Effects of partial meridional barriers on the Antarctic Circumpolar Current—Buoyancy-driven three-layer model

The transport and vertical structure of the Antarctic Circumpolar Current (ACC) are examined, especially the component of the current driven by buoyancy, by using a three-layer model. We investigate the effects of the South American peninsula, the island arc to the east, and the Macquarie ridge, which are modeled as partial meridional barriers overlapping meridionally each other. We found that the buoyancy-driven component is given as a function of the transport out of the Weddell Sea (S _W ) and the sum of the transports into the North Atlantic (S _A ) and the North Pacific (S _P ) out of the Southern Ocean. The buoyancy-driven current flows westward, ifS _W andS _A +S _P are positive. The transport depends on the value ofS _W more thanS _A +S _P by one order of magnitude within a realistic range of parameters. The most predominant term in the transport equation is inversely proportional to the difference between the Coriolis parameters at the tips of the partial meridional barriers. Thus, the magnitude of the transport strongly depends on the overlapping length of the meridional barriers. The eastward current of the ACC is driven by the predominant eastward wind stress in the Southern Ocean, although a part of the wind-driven component is canceled by the westward buoyancy-driven component. The vertical structure of the ACC is found to be attributed to the surface wind-driven circulation and the deep and bottom buoyancy-driven circulation.     

Long-term monitoring of the sedimentary processes in the central part of Sagami Bay, Japan: rationale, logistics and overview of results

Deep-sea benthic ecosystems are mainly sustained by sinking organic materials that are produced in the euphotic zone. “Benthic-pelagic coupling” is the key to understanding both material cycles and benthic ecology in deep-sea environments, in particular in topographically flat open oceanic settings. However, it remains unclear whether “benthic-pelagic coupling” exists in eutrophic deep-sea environments at the ocean margins where areas of undulating and steep bottom topography are partly closely surrounded by land. Land-locked deep-sea settings may be characterized by different particle behaviors both in the water column and in relation to submarine topography. Mechanisms of particle accumulation may be different from those found in open ocean sedimentary systems. An interdisciplinary programme, “Project Sagami”, was carried out to understand seasonal carbon cycling in a eutrophic deep-sea environment (Sagami Bay) with steep bottom topography along the western margin of the Pacific, off central Japan. We collected data from ocean color photographs obtained using a sea observation satellite, surface water samples, hydrographic casts with turbidity sensor, sediment trap moorings and multiple core samplings at a permanent station in the central part of Sagami Bay between 1997 and 1998. Bottom nepheloid layers were also observed in video images recorded at a real-time, sea-floor observatory off Hatsushima in Sagami Bay. Distinct spring blooms were observed during mid-February through May in 1997. Mass flux deposited in sediment traps did not show a distinct spring bloom signal because of the influence of resuspended materials. However, dense clouds of suspended particles were observed only in the spring in the benthic nepheloid layer. This phenomenon corresponds well to the increased deposition of phytodetritus after the spring bloom. A phytodetrital layer started to form on the sediment surface about two weeks after the start of the spring bloom. Chlorophyll-a was detected in the top 2 cm of the sediment only when a phytodetritus layer was present. Protozoan and metazoan meiobenthos increased in density after phytodetritus deposition. Thus, “benthic-pelagic coupling” was certainly observed even in a marginal ocean environment with undulated bottom topography. Seasonal changes in features of the sediment-water interface were also documented.     

Sterols of the suspended matters in sea water

The present study deals with the content and composition of sterols of the marine-suspended matter in the surface sea water of Kagoshima-Bay. The suspended matters were separated into 3 fractions by filtration with the glass and millipore filters. The sterols in each fraction were analyzed by gas-liquid chromatography on 1.5 % SE-30. The results obtained are as follows: 1) The sterol contents ranged from 2.3 to 20.5g/liter of sea water. 2) Cholesterol was usually present as a predominant sterol in both suspended and dissolved matters. 3) A relatively large amount of other sterols such as 22-dehydro-cholesterol, campesterol, stigmasterol,-sitosterol, and fucosterol were occasionally detected.     

