The Asymmetry of Recirculation of a Double Gyre in a Two Layer Ocean

Using an eddy-resolving two layer primitive-equation model forced by symmetric wind stress, we investigate the asymmetry of the recirculation of a double gyre (subpolar gyre and subtropical gyre). In the case where the layer thickness change is large, cyclonic recirculation becomes unstable and splits into meso-scale vortices more easily than anti-cyclonic recirculation in their developing stage. The subpolar gyre is, therefore, filled with more vortices than the subtropical gyre. Moreover, the effect of the upper layer potential vorticity on the lower layer potential vorticity in the subpolar gyre is stronger than that in the subtropical gyre. The characteristic of turbulence in the subpolar gyre is different from that found in the subtropical gyre and, therefore, the asymmetry of the recirculation of the double gyre is maintained by this difference. The asymmetry can not be produced in a quasi-geostrophic model because it ignores the nonlinearity associated with layer thickness change. Moreover, we investigate the effects of layer thickness and lateral viscosity on the asymmetry of the recirculation of the double gyre. In the case of realistic physical parameters, the asymmetry of the recirculation of the double gyre is noticeable from the view of the activities of the eddies. In the case with the shallowest upper layer, the position of separation of the western boundary current moves further southward.     

Effects of coastal erosion on landslide activity revealed by multi-sensor observations

Coastal erosion is becoming an increasingly serious consequence of climate change. This study demonstrates the effects of coastal erosion on landslide activity while considering the amount of erosion and changes in pore water pressure. To determine the factors related to landslide slip generation, we specifically measured the displacement, deformation, pore water pressure, and amount of erosion with high temporal resolution (1 s–1 h) for a coastal landslide in Hokkaido, north-eastern Japan, for 7 months. It has been determined that landslides occur simultaneously with high pore water pressure. Toe erosion events also occurred several times, while the landslide exhibited major displacement. Because toe erosion and the increase in pore water pressure occurred simultaneously, we tried to determine which of the two contributed majorly to the landslide displacement by conducting a stability analysis that incorporates the effects of the two factors. From the actual observed data, toe erosion and the increase in pore water pressure had comparable effects on the destabilization of the studied landslide. Specifically, the time series for the safety factor shows that the landslide in the case with toe erosion was destabilized more than that in the case with no erosion, with a difference of more than 5% in the safety factor. The model with toe erosion provided a better explanation for the landslide displacement. Furthermore, the inclination data suggested that erosion took place at least 1 month before the landslide displacement. This implied that coastal erosion played a role in the preparation and ongoing displacement of the coastal landslide. Inland landslides with toes that are subject to undercutting due to river incision or artificial construction have geomorphological settings that are similar to those of the studied landslide. The knowledge obtained here can contribute to the understanding of destabilization mechanisms and terrain changes related to such landslides. © 2020 John Wiley & Sons, Ltd.     

The Southern Annular Mode and opposite-phased Basin Mode of the Southern Ocean circulation

Over the Southern Ocean the dominant modes of the atmospheric field are known as the Southern Annular Mode (SAM) or Antarctic Oscillation, and the Pacific South American (PSA) pattern. Statistical analysis of sea surface wind (SSW) from satellite observation revealed two leading modes of SAM-like and PSA patterns. In the high latitudes, the SAM-like pattern of the SSW had a large amplitude over the Bellingshausen Basin and Australian-Antarctic Basin, with opposite phase between the two basins. On the intraseasonal time scale, large-scale sea surface height (SSH) also had notable variability, showing a basin-scale anti-phase mode over the two basins. To explain the response of oceanic variations to these atmospheric modes, we analyzed the relationship between the dominant modes of wind stress and large-scale SSH on the intraseasonal time scale. The SAM-like pattern of wind stress was correlated with the SSH variation over the two basins. The SSH basin mode was most simply explained by a simple barotropic response to the SAM-like mode of wind stress, with the curl of opposite phase between the two basins. We conclude that the zonal asymmetry of the wind field of the SAM plays an important role in driving the antiphase SSH basin modes.     

Collision of comets with meteoroids

Statistical analysis of the quantity of dust in the cometary atmosphere in relation to the direction of motion of the comet about the Sun suggests an excess of dust for the retrograde comets. This excess is analyzed in the light of Harwit's theory of the cloud of “boulders” and of Öpik's impact theory. A comparison is also made between these excesses and other cometary phenomena such as splittings and outbursts.     

Polarization changes with time during solar microwave impulsive bursts

The evolution with time of circular polarization (t) from solar bursts at 7 GHz presents, in the majority of cases, a polarization degree peak before the maximum flux time. The subsequent evolution of (t) is continuous and usually increasing. The changes could be caused by superimposed polarization effects, due to the fast emissive electrons (dominant in the first phase), and to the propagation effects caused by the coronal condensation where the event occurred (dominant in the second phase). In an approximate approach, (t) is connected to the movement of the source in the second phase, being qualitatively sound, but limited to the lack of knowledge on acceleration processes and on magnetic field topology in the active region where the flares take place.     

