Using a new algorithm to track mixed-Rossby gravity waves (MRG) waves in reanalysis data
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 =-20 to 0), asymmetric meridional wind data at the 850hPa level from the NCEP CFS reanalysis, the algorithm finds locations with Gaussian-shaped meridional wind structure stated in SWE solutions through space and time.
Two groups of MRG waves were found: one with higher wavenumber (11) and another one with lower-wavenumber (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 6 waves were found in the eastern Pacific. The fact that both 6 and 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, the high wavenumber wave activities mainly appear west of the dateline, meaning that some triggering process may have taken place there.
Northwestward wave train movement was found west of 140E in the low-level wind composites. On the other hand, negative correlation between meridional wind and temperature (negative) 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.
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 precusors for tropical cyclone formation, recording the space-time properties of MRG waves in an instantaneous sense should be useful for studying possible formation mechanisms.