Supergranule and Mesogranule Evolution

The MDI instrument on the SOHO satellite obtained a nearly continuous 45.5-hr run in high-resolution mode on 17–18 January 1997, collecting continuum, Dopplergram, and magnetogram images once per minute. This is one of the longest data sets yet obtained in this mode and shows significant evolution of the supergranulation pattern. After allowing for solar rotation within the fixed field of view, an area spanning 17° in latitude and 11° in longitude was extracted that covers the same area of the solar surface for the entire run. From the de-rotated continuum images, we computed flow maps of photospheric motions using local correlation techniques (LCT). Horizontal divergence maps constructed from the flow maps show local maxima of the size of mesogranules (5–10). We interpret these as mesogranules although the LCT flow map resolution (4.8 FWHM) may not completely resolve smaller mesogranules. Movies made from the divergence maps clearly show the outward convection (advection) of these mesogranules within each supergranule, and narrow boundaries of negative divergence outlining the supergranules. Several new supergranules are observed forming. These appear as areas of strong divergence that pop up between pre-existing supergranules and grow, pushing their neighbors apart. Others seem to perish between growing neighbors. We also computed the vertical component of vorticity from the flow maps. Movies of this vorticity do not show any obvious patterns.     

Estimating climatic temperature change in the ocean with syntheticacoustic apertures

An acoustic tomography simulation is carried out in the eastern North Pacific ocean to assess whether climate trends are better detected and mapped with mobile or fixed receivers. In both cases, acoustic signals from two stationary sources are transmitted to ten receivers. Natural variability of the sound-speed field is simulated with the Naval Research Laboratory (NRL) layered-ocean model. A sequential Kalman-Bucy filter is used to estimate the sound speed field, where the a priori error covariance matrix of the parameters is estimated from the NRL model. A spatially homogeneous climate trend is added to the NRL fluctuations of sound speed, but the trend is not parameterized in the Kalman filter. Acoustic travel times are computed between the sources and receivers by combining sound speeds from the NRL model with those from the unparameterized climate trend. The effects of the unparameterized climate trend are projected onto parameters which eventually drift beyond acceptable limits. At that time, the unparameterized trend is detected. Mobile and fixed receivers detect the trend at about the same time. At detection time, however, maps from fixed receivers are less accurate because some of the unparameterized climate trend is projected onto tile spatially varying harmonics of the sound-speed field. With mobile receivers, the synthetic apertures suppress the projection onto these harmonics. Instead, the unparametrized trend is correctly projected onto the spatially homogeneous portion of the parameterized sound-speed field     

Acoustic tomography at basin scales and clock errors

A basin-scale acoustic tomography simulation is carried out for the northeast Pacific Ocean to determine the accuracy with which time must be kept at the sources when clocks at the receivers are accurate. A sequential Kalman filter is used to estimate sound-speed fluctuations and clock errors. Sound-speed fluctuations in the simulated ocean are estimated from an eddy-resolving hydrodynamic model of the Pacific forced by realistic wind fields at daily resolution from 1981-1993. The model output resembles features associated with El Nino and the Southern Oscillation, as well as many other features of the ocean's circulation. Using a Rossby-wave resolving acoustic array of four fixed sources and twenty drifting receivers, the authors find that the percentage of the modeled ocean's sound-speed variance accounted for with tomography is 92% at 400-km resolution, regardless of the accuracy of the clocks. Clocks which drift up to hundreds of seconds of error or more for a year do not degrade tomographic images of the model ocean. Tomographic reconstructions of the sound-speed field are insensitive to clock error primarily because of the wide variety of distances between the receivers from each source. Every receiver “sees” the same clock error from each source, regardless of section length, but the sound-speed fluctuations in the modeled ocean cannot yield travel times which lead to systematic changes in travel time that are independent of section length. The Kalman filter is thus able to map the sound-speed field accurately in the presence of large errors at the source's clocks     

What controls the extreme flow through the Kerama Gap: a global HYbrid Coordinate Ocean Model reanalysis point of view

Ocean Dynamics - The temporal variability of volume transport from the North Pacific Ocean to the East China Sea (ECS) through the Kerama Gap (between Okinawa Island and Miyakojima Island—a...     

