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Mara A. Freilich and Amala Mahadevan

Abstract

Within the pycnocline, where diapycnal mixing is suppressed, both the vertical movement (uplift) of isopycnal surfaces and upward motion along sloping isopycnals supply nutrients to the euphotic layer, but the relative importance of each of these mechanisms is unknown. We present a method for decomposing vertical velocity w into two components in a Lagrangian frame: vertical velocity along sloping isopycnal surfaces and the adiabatic vertical velocity of isopycnal surfaces . We show that , where is the isopycnal slope and is the geometric aspect ratio of the flow, and that accounts for 10%–25% of the total vertical velocity w for isopycnal slopes representative of the midlatitude pycnocline. We perform the decomposition of w in a process study model of a midlatitude eddying flow field generated with a range of isopycnal slopes. A spectral decomposition of the velocity components shows that while is the largest contributor to vertical velocity, is of comparable magnitude at horizontal scales less than about 10 km, that is, at submesoscales. Increasing the horizontal grid resolution of models is known to increase vertical velocity; this increase is disproportionately due to better resolution of , as is shown here by comparing 1- and 4-km resolution model runs. Along-isopycnal vertical transport can be an important contributor to the vertical flux of tracers, including oxygen, nutrients, and chlorophyll, although we find weak covariance between vertical velocity and nutrient anomaly in our model.

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Amala Mahadevan, Joseph Oliger, and Robert Street

Abstract

The nonhydrostatic model with a free surface is numerically implemented in boundary-fitted curvilinear coordinates to model the mesoscale circulation in an ocean basin with natural topography. A semi-implicit numerical scheme is used, and the directional inhomogeneity in the elliptic equation for pressure is exploited to speed up the computation of its solution while using the multigrid method.

The model is used to simulate the circulation in the Gulf of Mexico. We observe the formation of the Loop Current and several eddies. The flow is very strongly controlled by the topography and our numerical experiments reveal that in the bottom layers, the flow along topographic contours is in the opposite direction of the anticyclonic circulation in the top layers.

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Gualtiero Badin, Amit Tandon, and Amala Mahadevan

Abstract

Using a process study model, the effect of mixed layer submesoscale instabilities on the lateral mixing of passive tracers in the pycnocline is explored. Mixed layer eddies that are generated from the baroclinic instability of a front within the mixed layer are found to penetrate into the pycnocline leading to an eddying flow field that acts to mix properties laterally along isopycnal surfaces. The mixing of passive tracers released on such isopycnal surfaces is quantified by estimating the variance of the tracer distribution over time. The evolution of the tracer variance reveals that the flow undergoes three different turbulent regimes. The first regime, lasting about 3–4 days (about 5 inertial periods) exhibits near-diffusive behavior; dispersion of the tracer grows nearly linearly with time. In the second regime, which lasts for about 10 days (about 14 inertial periods), tracer dispersion exhibits exponential growth because of the integrated action of high strain rates created by the instabilities. In the third regime, tracer dispersion follows Richardson’s power law. The Nakamura effective diffusivity is used to study the role of individual dynamical filaments in lateral mixing. The filaments, which carry a high concentration of tracer, are characterized by the coincidence of large horizontal strain rate with large vertical vorticity. Within filaments, tracer is sheared without being dispersed, and consequently the effective diffusivity is small in filaments. While the filament centers act as barriers to transport, eddy fluxes are enhanced at the filament edges where gradients are large.

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Amala Mahadevan, Joseph Oliger, and Robert Street

Abstract

The incompressibility and hydrostatic approximations that are traditionally used in large-scale oceanography to make the hydrodynamic equations more amenable to numerical integration result in the primitive equations. These are ill-posed in domains with open boundaries and hence not well-suited to mesoscale or regional modeling. Instead of using the hydrostatic approximation, the authors permit a greater deviation from hydrostatic balance than what exists in the ocean to obtain a system of equations that is well-posed with the specification of pointwise boundary conditions at open or solid boundaries. These equations, formulated with a free-surface boundary, model the mesoscale flow field accurately in all three-dimensions, even the vertical. It is essential to retain the vertical component of the Coriolis acceleration in the model since it is nonhydrostatic.

