Trends In Negative Pressure Research Articles

The South Pacific Pressure Trend Dipole and the Southern Blob

AbstractDuring the last four decades, the sea level pressure has been decreasing over the Amundsen–Bellingshausen Sea (ABS) region and increasing between 30° and 40°S from New Zealand to Chile, thus forming a pressure trend dipole across the South Pacific. The trends are strongest in austral winter and have influenced the climate of West Antarctica and South America. The pressure trends have been attributed to decadal variability in the tropics, expansion of the Hadley cell, and an associated positive trend of the southern annular mode, but these mechanisms explain only about half of the pressure trend dipole intensity. Experiments conducted with two atmospheric models indicate that upper ocean warming over the subtropical southwest Pacific (SSWP), termed the Southern Blob, accounts for about half of the negative pressure trend in the ABS region and nearly all the ridging/drying over the eastern subtropical South Pacific, thus contributing to the central Chile megadrought. The SSWP warming intensifies the pressure trend dipole through warming the troposphere across the subtropical South Pacific and shifting the midlatitude storm track poleward into the ABS. Multidecadal periods of strong SSWP warming also appear in fully coupled preindustrial simulations, associated with a pressure trend dipole and reduction in rainfall over the central tropical Pacific, thus suggesting a natural origin of the Southern Blob and its teleconnection. However, the current warming rate exceeds the range of natural variability, implying a likely additional anthropogenic contribution.

OH defects in coesite and stishovite during ultrahigh-pressure metamorphism of continental crust

The high-pressure silica polymorphs coesite and stishovite were synthesized under water-saturated conditions from a natural granitic composition doped with Li and B. Experiments were performed in a Multi-Anvil apparatus between 4 and 9.1 GPa and 900 and 950 °C, based on the conditions of a subducting continental crust as realistic for the ultrahigh-pressure metamorphic units Dora Maira and Kochetav massifs. Run products consisted of coesite/stishovite + kyanite ± phengite ± omphacite, and quench material. The synthesized silica polymorphs were successively analyzed by infrared spectroscopy, electron microprobe, and Secondary-Ion Mass Spectrometry (SIMS). No hydrous defects were observed in coesite synthesized at 4 GPa and 900 °C, whereas coesite grown at higher pressures revealed a triplet of infrared absorptions bands at 3575, 3523, and 3459 cm− 1, two minor bands at 3535 and 3502 cm− 1, and a small band at 3300 cm− 1 that was only visible at 7.7 GPa. The total amount of Al was charge-balanced by H and the other monovalent cations. However, the band triplet could not be associated with AlOH defects, while the band doublet was inferred to BOH defects and the small band probably corresponded to interstitial H. Stishovite displayed one dominant band at 3116 cm− 1 with a shoulder at 3170 cm− 1, and a minor band at 2665 cm− 1, probably all associated with AlOH defects. BOH defects were not observed in stishovite, and LiOH defects were neither observed in coesite nor stishovite, probably because of preferentially partition of Li in other phases such as omphacite. The total amount of defect protons increased with pressure and with metal impurity concentrations. The general increase in OH defects in silica polymorphs with increasing pressure (this study) contrasted the negative pressure trend of OH in quartz observed previously from the same starting material, and revealed an incorporation minimum of OH in silica polymorphs around the quartz/coesite phase transition.

Austral Spring Southern Hemisphere Circulation and Temperature Changes and Links to the SPCZ

