Masaru Chiba

A general circulation model study of the solar and QBO modulation of the stratospheric circulation during the northern hemisphere winter

A general circulation model has been used to study the modulation of north-polar temperatures during winter by both solar activity and the equatorial quasi-biennial oscillation (QBO). The variation of solar activity was simulated by changing the heating rate due to the absorption of ultraviolet (UV) radiation by ozone, while the QBO zonal wind fields were reproduced by incorporating zonal-momentum sources in the equatorial stratosphere. A total of 10 experiments were conducted by changing the heating rate from 70 to 110% for each of the simulated QBO easterly and westerly cases. The results of the numerical experiments show modulation effects similar to those found by Labitzke (1987) in the 30-mb temperatures at the North Pole.

Tropospheric circulation changes associated with stratospheric sudden warmings: A case study

It was theoretically demonstrated by Matsuno that stratospheric warmings are caused by an intensified vertical propagation of tropospheric planetary waves. However, the question of how the resultant changes in the stratospheric circulation affect the troposphere in return is left unanswered. In the present study, a case study on the 1984–1985 stratospheric warming event is conducted to clarify the changes in the tropospheric circulation associated with stratospheric sudden warmings. The results of the present study indicate that during stratospheric warmings, not only the intensification of the upward propagation of planetary waves is found in the stratosphere, but also changes in the direction of the meridional propagation of waves occur in the troposphere as well as in the stratosphere. Changes in the meridional phase structure of tropospheric planetary waves produce enhanced cold surges over the oceans, which in turn generate intense synoptic eddies. Further disturbances, such as blockings, can be produced through interactions between the planetary waves and synoptic eddies, but this may be only indirectly related with the stratospheric warmings. Comparisons between the observed changes in circulation and results of numerical model experiments suggest a potential role of the stratosphere in the tropospheric circulation through changes in meridional propagation of planetary waves.