ALBERT

Description: Abstract The data assimilation scheme used in the Met Office's OSTIA (Operational Sea Surface Temperature and Ice Analysis) system has been updated from an OI‐type scheme to a variational assimilation scheme, NEMOVAR. The updated system includes a dual length scale background error correlation operator, and a flow‐dependent component to adjust the length scale combination in favour of the short scale in regions of high sea surface temperature (SST) variability. The NEMOVAR assimilation scheme improves both the analysis performance and the representation of SST features in the OSTIA analysis compared to the OI scheme of the original system. The results of spectral analysis, assessment of horizontal SST gradients and the response of an atmospheric model to the OSTIA SST analysis as a boundary condition indicate that the flow‐dependent formulation successfully contributes to improvements in the feature resolution capability of the analysis. Overall, using a short length scale of 15 km and including a flow‐dependent adjustment component produces the best results compared to using either 40 km or the first Rossby radius of deformation as the short length scale. The new system successfully captures realistic ocean variability without introducing noise into the analysis, allowing the feature resolution capability of the new system to out‐perform that of other comparable SST analysis products. This article is protected by copyright. All rights reserved.

Description: The equatorial region is a particularly challenging area for ocean data assimilation. In this region the dominant balance near the surface is between the ocean pressure gradients and the applied wind-stress. When increments are applied to an ocean model near the equator, the pressure gradients are modified and this can cause an imbalance with the unchanged wind-stress. This can lead to an initialisation shock where spurious equatorial waves and vertical velocities are triggered. The equatorial waves can degrade the performance of the ocean model while the increased vertical velocities have a significant negative impact on biogeochemical models forced by these physical ocean fields. Previous studies have proposed a scheme to reduce the ocean biases associated with slowly varying errors in the applied wind stress by applying a pressure correction calculated from accumulated temperature and salinity increments. This scheme has previously been shown to reduce the mean vertical velocities and improve modelled circulation. However, when applying this scheme we still see equatorial imbalances from data assimilation on shorter time scales. We consider a new method which is an extension of the pressure correction and aims to act as a balance to the equatorial increments and reduce intialisation shock. We refer to this as an incremental pressure correction. We provide a description of this method and an analysis of its energetics. In initial experiments with a global ocean data assimilation system, this new method is shown to suppress the generation of equatorial waves in a single observation experiment and to significantly reduce the standard deviation of vertical velocities in a re-analysis experiment.

Description: The diffusion and energization of electrons in the equatorial plane of Earth's magnetosphere by ULF waves under different ionosphere boundary conditions is examined. Using test-particle simulations and considering intervals of weak geomagnetic activity, we find that the highest energization and minimum diffusion rates correspond to nightside ionosphere conditions. Conversely, the highest diffusion rates and minimum energization are seen for a perfectly reflecting ionosphere boundary. The maximum energies gained under dayside conditions, when Hall conductivity is included, are slightly greater than the maximum energy for similar conditions without Hall conductivity. The diffusion rates for dayside ionosphere conditions with only Pedersen conductivity are greater than the diffusion rates when Hall conductivity is included. These findings show that ULF-wave-particle interactions in Earth's magnetosphere depend on the ionosphere conductance.

Description: We use test particle simulations incorporating an MHD model of ULF wave propagation in the magnetosphere with realistic ionosphere boundary conditions to study electron energization in the dayside outer Van Allen radiation belt, referenced to in situ particle and wave observations. On 7 January 2011 the THEMIS spacecraft detected 3 and 4 – 5 mHz waves simultaneous with flux enhancement of 〉10  keV electrons during the early recovery phase of a moderate geomagnetic storm. We find that internal energization of equatorially mirroring electrons via non-resonant ULF wave-particle interactions can explain these observations. The wave poloidal components cause radial drift of electrons, increasing (decreasing) their kinetic energy as they move inward (outward). Electrons with initial kinetic energies of a few keV can be energized to double these values within an hour by interaction with the 3 mHz waves. The energization rate is somewhat less for the 4 -5 mHz waves. An increase in the ionospheric conductance decreases the power of the fast mode wave, reducing radial drift velocities and hence decreasing the rate of energization. The fast mode poloidal field varies with radial distance and longitude and this also affects energization. Electrons which drift outward encounter a region where the toroidal field due to the field line resonance becomes dominant and produces strong azimuthal drift. These electrons become trapped in an L -shell range just outward of the resonance region, and are not energized.

