Antarctic sea-ice relationships with indices of the atmospheric circulation of the Southern Hemisphere

A link between the Antarctic sea-ice extent and low-frequency atmospheric variations, particularly ENSO, has been suggested by recent modeling and empirical studies. This question is examined here using a high-resolution (by week, by region) data base of Antarctic sea-ice extent for the 1973–1982 period. Although of relatively short duration by Northern Hemisphere standards, such a data base offers an opportunity rare in Southern Hemisphere climate studies. The seaice variations are examined in the context of longer-term indices of the large-scale atmospheric circulation. These are a Southern Oscillation Index (SOI) and an index of sea-level pressure (SLP) wavenumber one in the Southern Hemisphere extratropics. The indices are updated through 1982, and their associations with regional-scale pressure indices in the Australia-New Zealand sector are also examined. The 1973–1982 period is anomalous when compared with the period 1951–1972. Correlation analysis of the monthly sea ice and circulation index values reveals that much of the apparent link between the ice and the SOI suggested in previous studies arises from autocorrelations present in both data sets and the strong annual cycle of sea-ice extent. Removing these effects from the data and re-running the correlations reveals that most of the resulting significant associations between the ice and one or other of the circulation indices can probably be explained on the basis of chance. In order to reconcile these findings with previous studies that show some strong ice-circulation interactions on regional scales, only those months in which significant correlations occur between both largescale circulation indices and the sea ice are examined further. These occur preferentially in the Ross and Weddell sectors, which constitute the regions contributing most to the variability of Antarctic sea ice. The analysis suggests that the sea-ice-extent changes lag the SOI by several months but may precede changes in extratropical SLP wavenumber one. Confirmation of these tentative regional ice extent-circulation teleconnections necessarily awaits the forward extension of the high-resolution sea-ice data base beyond the 10 years available here.This paper is based on material presented at the Conference on Mechanisms of Interannual and Longer-Term Climatic Variations held at the University of Melbourne, Australia: December 8–12, 1986.     

Lessons Learned from 25 Years of Research at the MADE Site

Field studies at well‐instrumented research sites have provided extensive data sets and important insights essential for development and testing of transport theories and mathematical models. This paper provides an overview of over 25 years of research and lessons learned at one of such field research sites on the Columbus Air Force Base in Mississippi, commonly known as the Macrodispersion Experiment (MADE) site. Since the mid‐1980s, field data from the MADE site have been used extensively by researchers around the world to explore complex contaminant transport phenomena in highly heterogeneous porous media. Results from field investigations and modeling analyses suggested that connected networks of small‐scale preferential flow paths and relative flow barriers exert dominant control on solute transport processes. The classical advection‐dispersion model was shown to inadequately represent plume‐scale transport, while the dual‐domain mass transfer model was found to reproduce the primary observed plume characteristics. The MADE site has served as a valuable natural observatory for contaminant transport studies where new observations have led to better understanding and improved models have sprung out analysis of new data.     

Generation of Porphyritic and Equigranular Mafic Enclaves During Magma Recharge Events at Unzen Volcano, Japan

Mafic to intermediate enclaves are evenly distributed throughoutthe dacitic 1991–1995 lava sequence of Unzen volcano,Japan, representing hundreds of mafic recharge events over thelife of the volcano. This study documents the morphological,textural, chemical, and petrological characteristics of theenclaves and coexisting silicic host lavas. The eruptive productsdescribed in this study appear to be general products of magmamingling, as the same textural types are seen at many othervolcanoes. Two types of magmatic enclaves, referred to as Porphyriticand Equigranular, are easily distinguished texturally. Porphyriticenclaves display a wide range in composition from basalt toandesite, are glass-rich, spherical and porphyritic, and containlarge, resorbed, plagioclase phenocrysts in a matrix of acicularcrystals and glass. Equigranular enclaves are andesitic, non-porphyritic,and consist of tabular, medium-grained microphenocrysts in amatrix glass that is in equilibrium with the host dacite magma.Porphyritic enclaves are produced when intruding basaltic magmaengulfs melt and phenocrysts of resident silicic magma at theirmutual interface. Equigranular enclaves are a product of a moreprolonged mixing and gradual crystallization at a slower coolingrate within the interior of the mafic intrusion. KEY WORDS: mafic enclaves; quenched mafic inclusions; magma mingling; Unzen volcano; Unzen Scientific Drilling Project; resorbed plagioclase     

Assessing climate change impacts on river hydrology – A case study in the Western Ghats of India