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Rui M. Ponte, Amala Mahadevan, Jayendran Rajamony, and Richard D. Rosen

Abstract

Changes in axial atmospheric angular momentum M are related to zonal torques on the atmosphere, but studies reveal large imbalances between the estimated torques and M variations on seasonal timescales. The observed imbalances are commonly attributed to uncertainties in the torque estimates. One particularly important torque component at the seasonal period is that due to zonal wind stresses over the ocean T O. The uncertainties in T O are explored by calculating different multiyear time series based on surface wind products derived from passive and active microwave satellite data. The satellite-based T O are compared to available reanalysis products. Results indicate that there are indeed substantial uncertainties in the seasonal T O, and that these uncertainties are related mostly to the wind fields rather than to the particular parameterizations of the surface stress in the boundary layer. Regional analyses point to the need to improve knowledge of the wind fields over extensive areas of the ocean, particularly in many tropical and southern latitude regions. Resolving subweekly variability in surface winds is also found to be important when determining the seasonal cycle in T O. The current satellite-based T O estimates can lead to a better seasonal momentum budget, but results are tempered by the uncertain effects of gravity wave torque in that budget.

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Sebastian Essink, Verena Hormann, Luca R. Centurioni, and Amala Mahadevan

Abstract

A cluster of 45 drifters deployed in the Bay of Bengal is tracked for a period of four months. Pair dispersion statistics, from observed drifter trajectories and simulated trajectories based on surface geostrophic velocity, are analyzed as a function of drifter separation and time. Pair dispersion suggests nonlocal dynamics at submesoscales of 1–20 km, likely controlled by the energetic mesoscale eddies present during the observations. Second-order velocity structure functions and their Helmholtz decomposition, however, suggest local dispersion and divergent horizontal flow at scales below 20 km. This inconsistency cannot be explained by inertial oscillations alone, as has been reported in recent studies, and is likely related to other nondispersive processes that impact structure functions but do not enter pair dispersion statistics. At scales comparable to the deformation radius L D, which is approximately 60 km, we find dynamics in agreement with Richardson’s law and observe local dispersion in both pair dispersion statistics and second-order velocity structure functions.

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Takeyoshi Nagai, Amit Tandon, Eric Kunze, and Amala Mahadevan

Abstract

While near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10–100-km-scale frontal meanders, accompanied by O(10) mW m−2 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation of near-inertial waves represents a redistribution of quasigeostrophic energy rather than a significant sink.

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Sebastian Essink, Verena Hormann, Luca R. Centurioni, and Amala Mahadevan

Abstract

Horizontal kinematic properties, such as vorticity, divergence, and lateral strain rate, are estimated from drifter clusters using three approaches. At submesoscale horizontal length scales O(1-10)-km, kinematic properties become as large as planetary vorticity ƒ, but are challenging to observe because they evolve on short O(hours-days) timescales. By simulating surface drifters in a model flow field, we quantify the sources of uncertainty in the kinematic property calculations due to the deformation of cluster shape. Uncertainties arise primarily due to (i) violation of the linear estimation methods and (ii) aliasing of unresolved scales. Systematic uncertainties (iii) due to GPS errors, are secondary but can become as large as (i) and (ii) when aspect ratios are small. Ideal cluster parameters (number of drifters, length scale, and aspect ratio) are determined and error functions due to cluster deformations are estimated empirically and theoretically. We apply the most robust method – a two-dimensional, linear least-squares fit – to the first few days of a drifter data set from the Bay of Bengal. Application of the length-scale and aspect-ratio criteria minimizes errors (i) and (ii), and reduces the number of clusters and so computational cost. The drifter-estimated kinematic properties map out a cyclonic mesoscale eddy with a surface, submesoscale fronts at its perimeter. Our analyses suggest methodological guidance for computing the two-dimensional kinematic properties in submesoscale flows, given the recently increasing quantity and quality of drifter observations, while also highlighting challenges and limitations.

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Mathieu Dever, Mara Freilich, J. Thomas Farrar, Benjamin Hodges, Tom Lanagan, Andrew J. Baron, and Amala Mahadevan

Abstract

The study of ocean dynamics and biophysical variability at submesoscales of O(1) km and O(1) h raises several observational challenges. To address these by underway sampling, we recently developed a towed profiler called the EcoCTD, capable of concurrently measuring both hydrographic and bio-optical properties such as oxygen, chlorophyll fluorescence, and optical backscatter. The EcoCTD presents an attractive alternative to currently used towed platforms due to its light footprint, versatility in the field, and ease of deployment and recovery without cranes or heavy-duty winches. We demonstrate its use for gathering high-quality data at submesoscale spatiotemporal resolution. A dataset of bio-optical and hydrographic properties, collected with the EcoCTD during field trials in 2018, highlights its scientific potential for the study of physical–biological interactions at submesoscales.

Open access
Amala Mahadevan, Ananda Pascual, Daniel L. Rudnick, Simón Ruiz, Joaquín Tintoré, and Eric D’Asaro
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