Observed dispersion characteristics and evolution of tropical Pacific mixed-Rossby gravity waves in boreal summer

Abstract

A new algorithm has been developed to track westward travelling mixed-Rossby gravity waves (MRG) waves in the western Pacific based on the theoretical meridional wind structure stated in the shallow water equation (SWE) solutions. Applied to space-time filtered (period=3 to 8 days and wavenumber k=-20 to 0), asymmetric meridional wind data at the 850hPa level from a reanalysis dataset, the algorithm finds locations with Gaussian-shaped meridional wind structure stated in SWE solutions through space and time. Local and instantaneous wave properties such as occurrence time, wavenumber, intrinsic frequency and magnitude, are then examined. It was found that the tracked low-level MRG waves can be classified into two groups: one with higher wavenumber (k~11) and another one with lower-wavenumber (k~6). Moreover, the MRG waves show very different dispersive properties and occurrence behavior in the western and eastern equatorial Pacific. While both groups appear in the western Pacific, mainly k~6 waves were found in the eastern Pacific. The fact that both k~6 and k~11 waves appear in the western Pacific agrees with past discussions on the co-existence of MRG waves and tropical-disturbance type (TD-type) waves. Also, MRG waves in the western Pacific were observed to have larger variation in their dispersive properties, implying that they do not have a very regular waveform in terms of their wavenumber and frequencies and it is better to track them without assuming any waveform in space. Furthermore, the high-wavenumber wave activities mainly appear west of the dateline, meaning that some triggering process may have taken place there. Low-level wind composites found east of ~140°E agree with the alternating cyclonic and anti-cyclonic patterns given by SWE solutions. Northwestward wave train movement was found west of ~140°E. On the other hand, negative correlation between meridional wind and temperature (negative v'T') found in the vertical composites indicates downward vertical wave activity flux prior to the occurrences of MRG waves. Finally, in the western Pacific, kinetic energy energetics suggests that energy source of transient eddies is from the confluent background flow and also the zonal wind shear terms in the western Pacific, while the energy source in eastern Pacific mainly comes from the meridional shear in the barotropic convergence term. Overall, our method provides a way to identify the MRG waves instantaneously; in contrast, most of the methods employed in the past (e.g., spectral analysis or lag correlation/regression) are based on aggregates of data and they can only examine wave properties averaged over a certain period of time. Since MRG waves can serve as precursors for tropical cyclone formation, recording the space-time properties of MRG waves in an instantaneous sense should be useful for studying possible formation mechanisms.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Yung Francis TAM (Supervisor) & Chung Leung Johnny CHAN (Supervisor)