Abstract Significant austral spring trends have previously been observed in West Antarctica and Antarctic Peninsula temperatures and in atmospheric circulation across the southern Pacific and Atlantic. Here, physical mechanisms for the observed trends are investigated through analysis of monthly circulation and temperatures from the ERA-Interim dataset and outgoing longwave radiation (OLR) data. The negative pressure trend over the South Pacific during spring is strongest in September, while the positive pressure trend over the South Atlantic is strongest in October. Pressure trends in November are generally nonsignificant. The authors demonstrate that a significant September trend toward increased convection (reduced OLR) in the poleward portion of the South Pacific convergence zone (SPCZ) is statistically related to Rossby wave–like circulation changes across the southern oceans. The wave response is strongest over the South Pacific in September and propagates eastward to the South Atlantic in October. OLR-related changes are linearly congruent with around half of the observed total changes in circulation during September and October and are consistent with observed trends in South Pacific sea ice concentration and surface temperature over western West Antarctica and the western Antarctic Peninsula. These results suggest SPCZ variability in early spring, especially on the poleward side of the SPCZ, is an important contributor to circulation and surface temperature trends across the South Pacific/Atlantic and West Antarctica.

Chronic Tympanic Membrane Perforation: Middle Ear Pressure and Tubal Function

Objective --To monitor the middle ear (ME) pressure continuously, day and night, in subjects with chronic central perforation (CCP) whose ears were temporarily sealed off from ambient air. Material and Methods --Twenty-three consecutive patients at our outpatient clinic, considered for myringoplasty, were studied. Results --The measurements revealed slow pressure changes with a similar pattern in 21/23 subjects. The pattern could be divided into four periods: coupling; erect body position; sleep; and the period after getting up in the morning. The coupling period was characterized by an immediate fall in pressure in 19/23 subjects and the negative pressure trend continued during most of the time in the erect position. During sleep the ME pressure initially rose and then stabilized. When the subjects got up in the morning, the ME pressure declined again. Conclusion --The different pressure periods observed in this study confirm that ME pressure regulation is a dynamic process. There is clearly a difference in the ME pressure of ears with CCP compared to previous results obtained from continuous measurements in 20 healthy ears.

Stagnations of increasing trends in negative pressure with repeated cavitation in water/metal Berthelot tubes as a result of mechanical sealing

To investigate effects of mechanical sealing on negative pressures in water/metal tube Berthelot systems, trends in negative pressure are observed through runs of temperature cycles below 90°C in two systems made of metals having small amounts of gas inclusions. The first system is a pre-degassed all-stainless-steel tube/plug system. The steel is a special product for vacuum engineering. The second is the same tube sealed with plugs made of silver solidified one-dimensionally in a vacuum furnace. A new type of trend, stagnation for intermediate cycles is found in both systems so long as sealing distortion of each plug is small in amount. The stagnation period for the first system is longer than that for the second one. A metallurgical mechanism of a gas-being-replenished crevice model is proposed: distorted parts of metals undergo heat-treatment during runs of temperature cycles, and the heat-treatment enhances the rates of impurity gas transports to crevices on the metal surface where cavitation occurs, and the transport causes the stagnation for cycles during which the rates are still high.

Effects of gas transports in metals on negative pressures in water inMo/Cu Berthelot tubes

Two series of trends in negative pressure with cavitation in waterwere observed by the Berthelot method using single- and poly-crystallinemolybdenum tubes sealed with single-crystalline copper plugs byrepeating runs of temperature cycles at rates of 70-100 cycles/daybetween 50 and 98 °C, where a run means successive cycles with thesame water. The first series were observed while both tubes and plugs werepre-degassed, and the second after exposure of the both tubes alone to N2gas.The all-single-crystalline-metal tube in the pre-degassed state yielded asteady increase from -120 to -140 bar (1 bar≃0.1 MPa) in theinitial 1250 cycles. After its gas exposure the single-crystalline tubeyielded a trend having an initial peak of -160 bar followed by a steady fallto -110 bar in the first run, and recovered the capability of yielding atrend levelling around -165 bar throughout the sixth run after a total ofabout 5000 cycles. In any run cavitation occurred within about ±12 bararound the current average.In contrast, the poly-crystalline Mo tube in the pre-degassed state yieldedwidely-scattered negative pressures; the higher envelope increased graduallyto -140 bar while lower envelope remained to be about -40 bar in thesecond run to a total of initial 4000 cycles. After similar gas exposure ofthe tube alone, the deteriorated capability of the poly-crystalline tube wasrecovered only to -120 bar after a total of 1.2×104 cycles.The results form qualitative evidence of the crucial effects of impurity gastransport in metal bulks on trends in negative pressures of water in metaltubes. The thermodynamic and metallurgical causes of the time consumptionfor achieving high negative pressure in liquid/metal Berthelot tube systemsare discussed.

