Numerical Ocean Model Research Articles

Observations and modeling of tidally generated high-frequency velocity fluctuations downstream of a channel constriction

Abstract. We investigate data from an acoustic Doppler current profiler deployed in a constricted ocean channel showing a tidally dominated flow with intermittent velocity extrema during outflow from the constriction but not during inflow. A 2D numerical ocean model forced by tides is used to examine the spatial flow structure and underlying dynamical processes. We find that flow-separation eddies generated near the tightest constriction point form a dipole pair which propagates downstream and drives the observed intermittent flow variability. The eddies, which are generated by an along-channel adverse pressure gradient, spin up for some time near the constriction until they develop local low pressures in their centers that are strong enough to modify the background along-channel pressure gradient significantly. When the dipole has propagated some distance away from the constriction, the conditions for flow separation are recovered, and new eddies are formed.

Long-term changes in bloom dynamics of Southern and Central Baltic cold-water phytoplankton

In the Baltic Sea, cold-water adapted dinoflagellates and diatoms dominate the phytoplankton spring bloom of the Northern and Eastern Basins of the Baltic Sea. In the Central and Southern parts, where such species are less prominent, they cause occasional biomass peaks. We hypothesized that these dynamics correlate with ice cover, sea surface temperature (SST), and water transport processes, as the large Basins of the Central Baltic Sea are too deep to build-up blooms from their own seed banks. Long-term monitoring data from the past 40 and 20 years in the central and southern Baltic Sea, respectively, were analyzed here for biomass development of five cold-adapted taxa: the diatoms Pauliella taeniata, Thalassiosira baltica, Thalassiosira levanderi and Melosira spp. and the dinoflagellate Peridinella catenata. Results show that diatoms generally reached high biomass peaks in the 1980s and in shorter periods from 1995-1997, 2003- 2006, and 2010-2013 in all areas. We detected good correlations with the length of the ice cover period as well as low minimum and mean winter and spring SSTs. In contrast, biomass dynamics of the dinoflagellate P. catenata are more independent from these factors but have decreased strongly since the beginning of the 21st century. A numerical ocean model analysis confirmed the hypothesis that large blooms in the deep basins are seeded through water transport from adjacent shallow, ice-covered coastal areas such as the Gulf of Finland and the Gulf of Riga. Our results show that under ongoing climate warming, the common cold-water species may disappear from spring blooms in southern and central areas with unknown consequences for the ecosystem.

Clarifying the Distinction between Steric and Baroclinic Sea Surface Height

Abstract One of the most fundamental uses of ocean models is for the prediction of sea level. Vertical integration of the hydrostatic equation leads to the partitioning of sea level in terms of atmospheric pressure, steric height, and bottom pressure. In an effort to validate the baroclinic wave dynamics of numerical ocean models, some researchers have compared the steric height from models with the sea level anomaly derived from satellite altimetry. The use of steric height in these comparisons captures the qualitative aspects of the baroclinic waves, but it neglects a nonnegligible contribution from bottom pressure. A more accurate evaluation of baroclinic wave dynamics using sea level would involve projecting the pressure field onto orthogonal barotropic and baroclinic components to isolate the baroclinic sea level anomaly. This note illustrates the quantitative difference between steric height and baroclinic sea level, which amounts to approximately a 20% bias of steric height over baroclinic sea level, depending on location. Significance Statement The bottom pressure variability associated with superinertial internal waves is not negligible, but it has been neglected in a recent series of ocean model validation studies. Baroclinic surface pressure, and its associated sea level anomaly, should be used instead of steric height for model–data comparisons of baroclinic sea level variability.

