Hisashi Nakamura - Academia.edu (original) (raw)

Papers by Hisashi Nakamura

1. Preface Starting Journal of the Earth Simulator – Dawning of New Era in Simulation Science – .... more 1. Preface Starting Journal of the Earth Simulator – Dawning of New Era in Simulation Science – ............................................... 2 Tetsuya Sato ... 2. The Current Status of the Earth Simulator ..................................................... 6 ... 3. Atmospheric and Oceanic Simulation 1) 10-km Mesh Meso-scale Resolving Simulations of the Global Atmosphere on the Earth Simulator – Preliminary Outcomes of AFES (AGCM for the Earth Simulator) – ..... 8 Wataru Ohfuchi, Hisashi Nakamura, Mayumi K. Yoshioka, Takeshi Enomoto, Koutarou Takaya, Xindong Peng, ...

Journal of Climate, 2010

Ocean–atmosphere interaction over the Northern Hemisphere western boundary current (WBC) regions ... more Ocean–atmosphere interaction over the Northern Hemisphere western boundary current (WBC) regions (i.e., the Gulf Stream, Kuroshio, Oyashio, and their extensions) is reviewed with an emphasis on their role in basin-scale climate variability. SST anomalies exhibit considerable variance on interannual to decadal time scales in these regions. Low-frequency SST variability is primarily driven by basin-scale wind stress curl variability via the oceanic Rossby wave adjustment of the gyre-scale circulation that modulates the latitude and strength of the WBC-related oceanic fronts. Rectification of the variability by mesoscale eddies, reemergence of the anomalies from the preceding winter, and tropical remote forcing also play important roles in driving and maintaining the low-frequency variability in these regions. In the Gulf Stream region, interaction with the deep western boundary current also likely influences the low-frequency variability. Surface heat fluxes damp the low-frequency SST...

… , Baltimore, MD, USA, 2004

... A Review Shoshiro Minobe1, Niklas Schneider2, Clara Deser3, Zhengyu Liu4, Nathan Mantua5, His... more ... A Review Shoshiro Minobe1, Niklas Schneider2, Clara Deser3, Zhengyu Liu4, Nathan Mantua5, Hisashi Nakamura6,7, and Masami Nonaka7 1: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N-10, W-8, 060-0810, Sapporo, Japan. ...

Geophysical Research Letters, 2000

Time evolution of a prominent wintertime blocking event over East Antarctica and the associated d... more Time evolution of a prominent wintertime blocking event over East Antarctica and the associated drastic changes in weather conditions observed at an inland station are documented. A strong blocking ridge was formed at the leading edge of a quasi-stationary Rossby wavetrain where a wave activity flux emanating from the subtropics was convergent. The ridge pumped up heat and moisture from lower latitudes into inland Antarctica with a strong poleward flow along its upstream flank. The station was situated under this flow just for two days, during which the normal condition with cold and clear weather was markedly disturbed. The flow climbing up the continental slope led to cloud formation above the inland station. Enhanced downward longwave radiation from the clouds and enhanced vertical mixing associated with the record-setting wind speeds resulted in a sudden, drastic increase in surface air temperature and the breakdown of the developed surface inversion layer.

IEEE Pacific Rim Conference on Communications, Computers, and Signal Processing. Proceedings, 1995

Exploratory data mining and analysis requires an extensible environment which provides facilities... more Exploratory data mining and analysis requires an extensible environment which provides facilities for the user-friendly expression and rapid execution of “scientific queries”. The authors present the CONQUEST (parallel query processing system) environment and illustrate its use for exploratory data analysis and data mining of spatio-temporal phenomena from geophysical datasets

Quarterly Journal of the Royal Meteorological Society

Behaviour of quasi-stationary circulation anomalies observed in the lower stratosphere of the ext... more Behaviour of quasi-stationary circulation anomalies observed in the lower stratosphere of the extratropical Southern Hemisphere during austral late winter of 1997 is studied. The anomalies are defined as daily low-pass-filtered departures from the circulation varying slowly with the seasonal cycle. A wave-activity flux and refractive index for stationary Rossby waves are utilized in our analysis, each of which was defined locally for the zonally varying westerlies. Subseasonal fluctuations in the lower stratosphere were often associated with zonally confined wave trains emanating upward from localized, quasi-stationary anomalies in the troposphere including blocking ridges. The three-dimensional propagation of the waves was found sensitive to the structure of a local waveguide. Upward injection of Rossby wave activity into the stratosphere tended to occur slightly upstream or just beneath of a lower-stratospheric waveguide associated with the developed polar-night jet (PNJ), which led to the subsequent formation of a well-defined wave train downstream along that jet. The distribution of the lower-stratospheric subseasonal variability thus exhibits significant zonal asymmetries, reflecting those in the PNJ structure and the distribution of tropospheric disturbances. Seasonal evolution of the PNJ and that of the tropospheric intraseasonal variability substantially modulated the lower-stratospheric activity of subseasonal fluctuations.

