Seasonal prediction of global sea level anomalies using an ocean–atmosphere dynamical model

Advanced warning of extreme sea level events is an invaluable tool for coastal communities, allowing the implementation of management policies and strategies to minimise loss of life and infrastructure damage. This study is an initial attempt to apply a dynamical coupled ocean–atmosphere model to the prediction of seasonal sea level anomalies (SLA) globally for up to 7 months in advance. We assess the ability of the Australian Bureau of Meteorology’s operational seasonal dynamical forecast system, the Predictive Ocean Atmosphere Model for Australia (POAMA), to predict seasonal SLA, using gridded satellite altimeter observation-based analyses over the period 1993–2010 and model reanalysis over 1981–2010. Hindcasts from POAMA are based on a 33-member ensemble of seasonal forecasts that are initialised once per month for the period 1981–2010. Our results show POAMA demonstrates high skill in the equatorial Pacific basin and consistently exhibits more skill globally than a forecast based on persistence. Model predictability estimates indicate there is scope for improvement in the higher latitudes and in the Atlantic and Southern Oceans. Most characteristics of the asymmetric SLA fields generated by El-Nino/La Nina events are well represented by POAMA, although the forecast amplitude weakens with increasing lead-time.     

Geochemical and isotope data from the Acatlán Volcanic Field, western Trans-Mexican Volcanic Belt: Origin and evolution

The Quaternary Acatlán Volcanic Field (AVF) is located at the western edge of the Trans-Mexican Volcanic Belt (TMVB). This region is related to the subduction of the Pacific Cocos and Rivera plates beneath the North American plate since the late Miocene. AVF rocks are products of Pleistocene volcanic activity and include lava flows, domes, erupted basaltic andesite, trachyandesite, trachydacite, and rhyolite of calc–alkaline affinity. Most rocks show depletion in high field-strength elements and enrichment in large ion lithophile elements and light rare earth elements as is typical for magmas in subduction-related volcanic arcs. ⁸⁷Sr/⁸⁶Sr values range from 0.70361 to 0.70412, while Nd values vary from +2.3 to +5.2. Sr–Nd isotopic data plot along the mantle array. On the other hand, lead isotope compositions (²⁰⁶Pb/²⁰⁴Pb=18.62–18.75, ²⁰⁷Pb/²⁰⁴Pb=15.57–15.64, and ²⁰⁸Pb/²⁰⁴Pb=38.37–38.67) give evidence for combined influences of the upper mantle, fluxes derived from subducted sediments, and the upper continental crust involved in magma genesis at AVF. Additionally δ¹⁸O whole rock analyses range from +6.35‰ in black pumice to +10.9‰ in white pumice of the Acatlán Ignimbrite. A fairly good correlation is displayed between Sr as well as O isotopes and SiO₂ emphasizing the effects of crustal contamination. Compositional and isotopic data suggest that the different AVF series derived from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source.     

Halite and sylvite as solid inclusions in high-grade metamorphic rocks

Solid inclusions of halite and sylvite, formed during regional and contact metamorphism have been identified by microscopy and by electron microprobe analysis in rocks from Campolungo, Switzerland and Cornone di Blumone, Italy. The solid inclusions occur in several of the major minerals crystallized during metamorphism and have been observed as idiomorphic crystals and dendrites. The compositions measured in 100 analyses from Campolungo, Switzerland and 40 analyses from Cornone di Blumone, Italy extend across the two-phase region in the system, KCl-NaCl, indicating that the salt inclusions are high temperature precipitates. In both localities compositionally zoned and unzoned crystals have been found. Measured compositions on the temperature maximum of the two-phase region indicate at least 500° C which can be compared with 500°±20° C determined by Mercolli (1982) and Walther (1983) from the Mg content of calcites from Campolungo. The solid inclusions have been trapped apart from CO₂-rich and saline, H₂O-rich fluid inclusions which have been described by Mercolli (1982) as the earliest preserved fluid inclusions in the rocks. The early precipitation of salt minerals at Campolungo indicates that fluids were saturated with NaCl and KCl at 500° C and pressures of 2,000 bars or higher. Similar relationships exist between solid and fluid inclusions in the rocks of Cornone di Blumone which formed at temperatures as high as 800° C and pressures between 0.5 and 1 kilobar (Ulmer 1983). The entrapment of halite and sylvite as solid inclusions preserves the composition of the minerals which may therefore be useful as geothermometers.     

