Contribution of sea-surface wind curl to the maintenance of the SST gradient along the upstream Kuroshio Extension in early summer     

Naoki Sato  Masami Nonaka  Yoshikazu Sasai  Hideharu Sasaki  Youichi Tanimoto  Ryuichi Shirooka 《Journal of Oceanography》2016,72(5):697-705

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Multiple-tracers-aided surface-subsurface hydrological modeling for detailed characterization of regional catchment water dynamics in Kumamoto area,southern Japan     

Rahman  A. T. M. Sakiur  Hosono  Takahiro  Tawara  Yasuhiro  Fukuoka  Youichi  Hazart  Aurelien  Shimada  Jun 《Hydrogeology Journal》2021,29(5):1885-1904

Hydrogeology Journal - Integrated watershed modeling techniques have been applied in recent years to examine surface and subsurface interactions. Model performance is often evaluated by best fit of...  相似文献   

Multi-spacecraft observations of heliographic structure of the solar wind speed     

W. Miyake

K. Kobayashi

K. -I. Oyama  T. Mukai

T. Abe

T. Terasawa

K. Yumoto  T. Saito

K. Hirao

A. J. Lazarus

A. D. Johnstone 《Planetary and Space Science》1988,36(12):1329-1342

Multi-spacecraft observations in the interplanetary space are used to build up a picture of the distribution of solar wind velocities in heliographic latitude and longitude. Analyses are made for the solar wind data obtained by Sakigake, Suisei, IMP-8 and Giotto between late 1985 and early 1987. Until Janaury 1986, high-speed streams were extended across the equator from the high latitudes of the heliosphere. After March 1986, high-speed streams were rarely seen on the equator. Although there remained a slight wavy pattern in latitude-longitude structure, low-speed streams were basically ranged along the equator. After January 1987, the amplitude of this wavy pattern was further diminished and low-speed regions were completely aligned to the equator.  相似文献   

Annual and seasonal variations in the low-latitude topside ionosphere     

Y. Z. Su  G. J. Bailey  K.-I. Oyama 《Annales Geophysicae》1998,16(8):974-985

Annual and seasonal variations in the low-latitude topside ionosphere are investigated using observations made by the Hinotori satellite and the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The observed electron densities at 600 km altitude show a strong annual anomaly at all longitudes. The average electron densities of conjugate latitudes within the latitude range ±25° are higher at the December solstice than at the June solstice by about 100% during daytime and 30% during night-time. Model calculations show that the annual variations in the neutral gas densities play important roles. The model values obtained from calculations with inputs for the neutral densities obtained from MSIS86 reproduce the general behaviour of the observed annual anomaly. However, the differences in the modelled electron densities at the two solstices are only about 30% of that seen in the observed values. The model calculations suggest that while the differences between the solstice values of neutral wind, resulting from the coupling of the neutral gas and plasma, may also make a significant contribution to the daytime annual anomaly, the E × B drift velocity may slightly weaken the annual anomaly during daytime and strengthen the anomaly during the post-sunset period. It is suggested that energy sources, other than those arising from the 6% difference in the solar EUV fluxes at the two solstices due to the change in the Sun-Earth distance, may contribute to the annual anomaly. Observations show strong seasonal variations at the solstices, with the electron density at 600 km altitude being higher in the summer hemisphere than in the winter hemisphere, contrary to the behaviour in NmF2. Model calculations confirm that the seasonal behaviour results from effects caused by transequatorial component of the neutral wind in the direction summer hemisphere to winter hemisphere.  相似文献   

Properties of solar wind turbulence from radio occultation experiments with the NOZOMI spacecraft     

A. I. Efimov  T. Imamura  K. -I. Oyama  K. Noguchi  L. N. Samoznaev  A. S. Nabatov  M. K. Bird  I. V. Chashei 《Astronomy Reports》2010,54(11):1032-1041

