Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Atsushi Goto x
  • Refine by Access: All Content x
Clear All Modify Search
Takeaki Sampe
,
Hisashi Nakamura
,
Atsushi Goto
, and
Wataru Ohfuchi

Abstract

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.

Full access
Yayoi Harada
,
Atsushi Goto
,
Hiroshi Hasegawa
,
Norihisa Fujikawa
,
Hiroaki Naoe
, and
Toshihiko Hirooka

Abstract

The major stratospheric sudden warming (SSW) event of January 2009 is analyzed using the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS). This SSW event is characterized by the extraordinary predominance of the planetary-scale wave of zonal wavenumber 2 (wave 2). The total amount of the upward Eliassen–Palm (EP) flux for wave 2 was the strongest since the winter of 1978/79.

It is found that the remarkable development of the upper troposphere ridge over Alaska played important roles in the SSW in January 2009. During the first development stage, the ridge excited wave packets upward as well as eastward over around Alaska. The eastward-propagating packets intensified a trough over eastern Siberia, which led to the development of the planetary wave over eastern Siberia during the second development stage. The results of this study indicate that the pronounced wave-2 pattern observed in the stratosphere was brought about by accumulative effects of rather localized propagation of wave packets from the troposphere during the course of this SSW event rather than by the ubiquitous propagation of planetary-scale disturbances in the troposphere.

The features of the SSW in January 2009 are quite similar to those during the major stratospheric warming event in February 1989: both SSWs are characterized by the predominance of wave 2, the remarkable development of the upper troposphere ridge over around Alaska, and positive SSTs in the eastern part of the North Pacific corresponding to a La Niña condition.

Full access
Bunmei Taguchi
,
Hisashi Nakamura
,
Masami Nonaka
,
Nobumasa Komori
,
Akira Kuwano-Yoshida
,
Koutarou Takaya
, and
Atsushi Goto

Abstract

Potential impacts of pronounced decadal-scale variations in the North Pacific sea surface temperature (SST) that tend to be confined to the subarctic frontal zone (SAFZ) upon seasonally varying atmospheric states are investigated, by using 48-yr observational data and a 120-yr simulation with an ocean–atmosphere coupled general circulation model (CGCM). SST fields based on in situ observations and the ocean component of the CGCM have horizontal resolutions of 2.0° and 0.5°, respectively, which can reasonably resolve frontal SST gradient across the SAFZ. Both the observations and CGCM simulation provide a consistent picture between SST anomalies in the SAFZ yielded by its decadal-scale meridional displacement and their association with atmospheric anomalies. Correlated with SST anomalies persistent in the SAFZ from fall to winter, a coherent decadal-scale signal in the wintertime atmospheric circulation over the North Pacific starts emerging in November and develops into an equivalent barotropic anomaly pattern similar to the Pacific–North American (PNA) pattern. The PNA-like signal with the weakened (enhanced) surface Aleutian low correlated with positive (negative) SST anomalies in the SAFZ becomes strongest and most robust in January, under the feedback forcing from synoptic-scale disturbances migrating along the Pacific storm track that shifts northward (southward) in accord with the oceanic SAFZ. This PNA-like signal, however, breaks down in February, which is suggestive of a particular sensitivity of that anomaly pattern to subtle differences in the background climatological-mean state. Despite its collapse in February, the PNA-like signal recurs the next January. This subseasonal evolution of the signal suggests that the PNA-like anomaly pattern may develop as a response to the persistent SST anomalies that are maintained mainly through ocean dynamics.

Full access
Manabu Nemoto
,
Takahiro Hamasaki
,
Ryoji Sameshima
,
Etsushi Kumagai
,
Hiroyuki Ohno
,
Yasuyuki Wakiyama
,
Atsushi Maruyama
,
Shinkichi Goto
, and
Kiyoshi Ozawa

Abstract

A framework for the probabilistic risk assessment of the rice (Oryza sativa L.) cropping schedule (PRARCS) is presented. The method accounts for interannual meteorological variation, as opposed to the traditional cultivation schedule planning method, which is based on the seasonal change in long-term average air temperature. PRARCS uses an arbitrary developmental index model to estimate the timing of the heading stage, which is required to assess the risks of cold or heat damage and productivity. All results of risk assessment and productivity are linked to transplanting date as the most important cultivation practice for irrigated rice paddies. The results of assessments using PRARCS at Iwamizawa, central Hokkaido, Japan, indicated that for the current climate the optimal transplanting period is the end of May, and this corresponds to the actual transplanting dates used and the increasing risks and decreasing productivity with earlier or later transplanting. Assessment using projected climatic data for the period 2031–50 showed that Akitakomachi and Hitomebore, present-day cultivars in the Tohoku region, could achieve satisfactory productivity in central Hokkaido. For the term 2081–2100, assessment using an extreme warming projection (4.7°C higher than at present) indicated that the probability of heat damage (white-based rice and heat sterility) is once every 2 yr for some transplanting dates and that cold risks (transplanting damage, cold injury, and immature ripening) are reduced. It was also found that the cultivar Koshihikari, at present suited to more southern regions, could be cultivated in central Hokkaido in 2081–2100 if transplanting is conducted in early May.

Full access