Polar Vortex Oscillation Viewed in an Isentropic Potential Vorticity Coordinate

The stratospheric polar vortex oscillation （PVO） in the Northern Hemisphere is examined in a semiLagrangian θ-PVLAT coordinate constructed by using daily isentropic potential vorticity maps derived from NCEP/NCAR reanalysis Ⅱdataset covering the period from 1979 to 2003. In the semi-Lagrangian θ-PVLAT coordinate, the variability of the polar vortex is solely attributed to its intensity change because the changes in its location and shape would be naturally absent by following potential vorticity contours on isentropic surfaces. The EOF and regression analyses indicate that the PVO can be described by a pair of poleward and downward propagating modes. These two modes together account for about 82% variance of the daily potential vorticity anomalies over the entire Northern Hemisphere. The power spectral analysis reveals a dominant time scale of about 107 days in the time series of these two modes, representing a complete PVO cycle accompanied with poleward propagating heating anomalies of both positive and negative signs from the equator to the pole. The strong polar vortex corresponds to the arrival of cold anomalies over the polar circle and vice versa. Accompanied with the poleward propagation is a simultaneous downward propagation. The downward propagation time scale is about 20 days in high and low latitudes and about 30 days in mid-latitudes. The zonal wind anomalies lag the poleward and downward propagating temperature anomalies of the opposite sign by 10 days in low and high latitudes and by 20 days in mid-latitudes. The time series of the leading EOF modes also exhibit dominant time scales of 8.7, 16.9, and 33.8 months. They approximately follow a double-periodicity sequence and correspond to the 3-peak extratropical Quasi-Biennial Oscillation （QBO） signal.

Influences of the 11-yr Sunspot Cycle and Polar Vortex Oscillation on Observed Winter Temperature Variations in China

Using the NCEP-2 reanalysis data in 1979-2015, we analyze variations in the coupled stratosphere-troposphere system and attribute them to the polar vortex oscillation(PVO) and the 11-yr sunspot cycle(SC). Subsequently, influences of PVO and SC on the near-ground temperature and extreme temperatures are diagnosed based on observations at 2419 surface stations in China over the same period. Empirical Orthogonal Function(EOF) analysis of geopotential height(GH) anomalies indicates that the first and second EOF modes together can explain nearly 50% of the total variance and they have different driving sources, active periods, and regions. The first EOF mode mainly represents variation characteristics of the polar vortex, and its active periods appear in late winter. It is found that a weakened polar vortex(larger amplitude in the positive time series of the first mode) corresponds to lower daily mean, minimum, and maximum temperatures and more frequent cold nights and days. This cooling effect mainly occur in northeastern China. The second EOF mode is closely related to the SC, and its major active periods are late autumn and early winter. The results reveal that strong solar activity(larger amplitude in the positive time series of the second mode) leads to cooling effects in northern China through accelerating seasonal transformation of the stratospheric circulation and enhancing intensity of the subtropical westerly jet in the upper troposphere and lower stratosphere. The near-ground temperature is lower than usual, especially for daily mean and minimum temperatures. The number of warm nights and days is significantly reduced, and cold nights and days become more frequent. Therefore,the first and second EOF mode time series of GH anomalies can be used as indices of PVO and solar activity, respectively; and can provide indications of winter cooling processes in China.

Multi-scale variation characteristics of polar vortex at 10 hPa in the Southern Hemisphere

By use of 1948-2007 NCEP/NCAR reanalysis monthly geopotential data, a set of circulation indices are defined to characterize the polar vortex at 10 hPa in the Southern Hemisphere, including area-（S）, intensity-（P） and centre position-（λc , φc） indices. Sea-sonal variation, interannual anomalies and their possible causes of 10 hPa polar vortex in the Southern Hemisphere are analyzed by using these indices, the relationship between 10 hPa polar vortex strength and the Antarctic Oscillation are analyzed as well. The results show that： （1） the polar region at 10 hPa in the Southern Hemisphere is controlled by anticyclone （cyclone） from Dec. to Jan. （from Mar. to Oct.）, Feb. and Nov. are circulation transition seasons. （2） Intensity index （P） and area index （S） of anticy-clone （cyclone） in Jan. （Jul.） show a significant spike in the late 1970s, the anticyclone （cyclone） enhances （weakens） from ex-tremely weak （strong） oscillation to near the climatic mean before a spike, anticyclone tends to the mean state from very strong oscillation and cyclone oscillates in the weaker state after the spike. （3） There is significant interdecadal change for the anticyclone center in Jan., while markedly interannual variation for cyclone center in July. （4） The ozone anomalies can cause the interannual anomaly of the polar anticyclone at 10 hPa in the Southern Hemisphere in Jan. （positive correlation between them）, but it is not related to the polar cyclone anomalies. （5） There is notable negative correlation between the polar vortex intensity index P and the Antarctic Oscillation index （AAOI）, thus AAOI can be represented by P.

Parallel Comparison of the Northern Winter Stratospheric Circulation in Reanalysis and in CMIP5 Models

A parallel comparison is made of the circulation climatology and the leading oscillation mode of the northern winter stratosphere among six reanalysis products and 24 CMIP5 （Coupled Model Intercomparison Project Phase 5） models. The results reveal that the NCEP/NCAR, NECP/DOE, ERA40, ERA-Interim and JRA25 reanalyses are quite consistent in describ- ing the climatology and annual cycle of the stratospheric circulation. The 20CR reanalysis, however, exhibits a remarkable ＂cold pole＂ bias accompanied by a much stronger stratospheric polar jet, similar as in some CMIP5 models. Compared to the 1-2 month seasonal drift in most coupled general circulation models （GCMs）, the seasonal cycle of the stratospheric zonal wind in most earth system models （ESMs） agrees very well with reanalysis. Similar to the climatology, the amplitude of Polar Vortex Oscillation （PVO） events also varies among CMIP5 models. The P^O amplitude in most GCMs is relatively weaker than in reanalysis, while that in most of the ESMs is more realistic. In relation to the ＂cold pole＂ bias and the weaker oscillation in some CMIP5 GCMs, the frequency of PVO events is significantly underestimated by CMIP5 GCMs; while in most ESMs, it is comparable to that in reanalysis. The PVO events in reanalysis （except in 20CR） mainly occur from mid-winter to early spring （January-March）; but in some of the CMIP5 models, a l-2 month delay exists, especially in most of the CMIP5 GCMs. The long-term trend of the PVO time series does not correspond to long-term changes in the frequency of PVO events in most of the CMIP5 models.