Intraseasonal variability of air temperature over the mid-high latitude Eurasia in boreal winter
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The intraseasonal oscillation (ISO) of air temperature over the mid- and high-latitude Eurasia in boreal winter was investigated by NCEP-NCAR reanalysis data. It is found that the intraseasonal temperature disturbances exhibit maximum variability near the surface in the region of 50°–75°N, 80°‒120°E and they propagate southeastwards at average zonal and meridional phase speeds of 3.2 and 2.5 m s−1, respectively. The low-level temperature signal is tightly coupled with upper-tropospheric height anomalies, and both propagate southeastward in a similar phase speed. A diagnosis of the temperature budget reveals that the southeastward propagation is primarily attributed to the advection of the temperature anomaly by the mean wind. A wave activity flux analysis indicates that the southeastward propagating wave train is likely a result of Rossby wave energy propagation. The source of the Rossby wave train appears at the high latitude Europe/Atlantic sector, where maximum wave activity flux convergence resides. During its southeastward journey, the ISO perturbation gains energy from the mean flow through both kinetic and potential energy conversions. A physics-based empirical model was constructed to predict the intraseasonal temperature anomaly over southeast China. The major predictability source is the southeastward-propagating ISO signal. The data for 1979‒2003 were used as a training period to construct the empirical model. A 10-yr (2004‒2013) independent forecast shows that the model attains a useful skill of up to 25 days.
KeywordsMid-high latitude intraseasonal temperature variability Southeastward propagation Extended-range forecast
The authors are grateful to anonymous reviewers for their constructive comments. This work was supported by China National 973 project 2015CB453200, NSFC grant 41475084, NRL grant N00173-13-1-G902, Jiangsu Shuang-Chuang Team, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is SOEST contribution number 9553, IPRC contribution number 1163, and ESMC contribution number 82.
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