An observing system experiment framework for the tropical Indian Ocean salinity: A case study using a constellation of three satellites

In this study impact of assimilating Sea Surface Salinity (SSS) from multi-satellites (SMOS, Aquarius and SMAP) on numerical ocean model simulations in the north Indian Ocean has been analysed under the observing system experiment (OSE) framework. Daily data sets of Aquarius, SMAP and SMOS, which were available for a common period of April–May 2015, are used to constrain the ocean model using ensemble optimal interpolation technique. Apart from the control simulation in which satellite data were not assimilated, a total of seven assimilation experiments using different combinations of satellite SSS were conducted. The impact of assimilation experiments is analysed by comparing the model-simulated variables with in situ observations. Assimilating satellite SSS results in a reduction in Root Mean Square Error (RMSE) in SSS (∼ 54%) and also in subsurface salinity (∼ 21%) over the control run. The impact of assimilating SMAP observations is maximum on model simulations with the errors reducing by ∼ 54%. Subsurface salinity improvement is better with three satellites with ∼31% improvement in RMSE in the halocline region, which was ∼11% more than single satellite assimilation. Assimilation of SSS also resulted in improved simulations of the model surface, subsurface temperature and mixed layer depth. Model results show the ability of SSS observations to complement other ocean observation networks. One important observation from this study is that while the impact of assimilating SSS observations from a single satellite was on par with the impact of assimilating SSS observations from two or three satellites in correcting simulated surface salinity, assimilation from more than one satellite had a larger impact in the salinity of deeper layers of the ocean.

Data Assimilation of Range- and Depth-Averaged Sound Speed from Acoustic Tomography Measurements in Fram Strait

Abstract The 2010–12 Acoustic Technology for Observing the Interior of the Arctic Ocean (ACOBAR) experiment provided acoustic tomography data along three 167–301-km-long sections in Fram Strait between Greenland and Spitsbergen. Ocean sound speed data were assimilated into a regional numerical ocean model using the Massachusetts Institute of Technology General Circulation Model–Estimating the Circulation and Climate of the Ocean four-dimensional variational (MITgcm-ECCO 4DVAR) assimilation system. The resulting state estimate matched the assimilated sound speed time series; the root-mean-squared (RMS) error of the sound speed estimate (∼0.4 m s−1) is smaller than the uncertainty of the measurement (∼0.8 m s−1). Data assimilation improved modeled range- and depth-averaged ocean temperatures at the 78°50′N oceanographic mooring section in Fram Strait. The RMS error of the state estimate (0.21°C) is comparable to the uncertainty of the interpolated mooring section (0.23°C). Lack of depth information in the assimilated ocean sound speed measurements caused an increased temperature bias in the upper ocean (0–500 m). The correlations with the mooring section were not improved as short-term variations in the mooring measurements and the ocean state estimate do not always coincide in time. This is likely due to the small-scale eddying and nonlinearity of the ocean circulation in Fram Strait. Furthermore, the horizontal resolution of the state estimate (4.5 km) is eddy permitting, rather than eddy resolving. Thus, the state estimate cannot represent the full ocean dynamics of the region. This study is the first to demonstrate the usefulness of large-scale acoustic measurements for improving ocean state estimates at high latitudes. Significance Statement Acoustic tomography measurements allow one to observe ocean temperature in large ocean volumes under the Arctic sea ice by measuring sound speed, which is hard to synoptically observe by other methods. This study has established methods for assimilation of depth- and range-averaged ocean sound speed from an acoustic tomography experiment in Fram Strait. For the first time, a 2-yr time series of ocean sound from acoustic tomography has been assimilated into an ocean state estimate. The results highlight the use of ocean tomography in ice-covered regions to improve state estimates of ocean temperature.

How fine is fine enough? Effect of mesh resolution on hydrodynamic simulations in coral reef environments

Coral reef environments are biodiversity hotspots that provide many services to coastal communities. They are currently facing an increasing anthropogenic pressure that jeopardises their survival. Numerical ocean models are an important tool to understand the functioning of coral ecosystems and the mechanisms ensuring their resilience. However, simulating the water circulation through reef systems is challenging because of their naturally complex topography and bathymetry. Many ocean models used for such applications have a spatial resolution coarser than the scale of individual reefs, which puts into question their suitability for the task. Here, we assess the sensitivity of a coastal ocean model’s outputs to its spatial resolution when simulating the water circulation in the entire Great Barrier Reef (Australia), the largest coral reef system in the world. We consider the same model with five different resolutions near reefs, ranging from 250 m to 4 km, and compare the model outputs at different locations around reefs and in the open ocean. We also simulate the transport of passive particles released from those different locations. Our results show that the simulated tidal signal is similar for all five resolutions. However, strong discrepancies (> 10 cm/s) in the current velocity are observed near the reefs and along the rugged coastline. When using a coarse-resolution model, the amplitude of the currents is overestimated over reefs, and underestimated between them. We find that validating the model at deep water mooring sites is not sufficient to ensure it performs well close to reefs. Discrepancies in currents lead to more directional and uniform tracer dispersal patterns on coarse-resolution meshes that contrast with the more dispersive patterns observed on fine-resolution meshes. Those differences at the reef level have a large cumulative effect when simulating transport processes over several weeks. Our results suggest that ocean circulation and transport simulations in coral reef environments should be based on model resolutions finer than the reef scale, which generally means a maximum resolution of about 250–500 m.

