surface salt balance but rather to the accuracy with which those models reproduce the ocean stratification. To substantiate this behavior, we apply a simple conceptual model, which simulates the fall/winter month-to-month evolution of the mixed layer depth in ice-covered regions. In almost fully sea ice-covered regions such as the central Arctic Ocean, Beaufort, and Chukchi Seas, this simplified dynamics captures very well the behavior of the general circulation models, and this highlights that the main difference between the models is the ocean stratification. At the same time, in the East Siberian, Laptev, and Kara Seas, inter-model variations are not explained by the differences in ocean stratification, even though they contain a significant concentration of sea ice. In not fully sea ice-covered regions, such as the Barents Sea, the mixed layer depth dynamics is different: the retreat of the ice cover during summer is more significant than in fully covered regions, hence favoring exchanges with the atmosphere.

How to cite: Allende, S., Fichefet, T., Goosse, H., and Tréguier, A.-M.: On the ability of OMIP models to simulate the seasonal cycle of the ocean mixed layer depth in pan-Arctic Seas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8577, https://doi.org/10.5194/egusphere-egu23-8577, 2023.

X5.293

EGU23-14268

OS1.3

ECS

Observations of Sea Ice Melt and Ice-Ocean Boundary Layer Heat Fluxes in the Marginal Ice Zone North of Fram Strait

Simon F. Reifenberg, Wilken-Jon von Appen, Ilker Fer, Christian Haas, Mario Hoppmann, and Torsten Kanzow

Given the prospect of a merely seasonally ice-covered Arctic Ocean in the future and a consequential new quality of atmosphere-ocean coupling, understanding and quantifying oceanic processes that contribute to sea ice melt is of particular relevance.

A region of intense melting is the marginal ice zone (MIZ) north of Fram Strait, where inflowing warm Atlantic Water meets sea ice advected southward by the Transpolar Drift. We present observations of the ice-ocean boundary layer (IOBL) from a cruise of German research vessel Polarstern to that region in summer 2022, where we gathered continuous-in-time hydrographic observations from autonomous drifting stations on three separate ice floes, supplemented by intense observation periods of vertical microstructure profiles and ice cores from crewed stations during three revisits per floe throughout the drifting period.

The three occupied floes were oriented on a line approximately perpendicular to the ice edge, initially about 25 km apart from each other, with the southernmost floe located 75 km away from the edge. The drifting instrument platforms cover a common time period of approximately two weeks, under relatively quiescent atmospheric conditions. First results show that, while the floes exhibited similar drift trajectories dominated by superimposed diurnal and semidiurnal oscillations, the evolution of key IOBL variables, such as stratification, melt rates, friction velocity, and turbulent fluxes, varied considerably – both in time and among the occupied floes.

We plan to assess how this observed variability relates to other measured properties of sea ice (e.g., ice roughness, ice thickness distribution, floe size distribution) and of the upper ocean (e.g., ice-ocean velocity shear, turbulence, surface waves, internal waves and tides) and their interaction, in order to put our preliminary findings into the broader context: ocean controls on sea ice melt in the marginal ice zone north of Fram Strait.

How to cite: Reifenberg, S. F., von Appen, W.-J., Fer, I., Haas, C., Hoppmann, M., and Kanzow, T.: Observations of Sea Ice Melt and Ice-Ocean Boundary Layer Heat Fluxes in the Marginal Ice Zone North of Fram Strait, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14268, https://doi.org/10.5194/egusphere-egu23-14268, 2023.