Radio-sounding experiments using signals from the Japanese NOZOMI spacecraft to probe the circum solar plasma were performed from December 2000 through January 2001. They can be used to obtain information about the properties of the solar wind plasma in the region where it is accelerated at heliocentric distances of 12.8–36.9R _s (where R _s is the radius of the Sun). Measurements of the intensity and frequency of the received signals were carried out with high time resolution (～0.05 s for the frequency and ～0.0064 s for the intensity), making it possible to investigate the anisotropy of inhomogeneities and the spatial spectrum of the turbulence of the circum solar plasma. Analysis of these radio-sounding data has shown that the scintillation index and intensity of the frequency fluctuations decrease approximately according to a power law with increasing distance of the line of sight from the Sun. Measurements of the amplitude fluctuations and estimates of the solar wind velocity derived from spatially separated observations indicate the presence of small-scale inhomogeneities with sizes of the order of 50 km at heliocentric distances less than 25R _s , which are elongated in the radial direction with anisotropy coefficients from 2.3 to 3.0. The inhomogeneities at heliocentric distances exceeding 30R _s become close to isotropic.  相似文献   

Projected future changes in the contribution of Indo-Pacific sea surface height variability to the Indonesian throughflow     

Shilimkar  Vivek  Abe  Hiroto  Roxy  Mathew Koll  Tanimoto  Youichi 《Journal of Oceanography》2022,78(5):337-352

The Indonesian throughflow (ITF) transports a significant amount of warm freshwater from the Pacific to the Indian Ocean, making it critical to the global climate system. This study examines decadal ITF variations using ocean reanalysis data as well as climate model simulations from the Coupled Model Inter-comparison Project Phase 5 (CMIP5). While the observed annual cycle of ITF transport is known to be correlated with the annual cycle of sea surface height (SSH) difference between the Pacific and Indian Oceans, ocean reanalysis data (1959–2015) show that the Pacific Ocean SSH variability controls more than 85% of ITF variation on decadal timescales. In contrast, the Indian Ocean SSH variability contributes less than 15%. While those observed contributions are mostly reproduced in the CMIP5 historical simulations, an analysis of future climate projections shows a 25–30% increase in the Indian Ocean SSH variability to decadal ITF variations and a corresponding decrease in the Pacific contribution. These projected changes in the Indian Ocean SSH variability are associated with a 23% increase in the amplitudes of negative zonal wind stress anomalies over the equatorial Indian Ocean, along with a 12º eastward shift in the center of action in these anomalies. This combined effect of the increased amplitude and eastward shift in the zonal wind stress increases the SSHA variance over the Indian Ocean, increasing its contribution to the ITF variation. The decadal ITF changes discussed in this study will be crucial in understanding the future global climate variability, strongly coupled to Indo-Pacific interactions.

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Intraseasonal SST-precipitation relationship and its spatial variability over the tropical summer monsoon region   总被引：1，自引：1，他引：0  

Mathew Roxy  Youichi Tanimoto  B. Preethi  Pascal Terray  R. Krishnan 《Climate Dynamics》2013,41(1):45-61