Impact of 1/8° to 1/64° resolution on Gulf Stream model–data comparisons in basin-scale subtropical Atlantic Ocean models

We investigate the impact of 1/8°, 1/16°, 1/32°, and 1/64° ocean model resolution on model–data comparisons for the Gulf Stream system mainly between the Florida Straits and the Grand Banks. This includes mean flow and variability, the Gulf Stream pathway, the associated nonlinear recirculation gyres, the large-scale C-shape of the subtropical gyre and the abyssal circulation. A nonlinear isopycnal, free surface model covering the Atlantic from 9°N to 47°N or 51°N, including the Caribbean and Gulf of Mexico, and a similar 1/16° global model are used. The models are forced by winds and by a global thermohaline component via ports in the model boundaries. When calculated using realistic wind forcing and Atlantic model boundaries, linear simulations with Munk western boundary layers and a Sverdrup interior show two unrealistic mean Gulf Stream pathways between Cape Hatteras and the Grand Banks, one proceeding due east from Cape Hatteras and a second one continuing northward along the western boundary until forced eastward by the regional northern boundary. The northern pathway is augmented when a linear version of the upper ocean global thermohaline contribution to the Gulf Stream is added as a Munk western boundary layer. A major change is required to obtain a realistic pathway in nonlinear models. Resolution of 1/8° is eddy-resolving but mainly gives a wiggly version of the linear model Gulf Stream pathway and weak abyssal flows except for the deep western boundary current (DWBC) forced by ports in the model boundaries. All of the higher resolution simulations show major improvement over the linear and 1/8° nonlinear simulations. Additional major improvement is seen with the increase from 1/16° to 1/32° resolution and modest improvement with a further increase to 1/64°. The improvements include (1) realistic separation of the Gulf Stream from the coast at Cape Hatteras and a realistic Gulf Stream pathway between Cape Hatteras and the Grand Banks based on comparisons with Gulf Stream pathways from satellite IR and from GEOSAT and TOPEX/Poseidon altimetry (but 1/32° resolution was required for robust results), (2) realistic eastern and western nonlinear recirculation gyres (which contribute to the large-scale C-shape of the subtropical gyre) based on comparisons with mean surface dynamic height from the generalized digital environmental model (GDEM) oceanic climatology and from the pattern and amplitude of sea surface height (SSH) variability surrounding the eastern gyre as seen in TOPEX/Poseidon altimetry, (3) realistic upper ocean and DWBC transports based on several types of measurements, (4) patterns and amplitude of SSH variability which are generally realistic compared to TOPEX/Poseidon altimetry, but which vary from simulation to simulation for specific features and which are most realistic overall in the 1/64° simulation, (5) a basin wide explosion in the number and strength of mesoscale eddies (with warm core rings (WCRs) north of the Gulf Stream, the regional eddy features best observed by satellite IR), (6) realistic statistics for WCRs north of the Gulf Stream based on comparison to IR analyses (low at 1/16° resolution and most realistic at 1/64° resolution for mean population and rings generated/year; realistic ring diameters at all resolutions), and (7) realistic patterns and amplitude of abyssal eddy kinetic energy (EKE) in comparison to historical measurements from current meters.     

