太平洋高压移动规律性的探讨

夏半年太平洋高压的平均变化,带著突变性质的增强和减弱,突然的增强发生6月到7月,而4月至6月更有逐渐减弱的趋向,到9月中旬以後则突然的减退.夏季西太平洋高压脊的东西进退和整个西风环流变化及西风环流的情况密切关连,同时太平洋高压本身也是西风环流及其变化的重要因素之一.当西风环流强时,太平洋高压比较强大,这时太平洋高压脊的变化很少,在西风环流小波动东移的过程中,太平洋高压脊只有微小的摆动.但西风带波动较大时,随着西风槽和高压脊的东移,太平洋高压脊便有了比较长时期的和明显的东退和西伸,它的周期一般都在5、6天左右.如果西风环流起了大型的变化,即西风环流由强转弱,再由弱转强,太平洋高压脊随之不断的东退和西伸,进退的范围可达30-40经距.它的周期较长,一般都在半个月的光景,最长的可达一个月,10天以内的很少.太平洋高压脊除了和西风带的环流,极地高压和低压槽有关外,也和它南边的气压系统,如台风和热带风暴有关.     

海陆分布对于大气环流的地形与热力的影响

关於海陆分布对大气环流的影响,有两派意见.一派著重於热力的影响,另一派则着重於动力的影响.对海洋而言,在冬季大陆可以看作冷源,在夏季则好似一热源.由於海洋上与大陆上受热不同的结果,冬季在西伯利亚可长期维持着一强大高压.关於冬夏季风的产生,一般也都认为是由於海陆受热不同的效果.     

Fluid flow in sedimentary basins: model of pore water flow in a vertical fracture

Open fractures provide high-permeability pathways for fluid flow in sedimentary basins. The potential for flow along permeable or open fractures and faults depends on the continuity of flow all the way to the surface except in the case of convective flow. Upward flowing fluid cools and may cause cementation due to the prograde solubility of quartz, but in the case of carbonates such flow may cause dissolution. The rate and duration of these processes depend on the mechanisms for sustaining fluid flow into the fracture, the geometries of fracture and sedimentary beds intersected, permeability, pressure and temperature gradients. Heat loss to the adjacent sediments causes sloping isotherms which can induce non-Rayleigh convection. To analyse these problems we have used a simple model in which a single fracture acts as a pathway for vertically moving fluid and there is no fluid transport across the walls of the fracture except near its inlet and outlet. Four mechanisms for fluid flow into the lower part of the fracture are considered: decompression of pore water; compaction of intersected overpressared sediments; focusing of compaction water derived from sediments beneath the fracture; and finally focusing of pore water moving through an aquifer. Water derived from the basement is not considered here. We find that sustained flow is unlikely to have velocities much higher than 1–100 m/yr, and the flow is laminar. The temperature of the fluid expelled at the top of the fracture increases by less than 1% and the vertical temperature gradient in the fracture remains close to the geothermal gradient. Where hot water is introduced from basement fractures (hydrothermal water) during tectonic deformation, much higher velocities may be sustained in the overlying sediments, but here also this depends on the permeability near the surface. Most of the cooling of water with (ore) mineral precipitation will then occur near the surface. In most cases, pore water decompression and sediment compaction will yield only very limited pore water flux with no significant potential for cementation or heating of the sediments adjacent to the fracture. Focusing of compaction water from sediments beneath the fracture or from an intersected aquifer can yield fluxes high enough to cement an open fracture significantly but the flow must be sustained for a very long time. For velocities of 1–100 m/yr, it takes typically 0.3–30 Myr to cement a fracture by 50%. The highest velocities may be obtained when a fracture extends all the way to the surface or sea floor. When a fracture does not reach the sediment surface, the flow velocity is reduced by the displacement of water in the sediments near the top of the fracture. The flow into the fracture from the sediments may often be rate limiting rather than the flow on the fracture. Sedimentary rocks only a few metres from the fracture will receive a much lower flux than the fracture. The fracture will therefore close due to cementation before significant amounts of silica can be introduced into adjacent sandstones. The isotherm slope in the adjacent sediments will in most cases be less than 10–20°. Non-Rayleigh convection velocities in the sediments adjacent to the fracture are too small to cause any significant diagenetic reactions such as quartz cementation. These quantifications of fluid flow in fractures in sedimentary basins are important in terms of constraining models for diagenesis, heat transport and formation of ore minerals in a compaction-driven system.     

