Characteristics Of Low Frequency Mode On An Intraseasonal Time Scale At Middle-high Latitudes Over The Eurasia In Winter And Its Influence On Air Temperature In China

Based on the daily NCEP/DOE 2 reanalysis data and the surface air temperature station data over China,the methods of Spatiotemporal spectral analysis,Lanczos filtering,Empirical Orthogonal Function(EOF),regression analysis and phase composite are used to study the issues of the variation features in winter intraseasonal oscillation over Eurasian middle-high latitudes in the Northern Hemisphere(NH)as well as their influences on air temperature over China,which gets less attention at present.The characteristics of distribution and propagation as well as the vertical structures of winter intraseasonal low-frequency predominant mode over Eurasian middle-high latitudes in the NH are discussed first.Secondly,the relationship between the low-frequency predominant mode and the circulation conditions are analyzed.The influences of low-frequency predominant mode on temperature over China are finally studied,including the daily average temperature,daily minimum temperature,and daily maximum temperature.The main conclusions are summarized as follows(1)The intraseasonal oscillation over Eurasian middle-high latitudes in the NH has mainly quasi period about 20-40 days,and it is stronger in winter than in summer.The first two EOF modes show the intraseasonal oscillation over Eurasian middle-high latitude in the northern hemisphere has the distribution characteristic of zonal one wave at 500 hPa.From the results of phase composites,we can see the intraseasonal oscillation over Eurasian middle-high latitudes mainly propagates westward,the speed of zonally propagation is about 5-10 longitude/day,and the quasi-barotropic structure is shown in vertical direction.(2)The correlation coefficients between the winter intraseasonal low-frequency predominant mode over Eurasian middle-high latitudes in the NH and EU teleconnection as well as the Siberian high show the significant positive value.The intraseasonal low-frequency predominant mode over the Eurasian middle-high latitudes has impacts on short wave troughs over middle-high latitudes in the mid tropospheric and the activities of the Siberian high in the lower troposphere through EU wave column,and then influences the temperature change over Eurasian regions.Manifested in phase 1 to 4,the Siberian high shows the stronger phase,while the short wave trough controls the Lake Baykal,which is beneficial for the southward cold air.As for the phase 5 to 8,they are corresponding to the weak Siberian high,and lead to the weakening of cold air.(3)The intraseasonal oscillation over Eurasian middle-high latitudes in the NH has the remarkable adjustment function of daily average temperature,daily maximum temperature,and daily minimum temperature.The phases 1 to 4 are shown as the cold phases,while phases 5 to 8 are warm phases.The largest negative anomallies appear in phase 2 and 3,and the highest positive anomalies show in phase 6 and 7.In terms of the daily average temperature,there is no obvious difference for the day numbers in phases,about 3.7 days per year at the most.As for the strong event whose amplitude is more than doubling standard,the winter intraseasonal low-frequency over Eurasian middle-high latitudes has an impact of more than 0.5? on daily average temperature over most regions of China,and more than 1.0 ? on daily maximum temperature,and the South China and the North China are the mainly salient regions.Compared with the daily average temperature and the daily maximum temperature,the impact on daily minimum temperature over southern China is weaker,and will have an impact of more than 0.5? on daily minimum temperature over most north parts of China when the amplitude is more than doubling standard.(4)The intraseasonal oscillation over Eurasian middle-high latitudes in the NH prefers to affect the extreme low temperature events,no matter daily average,maximum or minimum temperatures;extreme low temperature events mainly occur in phase 2 and phase 5,while extreme high temperature events mainly occur in phase 5 and phase 7.For different regions,strong intraseasonal oscillation over Eurasian middle-high latitudes may lead to a lower daily maximum temperature and a higher daily minimum temperature in South China,while in North China it may lead to a higher daily maximum temperature and a lower minimum temperature.