An Observational Study of Tropical–Midlatitude Interaction on Intraseasonal Time Scales during Winter

Eight Northern Hemisphere winters of five- and ten-day average midlatitude 500 mb heights and tropical outgoing IR are used in a correlative study of tropical-midlatitude interaction. The seasonal cycle and interannual variability are removed so that only intraseasonal variability remains. Results indicate that energy predominantly propagates from midlatitudes to the tropics for both five- and ten-day averaged data, although the propagation is more apparent in five-day averaged data. This is evidenced by the fact that the largest tropical IR patterns are southeastward of the 500 mb point with which the IR field is correlated. The result is interpreted in terms of a quasi-stationary Rossby wave which has an eastward component of group velocity. The southwest-northeast tilt of the 500 mb height correlation patterns, indicating poleward momentum transport or equatorward wave propagation, also supports the hypothesis that midlatitude flow drives the tropics. Lead and lag correlations show that when 500 mb heights lead IR, an upstream development appears in the 500 mb correlation pattern. The field is nearly featureless, however, when 500 mb heights lag IR. Well-defined time evolution is more evident over the eastern Pacific than over the western Pacific. The only indication of possible forcing of the midlatitude flow by the tropics is from IR anomalies in the region of winter monsoon rainfall over the far western Pacific, which are associated with a pattern of correlations in the 500 mb field of nearly global extent. The pattern may be related to that produced by Simmons et al. with a barotropic model, when steady forcing is used to perturb a zonally-varying basic state. They hypothesize that the large global anomalies are the result of the barotropic instability of the basic state. Although the global correlation pattern is statistically significant, it explains only a small fraction of the total variance.