Thermoelastic properties of chromium oxide Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> (eskolaite) at high pressures and temperatures

A new synchrotron X-ray diffraction study of chromium oxide Cr₂O₃ (eskolaite) with the corundum-type structure has been carried out in a Kawai-type multi-anvil apparatus to pressure of 15 GPa and temperatures of 1873 K. Fitting the Birch–Murnaghan equation of state (EoS) with the present data up to 15 GPa yielded: bulk modulus (K _0,T0), 206 ± 4 GPa; its pressure derivative K′_0,T , 4.4 ± 0.8; (?K _0,T /?T) = ?0.037 ± 0.006 GPa K^?1; a = 2.98 ± 0.14 × 10^?5 K^?1 and b = 0.47 ± 0.28 × 10^?8 K^?2, where α _0,T  = a + bT is the volumetric thermal expansion coefficient. The thermal expansion of Cr₂O₃ was additionally measured at the high-temperature powder diffraction experiment at ambient pressure and α _0,T0 was determined to be 2.95 × 10^?5 K^?1. The results indicate that coefficient of the thermal expansion calculated from the EoS appeared to be high-precision because it is consistent with the data obtained at 1 atm. However, our results contradict α ₀ value suggested by Rigby et al. (Brit Ceram Trans J 45:137–148, 1946) widely used in many physical and geological databases. Fitting the Mie–Grüneisen–Debye EoS with the present ambient and high-pressure data yielded the following parameters: K _0,T0 = 205 ± 3 GPa, K′_0,T  = 4.0, Grüneisen parameter (γ ₀) = 1.42 ± 0.80, q = 1.82 ± 0.56. The thermoelastic parameters indicate that Cr₂O₃ undergoes near isotropic compression at room and high temperatures up to 15 GPa. Cr₂O₃ is shown to be stable in this pressure range and adopts the corundum-type structure. Using obtained thermoelastic parameters, we calculated the reaction boundary of knorringite formation from enstatite and eskolaite. The Clapeyron slope (with \({\text{d}}P/{\text{d}}T = - 0.014\) GPa/K) was found to be consistent with experimental data.     

A note on the horizontal momentum exchange in combined waves and current

The horizontal exchange of momentum due to the organized motion in combined waves and current has been analyzed. The combination of the vertical orbital wave motion and the mean current gives a periodic variation in the horizontal velocity in addition to the wave orbital motion. This periodic variation, combined with the wave orbital motion, gives a significant contribution to the momentum exchange. Two examples are considered, the interaction of a pure wave motion and a current normal to the direction of wave propagation, and a wave driven longshore current with an undertow velocity profile. It is demonstrated that the new contribution changes the resulting momentum exchange considerably.     

Cryptosporidium and Giardia in Natural,Drinking, and Recreational Water of Northwestern Greece

An investigation about distribution of Giardia cysts and Cryptosporidium oocysts in natural, drinking, and recreational water in Northwestern Greece was performed. Five rivers (Aoos, Arachthos, Kalamas, Louros, and Voidomatis) and one lake (Pamvotis Ioannina Lake) in Northwestern Greece were investigated during a 10‐month period. Drinking and recreational water (swimming pools) from the area were also examined. Samples were collected from prefixed sampling stations and processed following a modification of standard methods for the microbiological examination of water, as suggested by the APHA/AWWA/WEF. Both Giardia cysts and Cryptosporidium oocysts were isolated from Pamvotis Ioannina Lake (15 positive/27 examined samples). Significantly lower numbers of Cryptosporidium oocysts were detected in Arachthos River (1/5), Voidomatis River (1/5), drinking water (1/7), and pool water samples (1/9). No Giardia cysts were detected, neither in river water, nor in drinking, and pool water samples. The results clearly show that, with the exception of Pamvotis Ioannina Lake, where contamination of high level was observed, natural water sources of the investigated area have low pollution, resulting in low contamination with parasites.     

The structure and behavior of the arctic cyclone in summer analyzed by the JRA-25/JCDAS data

In this study, three-dimensional structures and the life-time behavior of arctic cyclones are investigated as case studies, using reanalysis data of JRA-25 and JCDAS. In recent years, arctic region has undergone drastic warming in conjunction with the reduced sea ice concentration in summer. The rapid reduction of the sea ice concentration is explained, to some extent, by a pressure dipole of the arctic cyclone and Beaufort high over the Arctic Ocean. This paper presents some case studies for the structure of the arctic cyclone.It is found by the analysis of this study that the arctic cyclone indicates many differences in structure and behavior compared with the mid-latitude cyclone. The arctic cyclones move rather randomly in direction over the Arctic Ocean. The arctic cyclone has a barotropic structure in the vertical from the surface to the stratosphere. The arctic cyclone detected at the sea level pressure is connected with the polar vortex at the 500 hPa level and above. Importantly, the arctic cyclone has a cold core in the troposphere and a warm core around the 200 hPa level. The mechanism of the formation is discussed based on the analyzed structure of the arctic cyclones.     