The Iceland–Faroe warm-water flow towards the Arctic estimated from satellite altimetry and in situ observations

Abstract. The inflow of warm and saline Atlantic water to the Arctic Mediterranean (Nordic Seas and Arctic Ocean) between Iceland and the Faroes (IF inflow) is the strongest Atlantic inflow branch in terms of volume transport and is associated with a large transport of heat towards the Arctic. The IF inflow is monitored in a section east of the Iceland–Faroe Ridge (IFR) by use of sea level anomaly (SLA) data from satellite altimetry, a method that has been calibrated by in situ observations gathered over 2 decades. Monthly averaged surface velocity anomalies calculated from SLA data were strongly correlated with anomalies measured by moored acoustic Doppler current profilers (ADCPs) with consistently higher correlations when using the reprocessed SLA data released in December 2021 rather than the earlier version. In contrast to the earlier version, the reprocessed data also had the correct conversion factor between sea level slope and surface velocity required by geostrophy. Our results show that the IF inflow crosses the IFR in two separate branches. The Icelandic branch is a jet over the Icelandic slope with average surface speed exceeding 20 cm s−1, but it is narrow and shallow with an average volume transport of less than 1 Sv (106 m3 s−1). Most of the Atlantic water crosses the IFR close to its southernmost end in the Faroese branch. Between these two branches, water from the Icelandic branch turns back onto the ridge in a retroflection with a recirculation over the northernmost bank on the IFR. Combining multi-sensor in situ observations with satellite SLA data, monthly mean volume transport of the IF inflow has been determined from January 1993 to December 2021. The IF inflow is part of the Atlantic Meridional Overturning Circulation (AMOC), which is expected to weaken under continued global warming. Our results show no weakening of the IF inflow. Annually averaged volume transport of Atlantic water through the monitoring section had a statistically significant (95 % confidence level) increasing trend of (0.12±0.10) Sv per decade. Combined with increasing temperature, this caused an increase of 13 % in the heat transport, relative to 0 ∘C, towards the Arctic of the IF inflow over the 29 years of monitoring. The near-bottom layer over most of the IFR is dominated by cold water of Arctic origin that may contribute to the overflow across the ridge. Our observations confirm a dynamic link between the overflow and the Atlantic water flow above. The results also provide support for a previously posed hypothesis that this link may explain the difficulties in reproducing observed transport variations in the IF inflow in numerical ocean models, with consequences for its predictability under climate change.

On dynamical downscaling of ENSO-induced oceanic anomalies off Baja California Peninsula, Mexico: role of the air-sea heat flux

The El Niño Southern Oscillation (ENSO) phenomenon is responsible for important physical and biogeochemical anomalies in the Northeastern Pacific Ocean. The event of 1997-98 has been one of the most intense in the last decades and it had large implications for the waters off Baja California (BC) Peninsula with a pronounced warm sea surface temperature (SST) anomaly adjacent to the coast. Downscaling of reanalysis products was carried out using a mesoscale-resolving numerical ocean model to reproduce the regional SST anomalies. The nested model has a 9 km horizontal resolution that extend from Cabo Corrientes to Point Conception. A downscaling experiment that computes surface fluxes online with bulk formulae achieves a better representation of the event than a version with prescribed surface fluxes. The nested system improves the representation of the large scale warming and the localized SST anomaly adjacent to BC Peninsula compared to the reanalysis product. A sensitivity analysis shows that air temperature and to a lesser extent wind stress anomalies are the primary drivers of the formation of BC temperature anomaly. The warm air-temperature anomalies advect from the near-equatorial regions and the central north Pacific and is associated with sea-level pressure anomalies in the synoptic-scale atmospheric circulation. This regional warm pool has a pronounced signature on sea level anomaly in agreement with observations, which may have implications for biogeochemistry.

