A four-dimensional survey of the Almeria-Oran front by underwater gliders: Tracers and circulation

A four-dimensional survey by a fleet of 7 underwater gliders was used to identify pathways of subduction at the Almeria-Oran front in the western Mediterranean Sea. The combined glider fleet covered nearly 9000 km over ground while doing over 2500 dives to as deep as 700 m. The gliders had sensors to measure temperature, salinity, velocity, chlorophyll fluorescence and acoustic backscatter. Data from the gliders were analyzed through objective maps that were functions of across-front distance, along-front distance, and time on vertical levels separated by 10 m. Geostrophic velocity was inferred using a variational approach, and the quasigeostrophic omega equation was solved for vertical and ageostrophic horizontal velocities. Peak downward vertical velocities were near 25 m day-1 in an event that propagated in the direction of the frontal jet. An examination of an isopycnal surface that outcropped as the front formed showed consistency between the movement of the tracers and the inferred vertical velocity. The vertical velocity tended to be downward on the dense side of the front and upward on the light side so as to flatten the front in the manner of a baroclinic instability. The resulting heat flux approached 80 W m-2 near 100 m depth with a structure that would cause restratification of the front. One glider was used to track an isotherm over a day for a direct measure of vertical velocity as large as 50 m day-1, with a net downward displacement of 15 m over the day.

Numerical simulation of jets generated by a sphere moving vertically in a stratified fluid

The flow past a sphere moving vertically at constant speeds in a salt-stratified fluid is investigated numerically at moderate Reynolds numbers $\mathit{Re}$. Time development of the flow shows that the violation of density conservation is the key process for the generation of the buoyant jet observed in the experiments. For example, if the sphere moves downward, isopycnal surfaces are simply deformed and dragged down by the sphere while the density is conserved along the flow. (The flow pattern is inverted if the sphere moves upward. Some explanations are given in the introduction.) Then, the flow will never become steady. As density diffusion becomes effective around the sphere surface and generates a horizontal hole in the isopycnal surface, fluid with non-conserved density is detached from the isopycnal surface and moves upward to generate a buoyant jet. These processes will constitute a steady state near the sphere. With lengths scaled by the sphere diameter and velocities by the downward sphere velocity, the duration of density conservation at the rear/upper stagnation point, or the maximum distance that the isopycnal surface is dragged downward, is proportional to the Froude number $\mathit{Fr}$, and estimated well by ${\rm\pi}\mathit{Fr}$ for $\mathit{Fr}\gtrsim 1$ and $\mathit{Re}\gtrsim 200$, corresponding to a constant potential energy. The radius of a jet defined by the density and velocity distributions, which would have correlations with the density and velocity boundary layers on the sphere, is estimated well by $\sqrt{\mathit{Fr }/2\mathit{Re }\mathit{ Sc}}$ and $\sqrt{\mathit{Fr }/2\mathit{Re}}$ respectively for $\mathit{Fr}\lesssim 1$, where $\mathit{Sc}$ is the Schmidt number. Numerical results agree well with the previous experiments, and the origin of the conspicuous bell-shaped structure observed by the shadowgraph method is identified as an internal wave.

Adjoint-Based Estimation of Eddy-Induced Tracer Mixing Parameters in the Global Ocean

Using the German Estimating the Circulation and Climate of the Ocean (GECCO) synthesis framework, four separate eddy tracer mixing coefficients are adjusted jointly with external forcing fields, such as to reduce a global misfit between the model simulations and ocean observations over a single 10-yr period and weighted by uncertainties. The suite of the adjusted eddy tracer mixing coefficients includes the vertical diffusivity kz, the along-isopycnal surface diffusivity kredi, the isopycnal layer thickness diffusivity kgm, and the along-iso-thickness advection coefficient kgmskew. Large and geographically varying adjustments are found in all four parameters, which all together lead to an additional 10% reduction of the total cost function, as compared to using only surface flux parameters. However, their relative contribution to the cost reduction varies from 1% to 50% among the four coefficients, with the adjusted kgm contributing most. Regionally, the estimated kgm ranges from less than −800 to about 2500 m2 s−1. Largest adjustments in kgm reside in the vicinity of large isopycnal slopes and support a mixing length hypothesis; they also likewise support the hypothesis of a critical layer enhancement and high potential density gradient suppression. In a few occasions, resulting negative net kgm values can be found in the core of main currents, suggesting the potential for an inverse energy cascade transfer there. Large adjustments of kredi and kgmskew are found in the vicinity of isopycnal slopes. The adjustments of kz in the tropical thermoclines suggest deficiencies of the mixed layer parameterization.

Formation Mechanism for Isopycnal Temperature–Salinity Anomalies Propagating from the Eastern South Pacific to the Equatorial Region

Equatorward propagation of temperature–salinity (or spiciness) anomalies on an isopycnal surface emanating from the eastern subtropical South Pacific and their formation mechanism are investigated based on a hindcast simulation with an eddy-resolving quasi-global ocean general circulation model. Because of density-compensating meridional distributions of temperature and salinity, the meridional density gradient is weak at the sea surface in the eastern subtropical South Pacific. With these mean fields, cool sea surface temperature anomalies (SSTAs) can make the outcrop line of an isopycnal surface migrate equatorward more than 5° and induce warm and salty anomalies on the isopycnal surface. Subducted warm, salty anomalies propagate to the equatorial region over approximately 5 yr and may influence equatorial isopycnal temperature–salinity anomalies. Although the associated effects are unclear, if these anomalies could further induce warm eastern equatorial SSTAs that are positively correlated with eastern South Pacific SSTAs, opposite sign temperature–salinity anomalies would be formed in the subtropical South Pacific, and a closed cycle having a decadal time scale might be induced.