Horizontal Structure of 500 mb Height Fluctuations with Long, Intermediate and Short Time Scales

Abstract

Maps of standard deviations and one-point correlation maps based on twice-daily data subjected to a variety of temporal filters are presented, in order to document the horizontal structure of 500 mb height fluctuations with different time scales. The filters have been chosen to isolate fluctuations with long time scales (periods much longer than 30 days), intermediate time scales (10?30 day periods) and short time scales (2.5?6 day periods). The one-point correlation maps for long time scales resemble the teleconnection patterns described by Wallace and Cutzler. These patterns are strongly regionally dependent, with meridionally oriented dipole structures in the jet exit regions over the oceans, indicative of strong fluctuations in the zonal wind. For intermediate time scales, the dominant patterns consist of more zonally oriented wave trains which originate in the jet entrance regions and cross the jet streams as they curve southeastward into the tropics. Another important distinction between the one-point correlation patterns for long and intermediate time scales is that the former tend to be geographically fixed, whereas the latter tend to have a more universal shape when viewed in a coordinate system relative to the base grid-point. Hence, as the base grid-point is moved, say eastward along a latitude circle, the long time scale pattern may undergo abrupt changes as this grid-point moves out of the domain of one geographically fixed pattern and into the domain of another. In contrast, the intermediate lime scale patterns are mobile in the sense that they retain more or less the same shape, and they simply translate eastward with the base grid-point. For short time scales the one-point correlation patterns show evidence of simple, zonally oriented wave trains dominated by zonal wavenumbers 6?7 which are most clearly defined near and just to the north of the jet streams. The roles of two-dimensional Rossby-wave dispersion, baroclinic instability and barotropic instability in accounting for these structural characteristics are discussed.