Changes in climate classification and extreme climate indices from a high-resolution future projection in Korea

We investigate the future changes in the climate zone and six extreme temperature indices in Korea, using the 20-km high-resolution atmospheric general circulation model (MRI-AGCM3.1S). The Trewartha and K?ppen climate classification schemes are applied, and four summer-based extreme temperature indices (i.e., summer days, tropical nights, growing degree days, and cooling degree days (CDD) and two winter-based indices (frost days and heating degree days (HDD) are analyzed. To represent significantly the change in threshold indices, the monthly mean bias is corrected in model. The model result reasonably captures the temporal and spatial distribution of the present-day extreme temperatures associated with topography. It was found that in the future climate, the area of the subtropical climate zone in Korea expands northward and increases by 21% under the Trewartha classification scheme and by 35% under the K?ppen classification scheme. The spatial change in extreme climate indices is significantly modulated by geographical characteristics in relation to land-ocean thermal inertia and topographical effects. The change is manifested more in coastal regions than in inland regions, except for that in summer days and HDD. Regions with higher indices in the present climate tend to reveal a larger increase in the future climate. The summer-based indices display an increasing trend, while the winter-based indices show a decreasing trend. The most significant increase is in tropical nights (+452%), whereas the most significant decrease is in HDD (?25%). As an important indicator of energy-saving applications, the changes in HDD and CDD are compared in terms of the frequency and intensity. The future changes in CDD reveal a higher frequency but a lower temperature than those in HDD. The more frequent changes in CDD may be due to a higher and less dispersed occurrence probability of extreme temperatures during the warm season. The greater increase in extreme temperature events during the summer season remains an important implication of projecting future changes in extreme climate events.     

Evaluation of coupled ocean–atmosphere simulations of the mid-Holocene using palaeovegetation data from the northern hemisphere extratropics

We have used the BIOME4 biogeography–biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean–atmosphere general circulation models to mid-Holocene changes in orbital forcing in the mid- to high-latitudes of the northern hemisphere. All the models produce: (a) a northward shift of the northern limit of boreal forest, in response to simulated summer warming in high-latitudes. The northward shift is markedly asymmetric, with larger shifts in Eurasia than in North America; (b) an expansion of xerophytic vegetation in mid-continental North America and Eurasia, in response to increased temperatures during the growing season; (c) a northward expansion of temperate forests in eastern North America, in response to simulated winter warming. The northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in North America are supported by palaeovegetation data. The expansion of xerophytic vegetation in mid-continental North America is consistent with palaeodata, although the extent may be over-estimated. The simulated expansion of xerophytic vegetation in Eurasia is not supported by the data. Analysis of an asynchronous coupling of one model to an equilibrium-vegetation model suggests vegetation feedback exacerbates this mid-continental drying and produces conditions more unlike the observations. Not all features of the simulations are robust: some models produce winter warming over Europe while others produce winter cooling. As a result, some models show a northward shift of temperate forests (consistent with, though less marked than, the expansion shown by data) and others produce a reduction in temperate forests. Elucidation of the cause of such differences is a focus of the current phase of the Palaeoclimate Modelling Intercomparison Project.     

Slab decarbonation and CO2 recycling in the Southwestern Colombian volcanic arc

The contribution of subducted carbonate sediments to the genesis of the Southwestern Colombian arc magmas was investigated using a comprehensive petrography and geochemical analysis, including determination of major and trace element contents and Sr, Nd, Hf and Pb isotope compositions. These data have been used to constrain the depth of decarbonation in the subducted slab, indicating that the decarbonation process continues into the sub-arc region, and ultimately becomes negligible in the rear arc. We propose on the basis of multi-isotope approach and mass balance calculations, that the most important mechanism to induce the slab decarbonation is the infiltration of chemically reactive aqueous fluids from the altered oceanic crust, which decreasingly metasomatize the mantle wedge, triggering the formation of isotopically different primary magmas from the volcanic front (VF) with relatively high ¹⁷⁶Hf/¹⁷⁷Hf, high ⁸⁷Sr/⁸⁶Sr, negative values of εNd and lower Pb isotopes compared to the rear arc (RA).The presence of more aqueous fluids at the volcanic front may increase the degree of decarbonation into carbonate-bearing lithologies. Moreover, with increasing pressure and temperature in the subduction system, the decrease in dehydration of the slab, leads to cessation of fluid-induced decarbonation reactions at the rear arc. This development allows the remaining carbonate materials to be recycled into the deep mantle.