Artificial Surface Pressure Trends in the NCEP–NCAR Reanalysis over the Southern Ocean and Antarctica*

An examination of 50 years of the National Centers for Environmental Prediction (NCEP)‐National Center for Atmospheric Research (NCAR) reanalysis from 1949 to 1998 reveals that significant spurious trends occur in the surface pressure field. Long-term surface pressure reductions are apparent south of 458S. The largest trend in surface pressure is near 658S where an approximately steady long-term pressure reduction of about 0.20 hPa yr21 (10 hPa in 50 yr) is located. The negative pressure trend represents a gradual reduction in a positive bias for the reanalysis. Observations at Antarctic stations do not support this long-term trend, although short-term interannual variations are reasonably well captured after about 1970. The negative pressure tendency near 65 8S continues well into the 1990s although a reasonable number of stations between 658 and 708S began taking observations along the coast of east Antarctica during the 1950s and 1960s. Few Antarctic observations, however, are used by the reanalysis until about 1968, and the quality of the pressure field for the reanalysis appears poor in high southern latitudes prior to then. The trend in high southern latitudes appears to be a component of global temporal variations in the reanalysis, some of which are supported by observations but others are not. In the Southern Hemisphere, the sea level pressure difference between 408 and 608S, an indicator of westerly wind intensity, increases approximately from 20 hPa in the early 1950s to 25 hPa in the early 1970s and 28 hPa in recent years. The relatively high density of observing stations along the Antarctic Peninsula, however, results in an approximately steady local surface pressure after the pressure fell about 4 hPa during the late 1950s. Based upon these findings, researchers should account for jumps and long-term trends when making use of the NCEP‐NCAR reanalysis.

Raising of negative pressure to around -200 bar for some organic liquids in a metal Berthelot tube

Trends in negative pressure achieved over a few thousand cavitation events were observed for ethanol, benzene and xylene in a stainless steel Berthelot tube sealed with a pre-de-gassed Ni plug. When the system was repeatedly heated and then cooled alternately over a temperature range between 60 degrees C and 10 degrees C (temperature cycle), negative pressure increased steeply for earlier cycles and levelled off eventually as in a water-metal tube system. Owing to the cavitation history effect, negative pressures of around -200 bar, the highest ever attained for organic liquids in the Berthelot method, were generated at around 15 degrees C in a useful volume ( approximately 1 cm3). It has become feasible to measure thermodynamic properties of organic liquids under negative pressure, since the pressures were up to half of the homogeneous nucleation limits of cavitation.

Effects on trends in negative pressure of surface pre-treatments for the sealing plugs of a water-metal Berthelot tube system

Trends in negative pressure over a few thousand cavitation events have been investigated for a Berthelot system comprising water/stainless steel/tube/sealing Ni plug in order to study the effects and extents of surface pre-treatments for the sealing metal, polishing, etching, and electroplating. The polishing and etching pre-treatments separated two kinds of cavitation nuclei on the metal surface; gases trapped within pores (crevices) (i) in the metal surface, and (ii) extending deep into the metal bulk. The latter, although much tougher, influenced negative pressure for a thousand cavitation events at most. The rise in negative pressure after a total of 1800 temperature cycles levelled off at around -115 bar (-11.5 MPa) irrespective of the surface finishes of the plugs. Electroplated plugs gave poor increasing trends owing to the co-deposited hydrogen. Impurity gases in the metal did limit the rise in negative pressure. These results, together with those reported before, have validated the authors extended gas-trapping crevice model, negative pressure is eventually limited by a supply of gas from sources in the metal bulk to surface crevices.