Description: Petrographic analysis of peraluminous metapelites from two separate regions of the Karakoram metamorphic complex, North Pakistan, has produced new insights into the P–T–t evolution of the deep crust along the south Asian margin before and after the India-Asia collision. Average P–T estimates and pseudosection construction in the MnO–Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 (MnNCKFMASHTO) system using THERMOCALC have provided prograde and peak metamorphic conditions and U–Pb geochronology of metamorphic monazite has provided age constraints. Two new events in the tectonothermal evolution of the Hunza Valley have been documented; an andalusite-grade contact metamorphic event at 105.5 ± 0.8 Ma, at unknown P–T conditions, associated with the widespread subduction-related granite magmatism before the India-Asia collision, and a kyanite-grade overprint of sillimanite-grade rocks with peak P–T conditions of ∼7.8 kbar, 645 °C at 28.2 ± 0.8 Ma associated with the ongoing India-Asia collision. A kyanite-grade event observed in the Baltoro region with similar peak P–T conditions (∼7.4–8.0 kbar, ∼640–660 °C) is interpreted to have occurred sometime after 21.8 ± 0.6 Ma, however, previous studies have suggested that this event commenced in the Baltoro as early as c. 28 Ma. A calculated prograde P–T path for this kyanite-grade event in the Baltoro indicates that garnet first nucleated on an initially high geothermal gradient (∼30 °C km −1 ) and grew during a significant increase in pressure of ∼2.6 kbar over a temperature increase of ∼100 °C. This event is thought to represent evidence for conductive heating of the middle crust during early stages of intrusion and lateral migration of the Baltoro batholith, with thermal conditions comparable with tectonic models of magmatic over-accretion.

Description: [1]  Infrastructures such as pipelines and power networks at low-middle latitude regions have historically been considered relatively immune to geomagnetically induced currents (GICs). Over the past decade there have been an increasing number of investigations into the impact of GICs in long grounded conductors at these latitudes. The Australian region power network spans thousands of kilometers from low to middle latitudes. The approaching maximum of solar cycle 24 and recent findings of studies into power networks located at similar latitudes have stimulated the Australian power industry to better understand this phenomenon in their region. As a result, a pilot study to compare space weather activity with in situ GIC monitors at strategic locations within the power network was initiated. This paper provides some results from the first of these operational GIC monitors during a modest geomagnetic storm, showing the first observational evidence of space weather well correlated with GICs measured in the Australian power network. Transformer neutral currents show a high degree of similarity with the geoelectric field derived from the closest available geomagnetic observatory. Current maxima of 4–5 amperes were observed in association with geoelectric field values of ~0.06–0.07 volts per kilometer. This paper also discusses the GIC measurements obtained during this storm in terms of the space weather drivers and the considerably larger geoelectric field values anticipated during larger geomagnetic storms.

Description: There has been a widespread increase in the reporting of harmful and ‘nuisance’ algal blooms in the coastal ocean over the past few decades. On the global scale this is suspected to be a consequence of coastal eutrophication, however, on a case-by-case basis there is usually insufficient evidence to discriminate between the effects of human and natural causal factors. Intense blooms of the ‘Brown Tide’ unicellular algae (Aureococcus anophagefferens) have occurred sporadically since 1985 in coastal waters of Eastern Long Island and have devastated the local commercial scallop fishery. Analysis of an 11-year time-series dataset from this region indicates that bloom intensity is correlated with higher salinities and inversely correlated with the discharge of groundwater. Laboratory and field studies suggest that whereas salinity is unlikely to represent a direct physiological control on Brown Tide blooms, the addition of inorganic nitrogen tends to inhibit Brown Tide blooms. Budget calculations indicate that the inorganic nitrogen supply from groundwater is 1–2 orders of magnitude higher than any other external source of nitrogen for this ecosystem. Biweekly time series data collected in 1995 demonstrate that Brown Tide blooms utilize dissolved organic nitrogen (DON) for growth, as evidenced by a large decrease in DON parallel with an increase in cell abundance. On an interannual basis, bloom intensity was also positively correlated with mean DON concentrations. We hypothesize that bloom initiation is regulated by the relative supply of inorganic and organic nitrogen, determined to a large extent by temporal variability in groundwater flow. The 1980s and 1990s were characterized by exceptionally high and interannually variable groundwater discharge, associated with a large-scale climate shift over the North Atlantic. This, coupled with the time-lagged discharge of groundwater with high nitrate concentrations resulting from increased fertilizer use and population increase during the 1960s and 1970s, may have been a key factor in the initiation of Brown Tide blooms in 1985