Comparison of techniques for isolating equatorial Rossby waves in synoptic studies

Interactions between many temporal and spatial scales drive the weather in the equatorial tropics. These complexities make synoptic analysis problematic. However, equatorial wave modes are an important part of this interaction. Once studied primarily in theory, several recent observational studies show the influence of equatorial waves on tropical convection and cyclogenesis. One key problem in these studies is isolating the equatorial wave modes from the other intricacies of the tropics. Various filtering techniques have been used to resolve this problem.

A time filter is one such technique. This filter isolates those disturbances that have a similar periodicity to the targeted equatorial wave. For example, a 15-40 day band-pass filter may be used to isolate equatorial Rossby (ER) waves, because they most commonly have a period within that range. Wheeler and Kiladis (1999) used another filtering technique to isolate equatorial wave modes. This technique filters the region of the space-time phase space that fits the shallow water solution for a chosen range of equivalent depths.

In this presentation, a case study of an equatorial Rossby (ER) wave packet will be used to examine the strengths and weaknesses of these filters. During October and November 1997, ER waves appeared to be associated with four instances of near-simultaneous tropical cyclogenesis in each hemisphere. ECMWF gridded analyses and outgoing longwave radiation are used to investigate these waves. The propagation characteristics and horizontal structure of these ER waves will be examined using unfiltered, time-filtered, and wavenumber-frequency filtered data. A comparison of these results will illustrate the differences between these methods for examining ER waves.