The viscous lee wave problem and its implications for ocean modelling

Ocean circulation models employ horizontal viscosity and diffusivity to represent unresolved sub-gridscale processes such as breaking internal waves. Computational power has now advanced sufficiently to permit regional ocean circulation models to be run at sufficiently high (100m-1km) horizontal resolution to resolve a significant part of the internal wave spectrum. Here we develop theory for boundary generated internal waves in such models, and in particular, where the waves dissipate their energy. We focus specifically on the steady lee wave problem where stationary waves are generated by a large-scale flow acting across ocean bottom topography. We generalise the energy flux expressions of Bell (1975) to include the effect of horizontal viscosity and diffusivity. Applying these results for realistic parameter choices we show that in the present generation of models with O(1)m2 s −1 horizontal viscosity/diffusivity boundary-generated waves will inevitably dissipate the majority of their energy within a few hundred metres of the boundary. This dissipation is essentially spurious since it is a direct consequence of the artificially high viscosity/diffusivity used in the numerical models and hence caution is necessary in comparing model results to ocean observations. Our theory further predicts that O(0.01)m2 s −1 horizontal viscosity/diffusivity is required to satisfactorily reduce the spurious dissipation and enable a realistic representation of wave dynamics in ocean modelsThe authors acknowledge funding from the ARC Centre of Excellence for Climate System Science grant number CE1101028

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

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

Asymmetric Internal Tide Generation in the Presence of a Steady Flow

The generation of topographic internal waves (IWs) by the sum of an oscillatory and a steady flow is investigated experimentally and with a linear model. The two forcing flows represent the combination of a tidal constituent and a weaker quasi-steady flow interacting with an abyssal hill. The combined forcings cause a coupling between internal tides and lee waves that impacts their dynamics of IWs as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream IW beams due to a quasi-Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a superbuoyancy (ω > N) downstream beam and an evanescent upstream beam are measured. Energy fluxes are measured and compared with the linear model, that has been extended to account for the coupling mechanism. The structure and amplitude of energy fluxes match well in most regimes, showing the relevance of the linear prediction for IW wave energy budgets, while the energy flux toward IW beams is limited by the generation of periodic vortices in a particular experiment. The upstream-bias energy flux-and consequently net horizontal momentum-described in Shakespeare (2020, https://doi.org/10.1175/JPO-D-19-0179.1) is measured in the experiments. The coupling mechanism plays an important role in the pathway to IW-induced mixing, that has previously been quantified independently for lee waves and internal tides. Hence, future parameterizations of IW processes ought to include the coupling mechanism to quantify its impact on the global distribution of mixing.This work was supported partly by theFrench PIA project LorraineUniversité d' Excellence, referenceANR-15-IDEX-04-LUE. Y. D.acknowledges support from theEmbassy of France in Australia. C. J. S.acknowledges support from an ARCDiscovery Early Career ResearcherAward DE180100087 and ANU Futures Scheme awar

Drivers of atmospheric and oceanic surface temperature variance: A frequency domain approach

Ocean–atmosphere coupling modifies the variability of Earth’s climate over a wide range of time scales. However, attribution of the processes that generate this variability remains an outstanding problem. In this article, air–sea coupling is investigated in an eddy-resolving, medium-complexity, idealized ocean–atmosphere model. The model is run in three configurations: fully coupled, partially coupled (where the effect of the ocean geostrophic velocity on the sea surface temperature field is minimal), and atmosphere-only. A surface boundary layer temperature variance budget analysis computed in the frequency domain is shown to be a powerful tool for studying air–sea interactions, as it differentiates the relative contributions to the variability in the temperature field from each process across a range of time scales (from daily to multidecadal). This method compares terms in the ocean and atmosphere across the different model configurations to infer the underlying mechanisms driving temperature variability. Horizontal advection plays a dominant role in driving temperature variance in both the ocean and the atmosphere, particularly at time scales shorter than annual. At longer time scales, the temperature variance is dominated by strong coupling between atmosphere and ocean. Furthermore, the Ekman transport contribution to the ocean’s horizontal advection is found to underlie the low-frequency behavior in the atmosphere. The ocean geostrophic eddy field is an important driver of ocean variability across all frequencies and is reflected in the atmospheric variability in the western boundary current separation region at longer time scales.This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant DGE 1256260. PEM also acknowledges the associated Graduate Research Opportunities Worldwide fellowship to conduct research at the Australian National University. Q-GCM and analysis were run on the National Computational Infrastructure (NCI), which is supported by the Australian Government. The codes are written in Python with the Pangeo environment. Specific software used includes NumPy (Harris et al. 2020), Matplotlib (Hunter 2007), xarray (Hoyer and Hamman 2017), and Dask (Dask Development Team 2016). PEM and BKA acknowledge support from NSF Grants OCE-0960820, OCE-1351837, and OCE-1851164, and the University of Michigan African Studies Center and M-Cubed program, the latter supported by the Office of the Provost and the College of Literature, Science, and the Arts

Response of the Southern Ocean Overturning Circulation to Extreme Southern Annular Mode Conditions