The passivation of calcite by acid mine water. Column experiments with ferric sulfate and ferric chloride solutions at pH 2

Column experiments, simulating the behavior of passive treatment systems for acid mine drainage, have been performed. Acid solutions (HCl or H₂SO₄, pH 2), with initial concentrations of Fe(III) ranging from 250 to 1500 mg L⁻¹, were injected into column reactors packed with calcite grains at a constant flow rate. The composition of the solutions was monitored during the experiments. At the end of the experiments (passivation of the columns), the composition and structure of the solids were measured. The dissolution of calcite in the columns caused an increase in pH and the release of Ca into the solution, leading to the precipitation of gypsum and Fe–oxyhydroxysulfates (Fe(III)–SO₄–H⁺ solutions) or Fe–oxyhydroxychlorides (Fe(III)–Cl–H⁺ solutions). The columns worked as an efficient barrier for some time, increasing the pH of the circulating solutions from 2 to 6–7 and removing its metal content. However, after some time (several weeks, depending on the conditions), the columns became chemically inert. The results showed that passivation time increased with decreasing anion and metal content of the solutions. Gypsum was the phase responsible for the passivation of calcite in the experiments with Fe(III)–SO₄–H⁺ solutions. Schwertmannite and goethite appeared as the Fe(III) secondary phases in those experiments. Akaganeite was the phase responsible for the passivation of the system in the experiments with Fe(III)–Cl–H⁺ solutions.     

The distribution of tritium and CFCs in the Weddell Sea during the mid-1980s

Transient tracer data (tritium, CFC11 and CFC12) from the southern, central and northwestern Weddell Sea collected during Polarstern cruises ANT III-3, ANT V-2/3/4 and during Andenes cruise NARE 85 are presented and discussed in the context of hydrographic observations. A kinematic, time-dependent, multi-box model is used to estimate mean residence times and formation rates of several water masses observed in the Weddell Sea.Ice Shelf Water is marked by higher tritium and lower CFC concentrations compared to surface waters. The tracer signature of Ice Shelf Water can only be explained by assuming that its source water mass, Western Shelf Water, has characteristics different from those of surface waters. Using the transient nature of tritium and the CFCs, the mean residence time of Western Shelf Water on the shelf is estimated to be approximately 5 years. Ice Shelf Water is renewed on a time scale of about 14 years from Western Shelf Water by interaction of this water mass with glacial ice underneath the Filchner-Ronne Ice shelf. The Ice Shelf Water signature can be traced across the sill of the Filchner Depression and down the continental slope of the southern Weddell Sea. On the continental slope, new Weddell Sea Bottom Water is formed by entrainment of Weddell Deep Water and Weddell Sea Deep Water into the Ice Shelf Water plume. In the northwestern Weddell Sea, new Weddell Sea Bottom Water is observed in two narrow, deep boundary currents flowing along the base of the continental slope. Classically defined Weddell Sea Bottom Water (θ ≤ −0.7°C) and Weddell Sea Deep Water (−0.7°C ≤ θ ≤ 0°C) are ventilated from the deeper of these boundary currents by lateral spreading and mixing. Model-based estimates yield a total formation rate of 3.5Sv for new Weddell Sea Bottom Water (θ = −1.0°C) and a formation rate of at least 11Sv for Antarctic Bottom Water (θ = −0.5°C).     

Nonhydrostatic effects and the determination of icy satellites’ moment of inertia

We compare the moment of inertia (MOI) of a simple hydrostatic, two layer body as determined by the Radau–Darwin Approximation (RDA) to its exact hydrostatic MOI calculated to first order in the parameter q = Ω²R³/GM, where Ω, R, and M are the spin angular velocity, radius, and mass of the body, and G is the gravitational constant. We show that the RDA is in error by less than 1% for many configurations of core sizes and layer densities congruent with those of solid bodies in the Solar System. We then determine the error in the MOI of icy satellites calculated with the RDA due to nonhydrostatic effects by using a simple model in which the core and outer shell have slight degree 2 distortions away from their expected hydrostatic shapes. Since the hydrostatic shape has an associated stress of order ρΩ²R² (where ρ is density) it follows that the importance of nonhydrostatic effects scales with the dimensionless number σ/ρΩ²R², where σ is the nonhydrostatic stress. This highlights the likely importance of this error for slowly rotating bodies (e.g., Titan and Callisto) and small bodies (e.g., Saturn moons other than Titan). We apply this model to Titan, Callisto, and Enceladus and find that the RDA-derived MOI can be 10% greater than the actual MOI for nonhydrostatic stresses as small as ∼0.1 bars at the surface or ∼1 bar at the core–mantle boundary, but the actual nonhydrostatic stresses for a given shape change depends on the specifics of the interior model. When we apply this model to Ganymede we find that the stresses necessary to produce the same MOI errors as on Titan, Callisto, and Enceladus are an order of magnitude greater due to its faster rotation, so Ganymede may be the only instance where RDA is reliable. We argue that if satellites can reorient to the lowest energy state then RDA will always give an overestimate of the true MOI. Observations have shown that small nonhydrostatic gravity anomalies exist on Ganymede and Titan, at least at degree 3 and presumably higher. If these anomalies are indicative of the nonhydrostatic anomalies at degree 2 then these imply only a small correction to the MOI, even for Titan, but it is possible that the physical origin of nonhydrostatic degree 2 effects is different from the higher order terms. We conclude that nonhydrostatic effects could be present to an extent that allows Callisto and Titan to be fully differentiated.     