Anomalous Nutricline Drives Marked Biogeochemical Contrasts During the Kuroshio Large Meander

AbstractThe large meander (LM) of the Kuroshio western boundary current is well known to influence the local climate, fisheries, and aquaculture by greatly modulating regional heat transport, but its impact on biogeochemical processes remains unclear. Using a high‐resolution numerical ocean model and long‐term observational datasets, we show that the path of the Kuroshio south of Japan determines the extent of the shallow nutricline region, where winter convective mixing replenishes nutrient availability for subsequent blooms of phytoplankton during spring. During the LM phase, this mechanism triggers offshore phytoplankton blooms that can be twice as strong as those during the non‐large meander phase. In contrast, spring blooms are weaker in the nearshore region where the nutricline depth is deeper, which reduces the nutrient supply by winter convective mixing during the LM. The LM also modulates the spatial distributions of primary production, air‐sea carbon flux, and export production. Hence, the Kuroshio path variability has broader implications for marine ecosystems and biogeochemical cycles. The biogeochemical impacts of the Kuroshio LM exert bottom‐up control on regional marine ecosystems that may be disproportionate to its thermal effect, and therefore need to be assessed to understand the LM's overall impacts on fisheries and aquaculture.

3-D Adaptive AUV Sampling for Classification of Water Masses

Autonomous underwater vehicles with onboard computing units foster innovative approaches for sampling oceanographic phenomena. Feedback of observations via the onboard model for planning algorithms enable adaptive sampling for such robotic units. In this work, we develop, implement, and test an adaptive sampling algorithm for efficient sampling of water masses in a 3-D frontal system. Focusing on a river plume, salinity variations are used to characterize the water masses. A threshold in salinity is assumed to distinguish the ocean and river waters, so that excursions below the threshold define river waters. The onboard model builds on a Gaussian random field representation of the salinity variations in (north, east, depth) coordinates. This model is initially trained from numerical ocean model data, and then updated with data gathered by the vehicle sensor. The Gaussian random field model further allows closed-form expressions of the expected spatially integrated Bernoulli variance of the salinity excursion set, which is used to reward sampling efforts. Combining these results with forward-looking planning algorithms, we suggest a workflow for 3-D adaptive sampling to map river plume systems. Simulation studies are used to compare the suggested approach with others. Results of field trials in the Nidelva river plume in Norway are presented and discussed.

Predicting bottom current deposition and erosion on the ocean floor

AbstractMapping sediment deposition and erosion by thermohaline ocean bottom currents is important for the development of ocean infrastructure, future geo resources and understanding the sedimentology of contourites and abyssal sediment wavefields. However, only a limited percentage (estimated 20%) of the ocean floor has been mapped directly through seismic or sonar imaging. To better delineate where zones of bottom current deposition and erosion exist, we develop a prediction from numerical model solutions and sedimentological measurements of the ocean floor. This is achieved by integrating three types of data, which include the following: (1) bottom current shear stress from a model run of the HYCOM numerical ocean model; (2) sedimentation rates from ocean lithospheric age and sediment thickness from the GlobSed Model; (3) the measured extents of bottom current deposits from sonar observations. Shear stresses and sedimentation rates inside and outside the mapped extents of bottom current deposits allow us to quantify the conditions that are conducive for bottom current deposition. These conditions are then extrapolated and displayed on a 1/12° arcsecond resolution map of the world's oceans and validated through comparison with known, mapped systems. Based on our prediction, around 12% of the ocean has significant deposition by bottom currents while only 1% has erosion. Most bottom current activity occurs where thermohaline currents impinge upon the ocean floor like on continental slopes or some areas of the abyssal plain. Deposition and erosion also occur where constriction of ocean bottom currents takes place as in straits and seaways. Inland basins (i.e. seas) and continental shelves are mostly disconnected from global‐ocean thermohaline bottom current conveyors and, therefore, have limited bottom current deposition and erosion. Mid‐ocean ridges also have little bottom current deposition due to low sediment supply.

