Eddy genesis and transformation of Stokes flow in a double-lid-driven cavity. Part 2: deep cavities

This paper extends an earlier work [1] on the development of eddies in rectangular cavities driven by two moving lids. The streamfunction describing Stokes flow in such cavities is expressed as a series of Papkovich-Faddle eigenfunctions. The focus here is deep cavities, i.e. those with large height-to-width aspect ratios, where multiple eddies arise. The aspect ratio of the fully developed eddies is found computationally to be 1.38 > 0.05, which is in close agreement with that obtained from Moffatt's [2] analysis of the decay of a disturbance between infinite stationary parallel plates. Extended control space diagrams for both negative and positive lid speed ratios are presented, and show that the pattern of bifurcation curves seen previously in the single-eddy cavity is repeated at higher aspect ratios, but with a shift in the speed ratio. Several special speed ratios are also identified for which the flow in one or more eddies becomes locally symmetric, resulting in locally symmetric bifurcation curves. By superposing two semi-infinite cavities and using the constant velocity damping factor found by Moffatt, a simple model of a finite multiple-eddy cavity is constructed and used to explain both the repetition of bifurcation patterns and the local symmetries. The speed ratios producing partial symmetry in the cavity are shown to be integer powers of Moffatt's velocity damping factor

Seasonal modulation of mesoscale processes alters nutrient availability and plankton communities in the Red Sea

Hydrographic and atmospheric forcing set fundamental constraints on the biogeochemistry of aquatic ecosystems and manifest in the patterns of nutrient availability and recycling, species composition of communities, trophic dynamics, and ecosystem metabolism. In the Red Sea, latitudinal gradients in environmental conditions and primary production have been ascribed to fluctuations in Gulf of Aden Water inflow, upwelling/mixing, and regenerated nutrient utilization i.e. rapidly recycled nitrogen in upper layers. However, our understanding of upper layer dynamics and related changes in plankton communities, metabolism and carbon and nitrogen export is limited. We surmised that stratification and mesoscale eddies modulate the nutrient availability and taxonomic identity of plankton communities in the Red Sea. Based on remote-sensing data of sea level anomalies and high resolution in situ measurements (ScanFish) we selected stations for hydrographic CTD profiles, water sampling (nutrients, seawater oxygen stable isotopes [δ18OSW]), phytoplankton and zooplankton collections. In fall 2014, strong stratification subjected the plankton community to an overall nitrogen and phosphorus shortage. The nutrient deficiency increased numbers of heterotrophic dinoflagellates, microzooplankton, and diazotrophs (Trichodesmium, diatom-diazotroph associations [DDAs]), albeit largely decreased phytoplankton and mesozooplankton abundances. In spring 2015, mesoscale eddies increased the nutrient availability, and the thermohaline characteristics and low δ18OSW point to the interaction of eddies with Gulf of Aden Surface Water (GASW). Cyclonic eddies and, most likely, the availability of nutrients associated with the GASW, increased the abundances of autotrophs (diatoms, Prasinophytes) and supported larger numbers of zooplankton and their larvae. We demonstrate that the interplay of stratification, advection of Gulf of Aden water and mesoscale eddies are key elements to better understand changes in plankton community composition, ecosystem metabolism, and macronutrient export in the Red Sea in space and time

Characterizing and following eddies in Drake Passage

Drake Passage, the gap between South America and Antarctica, is a very energetic region, with strong currents and numerous eddies. These eddies are important for mixing waters across the main three fronts, and for affecting the biological productivity in the region. We use weekly maps of colour images, sea level anomaly and geostrophic velocities to characterize physical and biological activity in the region surrounding Drake Passage. In particular, we note that the largest eddy kinetic energy occurs to the east of the passage and is principally in the area bounded by the mean Subantarctic Front (SAF) and the mean Polar Front, whilst the biological productivity is highest in coastal regions, with the SAF acting as a clear southern boundary. In a number of cases, cyclonic eddies detected by altimetry also show a strong signature in ocean color. Regular XBT (eXpendable BathyThermograph) surveys provide information on sub-surface structure, confirming the altimetric identification of features and showing that they often contain water masses originating from the other side of the front

Impact of sea surface height anomalies on cyclone track

Predicting accurate cyclone tracks is crucial for disaster management practices. The unusual westward movement of the 6-11 May 2002 Arabian Sea cyclone has been investigated through community mesoscale National Centre for Atmospheric Research model by giving different sea surface temperatures (SST) in different experiments keeping all other conditions same. In one experiment, we converted sea surface height anomalies (SSHAs) to SST. Oceanic eddies and SSHAs, representing the subsurface thermal structure, played a prominent role in the unusual westward movement of this cyclone. This is the first time that the effect of eddies and SSHAs on cyclone track has been reported