The SST-precipitation relationship in the intraseasonal variability (ISV) over the Asian monsoon region is examined using recent high quality satellite data and simulations from a state of the art coupled model, the climate forecast system version 2 (CFSv2). CFSv2 demonstrates high skill in reproducing the spatial distribution of the observed climatological mean summer monsoon precipitation along with its interannual variability, a task which has been a conundrum for many recent climate coupled models. The model also exhibits reasonable skill in simulating coherent northward propagating monsoon intraseasonal anomalies including SST and precipitation, which are generally consistent with observed ISV characteristics. Results from the observations and the model establish the existence of spatial variability in the atmospheric convective response to SST anomalies, over the Asian monsoon domain on intraseasonal timescales. The response is fast over the Arabian Sea, where precipitation lags SST by ~5 days; whereas it is slow over the Bay of Bengal and South China Sea, with a lag of ~12 days. The intraseasonal SST anomalies result in a similar atmospheric response across the basins, which consists of a destabilization of the bottom of the atmospheric column, as observed from the equivalent potential temperature anomalies near the surface. However, the presence of a relatively strong surface convergence over the Arabian Sea, due to the presence of a strong zonal gradient in SST, which accelerates the upward motion of the moist air, results in a relatively faster response in terms of the local precipitation anomalies over the Arabian Sea than over the Bay of Bengal and South China Sea. With respect to the observations, the ocean–atmosphere coupling is well simulated in the model, though with an overestimation of the intraseasonal SST anomalies, leading to an exaggerated SST-precipitation relationship. A detailed examination points to a systematic bias in the thickness of the mixed layer of the ocean model, which needs to be rectified. A too shallow (deep) mixed layer enhances (suppress) the amplitude of the intraseasonal SST anomalies, thereby amplifying (lessening) the ISV and the active-break phases of the monsoon in the model.  相似文献   

Erroneous evaluation of electron and ion temperatures from a contaminated retarding potential trap     

Oyama Koh-Ichiro  Hirao Kunio 《Planetary and Space Science》1975,23(9):1309-1312

Contaminated electrode effects of a retarding potential trap on the temperature measurement are investigated in laboratory plasma. It is shown that poorly conductive layers on the electrode surface modify the v-i curves. This contamination effect of RPT electrode can be overcome either by increasing the sweep-rate of the grid potential or by the use of a large effective sheath resistance.  相似文献   

A systematic investigation of several phenomena associated with contaminated Langmuir probes     

Oyama Koh-Ichiro 《Planetary and Space Science》1976,24(2):183-190

The first part of this paper uses a contamination layer model to discuss the effects of electrode contamination upon electron temperatures estimated from Langmuir probe measurements. The model assumes that the contamination layer is equivalent to a parallel capacitor and resistor. It predicts two main features associated with electrode contamination. One of them, the so-called “frequency dependence,” concerns the sweep-rate of the probe voltage and is well understood. The other is that the effect of a contaminated electrode is decreased as the density of the ambient plasma is decreased and this will be called “density dependence.” We present several experimental results which verify the above predictions.This paper also presents some other experimental results which may be useful in improving the accuracy of Langmuir probe measurements.Finally the effects of electrode contamination upon electron density estimates and energy distribution measurements are briefly discussed.  相似文献   

Perturbed physics ensemble using the MIROC5 coupled atmosphere–ocean GCM without flux corrections: experimental design and results     

Hideo Shiogama  Masahiro Watanabe  Masakazu Yoshimori  Tokuta Yokohata  Tomoo Ogura  James D. Annan  Julia C. Hargreaves  Manabu Abe  Youichi Kamae  Ryouta O’ishi  Rei Nobui  Seita Emori  Toru Nozawa  Ayako Abe-Ouchi  Masahide Kimoto 《Climate Dynamics》2012,39(12):3041-3056

In this study, we constructed a perturbed physics ensemble (PPE) for the MIROC5 coupled atmosphere–ocean general circulation model (CGCM) to investigate the parametric uncertainty of climate sensitivity (CS). Previous studies of PPEs have mainly used the atmosphere-slab ocean models. A few PPE studies using a CGCM applied flux corrections, because perturbations in parameters can lead to large radiation imbalances at the top of the atmosphere and climate drifts. We developed a method to prevent climate drifts in PPE experiments using the MIROC5 CGCM without flux corrections. We simultaneously swept 10 parameters in atmosphere and surface schemes. The range of CS (estimated from our 35 ensemble members) was not wide (2.2–3.2?°C). The shortwave cloud feedback related to changes in middle-level cloud albedo dominated the variations in the total feedback. We found three performance metrics for the present climate simulations of middle-level cloud albedo, precipitation, and ENSO amplitude that systematically relate to the variations in shortwave cloud feedback in this PPE.  相似文献