Some Effects of Rotation Rate on Planetary-Scale Wave Flows

A series of experiments were performed in a rotating annulus of fluid to study effects of rotation rate on planeta-ry-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth’s atmosphere, although magnitude of variation in earth’s rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number deter-mining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with the variation in earth’s rotation rale. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.     

Development of lava tubes in the light of observations at Mauna Ulu,Kilauea Volcano,Hawaii

During the 1969–1974 Mauna Ulu eruption on Kilauea's upper east rift zone, lava tubes were observed to develop by four principal processes: (1) flat, rooted crusts grew across streams within confined channels; (2) overflows and spatter accreted to levees to build arched roofs across streams; (3) plates of solidified crust floating downstream coalesced to form a roof; and (4) pahoehoe lobes progressively extended, fed by networks of distributaries beneath a solidified crust. Still another tube-forming process operated when pahoehoe entered the ocean; large waves would abruptly chill a crust across the entire surface of a molten stream crossing through the surf zone. These littoral lava tubes formed abruptly, in contrast to subaerial tubes, which formed gradually. All tube-forming processes were favored by low to moderate volume-rates of flow for sustained periods of time. Tubes thereby became ubiquitous within the pahoehoe flows and distributed a very large proportionof the lava that was produced during this prolonged eruption. Tubes transport lava efficiently. Once formed, the roofs of tubes insulate the active streams within, allowing the lava to retain its fluidity for a longer time than if exposed directly to ambient air temperature. Thus the flows can travel greater distances and spread over wider areas. Even though supply rates during most of 1970–1974 were moderate, ranging from 1 to 5 m³/s, large tube systems conducted lava as far as the coast, 12–13 km distant, where they fed extensive pahoehoe fields on the coastal flats. Some flows entered the sea to build lava deltas and add new land to the island. The largest and most efficient tubes developed during periods of sustained extrusion, when new lava was being supplied at nearly constant rates. Tubes can play a major role in building volcanic edifices with gentle slopes because they can deliver a substantial fraction of lava erupted at low to moderate rates to sites far down the flank of a volcano. We conclude, therefore, that the tendency of active pahoehoe flows to form lava tubes is a significant factor in producing the common shield morphology of basaltic volcanoes.     

Rates of erosion of till in the nearshore zone

Measurements of vertical erosion of till in water depths ranging from 1.1 m to 5 m have been obtained using a modified micro-erosion meter. Measurements in this environment are hindered by poor underwater visibility and losses due to high erosion rates and ice action. There is considerable spatial and temporal variability in the erosion rates measured but average values show a general increase from 11 mm y^?1 in 6 m of water to about 35 mm y^?1 in 2.3 m of water and even higher rates closer to shore. The measured values in shallow water are in good agreement with long-term rates extrapolated from shoreline recession. It is suggested that erosion occurs through abrasion and fluid stressing, and that these mechanisms are aided by softening of the upper surface of the till, possibly through cyclic ‘fatigue failure’.     

Le Volcan Popocatepetl (Mexique): structure,evolution pétrologique et risques

Volcan Popocatepetl, which lies 70 km southeast of Mexico City, is one of the most famous andesite composite volcanoes in the world. With 5,450 m of elevation, it is the second highest peak of Mexico. Located 320 km north of the Middle America Trench, at the centre of the Mexican Volcanic Belt, Volcano Popocatepetl forms the southern active part of a northsouth volcanic complex, the northern part consisting of the eroded Volcano Iztaccihuatl.Since its earliest reported eruption in 1519, Volcano Popocatepetl has had a continuous fumarolic activity in its crater, and in frequent small eruptions (1720, 1802–1804, 1920). In contrast with this light activity, C¹⁴ data indicate pre-historical cycles of intense volcanism with paroxysmal pyroclastic eruptions (ash and pumice-flows) alternating with effusive phases and plinian air-fall deposits.The results of a volcanological study and the petrological characteristics of the main volcanic units show that Volcano Popocatepetl is composed of a primitive composite-volcano on which a recent summit cone is superimposed. It has been built during 2 very dissimilar volcanic periods linked by a transitional phase.

