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Michael J. Bell, Adam T. Blaker, and Joël J.-M. Hirschi

Abstract

Large-amplitude [±100 Sv (1 Sv ≡ 106 m3 s−1)], high-frequency oscillations in the Pacific Ocean’s meridional overturning circulation within 10° of the equator have been found in integrations of the NEMO ocean general circulation model. Part I of this paper showed that these oscillations are dominated by two bands of frequencies with periods close to 4 and 10 days and that they are driven by the winds within about 10° of the equator. This part shows that the oscillations can be well simulated by small-amplitude, wind-driven motions on a horizontally uniform, stably stratified state of rest. Its main novelty is that, by focusing on the zonally integrated linearized equations, it presents solutions for the motions in a basin with sloping side boundaries. The solutions are found using vertical normal modes and equatorial meridional modes representing Yanai and inertia–gravity waves. Simulations of 16-day-long segments of the time series for the Pacific of each of the first three meridional and vertical modes (nine modes in all) capture between 85% and 95% of the variance of matching time series segments diagnosed from the NEMO integrations. The best agreement is obtained by driving the solutions with the full wind forcing and the full pressure forces on the bathymetry. Similar results are obtained for the corresponding modes in the Atlantic and Indian Oceans. Slower variations in the same meridional and vertical modes of the MOC are also shown to be well simulated by a quasi-stationary solution driven by zonal wind and pressure forces.

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Adam T. Blaker, Joël J.-M. Hirschi, Michael J. Bell, and Amy Bokota

Abstract

The great ocean conveyor presents a time-mean perspective on the interconnected network of major ocean currents. Zonally integrating the meridional velocities, either globally or across basin-scale domains, reduces the conveyor to a 2D projection widely known as the meridional overturning circulation (MOC). Recent model studies have shown the MOC to exhibit variability on near-inertial time scales, and also indicate a region of enhanced variability on the equator. We present an analysis of three integrations of a global configuration of a numerical ocean model, which show very large amplitude oscillations in the MOCs in the Atlantic, Indian, and Pacific Oceans confined to the equatorial region. The amplitude of these oscillations is proportional to the width of the ocean basin, typically about 100 (200) Sv (1 Sv ≡ 106 m3 s−1) in the Atlantic (Pacific). We show that these oscillations are driven by surface winds within 10°N/S of the equator, and their periods (typically 4–10 days) correspond to a small number of low-mode equatorially trapped planetary waves. Furthermore, the oscillations can be well reproduced by idealized wind-driven simulations linearized about a state of rest.

Open access
Xiaolong Yu, Alberto C. Naveira Garabato, Adrian P. Martin, and David P. Marshall

Abstract

The evolution of upper-ocean potential vorticity (PV) over a full year in a typical midocean area of the northeast Atlantic is examined using submesoscale- and mesoscale-resolving hydrographic and velocity measurements from a mooring array. A PV budget framework is applied to quantitatively document the competing physical processes responsible for deepening and shoaling the mixed layer. The observations reveal a distinct seasonal cycle in upper-ocean PV, characterized by frequent occurrences of negative PV within deep (up to about 350 m) mixed layers from winter to mid-spring, and positive PV beneath shallow (mostly less than 50 m) mixed layers during the remainder of the year. The cumulative positive and negative subinertial changes in the mixed layer depth, which are largely unaccounted for by advective contributions, exceed the deepest mixed layer by one order of magnitude, suggesting that mixed layer depth is shaped by the competing effects of destratifying and restratifying processes. Deep mixed layers are attributed to persistent atmospheric cooling from winter to mid-spring, which triggers gravitational instability leading to mixed layer deepening. However, on shorter time scales of days, conditions favorable to symmetric instability often occur as winds intermittently align with transient frontal flows. The ensuing submesoscale frontal instabilities are found to fundamentally alter upper-ocean turbulent convection, and limit the deepening of the mixed layer in the winter-to-mid-spring period. These results emphasize the key role of submesoscale frontal instabilities in determining the seasonal evolution of the mixed layer in the open ocean.