The positive trend of the Southern Annular Mode (SAM) will impact the Southern Ocean's role in Earth's climate; however, the details of the Southern Ocean's response remain uncertain. We introduce a methodology to examine the influence of SAM on the Southern Ocean and apply this method to a global ocean-sea ice model run at three resolutions (1◦, (1/4)◦, and (1/10)◦). Our methodology drives perturbation simulations with realistic atmospheric forcing of extreme SAM conditions. The thermal response agrees with previous studies; positive SAM perturbations warm the upper ocean north of the wind speed maximum and cool it to the south, with the opposite response for negative SAM. The overturning circulation exhibits a rapid response that increases/decreases for positive/negative SAM perturbations and is insensitive to model resolution. The longer-term adjustment of the overturning circulation, however, depends on the representation of eddies, and is faster at higher resolutions.Department of Education and Training | Australian Research Council (ARC). Grant Number: LP16010007

Heat and mass transport in geostrophic horizontal convection with surface wind stress

Direct Numerical Simulations are conducted to investigate heat and mass transport of flow with buoyancy forcing and surface wind stress. We use a re-entrant channel model with thermal and mechanical forcing similar to the Southern Ocean, with increasing surface wind stress. The model fully characterises convection and turbulence in the fluid. The presence of convection appears to significantly enhance the buoyancydriven overturning, resulting in an overturning cell which dominates the flow field compared with a relatively shallow and weak wind-driven cell. The vertical heat transport also indicates that the majority of vertical advective heat transport occurs in the convective zone, with strong upwelling of heat in this region. These results indicate that the presence of convection significantly enhances the impact of buoyancy forcing in driving mass and heat transport.This research was supported by the Australian Research Council grant DP140103706

Sequential changes in ocean circulation and biological export productivity during the last glacial-interglacial cycle: a model-data study

We conduct a model-data analysis of the marine carbon cycle to understand and quantify the drivers of atmospheric CO2 concentration during the last glacial-interglacial cycle. We use a carbon cycle box model, "SCP-M", combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and Southern Ocean biological export productivity across marine isotope stages spanning 130 000 years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea-ice cover and shallow-water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimized in each marine isotope stage against proxy data for atmospheric CO2, delta C-13 and Delta C-14 and deep-ocean delta C-13, Delta C-14 and CO32-. Our model-data results suggest that global overturning circulation weakened during Marine Isotope Stage 5d, coincident with a similar to 25 ppm fall in atmospheric CO2 from the last interglacial period. There was a transient slowdown in Atlantic meridional overturning circulation during Marine Isotope Stage 5b, followed by a more pronounced slowdown and enhanced Southern Ocean biological export productivity during Marine Isotope Stage 4 (similar to -30 ppm). In this model, the Last Glacial Maximum was characterized by relatively weak global ocean and Atlantic meridional overturning circulation and increased Southern Ocean biological export productivity (similar to -20 ppm during MIS 3 and MIS 2). Ocean circulation and Southern Ocean biological export productivity returned to modern values by the Holocene period. The terrestrial biosphere decreased by 385 Pg C in the lead-up to the Last Glacial Maximum, followed by a period of intense regrowth during the last glacial termination and the Holocene (similar to 600 Pg C). Slowing ocean circulation, a colder ocean and to a lesser extent shallow carbonate dissolution contributed similar to -70 ppm to atmospheric CO2 in the similar to 100 000-year leadup to the Last Glacial Maximum, with a further similar to -15 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological export productivity was one of the ingredients required to achieve the Last Glacial Maximum atmospheric CO2 level. We find that the incorporation of glacial-interglacial proxy data into a simple quantitative ocean transport model provides useful insights into the timing of past changes in ocean processes, enhancing our understanding of the carbon cycle during the last glacial-interglacial period

The financial cost of doctors emigrating from sub-Saharan Africa: human capital analysis

Objective To estimate the lost investment of domestically educated doctors migrating from sub-Saharan African countries to Australia, Canada, the United Kingdom, and the United States

The Extragalactic Distance Scale without Cepheids IV

The Cepheid period-luminosity relation is the primary distance indicator used in most determinations of the Hubble constant. The tip of the red giant branch (TRGB) is an alternative basis. Using the new ANU SkyMapper Telescope, we calibrate the Tully Fisher relation in the I band. We find that the TRGB and Cepheid distance scales are consistent.Comment: ApJ in press 201

Impact of increased resolution on Arctic Ocean simulations in Ocean Model Intercomparison Project phase 2 (OMIP-2)

This study evaluates the impact of increasing resolution on Arctic Ocean simulations using five pairs of matched low- and high-resolution models within the OMIP-2 (Ocean Model Intercomparison Project phase 2) framework. The primary objective is to assess whether a higher resolution can mitigate typical biases in low-resolution models and improve the representation of key climate-relevant variables. We reveal that increasing the horizontal resolution contributes to a reduction in biases in mean temperature and salinity and improves the simulation of the Atlantic water layer and its decadal warming events. A higher resolution also leads to better agreement with observed surface mixed-layer depth, cold halocline base depth and Arctic gateway transports in the Fram and Davis straits. However, the simulation of the mean state and temporal changes in Arctic freshwater content does not show improvement with increased resolution. Not all models achieve improvements for all analyzed ocean variables when spatial resolution is increased so it is crucial to recognize that model numerics and parameterizations also play an important role in faithful simulations. Overall, a higher resolution shows promise in improving the simulation of key Arctic Ocean features and processes, but efforts in model development are required to achieve more accurate representations across all climate-relevant variables.</p