A review of `synoptic to intraseasonal' tropical waves with relevance
to forecasting
Matthew C. Wheeler
Paper for the Proceedings of the International Workshop on the
Dynamics and Forecasting of Tropical Weather Systems, Darwin, January 2001.
Published by the Northern Territory Regional Office of the Bureau of Meteorology,
Pages 114-137.
Introduction
It has long been known that an important part of the synoptic and larger-scale
variability in the Tropics is due to propagating disturbances moving in the zonal
direction. Such disturbances organize individual convective elements
on a spatial scale that is larger than the size of the elements
themselves. A well-known example are the westward moving synoptic-scale
disturbances within the intertropical convergence zones
that were visible in time-longitude plots of
cloudiness viewed from early satellites (e.g., Fig.\,1,
as taken from Chang 1970). Another
well known example, but on an even larger scale, is the Madden-Julian
oscillation, that is often observed as a continental-scale organization
of convection moving slowly to the east across the Indian and western
Pacific ocean regions. Collectively, all such propagating disturbances are
commonly known as `tropical waves', and this paper reviews some
of the characteristics of the larger-scale tropical waves known from
observations, as well as presenting a technique for diagnosing and forecasting
them in real time.
For this purpose, I rely heavily on the data analysis
techniques of spectral analysis and filtering. The paper is not intended as
a complete review of the scientific literature, but instead draws mostly upon my
own research and perspective of the current state of knowledge.
References to theory will mostly be made to the standard text
books on atmospheric dynamics only, for example, Gill (1982) and
Holton (1992).