Zonal circulation in the North Atlantic ocean at 52°W from WOCE-WHP and CLIVAR sections: 1997, 2003 and 2012

The A20 line is a meridional hydrographic section located at 52°W that cuts through the western North Atlantic Subtropical Gyre (NASG). It encloses the main paths of the Atlantic Meridional Overturning Circulation (AMOC). Using data from three A20 hydrographic cruises carried out in 1997, 2003 and 2012 together with Acoustic Doppler Current Profiler data and the velocities from an inverse box model, the circulation of the western NASG is estimated. The Gulf Stream is the main poleward path of the AMOC, carrying 155.3 ± 11.1, 102.7 ± 13.5 and 181.1 ± 14.9 Sv in 1997, 2003 and 2012, respectively (1 Sv = 106 m3/s ∼ 109 kg/s). In opposite direction, the Deep Western Boundary Current crosses the section at the northern/southern boundaries with a mass transport of −21.2 ± 8.9/29.0 ± 9.1, −14.4 ± 10.8/14.2 ± 8.1 and −37.9 ± 10.2/44.5 ± 9.8 Sv in 1997, 2003 and 2012, respectively. A net heat exchange from the ocean to the atmosphere is estimated to be −0.7 ± 0.1 PW and −0.6 ± 0.1 PW in 1997 and 2012, respectively, but is negligible in 2003 (0.1 ± 0.1 PW). The freshwater flux is significantly lower in 2003 (0.3 ± 0.1 Sv) than in 1997 (0.6 ± 0.1 Sv) and 2012 (0.6 ± 0.1 Sv). Ocean numerical models such as ECCO, GLORYS and MOM are used to address the interannual variability between the three surveys, but only the GLORYS output roughly agrees with the hydrographic data. GLORYS suggests a heat transport from the ocean to the atmosphere throughout the year and a net freshwater flux supported by precipitation plus river runoff higher than evaporation in all seasons except the spring.

The role of preconditioning for extreme storm surges in the western Baltic Sea

Abstract. When natural hazards interact in compound events, they may reinforce each other. This is a concern today and in light of climate change. In the case of coastal flooding, sea-level variability due to tides, seasonal to inter-annual salinity and temperature variations, or larger–scale wind conditions modify the development and ramifications of extreme sea levels. Here, we explore how various prior conditions could have influenced peak water levels for the devastating coastal flooding event in the western Baltic Sea in 1872. We design numerical experiments by imposing a range of precondition circumstances as boundary conditions to numerical ocean model simulations. This allows us to quantify the changes in peak water levels that arise due to alternative preconditioning of the sea level before the storm surge. Our results show that certain preconditioning could have generated even more catastrophic impacts. As an example, a simulated increase in the water level of 36 cm compared to the 1872 event occurred in Køge just south of Copenhagen (Denmark) and surrounding areas – a region that was already severely impacted. The increased water levels caused by the alternative sea-level patterns propagate as long waves until encountering shallow and narrow straits, and after that, the effect vastly decreases. Adding artificial increases in wind speeds to each study point location reveals a near-linear relationship with peak water levels for all western Baltic locations, highlighting the need for good assessments of future wind extremes. Our research indicates that a more hybrid approach to analysing compound events and readjusting our present warning system to a more contextualised framework might provide a firmer foundation for climate adaptation and disaster risk management. In particular, accentuating the importance of compound preconditioning effects on the outcome of natural hazards may avoid under- or overestimation of the associated risks.

