Sustenance of phytoplankton in the subpolar North Atlantic during the winter through patchiness

This study investigates the influence of two factors that change the mixed layer depth and can potentially contribute to the phytoplankton sustenance over winter: 1) variability of air-sea fluxes and 2) three-dimensional processes arising from strong fronts. To study the role of these factors, we perform several three-dimensional numerical simulations forced with air-sea fluxes at different temporal averaging frequencies as well as different spatial resolutions. Results show that in the winter, when the average mixed layer is much deeper than the euphotic layer and the days are short, phytoplankton production is relatively insensitive to the high-frequency variability in air-sea fluxes. The duration of upper ocean stratification due to high-frequency variability in air-sea fluxes is short and hence has a small impact on phytoplankton production. On the other hand, slumping of fronts creates patchy, stratified, shallow regions that persist considerably longer than stratification caused by changes in air-sea fluxes. Simulations show that before spring warming, the average MLD with fronts is about 700 m shallower than the average MLD without fronts. Therefore, fronts increase the residence time of phytoplankton in the euphotic layer and contribute to phytoplankton growth. Results show that before the spring warming, the depth-integrated phytoplankton concentration is about twice as large as phytoplankton concentration when there are no fronts. Hence, fronts are important for setting the MLD and sustaining phytoplankton in the winter. Model results also show that higher numerical resolution leads to stronger restratification, shallower mixed layers, greater variability in the MLD and higher production of phytoplankton

Submesoscale dispersion in the vicinity of the Deepwater Horizon spill

Reliable forecasts for the dispersion of oceanic contamination are important for coastal ecosystems, society and the economy as evidenced by the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 and the Fukushima nuclear plant incident in the Pacific Ocean in 2011. Accurate prediction of pollutant pathways and concentrations at the ocean surface requires understanding ocean dynamics over a broad range of spatial scales. Fundamental questions concerning the structure of the velocity field at the submesoscales (100 meters to tens of kilometers, hours to days) remain unresolved due to a lack of synoptic measurements at these scales. \textcolor{black} {Using high-frequency position data provided by the near-simultaneous release of hundreds of accurately tracked surface drifters, we study the structure of submesoscale surface velocity fluctuations in the Northern Gulf Mexico. Observed two-point statistics confirm the accuracy of classic turbulence scaling laws at 200m$-$50km scales and clearly indicate that dispersion at the submesoscales is \textit{local}, driven predominantly by energetic submesoscale fluctuations.} The results demonstrate the feasibility and utility of deploying large clusters of drifting instruments to provide synoptic observations of spatial variability of the ocean surface velocity field. Our findings allow quantification of the submesoscale-driven dispersion missing in current operational circulation models and satellite altimeter-derived velocity fields.Comment: 9 pages, 6 figure

Oceanic three-dimensional Lagrangian Coherent Structures: A study of a mesoscale eddy in the Benguela ocean region

We study three dimensional oceanic Lagrangian Coherent Structures (LCSs) in the Benguela region, as obtained from an output of the ROMS model. To do that we first compute Finite-Size Lyapunov exponent (FSLE) fields in the region volume, characterizing mesoscale stirring and mixing. Average FSLE values show a general decreasing trend with depth, but there is a local maximum at about 100 m depth. LCSs are extracted as ridges of the calculated FSLE fields. They present a "curtain-like" geometry in which the strongest attracting and repelling structures appear as quasivertical surfaces. LCSs around a particular cyclonic eddy, pinched off from the upwelling front are also calculated. The LCSs are confirmed to provide pathways and barriers to transport in and out of the eddy

Acid rain: Mesoscale model

A mesoscale numerical model of the Florida peninsula was formulated and applied to a dry, neutral atmosphere. The prospective use of the STAR-100 computer for the submesoscale model is discussed. The numerical model presented is tested under synoptically undisturbed conditions. Two cases, differing only in the direction of the prevailing geostrophic wind, are examined: a prevailing southwest wind and a prevailing southeast wind, both 6 m/sec at all levels initially

A surface-aware projection basis for quasigeostrophic flow

Recent studies indicate that altimetric observations of the ocean's mesoscale eddy field reflect the combined influence of surface buoyancy and interior potential vorticity anomalies. The former have a surface-trapped structure, while the latter have a more grave form. To assess the relative importance of each contribution to the signal, it is useful to project the observed field onto a set of modes that separates their influence in a natural way. However, the surface-trapped dynamics are not well-represented by standard baroclinic modes; moreover, they are dependent on horizontal scale. Here we derive a modal decomposition that results from the simultaneous diagonalization of the energy and a generalisation of potential enstrophy that includes contributions from the surface buoyancy fields. This approach yields a family of orthonomal bases that depend on two parameters: the standard baroclinic modes are recovered in a limiting case, while other choices provide modes that represent surface and interior dynamics in an efficient way. For constant stratification, these modes consist of symmetric and antisymmetric exponential modes that capture the surface dynamics, and a series of oscillating modes that represent the interior dynamics. Motivated by the ocean, where shears are concentrated near the upper surface, we also consider the special case of a quiescent lower surface. In this case, the interior modes are independent of wavenumber, and there is a single exponential surface mode that replaces the barotropic mode. We demonstrate the use and effectiveness of these modes by projecting the energy in a set of simulations of baroclinic turbulence