Chuanxi Liu

Dynamical and chemical features of a cutoff low over northeast China in July 2007: Results from satellite measurements and reanalysis

The European Centre for Medium-Range Weather Forecasts Re-Analysis Interim (ERA-Interim) meteorology and measurements from the Microwave Limb Sounder, High Resolution Dynamics Limb Sounder, and Ozone Monitoring Instrument onboard the Earth Observing System Aura satellite were applied to analyze the dynamical and chemical features of a cutoff low (COL) event over northeast China in early July 2007. The results showed the polar stratospheric origin of an upper-level warm-core cyclone at 100–300 hPa, associated with a funnel-shaped tropopause intruding into the mid-troposphere just above the COL center. The impacts of the stratospheric intrusion on both column ozone and ozone profiles were investigated using satellite measurements. When the intensity of the COL peaked on 10 July 2007, the total column ozone (TCO) increase reached a maximum (40–70 DU). This could be dynamically attributed to both the descent of the tropopause (∼75%) and the downward transport of stratospheric ozone across the tropopause (∼25%). Analysis of the tropospheric ozone profiles provided evidence for irreversible transport/mixing of ozone-rich stratospheric air across the tropopause near the upper-level front region ahead of the COL center. This ozone intrusion underwent downstream transport by the upper tropospheric winds, leading to further increase in TCO by 12–16 DU over broad regions extending from east China toward the northern Japan Sea via South Korea. Meteorological analysis also showed the precedence of the stratospheric intrusion ahead of the development of cyclones in the middle and lower troposphere.

Middle Atmosphere Response to ENSO Events in Northern Hemisphere Winter by the Whole Atmosphere Community Climate Model

The influences of El Niño and La Niña sea surface temperature anomalies on the middle atmosphere are analyzed using a chemistry-climate model, the Whole Atmosphere Community Climate Model, version 3 (WACCM3), and the 40-year reanalysis dataset from the European Centre for Medium-range Weather Forecasts (ERA-40). Monthly mean data are used to perform statistical and dynamical analyses. Temperature anomalies are found in the stratosphere during both El Niño and La Niña years. These anomalies exhibit diverse distribution patterns, which we ascribe to the different scales of the planetary waves. During an El Niño winter, planetary waves are most active in December and January, leading to more disturbances of the stratospheric polar vortex and a higher frequency of stratospheric sudden warming (SSW) events. Moreover, on the basis of the three-dimensional Eliassen-Palm (E-P) flux, we find that the main wave action is located in the eastern hemisphere and displaces the polar vortex to the western hemisphere. These vortex displacements are closely connected to the weather and climate in the corresponding areas. In contrast, during a La Niña winter, planetary waves are most active in February and March, and most SSW events occur at the same time. The distribution of the E-P flux indicates that planetary wave fluctuations are located between 90°E and 180°E, resulting in a displacement of the polar vortex with some associated changes in the related atmospheric circulation. In addition, the mesospheric residual circulation (RC) reverses between February and March in both El Niño and La Niña years. Dynamical analyses using parameterized gravity waves show that the mesospheric meridional RC is closely connected to gravity-wave drag. This, in turn, is directly influenced by temperature and zonal wind patterns in the early and late winter of El Niño and La Niña years. These anomalies in the RC may influence the distribution of atmospheric constituents and should be explored further.

Satellite measurements of the Madden–Julian oscillation in wintertime stratospheric ozone over the Tibetan Plateau and East Asia

We investigate the Madden–Julian Oscillation (MJO) signal in wintertime stratospheric ozone over the Tibetan Plateau and East Asia using the harmonized dataset of satellite ozone profiles. Two different MJO indices—the all-season Real-Time multivariate MJO index (RMM) and outgoing longwave radiation-based MJO index (OMI)—are used to compare the MJO-related ozone anomalies. The results show that there are pronounced eastward-propagating MJO-related stratospheric ozone anomalies (mainly within 20–200 hPa) over the subtropics. The negative stratospheric ozone anomalies are over the Tibetan Plateau and East Asia in MJO phases 4–7, when MJO-related tropical deep convective anomalies move from the equatorial Indian Ocean towards the western Pacific Ocean. Compared with the results based on RMM, the MJO-related stratospheric column ozone anomalies based on OMI are stronger and one phase ahead. Further analysis suggests that different sampling errors, observation principles and retrieval algorithms may be responsible for the discrepancies among different satellite measurements. The MJO-related stratospheric ozone anomalies can be attributed to the MJO-related circulation anomalies, i.e., the uplifted tropopause and the northward shifted westerly jet in the upper troposphere. Compared to the result based on RMM, the upper tropospheric westerly jet may play a less important role in generating the stratospheric column ozone anomalies based on OMI. Our study indicates that the circulation-based MJO index (RMM) can better characterize the MJO-related anomalies in tropopause pressure and thus the MJO influence on atmospheric trace gases in the upper troposphere and lower stratosphere, especially over subtropical East Asia.

Dominant Modes of Tropospheric Ozone Variation over East Asia from GOME Observations

The variation in tropospheric ozone over East Asia was analyzed using tropospheric column ozone data measured by the Global Ozone Monitoring Experiment (GOME) satellite. An empirical orthogonal function (EOF) analysis was carried out to derive the dominant modes of the variation in the tropospheric ozone volume-mixing ratio (TOVMR). The EOF1 mode, which explained 61.5% of the total variance, showed a same-sign distribution over all of East Asia, with a belt of enhanced ozone concentrations around 40°N. The principal component of EOF1 (PC1) suggested that photochemical ozone production together with Brewer-Dobson circulation and subtropical westerly jet plays important roles in modulating the seasonal variation of the TOVMR; ozone-rich air produced by photochemical processes was transported from the stratosphere to the troposphere by BD circulation and this ozone-rich air was then blocked by the subtropical westerly jet and accumulated north of the jet. The EOF2 mode explained 29.2% of the total variance with an opposite-sign pattern on the north and south side of 35°N. When anticyclonic circulation transported ozone-poor air from the upwelling area over the Bay of Bengal towards the Tibetan Plateau during the onset of the Asian summer monsoon, tropospheric ozone in this region decreased dramatically.