Landscape Change and Environmental Histories

Historical geography was once a popular element of university curricula in New Zealand. It was also a conspicuous focus of research. Today however there is only one identifiable course in historical geography in New Zealand's university calendars – at Massey – and few writers have maintained an active research interest rooted in the sub‐discipline. This Comment suggests some reasons why now is a good time for New Zealand's geographers to reassess this state of affairs, and outlines five themes that might be pursued in the construction of more explicit historical geographies at the start of the third millennium.     

TEMPERATURE PROXY RECORDS COVERING THE LAST TWO MILLENNIA: A TABULAR AND VISUAL OVERVIEW

Proxy data are our only source of knowledge of temperature variability in the period prior to instrumental temperature measurements. Until recently, very few quantitative palaeotemperature records extended back a millennium or more, but the number is now increasing. Here, the first systematic survey is presented, with graphic representations, of most quantitative temperature proxy data records covering the last two millennia that have been published in the peer-reviewed literature. In total, 71 series are presented together with basic essential information on each record. This overview will hopefully assist future palaeoclimatic research by facilitating an orientation among available palaeotemperature records and thus reduce the risk of missing less well-known proxy series. The records show an amplitude between maximum and minimum temperatures during the past two millennia on centennial timescales ranging from c. 0.5 to 4°C and averaging c. 1.5–2°C for both high and low latitudes, although these variations are not always occurring synchronous. Both the Medieval Warm Period, the Little Ice Age and the 20th century warming are clearly visible in most records, whereas the Roman Warm Period and the Dark Age Cold Period are less clearly discernible.     

STORURDI: A LATE HOLOCENE ROCK-SLOPE FAILURE (STURZSTROM) IN THE JOTUNHEIMEN, SOUTHERN NORWAY

An extensive accumulation of rock debris at the mouth of Urdadalen in the Jotunheimen, southern Norway, is interpreted as the deposit of a rock-slope failure (sturzstrom). This is the first large-scale rock-slope failure to be recognised in the Jotunheimen but is unlikely to be the only one. The debris descended to the valley floor, passed across the valley axis and moved 40 m up the opposite side of the valley. Some debris passed over the lip of Urdadalen and now forms a boulder tongue that extends for c. 800 m into Utledalen. A maximum velocity of 80 m s⁻¹ for debris movement from crest of hillside to toe of boulder tongue has been estimated, and a minimum velocity of 28 m s⁻¹ was required for the debris to run up the opposite side of Urdadalen. The degree of boulder hardness/weathering as determined by Schmidt hammer suggests the sturzstrom occurred in the Late Holocene (1.825 ± 0.76 cal. ka bp ), during a period of climatic deterioration. The localised geomorphological impact of the failure event has been to dam the valley and create a small lake, to cause valley widening, to produce a north-facing hillside embayment at 1200–1400 m above sea level with the potential to collect and retain snow that might, in future, lead to niche glacier development and cirque initiation, and to provide a large volume of rock debris for entrainment and onward transport in future glacial cycles.     

NORTH ATLANTIC OSCILLATION SIGNATURES IN WESTERN IBERIAN TREE-RINGS

Exactly dated ring-width chronologies derived from Pyrenean oak and sweet chestnut trees growing in northern Extremadura, Spain, were evaluated for their potential as proxies for regional precipitation and North Atlantic Oscillation variability. The relationships among tree-rings, instrumental climatic records, and three versions of the NAO index were computed for different time subperiods over the last century. The results indicate that tree-ring records reflect, with variable intensity, both short-term and long-term variations in climate. Multiple correlation and regression analysis revealed that summer precipitation appeared to be the major factor affecting tree growth at inter-annual timescales. Moreover, since fluctuations in accumulated variability in annual rainfall over southwest Iberia are controlled by winter precipitation, the accumulated rainfall (August of the year n -1 to July of year n ) and winter NAO indices are also strongly correlated with tree-ring records at interdecadal timescales. This relationship appears to be especially strong during the second half of the 20th century, which is consistent with an increase of the NAO signal in the annual precipitation during the later part of the century. These results indicate that tree-rings from western Iberia are potential proxies of the NAO variability, useful to be included in palaeoclimatic model studies.     