Open access
Robin Waldman, Joël Hirschi, Aurore Voldoire, Christophe Cassou, and Rym Msadek

Abstract

This work aims to clarify the relation between the Atlantic meridional overturning circulation (AMOC) and the thermal wind. We derive a new and generic dynamical AMOC decomposition that expresses the thermal wind transport as a simple vertical integral function of eastern minus western boundary densities. This allows us to express density anomalies at any depth as a geostrophic transport in Sverdrups (1 Sv ≡ 106 m3 s−1) per meter and to predict that density anomalies around the depth of maximum overturning induce most AMOC transport. We then apply this formalism to identify the dynamical drivers of the centennial AMOC variability in the CNRM-CM6 climate model. The dynamical reconstruction and specifically the thermal wind component explain over 80% of the low-frequency AMOC variance at all latitudes, which is therefore almost exclusively driven by density anomalies at both zonal boundaries. This transport variability is dominated by density anomalies between depths of 500 and 1500 m, in agreement with theoretical predictions. At those depths, southward-propagating western boundary temperature anomalies induce the centennial geostrophic AMOC transport variability in the North Atlantic. They are originated along the western boundary of the subpolar gyre through the Labrador Sea deep convection and the Davis Strait overflow.

Open access
F. Sévellec, A. C. Naveira Garabato, and T. Huck

Abstract

The impact of mesoscale eddy turbulence on long-term, climatic variability in the ocean’s buoyancy structure is investigated using observations from a mooring deployed in the Drake Passage, Southern Ocean. By applying the temporal-residual-mean framework and characterizing the variance contributors and the buoyancy variance budget, we identify the main source and sink of long-term buoyancy variance. Long-term buoyancy variance amplitude is set by long-term vertical velocity fluctuations acting on the steady stratification. This baroclinic buoyancy flux is also the main source of the variance, indicative of the effect of large-scale baroclinic instability. This source is balanced by a sink of long-term buoyancy variance associated with the vertical advection of the steady stratification by the eddy-induced circulation. We conclude that mesoscale eddy turbulence acts as a damping mechanism for long-term, climatic variability in the region of the observations, consistent with an “eddy saturated” behavior of the Antarctic Circumpolar Current.

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Jenson V. George, P. N. Vinayachandran, and Anoop A. Nayak

Abstract

The inflow of high-saline water from the Arabian Sea (AS) into the Bay of Bengal (BoB) and its subsequent mixing with the relatively fresh BoB water is vital for the north Indian Ocean salt budget. During June–September, the Summer Monsoon Current carries high-salinity water from the AS to the BoB. A time series of microstructure and hydrographic data collected from 4 to 14 July 2016 in the southern BoB (8°N, 89°E) showed the presence of a subsurface (60–150 m) high-salinity core. The high-salinity core was composed of relatively warm and saline AS water overlying the relatively cold and fresh BoB water. The lower part of the high-salinity core showed double-diffusive salt fingering instability. Salt fingering staircases with varying thickness (up to 10 m) in the temperature and salinity profiles were also observed at the base of a high-salinity core at approximately 75–150-m depth. The average downward diapycnal salt flux out of the high-salinity core due to the effect of salt fingering was 2.8 × 10−7 kg m−2 s−1, approximately one order of magnitude higher than the flux if salt fingering was neglected.

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R. Pawlowicz

Abstract

Beaches, especially at or above the high tide line, are often covered in debris. An obvious approach to understanding the source of this debris elsewhere in the ocean is to use Lagrangian methods (observationally or in numerical simulations). However, the actual grounding of these floating objects, that is, the transition between freely floating near the coast and motionless on land, is poorly understood. Here, 800 groundings from a recent circulation project using expendable tracked drifters in the Salish Sea are statistically analyzed. Although the grounding process for individual drifters can be complex and highly variable, suitable analyses show that the complications of coastlines can be statistically summarized in meaningful ways. The velocity structure approaching the coastline suggests a quasi-steady “log-layer” associated with coastline friction. Although groundings are marginally more likely to occur at higher tides, there are many counterexamples and the preference is not overwhelming. The actual grounding process is then well modeled as a stationary process using a classical eddy-diffusivity formulation, and the eddy diffusivity that best matches observations is similar to that appearing in open waters away from the coast. A new parameter in this formulation is equivalent to a mean shoreward velocity for floating objects, which could vary with beach morphology and also (in theory) be measured offshore. Finally, it appears that currently used ad hoc beaching parameterizations should be reasonably successful in qualitative terms, but are unlikely to be quantitatively accurate enough for predictions of grounding mass budgets and fluxes.

