ON THE THERMAL INTERACTION BETWEEN THE SYNOPTIC-SCALE EDDIES AND THE INTRASEASONAL FLUCTUATIONS IN THE ATMOSPHERE

The characteristics of the Northern Hemisphere low-frequency temperatu re variability (periods 10 to 120 d) are examined using 700-hPa analys es for 5 winters. Of particular interest is the relationship between t he evolution of the low-frequency temperature field and that of the hi gher-frequency synoptic-scale flow The latter produces a low-frequency heat flux divergence that can interact with the low-frequency tempera ture field The nature of this interaction is investigated by examining the relative position, both in space and in time, of the low-frequenc y temperature and the heat flux divergence. The maximum temporal varia nce of the low-frequency temperature is found to occur over Canada and Siberia. Although not negligible in these regions, the low-frequency component of the heat flux divergence by the synoptic-scale flow reach es a maximum somewhat farther to the east, near the storm track region s. The low-frequency temperature variations are positively correlated in time with the in situ synoptic-scale heat flux divergence, implying that the synoptic-scale eddies act to dissipate the low-frequency tem perature variability. The dominant patterns of the low-frequency tempe rature am determined through an Empirical Orthogonal Function (EOF) an alysis. The first modes over East Asia and North America take the form of monopoles, one over each of these areas. The second and the third eigenvectors are more closely connected with the fluctuations of tempe rature near the east coast areas of Asia and North America. The patter n of the synoptic-scale eddy heat flux convergence associated with eac h of the leading EOF temperature modes is also identified. The importa nce of the synoptic-eddy heat flux convergence in determining the low- frequency temperature variability is compared with that of other proce sses through a thermal variance budget analysis. The main source of th e low-frequency thermal variance over the eastern portions of the Asia n and North American continents and the adjacent oceanic areas is the low-frequency heat flux down the time-mean temperature gradient. Both the heat flux divergence by the synoptic-scale eddies and the low-freq uency vertical motion tend to reduce the low-frequency thermal varianc e.