Spatiotemporal Inhomogeneity of Tropical Waves and Their Interactions

Florida State University, Tallahassee FL

Investigators

Abstract

Tropical waves, also referred to as equatorial waves, are a key component of the weather of the tropics, and are known to play important roles in monsoon variability, El Nino events, and the formation of hurricanes and typhoons. The basic characteristics of tropical waves, including their size, shape and frequency, can be derived by solving an idealized model of their fundamental dynamics. The resulting solutions are expressed in terms of sinusoidal waves, with successive ridges and troughs repeating uniformly around the equator. Analysis methods that assume such extended wave trains can detect equatorial waves, and these methods confirm that observed waves largely match theoretical predictions. But observed waves typically span a limited region and it can be desirable to study them using methods which make no assumptions regarding their global nature or longitudinal extent. In this project tropical waves are identified and studied using the Multi-dimensional Ensemble Empirical Mode Decomposition (MEEMD), a time series analysis method pioneered by the PI and others (see AGS-1139479). MEEMD is used to identify variability at each longitude along the equator at frequencies roughly corresponding to frequencies for particular waves. Additional information about the expected latitudinal shape of the waves is used to separate wave types which have similar frequencies. Once the waves are identified in the data their behavior is further analyzed using techniques to determine characteristics such as equivalent depth, a measure of the depth of wave motions that can be related to the generation of waves by condensational heating in clouds. The MEEMD technique has some advantages for representing the interaction of waves of different frequencies, and the MEEMD technique has been used to develop holo-spectral analysis, a method for quantifying wave interactions and studying their energetics. One application of holo-spectral analysis here is to examine the modulation of equatorial waves by the Madden-Julian Oscillation, a much larger form of tropical variability which fluctuates slowly over the course of a month or two. The work has broader impacts due to the importance of equatorial waves for the weather and climate of the tropics, as noted above. The MEEMD and holo-spectral analysis tools developed in the project will be made available to the research community to ensure the scientific broader impacts of the project. These tools have applicability in a variety of research fields beyond tropical meteorology, for example in the study of brain wave anomalies in neural disorders. In addition, the project would provide support and training for a graduate student and a postdoc, thereby promoting the next generation of scientists in this research area.