Open access
Olavo B. Marques, Matthew H. Alford, Robert Pinkel, Jennifer A. MacKinnon, Jody M. Klymak, Jonathan D. Nash, Amy F. Waterhouse, Samuel M. Kelly, Harper L. Simmons, and Dmitry Braznikov

Abstract

Mode-1 internal tides can propagate far away from their generation sites, but how and where their energy is dissipated is not well understood. One example is the semidiurnal internal tide generated south of New Zealand, which propagates over a thousand kilometers before impinging on the continental slope of Tasmania. In situ observations and model results from a recent process-study experiment are used to characterize the spatial and temporal variability of the internal tide on the southeastern Tasman slope, where previous studies have quantified large reflectivity. As expected, a standing wave pattern broadly explains the cross-slope and vertical structure of the observed internal tide. However, model and observations highlight several additional features of the internal tide on the continental slope. The standing wave pattern on the sloping bottom as well as small-scale bathymetric corrugations lead to bottom-enhanced tidal energy. Over the corrugations, larger tidal currents and isopycnal displacements are observed along the trough as opposed to the crest. Despite the long-range propagation of the internal tide, most of the variability in energy density on the slope is accounted by the spring–neap cycle. However, the timing of the semidiurnal spring tides is not consistent with a single remote wave and is instead explained by the complex interference between remote and local tides on the Tasman slope. These observations suggest that identifying the multiple waves in an interference pattern and their interaction with small-scale topography is an important step in modeling internal energy and dissipation.

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Dhruv Balwada, Joseph H. LaCasce, Kevin G. Speer, and Raffaele Ferrari

Abstract

Stirring in the subsurface Southern Ocean is examined using RAFOS float trajectories, collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), along with particle trajectories from a regional eddy permitting model. A central question is the extent to which the stirring is local, by eddies comparable in size to the pair separation, or nonlocal, by eddies at larger scales. To test this, we examine metrics based on averaging in time and in space. The model particles exhibit nonlocal dispersion, as expected for a limited resolution numerical model that does not resolve flows at scales smaller than ~10 days or ~20–30 km. The different metrics are less consistent for the RAFOS floats; relative dispersion, kurtosis, and relative diffusivity suggest nonlocal dispersion as they are consistent with the model within error, while finite-size Lyapunov exponents (FSLE) suggests local dispersion. This occurs for two reasons: (i) limited sampling of the inertial length scales and a relatively small number of pairs hinder statistical robustness in time-based metrics, and (ii) some space-based metrics (FSLE, second-order structure functions), which do not average over wave motions and are reflective of the kinetic energy distribution, are probably unsuitable to infer dispersion characteristics if the flow field includes energetic wave motions that do not disperse particles. The relative diffusivity, which is also a space-based metric, allows averaging over waves to infer the dispersion characteristics. Hence, given the error characteristics of the metrics and data used here, the stirring in the DIMES region is likely to be nonlocal at scales of 5–100 km.

Open access
Matthew S. Spydell, Falk Feddersen, and Jamie Macmahan

Abstract

Oceanographic relative dispersion Dr2 (based on drifter separations r) has been extensively studied, mostly finding either Richardson–Obukhov (Dr2~t3) or enstrophy cascade [Dr2~exp(t)] scaling. Relative perturbation dispersion Dr2 (based on perturbation separation rr 0, where r 0 is the initial separation) has a Batchelor scaling (Dr2~t2) for times less than the r 0-dependent Batchelor time. Batchelor scaling has received little oceanographic attention. GPS-equipped surface drifters were repeatedly deployed on the Inner Shelf off of Pt. Sal, California, in water depths ≤ 40 m. From 12 releases of ≈18 drifters per release, perturbation and regular relative dispersion over ≈4 h are calculated for 250 ≤ r 0 ≤ 1500 m for each release and the entire experiment. The perturbation dispersion Dr2 is consistent with Batchelor scaling for the first 1000–3000 s with larger r 0 yielding stronger dispersion and larger Batchelor times. At longer times, Dr2 and scale-dependent diffusivities begin to suggest Richardson–Obukhov scaling. This applies to both experiment averaged and individual releases. For individual releases, nonlinear internal waves can modulate dispersion. Batchelor scaling is not evident in Dr2 as the correlations between initial and later separations are significant at short time scaling as ~t. Thus, previous studies investigating Dr2(t) are potentially aliased by initial separation effects not present in the perturbation dispersion Dr2(t). As the underlying turbulent velocity wavenumber spectra is inferred from the dispersion power law time dependence, analysis of both Dr2 and Dr2 is critical.

Open access