Observations of the Labrador Sea eddy field

This paper is an observational study of small-scale coherent eddies in the Labrador Sea, a region of dense water formation thought to be of considerable importance to the North Atlantic overturning circulation. Numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat, yet their small size has hindered observational progress. A large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms, current meter moorings and satellite altimetry. Details of the current and water mass structure of individual eddy events, as they are swept past by an advecting flow, can then be extracted from the mooring data. A transition is seen during mid-1997, with long-lived boundary current eddies dominating the central Labrador Sea year-round after this time, and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time. The TOPEX / Poseidon altimeter covers the Labrador Sea with a loose “net” of observations, through which coherent eddies can seem to appear and disappear. By concentrating on locating and describing anomalous events in individual altimeter tracks, a portrait of the spatial and temporal variability of the underlying eddy field can be constructed. The altimeter results reveal an annual “pulsation” of energy and of coherent eddies originating during the late fall at a particular location in the boundary current, pinpointing the time and place of the boundary current-type eddy formation. The interannual variability seen at the mooring is reproduced, but the mooring site is found to be within a localized region of greatly enhanced eddy activity. Notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling. These eddy observations, as well as hydrographic evidence, suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region.     

Comment on “Complete Eulerian-mean tracer equation for coarse resolution OGCMs” by M. S. Dubovikov and V. M. Canuto

Dubovikov and Canuto (Dubovikov, M.S. and Canuto, V.M., Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. Astrophys. Fluid Dyn., 2006, 100, 197–214), after averaging the tracer conservation equation in density coordinates and transforming to height coordinates, concluded that present ocean models are missing important terms in their mean tracer equations. Here we point out some errors made by Dubovikov and Canuto (2006 Dubovikov, MS and Canuto, VM. 2006. Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. & Astrophys. Fluid Dynam., 100: 197–214. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) in their isopycnal averaging procedure. We draw on the temporal-residual-mean (TRM) theory and show that when the isopycnal averaging and coordinate transformation are performed correctly, the tracer equations of present ocean circulation models are recovered. We therefore conclude that present ocean circulation models are not neglecting the leading order terms identified by Dubovikov and Canuto (2006 Dubovikov, MS and Canuto, VM. 2006. Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. & Astrophys. Fluid Dynam., 100: 197–214. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]).     

Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration

Novel approaches to garnet analysis have been used to assess rates of intergranular diffusion between different matrix phases and garnet porphyroblasts in a regionally metamorphosed staurolite‐mica‐schist from the Barrovian‐type area in Scotland. X‐ray maps and chemical traverses of planar porphyroblast surfaces reveal chemical heterogeneity of the garnet grain boundary linked to the nature of the adjacent matrix phase. The garnet preserves evidence of low temperature retrograde exchange with matrix minerals and diffusion profiles documenting cation movement along the garnet boundaries. Garnet–quartz and garnet–plagioclase boundaries preserve evidence of sluggish Mg, Mn and Fe diffusion at comparable rates to volume diffusion in garnet, whereas diffusion along garnet–biotite interfaces is much more effective. Evidence of particularly slow Al transport, probably coupled to Fe³⁺ exchange, is locally preserved on garnet surfaces adjacent to Fe‐oxide phases. The Ca distribution on the garnet surface shows the most complex behaviour, with long‐wavelength heterogeneities apparently unrelated to the matrix grain boundaries. This implies that the Ca content of garnet is controlled by local availability and is thought likely to reflect disequilibrium established during garnet growth. Geochemical anomalies on the garnet surfaces are also linked to the location of triple junctions between the porphyroblasts and the matrix phases, and imply enhanced transport along these channels. The slow rates of intergranular diffusion and the characteristics of different boundary types may explain many features associated with the prograde growth of garnet porphyroblasts. Thus, minerals such as quartz, Fe‐oxides and plagioclase whose boundaries with garnet are characterized by slow intergranular diffusion rates appear to be preferentially trapped as inclusions within porphyroblasts. As such grain boundary diffusion rates may be a significant kinetic impediment to metamorphic equilibrium and garnet may struggle to maintain chemical and textural equilibrium during growth in pelites.     

Review: The Yucat��n Peninsula karst aquifer, Mexico

The Yucatán Peninsula karst aquifer is one of the most extensive and spectacular karst aquifer systems on the planet. This transboundary aquifer system extends over an area of approximately 165,000? km² in México, Guatemala and Belize. The Triassic to Holocene Yucatán limestone platform is located in the vicinity of the North American/Caribbean plate boundary and has been reshaped by a series of tectonic events over its long geologic history. At the end of the Cretaceous period, the Yucatán Peninsula was hit by a large asteroid, which formed the Chicxulub impact crater. The Yucatán Peninsula karst aquifer hosts large amounts of groundwater resources which maintain highly diverse groundwater-dependent ecosystems. Large parts of the aquifer are affected by seawater intrusion. Anthropogenic pollution of the aquifer has been increasing over the past few decades, owing to relentless economic development and population growth on the Peninsula. This review summarizes the state of knowledge on the Yucatán Peninsula karst aquifer and outlines the main challenges for hydrologic research and practical groundwater-resources management on the Peninsula.