Dependence of energy flux from the wind to surface inertial currents on the scale of atmospheric motions

AbstractAtmospheric features such as translating cold fronts and small lows with horizontal scales of about 100 km are traditionally thought to be most important in exciting near-inertial motions in the ocean. However, recent studies suggest that a significant fraction of energy flux from the wind to surface inertial currents may be supplied by atmospheric systems of larger scales. Here, the dependence of this energy flux on the scale of atmospheric motions is investigated using a high-resolution atmosphere reanalysis product and a slab model. It is found that mesoscale atmospheric systems with scales less than 1000 km are responsible for almost all the energy flux from the wind to near-inertial motions in the midlatitude North Atlantic and North Pacific. Transient atmospheric features with scales of ~100 km contribute significantly to this wind energy flux, but they are not as dominant as traditionally thought. Owing to the nonlinear nature of the stress law, energy flux from mesoscale atmospheric systems depends critically on the existence of the background, larger-scale wind field. Finally, accounting for relative motions in the stress calculation reduces the net wind energy flux to near-inertial motions by about one-fifth. Mesoscale atmospheric systems are found to be responsible for the majority of this relative wind damping effect.</jats:p

The annual cycle of surface eddy kinetic energy and its influence on eddy momentum fluxes as inferred from altimeter data

The annual cycle of surface eddy kinetic energy (EKE) and its influence on eddy momentum fluxes are investigated using an updated record of satellite altimeter data. It is found that there is a phase difference between the annual cycles of EKE in the western boundary current regions and EKE in the interior of the subtropical gyres, suggesting that different mechanisms may be at work in different parts of the subtropical gyres. The annual cycles of EKE averaged in the two hemispheres are found to be of similar magnitude but in opposite phase. As a result, the globally-averaged EKE shows little seasonal variability. The longer record of altimeter data used in this study has brought out a clearer and simpler picture of eddy momentum fluxes in the Gulf Stream and Kuroshio Extension. Considerable seasonal variations in eddy momentum fluxes are found in the western boundary current regions, which potentially play an important role in modulating the strength of the western boundary currents and their associated recirculation gyres on the seasonal time scale

Energetics of the global ocean: The role of mesoscale eddies

This article reviews the energy cycle of the global ocean circulation, focusing on the role of baroclinic mesoscale eddies. Two of the important effects of mesoscale eddies are: (i) the flattening of the slope of large-scale isopycnal surfaces by the eddy-induced overturning circulation, the basis for the Gent–McWilliams parametrization; and (ii) the vertical redistribution of the momentum of basic geostrophic currents by the eddy-induced form stress (the residual effect of pressure perturbations), the basis for the Greatbatch–Lamb parametrization. While only point (i) can be explained using the classical Lorenz energy diagram, both (i) and (ii) can be explained using the modified energy diagram of Bleck as in the following energy cycle. Wind forcing provides an input to the mean KE, which is then transferred to the available potential energy (APE) of the large-scale field by the wind-induced Ekman flow. Subsequently, the APE is extracted by the eddy-induced overturning circulation to feed the mean KE, indicating the enhancement of the vertical shear of the basic current. Meanwhile, the vertical shear of the basic current is relaxed by the eddy-induced form stress, taking the mean KE to endow the eddy field with an energy cascade. The above energy cycle is useful for understanding the dynamics of the Antarctic Circumpolar Current. On the other hand, while the source of the eddy field energy has become clearer, identifying the sink and flux of the eddy field energy in both physical and spectral space remains major challenges of present-day oceanography. A recent study using a combination of models, satellite altimetry, and climatological hydrographic data shows that the western boundary acts as a “graveyard” for the westward-propagating eddies

Sensitivity of Southern Ocean circulation to wind stress changes: Role of relative wind stress

The influence of different wind stress bulk formulae on the response of the Southern Ocean circulation to wind stress changes is investigated using an idealised channel model. Surface/mixed layer properties are found to be sensitive to the use of the relative wind stress formulation, where the wind stress depends on the difference between the ocean and atmosphere velocities. Previous work has highlighted the surface eddy damping effect of this formulation, which we find leads to increased circumpolar transport. Nevertheless the transport due to thermal wind shear does lose sensitivity to wind stress changes at sufficiently high wind stress. In contrast, the sensitivity of the meridional overturning circulation is broadly the same regardless of the bulk formula used due to the adiabatic nature of the relative wind stress damping. This is a consequence of the steepening of isopycnals offsetting the reduction in eddy diffusivity in their contribution to the eddy bolus overturning, as predicted using a residual mean framework

The Impact of Atmospheric Storminess on the Sensitivity of Southern Ocean Circulation to Wind Stress Changes