Journal of the Meteorological Society of Japan. Ser. II

The importance of West-Pacific pattern, a meridionally oriented dipole anomaly over the Pacific, ... more The importance of West-Pacific pattern, a meridionally oriented dipole anomaly over the Pacific, on the tropospheric circulation is well recognized. It strongly modulates the storm track activity and the occurrences of blockings over the Pacific, as well cold surges over the Far East. Here, we show that the positive phase of the West-Pacific pattern is a precursor of cold episodes in the polar stratosphere in winter. More precisely, we use composites and a case study to demonstrate that blocking-like flow configuration associated with this pattern is followed by cold stratospheric anomalies lasting about one month. Further analysis shows that the cold period in the stratosphere follows a reduction of meridional eddy heat fluxes involving the interaction of transient, westward-propagating anomalies with the background planetary wave trough over the North Pacific. The vertical propagation of wave activity flux in the stratosphere is hence reduced following the erosion of the background trough. The aim of this work is to unravel precursory disturbances in the troposphere, which are conducive to a cold winter polar stratosphere with a strong potential for ozone depletion.

Thesis University of Washington 1990 Source Dissertation Abstracts International Volume 51 09 Section B Page 4402, 1990

Journal of Climate, Jul 1, 2002

Interannual variability of the North Pacific storm track observed over 17 recent winters is docum... more Interannual variability of the North Pacific storm track observed over 17 recent winters is documented. The local storm track activity is measured by a meridional flux of sensible heat associated with the lower-tropospheric subweekly fluctuations. The interannual variability in the heat ...

The western North Pacific region is one of the centers of action of decadal time scale variations... more The western North Pacific region is one of the centers of action of decadal time scale variations in the atmosphere and ocean, and also oceanic ecosystem can be strongly influenced by the variations. One of the key variable of the decadal variations is sea surface temperature (SST) that varies as a result of interaction between the atmosphere and ocean. To improve our understanding of the key element, SST variaions in the two frontal zones in the western North Pacific Ocean, the Kuroshio Extension and subarctic frontal zones, are investigated on the basis of an in situ observational dataset. Interannual-to-decadal variations in these two frontal zones are not highly correlated, indicating that to some extent different mechanisms induce the SST variations in the frontal zones and those two frontal zone cannot be considered as a single frontal zone as has been done in most of previous studies. Meanwhile, the results are consistent with the recent studies that suggest different mechanisms for the variations in the two frontal zones on the basis of a solution to an eddy-resolving, i.e., very high horizontal resolution, ocean general circulation model.

Journal of the Atmospheric Sciences, Nov 1, 1987

... Appel aux cartes de régression basées sur les indices des modes Atlantique est (EA), Pacifiqu... more ... Appel aux cartes de régression basées sur les indices des modes Atlantique est (EA), Pacifique-Nord americain (PNA), Atlantique ouest (WA), Pacifique ouest (WP), et Eurasien (EU) définis par Wallace et Gutzler (1981), pour définir les figures fixées géographiquement. ...

Monthly Weather Review, Jul 1, 1993

In a set of idealized ''aquaplanet'' experiments with an atmospheric general circulation model to... more In a set of idealized ''aquaplanet'' experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature (SST) is prescribed globally as the lower boundary condition, an assessment is made of the potential influence of the frontal SST gradient upon the formation of a storm track and an eddy-driven midlatitude polar front jet (PFJ), and on its robustness against changes in the intensity of a subtropical jet (STJ). In experiments with the frontal midlatitude SST gradient as that observed in the southwestern Indian Ocean, transient eddy activity in each of the winter and summer hemispheres is organized into a deep storm track along the SST front with an enhanced low-level baroclinic growth of eddies. In the winter hemisphere, another storm track forms just below the intense STJ core, but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath. The near-surface westerlies are strongest near the midlatitude SST front as observed, consistent with westerly momentum transport associated with baroclinic eddy growth. The sharp poleward decline in the surface sensible heat flux across the SST frontal zone sustains strong near-surface baroclinicity against the relaxing effect by vigorous poleward eddy heat transport. Elimination of the midlatitude frontal SST gradient yields marked decreases in the activity of eddies and their transport of angular momentum into midlatitudes, in association with equatorward shifts of the PFJ-associated low-level westerlies and a subtropical high pressure belt, especially in the summer hemisphere. These impacts of the midlatitude frontal SST gradient are found to be robust against modest changes in the STJ intensity as observed in its interannual variability, suggesting the potential importance of midlatitude atmosphere-ocean interaction in shaping the tropospheric general circulation.

Scientific Reports, 2016

Predictability of atmospheric variability is known to be limited owing to significant uncertainty... more Predictability of atmospheric variability is known to be limited owing to significant uncertainty that arises from intrinsic variability generated independently of external forcing and/or boundary conditions. Observed atmospheric variability is therefore regarded as just a single realization among different dynamical states that could occur. In contrast, subject to wind, thermal and freshwater forcing at the surface, the ocean circulation has been considered to be rather deterministic under the prescribed atmospheric forcing, and it still remains unknown how uncertain the upper-ocean circulation variability is. This study evaluates how much uncertainty the oceanic interannual variability can potentially have, through multiple simulations with an eddy-resolving ocean general circulation model driven by the observed interannually-varying atmospheric forcing under slightly different conditions. These ensemble "hindcast" experiments have revealed substantial uncertainty due to intrinsic variability in the extratropical ocean circulation that limits potential predictability of its interannual variability, especially along the strong western boundary currents (WBCs) in mid-latitudes, including the Kuroshio and its eastward extention. The intrinsic variability also greatly limits potential predictability of mesoscale oceanic eddy activity. These findings suggest that multi-member ensemble simulations are essential for understanding and predicting variability in the WBCs, which are important for weather and climate variability and marine ecosystems.