Kunihiko Kodera

Transfer of the solar signal from the stratosphere to the troposphere: Northern winter

The atmospheric response to the solar cycle has been previously investigated with the Freie Universitat Berlin Climate Middle Atmosphere Model (FUB-CMAM) using prescribed spectral solar UV and ozone changes as well as prescribed equatorial, QBO-like winds. The solar signal is transferred from the upper to the lower stratosphere through a modulation of the polar night jet and the Brewer-Dobson circulation. These model experiments are further investigated here to show the transfer of the solar signal from the lower stratosphere to the troposphere and down to the surface during Northern Hemisphere winter. Analysis focuses on the transition from significant stratospheric effects in October and November to significant tropospheric effects in December and January. The results highlight the importance of stratospheric circulation changes for the troposphere. Together with the poleward-downward movement of zonal wind anomalies and enhanced equatorward planetary wave propagation, an AO-like pattern develops in the troposphere in December and January during solar maximum. In the middle of November, one third of eddy-forced tropospheric mean meridional circulation and surface pressure tendency changes can be attributed to the stratosphere, whereas most of the polar surface pressure tendency changes from the end of November through the middle of December are related to tropospheric mechanical forcing changes. The weakening of the Brewer-Dobson circulation during solar maximum leads to dynamical heating in the tropical lower stratosphere, inducing circulation changes in the tropical troposphere and down to the surface that are strongest in January. The simulated tropospheric effects are identified as indirect effects from the stratosphere because the sea surface temperatures are identical in the solar maximum and minimum experiment. These results confirm those from other simplified model studies as well as results from observations.

Solar cycle modulation of the North Atlantic Oscillation: Implication in the spatial structure of the NAO

[1] The spatial extent of the North Atlantic Oscillation (NAO) is investigated. The results suggest that the spatial structure of the NAO differs significantly according to the phase of the solar cycle. During the solar maximum phases, the NAO has a hemispherical structure extending into the stratosphere, which is similar to the Arctic Oscillation (AO) except for the Pacific sector. However, during the minimum phase, the NAO is confined to the eastern Atlantic sector in the troposphere. Whether or not the NAO is modulated by the solar cycle, these results may shed new light on the controversy of the physical reality of the two modes of variability, the NAO and the AO.

On the origin and nature of the interannual variability of the winter stratospheric circulation in the northern hemisphere

During the northern hemisphere winter, the stratospheric circulation exhibits quite large interannual variability. In the present study, the nature and origin of the interannual variation in the winter stratosphere is studied with two different approaches: one is composite analysis assuming a priori four possible causes: solar activity, equatorial quasi-biennial oscillation (QBO), volcanic aerosols, and unknown cause producing trends, while the other is an extended empirical orthogonal function analysis (E-EOF). The results of composite analysis show that in November, anomalous zonal-mean zonal winds and meridional temperature gradients are found in accordance with the assumed forcings in different regions in the stratosphere. As winter progresses, however, anomalous patterns in zonal wind become similar by forming a dipole-type pattern and extend down into the troposphere. The common features of the evolution of the anomalies during the winter can be well explained by the first mode of the E-EOF. The results of the present study suggest that interannual variability in the winter stratosphere in the northern hemisphere can be considered an internal mode of variability triggered by changes in forcing, such as solar activity, equatorial QBO, and volcanic aerosols. Although overall features of the evolution of zonal-mean zonal wind associated with trends are similar, the cause of the trends is still undetermined.

Influence of volcanic eruptions on the troposphere through stratospheric dynamical processes in the northern hemisphere winter

It has been suggested that the zonal-mean stratospheric zonal-wind anomalies can propagate downward into the troposphere, producing considerable changes. On the other hand, volcanic aerosols are thought to produce a stronger polar vortex in the winter stratosphere. If these suggestions are correct, the influence of volcanic eruptions can be expected to produce changes in the northern hemisphere winter troposphere through stratospheric process. In the present study, this idea is examined by analyzing changes in the January circulation after three recent volcanic eruptions. Regional features of observed changes in tropospheric temperatures due to volcanic eruptions can be explained by the changes in the propagation of stationary waves associated with dipole-type anomalies in the zonal-mean zonal winds, which are similar to those observed in a previous work on the influence of polar night jet.

Role of planetary waves in the stratosphere-troposphere coupled variability in the northern hemisphere winter

The role of planetary waves in stratosphere-troposphere coupled variability is investigated using an extended singular value decomposition analysis of zonal-mean zonal wind and the vertical component of the Eliassen-Palm (E-P) flux for the winters from 1979/80 to 1995/96. The results suggest a close relationship between anomalies of zonal-mean zonal wind and the convergence zone of E-P flux, which together shift poleward and downward from the stratosphere to the troposphere as time advances. Following enhanced vertical propagation of waves into the stratosphere, the Arctic Oscillation (AO) pattern is seen in the 500 hPa geopotential height field in association with an increased poleward propagation of tropospheric waves.

Variability of the polar night jet in the northern and southern hemispheres

The spatial and temporal characteristics of the month-to-month variability of the polar night jet and its relationship with tropospheric circulation is investigated for both the Northern and Southern Hemispheres. The variabilities of the hemispheres have many common characteristics of the Polar Night Jet Oscillation (PJO). These common characteristics include the following: (1) the anomalous zonal-mean zonal winds shift poleward and downward; (2) the anomalous polar temperatures propagate downward from the stratopause to the upper troposphere; and (3) they are made through a wave-mean flow interaction with mainly the planetary wave of zonal wave number one. Annular modes associated with the PJOs appear in both hemispheres when the zones of maximum polar temperature anomaly descend to the lowermost stratosphere and upper troposphere. The major difference in the PJOs of the two hemispheres is found in their temporal characteristics. In the Southern Hemisphere, the phase of the PJO is closely locked to the annual cycle, while in the Northern Hemisphere it exhibits quasiperiodic variability with its envelope controlled by the annual cycle. The origin of the differences between the PJOs is discussed based on the theory of the wave-mean flow interaction.

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.

The solar and equatorial QBO influences on the stratospheric circulation during the early northern‐hemisphere winter

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Tropospheric and stratospheric aspects of the Arctic oscillation

The role of the stratospheric variability in the Arctic Oscillation (AO) is investigated by selecting 18 large month-to-month variation events. It is shown that although tropospheric features are very similar, the vertical structure of the AO differs considerably depending on whether it is initiated by a downward propagation of zonal-mean zonal wind anomalies from the stratosphere (type S) or it is produced within the troposphere (type T). Type S events preferentially occur during February and March, while type T events are seen during November and December. The results of the analysis suggest that diverse aspects of the AO-related variability can more easily be understood as coupling and uncoupling of stratospheric and tropospheric modes of variability.

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.