Pulses of Cold Atlantic Water in the Arctic Ocean From an Ocean Model Simulation

AbstractThe intrusion of the warm and saline Atlantic Water (AW) has significant implications and feedback on the dynamics and thermodynamics of the Arctic Ocean. The AW enters the Arctic Ocean through two gateways: Fram Strait (FS) and the Barents Sea Opening (BSO). The AW is the dominant source of oceanic heat to the Arctic Ocean. In conjunction with the measurements in key hydrographic sections, numerical ocean modeling provides us with a useful tool to characterize and corroborate the temporal and spatial variability of the AW branches. Simulations are conducted using the regional configuration Arctic and North Hemispheric Atlantic (ANHA) of the ocean/sea‐ice model Nucleus for Modeling of the Ocean running at 1/4° and 1/12° resolution. Online passive tracers from the model configuration are used to trace the pathways of the AW inflow in the Arctic Ocean. While studying the variability of the AW through FS and the BSO, a cold mode of AW (Cold Atlantic Water [CAW]) with a temperature below 0℃ is detected along the rim of the eastern Eurasian Basin. It is formed primarily due to the intense sea surface cooling of the AW in the Barents and Kara Seas. The CAW exhibits the cascading process as it propagates downstream through the St. Anna Trough, leading to a considerable mixing with the ambient water. The CAW pulses eventually result in a significant heat content reduction in the eastern Arctic. This study provides insight into the changing AW property and its potential impact on the Arctic Basin.

Impacts of Tributary Inflows on the Circulation and Thermal Regime of the Green Bay Estuary of Lake Michigan

One of the largest freshwater estuarine systems on Earth, Green Bay receives freshwater runoff from its land watershed and cold-water intrusions from Lake Michigan. Southern Green Bay is a designated area of concern (AOC) due to ecosystem degradation, contaminated sediments, and poor water quality. Successful restoration of aquatic systems requires a clear understanding of their circulation and thermal regimes. Using a state-of-the-art numerical ocean model, this study examines the effect of variability in tributary inflows and the related lake intrusions on the circulation and thermal regime in Green Bay and on the mass and heat fluxes between lower- and upper–Green Bay areas. Our findings indicate that tributaries influence the circulation and thermal regime, locally and throughout the bay, and that the impact is more significant in hydrologically wet years. Model simulations showed evidence of small net transport out of lower Green Bay. This analysis of the simultaneous effects of tributary flows and lake intrusions in Green Bay can contribute to understanding the dynamics of freshwater estuaries and improve the planning of future restoration projects.

Combined physical and biogeochemical assessment of mesoscale eddy parameterisations in ocean models: Eddy induced advection at non-eddying resolutions

Ocean components of Earth System Models employed for climate projections do not routinely resolve mesoscale eddies for computational cost reasons, and the associated subgrid processes are still parameterised. While the performance of physics parameterisations in a numerical ocean model is normally assessed by examining the associated physical responses, biogeochemical responses are also important but often treated separately. Given recent advances in mesoscale eddy parameterisations, specifically for the eddy induced advection, this work systematically explores the joint consequences for physical as well as biogeochemical responses brought about by a more updated proposal for the eddy induced velocity coefficient, in the context of an idealised ocean relevant model. Relative to a high resolution mesoscale eddy resolving model, the more updated mesoscale eddy parameterisation is able to capture aspects of the model truth in the physical responses. The biogeochemical response is however rather more subtle, where a response with the conventional eddy parameterisation with a constant coefficient could arise from a physically inconsistent response. The present work highlights a need to assess both physical and biogeochemical aspects when judging the performance of eddy parameterisations, and additionally provides some important baseline model sensitivities that future assessments employing other parameterisations or in more complex settings could compare against.

Spatially varying anisotropy for Gaussian random fields in three-dimensional space

Isotropic covariance structures can be unreasonable for phenomena in three-dimensional spaces. In the ocean, the variability of a response may vary with depth, and ocean currents may lead to spatially varying anisotropy. We construct a class of non-stationary anisotropic Gaussian random fields (GRFs) in three dimensions through stochastic partial differential equations (SPDEs), where computations are done efficiently using Gaussian Markov random field approximations. A key novelty is the parametrization of the spatially varying anisotropy through vector fields.In a simulation study, we find that simple stationary models obtain reasonable parameter estimates with a moderate number of observations and a single realization, whereas the most complex non-stationary anisotropic model requires dense observations and multiple realizations. Further, we construct a stationary and a non-stationary GRF prior for salinity in an ocean mass outside Trondheim, Norway, based on simulations from the complex numerical ocean model SINMOD. These GRF priors are then evaluated using in-situ measurements collected with an autonomous underwater vehicle. We find that the new model outperforms the stationary anisotropic GRF prior for real-time prediction of unobserved locations both in terms of root mean square error and continuous rank probability score.