Modeled Sverdrup flow in the North Atlantic from 11 different wind stress climatologies

In studies of large-scale ocean dynamics, often quoted values of Sverdrup transport are computed using the Hellerman–Rosenstein wind stress climatology. The Sverdrup solution varies, however, depending on the wind set used. We examine the differences in the large-scale upper ocean response to different surface momentum forcing fields for the North Atlantic Ocean by comparing the different Sverdrup interior/Munk western boundary layer solutions produced by a 1/16° linear numerical ocean model forced by 11 different wind stress climatologies. Significant differences in the results underscore the importance of careful selection of a wind set for Sverdrup transport calculation and for driving nonlinear models. This high-resolution modeling approach to solving the linear wind-driven ocean circulation problem is a convenient way to discern details of the Sverdrup flow and Munk western boundary layers in areas of complicated geometry such as the Caribbean and Bahamas. In addition, the linear solutions from a large number of wind sets provide a well-understood baseline oceanic response to wind stress forcing and thus, (1) insight into the dynamics of observed circulation features, by themselves and in conjunction with nonlinear models, and (2) insight into nonlinear model sensitivity to the choice of wind-forcing product.The wind stress products are evaluated and insight into the linear dynamics of specific ocean features is obtained by examining wind stress curl patterns in relation to the corresponding high-resolution linear solutions in conjunction with observational knowledge of the ocean circulation. In the Sverdrup/Munk solutions, the Gulf Stream pathway consists of two branches. One separates from the coast at the observed separation point, but penetrates due east in an unrealistic manner. The other, which overshoots the separation point at Cape Hatteras and continues to flow northward along the continental boundary, is required to balance the Sverdrup interior transport. A similar depiction of the Gulf Stream is commonly seen in the mean flow of nonlinear, eddy-resolving basin-scale models of the North Atlantic Ocean. An O(1) change from linear dynamics is required for realistic simulation of the Gulf Stream pathway. Nine of the eleven Sverdrup solutions have a C-shaped subtropical gyre, similar to what is seen in dynamic height contours derived from observations. Three mechanisms are identified that can contribute to this pattern in the Sverdrup transport contours. Along 27°N, several wind sets drive realistic total western boundary current transport (within 10% of observed) when a 14 Sv global thermohaline contribution is added (COADS, ECMWF 10 m re-analysis and operational, Hellerman–Rosenstein and National Centers for Environmental Prediction (NCEP) surface stress re-analysis), a few drive transport that is substantially too high (ECMWF 1000 mb re-analysis and operational and Isemer–Hasse) and Fleet Numerical Meteorology and Oceanography Center (FNMOC) surface stresses give linear transport that is slightly weaker than observed. However, higher order dynamics are required to explain the partitioning of this transport between the Florida Straits and just east of the Bahamas (minimal in the linear solutions vs. 5 Sv observed east of the Bahamas). Part of the Azores Current transport is explained by Sverdrup dynamics. So are the basic path of the North Atlantic Current (NAC) and the circulation features within the Intra-Americas Sea (IAS), when a linear rendition of the northward upper ocean return flow of the global thermohaline circulation is added in the form of a Munk western boundary layer.     

Formation of magnetic flux tubes in cylindrical wedge geometry

Three-dimensional (3D) MHD numerical simulations have not been able to demonstrate convincingly the spontaneous formation of large vertical flux tubes. Two-dimensional (2D) magnetoconvection in axisymmetric cylinders forms a central magnetic flux tube surrounded by annular convection rings. To study the robustness of this type of solution in three dimensions, the nonlinear resistive MHD equations are solved numerically in a 3D cylindrical wedge from an initially uniform vertical magnetic field. It is shown that the 2D result is retrieved for small domain radii. However, for larger radii the central axis loses its importance and in this case many convection cells form in the numerical domain. Magnetic flux is captured between cells where flow converges and the reduced amount of flux that congregates at the central axis is eroded by the surrounding convection.     

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.     

Heliophysics Event Knowledgebase for?the?Solar Dynamics Observatory (SDO) and Beyond

The immense volume of data generated by the suite of instruments on the Solar Dynamics Observatory (SDO) requires new tools for efficient identifying and accessing data that is most relevant for research. We have developed the Heliophysics Events Knowledgebase (HEK) to fill this need. The HEK system combines automated data mining using feature-detection methods and high-performance visualization systems for data markup. In addition, web services and clients are provided for searching the resulting metadata, reviewing results, and efficiently accessing the data. We review these components and present examples of their use with SDO data.