X5.294

EGU23-9721

OS1.3

ECS

Effects of Ocean Mesoscale Processes on Biogeochemistry

Lucas Casaroli, Tatiana Ilyina, and Fatemeh Chegini

Mesoscale processes contemplate movements in the ocean ranging from tens to hundreds of kilometers. At this scale it is possible to observe phenomena such as eddies, vortices, fronts among others. These processes are of great importance to biogeochemical cycles as they, for example, can affect the transport of nutrients to the euphotic zone by vertical movements, alter the mixed layer depth through vertical displacement of isopycnals, as well as trap biological, chemical and physical properties inside eddies and meanders.
Ocean General Circulation Models (OGCM’s) either resolve the mesoscale eddies by increasing the model resolution or parameterize them. The chosen approach regarding the eddies comes with some caveats as it can lead to simulations limited to small time periods or substantial simplifications of the physical processes, which in turn can alter results and obtain a different configuration for ocean and atmosphere dynamics. Regarding biogeochemical tracers, how eddies are represented in the model bring different outcomes, showing solutions that are model dependent such as ocean regions acting as a net source or sink of nutrients, oxygen and carbon. Hence we can’t accurately constrain the ocean’s role as a carbon sink. Not only the results are model dependent, but also resolution dependent. Ocean models with higher resolutions indicate that the vertical profiles of salinity and temperature are substantially altered by mesoscale activity, thus it is expected that biogeochemical tracers are altered by eddy induced disturbances.
We present some preliminary results of the output of the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al 2013, Jungclaus et al 2020) in a 40 km and 10 km resolution on a global setup. As a result we show how changing the resolution affect on the upper ocean and mixed layer the major biogeochemical tracers and overall the carbon cycle.

How to cite: Casaroli, L., Ilyina, T., and Chegini, F.: Effects of Ocean Mesoscale Processes on Biogeochemistry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9721, https://doi.org/10.5194/egusphere-egu23-9721, 2023.

X5.295

EGU23-14300

OS1.3

ECS

Assessing the physical processes controlling oxygen subduction in CMIP6 models

Benedict Blackledge, Oliver Andrews, Esther Portela, Rory Bingham, and Damien Couespel

Approximately half of the total loss of upper ocean oxygen over the recent past has been driven by its reduced solubility in a warming ocean. The remainder can be explained by less well constrained changes to ocean circulation, mixing and biogeochemical processes. Model based studies have shown that the choice of mixing parameterisations as well as biogeochemical factors, can introduce substantial differences in the distribution of oxygen within an individual Earth System Model (ESM). Model Intercomparison Projects such as the latest Coupled Model Intercomparison Project Phase 6 provide an opportunity to explore processes controlling oceanic oxygen in a multi-model framework. Here we apply an oxygen transport decomposition to seven CMIP6 ESMs, using a well-established framework for the transport of tracers from the surface mixed layer into the ocean interior. We show that despite a close agreement in the oxygen concentration at the mixed-layer base, the transports to the ocean interior vary greatly between models. ESMs with similar physical ocean model components are clearly identifiable based on the spatial distribution of oxygen transport, both in the globally integrated transport terms and their inter-annual variability. Applying this decomposition to CMIP6 pre-industrial control experiments, we find the total oxygen subduction ranges between +0.6 to +1.1PMol yr-1, in agreement with an observationally based estimate. Despite broad agreement in the total magnitude of oxygen subduction, the inter-model range for individual transport terms is often large (+0.69 PMol yr-1 to -0.23 PMol yr-1 for vertical advection), implying a high degree of model uncertainty as to the physical processes controlling interior oxygen. We also characterise variability in oxygen transport terms and find that interannual variability in advective transport depends on the term and the model family. Lateral advection displays the greatest model-model difference in interannual variability, by a factor of ~6 between the most and least variable model. Mixed-layer entrainment of oxygen shows closer agreement between models, with interannual variability in this term differing by a factor of ~1.4. We recommend that future model intercomparisons including ocean biogeochemistry archive the relevant transport, production and consumption terms for key biogeochemical variables such as oxygen.

How to cite: Blackledge, B., Andrews, O., Portela, E., Bingham, R., and Couespel, D.: Assessing the physical processes controlling oxygen subduction in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14300, https://doi.org/10.5194/egusphere-egu23-14300, 2023.