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Le Volcan Popocatepetl (Mexique): structure, evolution pétrologique et risques

     

Hydrodynamic dispersion during absorption of water by soil: 1. Model soil-moisture profiles

Solute mixing during sorption of water is analysed for three model soil-water profiles: a step-function profile, an error function complement profile and a curvilinear profile.An analytical solution to the step-function profile was obtained and it was shown that the plane about which salt dispersion occurs is coincident with the plane of separation which assumes that all of the water initially present is pushed ahead by the infiltrating water. Similar results were obtained for the two other model profiles, using numerical analysis. A simplified approach for calculating these planes, based on the delta-function model, is presented for both horizontal and vertical infiltration.Proof is given that the estimate for the plane about which salt dispersion occurs should be made with λ⁰ (defined in the text) rather than with either λ′_c or λ′_θc (also defined in the text). It was also shown that the solute balance is maintained for all values of the dispersion coefficient for the flux-related boundary condition only.     

Modeling internal tides over Fieberling Guyot: resolution, parameterization, performance

Terrain-following ocean models are being used to simulate baroclinic tides and provide estimates of the tidal fields for circulation and mixing studies. These models have successfully reproduced elevations with most of the remaining inaccuracies attributed to topographic errors; however, the replication of barotropic and baroclinic velocity fields has not been as robust. Part of the problem is the lack of an adequate observational dataset in the simulated regions to compare the models. This problem was addressed using a dataset collected during the Flow over Abrupt Topography initiative at Fieberling Guyot. To evaluate the capability of the Regional Ocean Model System (ROMS) to simulate baroclinic tidal velocities, the combined tides for four constituents, M₂, S₂, K₁, and O₁, were modeled over Fieberling Guyot. Model inputs, numerical schemes, and parameterizations were varied to improve agreement with observations. These included hydrography, horizontal resolution, and the vertical mixing parameterization. Other factors were evaluated but are not included in this paper. With the best case, semidiurnal baroclinic tides were well replicated with RMS differences between the model estimates and the observations of 1.85 and 0.60 cm s⁻¹ for the major axes of the tidal ellipses for M₂ and S₂, respectively. However, diurnal K₁ baroclinic tides were poorly simulated with RMS differences of 4.49 cm s⁻¹. In the simulations, the K₁ baroclinic tides remained bottom-trapped unlike the observed fields, which had free waves due to the contribution of the mean velocity to the potential vorticity. The model did not adequately simulate the mean velocity, and the K₁ tides remained trapped. A resolution of 1 km most accurately reproduced the major axes and mean velocities; however, a 4-km resolution was sufficient for a qualitative estimate of where baroclinic tidal generation occurred. Nine vertical mixing parameterizations were compared. The vertical mixing parameterization was found to have minor effects on the velocity fields, with most effects occurring over the crown of guyot and in the lower water column; however, it had dramatic effects on the estimation of vertical diffusivity of temperature. Although there was no definitive best performer for the vertical mixing parameterization, several parameterizations could be eliminated based on comparison of the vertical diffusivity estimates with observations. The best performers were Mellor–Yamada and three generic length scale schemes.     

Benthic foraminifera in a deep-sea high-energy environment: the Moira Mounds (Porcupine Seabight,SW of Ireland)

Cold-water coral ecosystems represent unique and exceptionally diverse environments in the deep-sea. They are well developed along the Irish margin, varying broadly in shape and size. The Moira Mounds, numerous small-sized mounds, are nestled in the Belgica Mound Province (Porcupine Seabight, North-East Atlantic). The investigation of living (Rose Bengal stained) and dead benthic foraminiferal assemblages from these mounds allowed to describe their distribution patterns and to evaluate their response to environmental variability. Quantitative data was statistically treated to define groups of species/genera associated to specific habitats. The Moira Mounds differ from their larger neighbours by the reduced spatial variability of benthic foraminiferal assemblages, living assemblages only distinguishing coral-rich and coral-barren areas. The ecological needs of corals are highlighted by the abundance of Alabaminella weddellensis and Nonionella iridea, phytodetritus-feeding species in coral supporting sediments. Living foraminifera in sediments from the Moira Mounds concentrate in the upper first centimetre. Infaunal species may be affected by bioturbation and/or reworking by the strong currents in the area. Dead foraminiferal assemblages from the Moira Mounds resemble those described for the sandwave facies in adjacent giant mounds, suggesting similar processes in facies deposition.