GLACIATION OF MT ALLEN, STEWART ISLAND (RAKIURA): THE SOUTHERN MARGIN OF LGM GLACIATION IN NEW ZEALAND

The origin of two ridges on the eastern slopes of Mt Allen, southern Stewart Island, has remained equivocal, with differences of opinion over the exact process-mechanisms of formation. A variety of approaches was used to test a number of possible hypotheses about the origin of the ridges. These include topographic and spatial positioning, geomorphology, sedimentology and palaeoclimatological extrapolations to reconstruct two small former cirque glaciers with equilibrium line altitudes (ELAs) of c. 600 m. It would appear the two ridges reflect a glacial origin, the glaciers interpreted as forming during the Last Glacial Maximum (LGM) in New Zealand. Whilst glaciation during this time (18–19 ka) was extensive in the Southern Alps, the restricted nature of glaciation on Mt Allen suggests the low altitude restricted glaciation to niche sites on the lee side of upland areas.     

STUDIES ON THE GROWTH OF RHIZOCARPON GEOGRAPHICUM IN NW SCOTLAND, AND SOME IMPLICATIONS FOR LICHENOMETRY

Scotland, a maritime subpolar environment (55–60°N), has seen relatively few applications of lichenometry – even though it offers much potential. Perhaps surprisingly, direct measurements of Rhizocarpon geographicum growth rates in Scotland are so far lacking. This study reports on the growth of this crustose areolate species from two sites in Assynt, NW Scotland, between 2002 and 2009. Repeat photography of 23 non-competing thalli growing under identical environmental conditions on a single vertical surface over 5 years at Inchnadamph showed growth rates to be a function of size – with larger thalli (10–30 mm) growing significantly faster than the smallest thalli (<10 mm). Mean diametral growth rates in thalli >10 mm are 0.67 mm yr⁻¹ (s.d. = 0.16). Studies on a second vertical surface near Lochinver, over 7 years, yielded complex growth data on a more mature population of R. geographicum thalli (<50 mm in diameter). Here, mean diametral growth rates in the larger thalli (>10 mm) are slower (0.29 mm yr⁻¹; s.d. = 0.12) than those at Inchnadamph. However, at this site, competition with other species rules out any meaningful comparison of growth rates between the two sites. Other growth processes were monitored over the five to seven-year study period, including hypothallus growth, areolae development, thallus coalescence, and inter-species competition – all have important implications for the use of Rhizocarpon species in lichenometry.     

THE LAST GLACIATION OF SHETLAND, NORTH ATLANTIC

Evidence relating to the extent, dynamics, and relative chronology of the last glaciation of the Shetland Islands, North Atlantic, is presented here, in an attempt to better illuminate some of the controversies that still surround the glacial history of the archipelago. We appraise previous interpretations and compare these earlier results with new evidence gleaned from the interpretation of a high resolution digital terrain model and from field reconnaissance. By employing a landsystems approach, we identify and describe three quite different assemblages of landscape features across the main islands of Mainland, Yell and Unst. Using the spatial interrelationship of these landsystems, an assessment of their constituent elements, and comparisons with similar features in other glaciated environments, we propose a simple model for the last glaciation of Shetland. During an early glacial phase, a coalescent British and Scandinavian ice sheet flowed approximately east to west across Shetland. The terrestrial land‐forms created by this ice sheet in the north of Shetland suggest that it had corridors of relatively fast‐flowing ice that were partially directed by bed topography, and that subsequent deglaciation was interrupted by at least one major stillstand. Evidence in the south of Shetland indicates the growth of a local ice cap of restricted extent that fed numerous radial outlet glaciers during, or after, ice‐sheet deglaciation. Whilst the absolute age of these three landsystems remains uncertain, these new geo‐morphological and palaeoglaciological insights reconcile many of the ideas of earlier workers, and allow wider speculation regarding the dynamics of the former British ice sheet.