Sampling strategies and four-dimensional assimilation of altimetric data for ocean monitoring and prediction

Numerical experiments using simulated altimeter data were conducted in order to examine the assimilation of altimeter-derived sea surface heights into numerical ocean circulation models. A reduced-gravity, primitive equation circulation model of the Gulf of Mexico was utilized; the Gulf of Mexico was chosen because of its amenability to modeling and the ability of low vertical-mode models to reproduce the observed dynamical features of the Gulf circulation. The simulated data were obtained by flying an imaginary altimeter over the model ocean and sampling the model sea surface just as real altimeter would observe the true ocean. The data were used to initialize the numerical model and the subsequent forecast was compared to the true numerical solution. Results indicate that for a stationary, circular eddy, approximately three to four tracks (either ascending or descending) across the eddy are sufficient to ensure adequate spatial resolution

Numerical simulations of sunspots

The origin, structure and evolution of sunspots are investigated using a numerical model. The compressible MHD equations are solved with physical parameter values that approximate the top layer of the solar convection zone. A three dimensional (3D) numerical code is used to solve the set of equations in cylindrical geometry, with the numerical domain in the form of a wedge. The linear evolution of the 3D solution is studied by perturbing an axisymmetric solution in the azimuthal direction. Steady and oscillating linear modes are obtained

Converging and diverging convection around axisymmetric magnetic flux tubes

A numerical model of idealized sunspots and pores is presented, where axisymmetric cylindrical domains are used with aspect ratios (radius versus depth) up to 4. The model contains a compressible plasma with density and temperature gradients simulating the upper layer of the Sun's convection zone. Non-linear magnetohydrodynamic equations are solved numerically and time-dependent solutions are obtained where the magnetic field is pushed to the centre of the domain by convection cells. This central magnetic flux bundle is maintained by an inner convection cell, situated next to it and with a flow such that there is an inflow at the top of the numerical domain towards the flux bundle. For aspect ratio 4, a large inner cell persists in time, but for lower aspect ratios it becomes highly time dependent. For aspect ratios 2 and 3 this inner convection cell is smaller, tends to be situated towards the top of the domain next to the flux bundle, and appears and disappears with time. When it is gone, the neighbouring cell (with an opposite sense of rotation, i.e. outflow at the top) pulls the magnetic field away from the central axis. As this happens a new inner cell forms with an inflow which pushes the magnetic field towards the centre. This suggests that to maintain their form, both pores and sunspots need a neighbouring convection cell with inflow at the top towards the magnetic flux bundle. This convection cell does not have to be at the top of the convection zone and could be underneath the penumbral structure around sunspots. For an aspect ratio of 1, there is not enough space in the numerical domain for magnetic flux and convection to separate. In this case the solution oscillates between two steady states: two dominant convection cells threaded by magnetic field and one dominant cell that pushes magnetic flux towards the central axis

Methods

A descriptive study on unintentional drowning deaths in Alaska during 2000–2006 found that the statewide average annual drowning rate was 8.9 per 100,000 persons, which was over seven times the national rate of 1.2 per 100,000 persons during that period. 1,2 During 2001–2010, Alaska experienced a statistically significant decline in drowning death rates among both adults and children (Figure 1); 3 however, in 2010 Alaska still had the highest drowning rate in the nation. 2 This Bulletin provides an update on drowning deaths in Alaska

Foreign Illegal, Unreported, and Unregulated Fishing in Somali Waters Perpetuates Conflict

Somali waters have high fisheries production potential, but the sustainability of those fisheries is compromised by the presence of foreign fishing vessels, many of them fishing illegally. The Somali domestic fishing sector is small and relatively nascent, but foreign vessels have fished in Somali waters for at least seven decades. Some foreign vessels and their crew have been a direct, physical threat to Somali artisanal fishers. Many foreign vessels directly compete for fish, reducing fish populations and destroying marine habitat through bottom trawling. In this paper, we reconstruct foreign catch in Somali waters from 1981–2014 and classify the health of seventeen commercial fish stocks. Foreign fishing has increased more than twenty-fold since 1981, and the most rapid increase occurred during the 1990s after the collapse of the Federal government and ensuing civil war