The influence of changing the mean wind stress felt by the ocean through alteration of the variability of the atmospheric wind, as opposed to the mean atmospheric wind, on Southern Ocean circulation is investigated using an idealised channel model. Strongly varying atmospheric wind is found to increase the (parameterised) near-surface viscous and diffusive mixing. Analysis of the kinetic energy budget indicates a change in the main energy dissipation mechanism. For constant wind stress, dissipation of the power input by surface wind work is always dominated by bottom kinetic energy dissipation. However, with time-varying atmospheric wind, near surface viscous dissipation of kinetic energy becomes increasingly important as mean wind stress increases. This increased vertical diffusivity leads to thicker mixed layers and higher sensitivity of the residual circulation to increasing wind stress, when compared to equivalent experiments with the same wind stress held constant in time. This may have implications for Southern Ocean circulation in different climate change scenarios should the variability of the atmospheric wind change rather than the mean atmospheric wind

On the seasonal variability of eddy kinetic energy in the Gulf Stream region

In the Gulf Stream region, eddy kinetic energy (EKE) peaks in summer while, as measured by the baroclinic eddy growth time scale, the ocean is most baroclinically unstable in late winter. We argue that the seasonally-varying Ekman pumping is unlikely to be responsible for the seasonal variation in growth time, and that the summer peak in EKE results from a reduction in dissipation in summer compared to winter

Reduction of Near-Inertial energy through the dependence of wind stress on the Ocean-Surface velocity

A realistic primitive-equation model of the Southern Ocean at eddying spatial resolution is used to examine the effect of ocean-surface-velocity dependence of the wind stress on the strength of near-inertial oscillations. Accounting for the ocean-surface-velocity dependence of the wind stress leads to a large reduction of wind-induced near-inertial energy of approximately 40 percent and of wind power input into the near-inertial frequency band of approximately 20 percent. A large part of this reduction can be explained by the leading-order modification to the wind stress if the ocean-surface velocity is included. The strength of the reduction is shown to be modulated by the inverse of the ocean-surface-mixed-layer depth. We conclude that the effect of surface-velocity dependence of the wind stress should be taken into account when estimating the wind-power input into the near-inertial frequency band and when estimating near-inertial energy levels in the ocean due to wind forcing

Using GPT-4 to Augment Unbalanced Data for Automatic Scoring

Machine learning-based automatic scoring can be challenging if students' responses are unbalanced across scoring categories, as it introduces uncertainty in the machine training process. To meet this challenge, we introduce a novel text data augmentation framework using GPT-4, a generative large language model, specifically tailored for unbalanced datasets in automatic scoring. Our experimental dataset comprised student-written responses to two science items. We crafted prompts for GPT-4 to generate responses resembling student-written answers, particularly for the minority scoring classes, to augment the data. We then finetuned DistillBERT for automatic scoring based on the augmented and original datasets. Model performance was assessed using accuracy, precision, recall, and F1 score. We incorporate varied amounts of augmented data to examine scoring performance, and our findings revealed remarkedly improved model performance. The average maximum increase observed across two items is: 3.5% for accuracy, 30.6% for precision, 21.1% for recall, and 24.2% for F1 score. Notably, using just 5% of the augmented data led to substantial improvements: 2.6%, 29.2%, 15.1%, and 19.6%. Interestingly, the extent of improvement varied depending on specific datasets. Moreover, we found that a varying amount of augmented data (5%-40%) was needed to obtain a stable improvement. We also compare models trained with GPT-4 augmented data and those trained with additional student-written responses. The findings indicate that former ones match or even exceed the performance of the latter. Specifically, there is an average difference of 1.7%, 1.9%, 11.0%, and 7.8% for four metrics separately. This research underscores the potential and effectiveness of data augmentation techniques utilizing GPT-4 in addressing unbalanced datasets within automated assessment

Latitudinal dependence of wind-induced near-inertial energy

Mid-latitude storms, accounting for the majority of wind energy input to near-inertial motions in the ocean, are known to shift their track significantly from one year to another. The consequence of such storm track shifts on wind-induced near-inertial energy (NIE) is yet unknown. Here, the latitudinal dependence of wind-induced NIE is first analysed in the framework of the slab model and then tested using two numerical ocean models. It is found that the NIE input by pure inertial wind stress forcing, which dominates the wind energy input to near-inertial motions, is independent of latitude. As a consequence, the NIE generated by white-noise wind stress forcing is also latitudinally independent. In contrast, the NIE generated by red-noise wind stress forcing shows strong dependence on latitude owing to longer inertial periods at lower latitudes capable of sampling greater inertial wind stress forcing. Given that the observed surface wind stress spectra are red, results from this study suggest that an equatorward shift of the storm track is likely to result in an increase in wind-induced NIE in the ocean, while the opposite is true for a poleward shift