The vertical structure and intraseasonal variability of the deep currents in the Southern Philippine Basin

Based on the four-mooring array measurements along 130°E over 2015–2020, the vertical structure and variability of deep currents in the central Southern Philippine Basin (SPB) are revealed. The deep currents below 1500 m in the central SPB are characterized by alternating westward and eastward zonal jets with strong intraseasonal variability (ISV). These zonal jets have a weak mean velocity of less than 1.5 cm s−1 but vary with a relatively large amplitude of 10 cm s−1 at the intraseasonal period of 20–120 days. The full-depth zonal velocity in the central SPB has three typical vertical structures that are mainly controlled by the first baroclinic mode, the second and higher baroclinic modes, and the barotropic mode in descending order of occurrence frequency. These structures are vertically coherent below ∼2000–2500 m, below ∼1200 m, and over the whole water column, respectively. Further analysis of numerical ocean modeling results demonstrates that the ISV of deep currents is induced by the deep mesoscale eddy. The deep ISV intensifies when a deep eddy passes by and keeps at a low value when there is no deep eddy activity around. The deep eddy extends to the bottom with the upper boundaries at ∼1500 m below the subsurface layer or at ∼300 m including the subsurface layer. These deep eddies arise primarily from barotropic instability and secondarily from baroclinic instability.

Stochastic schemes for the perturbation of the atmospheric boundary conditions in ocean general circulation models

Advancing the representation of uncertainties in ocean general circulation numerical models is required for several applications, ranging from data assimilation to climate monitoring and extended-range prediction systems. The atmospheric forcing represents one of the main uncertainty sources in numerical ocean models. Here, we formulate and revise different approaches to perturb the air-sea fluxes used within the atmospheric boundary conditions. In particular, perturbation of the fluxes is performed either through i) stochastic modulation of the air-sea transfer coefficients; ii) stochastic modulation of the air-sea flux tendencies; iii) coarse-graining of stochastic sub-grid computation of the fluxes; or iv) multiple bulk formulas. The schemes are implemented and tested in the NEMO4 ocean model, implemented at an eddy-permitting resolution on a domain covering the North Atlantic and Arctic oceans and the Mediterranean Sea. A series of 22-year 4-member ensemble experiments with different stochastic schemes are performed and analyzed for the period 2000-2021, and results are compared in terms of the ensemble mean and, when applicable, ensemble spread of the principal oceanic diagnostics. Results indicate that the schemes, in general, can significantly improve some verification skill scores (e.g. against drifter current speed, SST analyses, and hydrographic profiles) and, in some cases, enhance the mesoscale activity and weaken the large-scale circulation. The response, however, is different depending on the specific scheme, whose choice thus depends on the target application, as detailed in the paper. These findings foster the adoption of these schemes in both extended-range operational ocean forecasts and coupled long-range climate prediction systems, where the boundary conditions perturbations may contribute to performance increases.

QuantifyingP-wave secondary microseisms events: a comparison of observed and modelled backprojection

SUMMARYSecondary microseisms are caused by nonlinear interactions between ocean waves of approximately equal wavelengths and opposite propagation directions. This seismic forcing is evaluated using ocean sea-state hindcast data and further modulated by the bathymetric effect. The numerical ocean model provides a global activity representation of the secondary microseisms, from which we isolate major events. We backprojected teleseismic P-wave propagation into the Earth's mantle to validate these events as effective seismic sources. The ocean model provides spectral amplitude information for modelling microseisms generated seismic wavefield. A comparison of the backprojection for P and PP phases from observed and synthetic microseisms forcing indicates high reliability in the ocean model, at least for major sources. A combination of P and PP phases detected across a global network of stations enables global ocean coverage. We improve backprojection images even further by introducing a two-step stacking for the P phase to address the problem of unbalanced station distribution. Thresholds of microseisms events forces valuable for seismic imaging are determined by comparing backprojections and ocean models for the years 2015 and 2020. Finally, we extracted a catalogue of microseisms events every 3-hr from 1994 to 2020 from the ocean hindcast data set. This catalogue is an intriguing resource for future applications of interferometric imaging at large scale.