Holocene East Asian monsoon variation inferred from species assemblage and shell chemistry of the ostracodes from Hulun Lake, Inner Mongolia

A sediment core from Hulun Lake, Inner Mongolia was analyzed for species assemblages and shell chemistry of ostracodes to investigate changes in the hydrology and climate of the East Asian summer monsoon margin during the Holocene. Darwinula stevensoni was abundant, Ilyocypris spp. scarce, littoral ostracodes absent and Mg/Ca, Sr/Ca and δ¹⁸O were low 11,100 to 8300 yr ago, indicating high lake levels and cool/fresh waters. Darwinula stevensoni declined largely, Ilyocypris spp. throve, littoral ostracodes were rare and chemical indicators remained in low values 8300 to 6200 yr ago, suggesting that the lake continued high stands but water became warm. The lake then contracted and water became cool/brackish 6200 to 4300 yr ago. Littoral ostracodes flourished 4300 to 3350 yr ago, marking the lowest lake levels of the entire Holocene. The lake level recovered and water salinity decreased 3350 to 1900 yr ago. From 1900 to 500 yr ago, the lake maintained the preceding status albeit lowered stands and increased salinities 1100 to 800 yr ago. During the recent 500 yr, the lake expanded and water salinity decreased. The data imply that the East Asian summer monsoon did not intensify until 8300 yr ago and weakened dramatically 4300 to 3350 yr ago.     

The effect of a tidal cycle on the dynamics of nutrients in a tidal estuary in the Seto Inland Sea, Japan

A 24 hour time series survey was carried out during a spring tide (tidal range ca.2 m) of May 1995 on a tidal estuary in the Seto Inland Sea, Japan, in the context of an integrated program planned to quantify the dynamics of biophilic elements (carbon, nitrogen and phosphorus) and the roles played by the macrobenthos on the processes. Three stations were set along a transect line of about 1.4 km, which linked the river to the rear to the innermost part of the subtidal zone. Every hour, at each station, measurements were made of surface water temperature, salinity and dissolved oxygen concentration, and surface water was collected for the determination of nutrients [NH₄ ⁺−N, (NO₃ ⁻+NO₂ ⁻)−N, PO₄ ³⁻−P and Si (OH)₄−Si]. During the ebb flow, riverine input of silicate and nitrate+nitrite significantly increased the concentrations of both the intertidal and the subtidal stations. Conversely, during the high tide, river nutrient concentrations were lowered by the mixing of fresh water with sea water. As a result, best (inverse) correlations were found at the river station for salinity against silicate (y=-2.9 Sal.+110.7,r ²=0.879) and nitrate+nitrite (y=-1.3 Sal.+48.4,r ²=0.796). In contrast, ammonium nitrogen concentrations were higher at intermediate salinities. Indeed, no significant correlation was found between salinity and ammonium. The effect of the macrobenthos, which is abundant on the intertidal flat, is discussed as a biological component that influences the processes of nutrient regeneration within the estuary. The effect of the tidal amplitude is an important one in determining the extent of the variations in nutrient concentrations at all three stations, which were stronger between the lower low tide and the higher high tide.     

The light curve of the semiregular variable L2 Puppis – I. A recent dimming event from dust

The nearby Mira-like variable L₂ Pup is shown to be undergoing an unprecedented dimming episode. The stability of the period rules out intrinsic changes to the star, leaving dust formation along the line of sight as the most likely explanation. Episodic dust obscuration events are fairly common in carbon stars but have not been seen in oxygen-rich stars. We also present a 10-μm spectrum, taken with the Japanese Infrared Telescope in Space satellite, showing strong silicate emission that can be fitted with a detached, thin dust shell, containing silicates and corundum.     

Obliquity variations of terrestrial planets in habitable zones

We have investigated obliquity variations of possible terrestrial planets in habitable zones (HZs) perturbed by a giant planet(s) in extrasolar planetary systems. All the extrasolar planets so far discovered are inferred to be jovian-type gas giants. However, terrestrial planets could also exist in extrasolar planetary systems. In order for life, in particular for land-based life, to evolve and survive on a possible terrestrial planet in an HZ, small obliquity variations of the planet may be required in addition to its orbital stability, because large obliquity variations would cause significant climate change. It is known that large obliquity variations are caused by spin-orbit resonances where the precession frequency of the planet's spin nearly coincides with one of the precession frequencies of the ascending node of the planet's orbit. Using analytical expressions, we evaluated the obliquity variations of terrestrial planets with prograde spins in HZs. We found that the obliquity of terrestrial planets suffers large variations when the giant planet's orbit is separated by several Hill radii from an edge of the HZ, in which the orbits of the terrestrial planets in the HZ are marginally stable. Applying these results to the known extrasolar planetary systems, we found that about half of these systems can have terrestrial planets with small obliquity variations (smaller than 10°) over their entire HZs. However, the systems with both small obliquity variations and stable orbits in their HZs are only 1/5 of known systems. Most such systems are comprised of short-period giant planets. If additional planets are found in the known planetary systems, they generally tend to enhance the obliquity variations. On the other hand, if a large/close satellite exists, it significantly enhances the precession rate of the spin axis of a terrestrial planet and is likely to reduce the obliquity variations of the planet. Moreover, if a terrestrial planet is in a retrograde spin state, the spin-orbit resonance does not occur. Retrograde spin, or a large/close satellite might be essential for land-based life to survive on a terrestrial planet in an HZ.