The objective of this study is to evaluate the hydrological impacts of climate change on rainfall, temperature and streamflow in a west flowing river originating in the Western Ghats of India. The long-term trend analysis for 110 yr of meteorological variables (rainfall and temperature) was carried out using the modified Mann–Kendall trend test and the magnitude of the trend was quantified using the Sen’s slope estimator. The Regional Climate Model (RCM), COordinated Regional climate Downscaling EXperiment (CORDEX) simulated daily weather data of baseline (1951–2005) and future RCP 4.5 scenarios (2006–2060) were used to run the hydrological model, Soil and Water Assessment Tool (SWAT), in order to evaluate the effect of climate change on rainfall, temperature and streamflow. Significant changes were observed with regard to rainfall, which have shown decreasing trend at the rate of 2.63 mm per year for the historical and 8.85 mm per year for RCP 4.5 future scenarios. The average temperature was found to be increasing at \(0.10\,^{\circ }\hbox {C}\) per decade for both historical and future scenarios. The impact of climate change on the annual streamflow yielded a decreasing trend at the rate of \(1.2\,\hbox {Mm}^{3}\) per year and 2.56 \(\hbox {Mm}^{3},\) respectively for the past and future scenarios. The present work also investigates the capability of SWAT to simulate the groundwater flow. The simulated results are compared with the recession limb of the hydrograph and were found to be reasonably accurate.     

Modes of climate variability as simulated by a coupled general circulation model. Part I: ENSO-like climate variability and its low-frequency modulation

The El Niño-Southern Oscillation (ENSO) is investigated in a multicentury integration conducted with the coupled general circulation model (CGCM) ECHAM3/LSG. The quasiperiodic interannual oscillations of the simulated equatorial Pacific climate system are due to subsurface temperature anomaly propagation and a positive atmosphere-ocean feedback. The gravest internal wave modes contribute to the generation of these anomalies. The simulated ENSO has a characteristic period of 5–8 years. Due to the coarse resolution of the ocean model the ENSO amplitude is underestimated by a factor of three as compared to observations. The model ENSO is associated with the typical atmospheric teleconnection patterns. Using wavelet statistics two characteristic interdecadal modulations of the ENSO variance are identified. The origins of a 22 and 35?y ENSO modulation as well as the characteristic ENSO response to greenhouse warming simulated by our model are discussed.     

Carbon-isotope records of the Early Jurassic (Toarcian) oceanic anoxic event from the Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps) sections: palaeoceanographic and stratigraphic implications

The Toarcian oceanic anoxic event ( ca 183 Ma) coincides with a global perturbation marked by enhanced organic carbon burial and a general decrease in calcium carbonate production, probably triggered by changes in the composition of marine plankton and elevated carbon dioxide levels in the atmosphere. This study is based on high-resolution sampling of two stratigraphic successions, located in Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps), Italy, which represent expressions of the Toarcian oceanic anoxic event in deep-water pelagic sediments. These successions are characterized by the occurrence of black shales showing relatively low total organic carbon concentrations (compared with coeval strata in Northern Europe), generally < 2%, and low hydrogen indices. On this basis, they are similar to other Toarcian black shales described from the Tethyan region. The positive and negative carbon-isotope records from the two localities permit a high-resolution correlation such that ammonite biostratigraphy information from Valdorbia can be transferred to those parts of the Monte Mangart section that lack these fossils. Spectral analyses of δ¹³C_org values and of CaCO₃ percentages from the sedimentary records of both the Valdorbia and Monte Mangart sections reveal a strong cyclic pattern, best interpreted as an eccentricity signal which hence implies a duration of ca 500 kyr for the negative carbon-isotope excursion. Based on the carbon-isotope curves obtained, the high-resolution correlation between the Italian successions and a section in Yorkshire (Northern Europe) confirms the supposition that the apparent mismatch between the dating of the Toarcian oceanic anoxic event in the Boreal and Tethyan realms is an artefact of biostratigraphy.     

Submarine groundwater discharge and benthic biogeochemical zonation in the Huanghe River Estuary

Submarine groundwater discharge(SGD) has received increasing attention by studies on coastal areas; however,its effects on biogeochemical zonation have not been investigated to date. The Huanghe River Estuary(HRE) is a world class river estuary with high turbidity, and heavy human regulation. This study investigated how SGD is related to the benthic biogeochemistry of the HRE. Based on the distribution of several parameters(e.g., salinity,temperature, dissolved oxygen(DO) levels, p H, radium iso...