| On the relation between stratosphere and troposphere, the traditional view is that the dynamical process, thermal structure and chemical components in the stratosphere are strongly dominated by the troposphere, and that the stratosphere is a passive recipient of energy and waves from weather systems in the underlying troposphere. Therefore, the stratosphere can not have important impact on the tropospheric weather and climate system. However, this direction has turned in the last two decades. Downward propagation of stratospheric anomalies to the troposphere has been observed. A new perspective of bidirectional coupling between stratosphere and troposphere has been established. This new idea will finally bring reformed assessments of ozone depletion, global warming and the role of atmospheric internal process in climate change.Currently, the feedback processes by the stratosphere are less studied; and the influences of stratospheric anomalies on the underlying weather and climate system, especially the East Asian monsoon, are less known. The strong circumpolar westerly vortex, known as the stratospheric polar vortex is a crucial component in the stratosphere-troposphere coupling system. It is directly related to the Northern Hemisphere Annular Mode (NAM). The polar stratosphere is a region of strong wave-flow interaction, and it is also a region of large variation of diabatic heating and ozone depletion. In this research, we focus on the anomalous variation of stratosphere polar vortex and study the polar vortex climatology, its seasonal transition, the interannual variation, downward propagation and its relation with East Asian winter monsoon. To further understand the variation mechanisms, the MAECHAM5 model (the Middle Atmosphere configuration of ECHAM5) from Max Planck Institute for Meteorology is used.The stratospheric polar vortex is a cold cyclone system formed during winter season when ultraviolet heating decreases in the polar region. It is maintained by the balance between diabatic cooling through emission of thermal infrared radiation and dynamical heating by planetary waves from troposphere. The planetary waves cause the deceleration of the circum-polar vortex, and induce substantial departure from radiative equilibrium state. In summer season, the polar region is characterized by circum-polar easterly. This seasonal transition of polar vortex usually happens in early spring. The average date when the polar vortex breaks up is about April 10. Composite of circulation and wave activities for extremely early and late breakup years shows that there are usually two periods of weakening of the polar vortex in the late breakup years while only one in the early breakup years. The first weakening in the late breakup years happens in middle January. This weakening is accompanied with strong wave activities in the polar stratosphere. However, the second weakening in the late breakup years is mainly the results of diabatic heating while the wave activities are weak. In the early breakup years, the transition from westerly to easterly is rapid, which is associated with a strong bidirectional dynamical coupling of the stratosphere and troposphere.The strength and stability of Northern Hemisphere wintertime stratospheric polar vortex is modulated by the equatorial quasi-biennial oscillation (QBO). This influence has been documented as Holton-Tan Oscillation. The analysis in this study reveals that the QBO is related to the first principal component of the polar vortex, and that the second principal component of PV is associated with sea surface temperature (SST) in the tropical eastern Pacific (known as ENSO). It is found that the Holton-Tan Oscillation pattern is more evident when the SST in the tropical eastern Pacific is cold. The Holton-Tan Oscillation becomes very weak when the SST in tropical eastern Pacific is warm. This weakening is suggested to be cuased by the anomalous propagation of planetary waves into the polar vortex in warm SST years. These strong wave activities in warm SST years offset the QBO-induced weak wave activities in the westerly phase, and therefore disrupt the H-T relation. The anomalous upward propagation seems has little influence on the polar vortex in the easterly QBO years. This unstability of Holton-Tan relation is also reproduced in our model simulation.In the winter half year, the dominant variability of the stratospheric polar vortex has the form of downward propagation of geopotential height anomaly. The time for the propagation from 1hPa to the upper troposphere is about 15 days. This downward propagation is evident also in polar temperature and zonal mean zonal wind at high latitude. The geopotential height anomaly is shown to come from the lower and middle latitude. The anomaly usually develops over Pacific section in upper stratosphere and moves northeastward, and then marches into the polar region from north of Greenland. Therefore, the stratospheric downward propagation is highly related to the activity of Aleutian High in the stratosphere.The downward propagation of the stratospheric polar vortex anomalies can influence the phase of Arctic Oscillation on one hand, and the activities of planetary waves on the other hand. In East Asian monsoon region, the Siberian High is influenced during propagating process, which causes the variability of the strength of East Asian winter monsoon and induces temperature fluctuation in China. This influence from the stratosphere polar vortex on winter temperature has the pattern of out-of-phase fluctuation between southern and northern China. This pattern is shown to relate to the second EOF of winter month (DJF) temperature from 160 China stations. To be specific, when positive polar geopotential height anomalies reach the surface, the phase of the Arctic Oscillation tends to be negative. Meanwhile, the Siberian High is weakened, which induces a weak monsoon and warms southern China and cools northern China at the same time. In contrast, when negative polar geopotential height anomalies reach the surface, the Arctic Oscillation is biased to the positive phase; and the Siberian High is strengthened, which induces strong monsoon in East Asia and causes cold and warm temperature anomaly in southern and northern China respectively. It is further revealed that the temperature seesaw pattern between southern and northern China is highly related to the strength of stratosphere polar vortex with a lag time of one month. This provides a new predictor for the East Asian winter monsoon.
|
PDF Full Text Request