An Observational Study of Northern Hemisphere Submonthly Retrograde Waves Using Reanalysis

Large-amplitude retrograde long waves in mid-latitudes of Northern Hemisphere were first identified by classical observational studies thirty years ago. Major retrograde wave events are distinguished by coherent phase propagation that lasts weeks, potentially enhancing the predictability of weather in the sub-monthly range. This study aims to update the classical observational analysis of Branstator (1987), Kushnir (1987), and Madden and Speth (1989) by establishing a catalog of major retrograde wave events to the present day. A set of criteria, modified from Madden and Speth (1989), are used to identify coherent retrograde waves based on the phase and amplitude of zonal wave-1 Fourier coefficients of geopotential height anomalies at multiple upper tropospheric levels. Using ERA-Interim as the principal data source and NCEP-DOE Reanalysis II for cross verification, the new catalog covers all major retrograde wave events from 1979-present. Based on the new catalog, the horizontal and vertical structures of the retrograde waves are established from composite and EOF analysis. An additional analysis establishes the statistics of the period, duration, and seasonal variation of the retrograde waves. The spatial structure and basic statistics of the waves for the post-1979 events are similar to their counterparts established by the classical studies. For example, large-amplitude retrograde waves usually occur in winter with a typical period of 3 weeks. They exhibit a deep equivalent barotropic structure with very little vertical phase shift. The amplitude of the wave-1 component increases with height into stratosphere, leaving a clear footprint of westward-propagating waves even at 30 mb level. A sequence of strong and long-lasting retrograde wave events in the 1979-80 winter, already identified by the classical studies, is found to remain the most extraordinary episode of retrograde waves in the last four decades. The new catalog is used to match strong retrograde wave events to particular features of the seasonal basic state and tropical forcing, potentially helping to clarify the mechanisms for the generation of coherent sub-monthly long waves.