Dispersion characteristics of low-level mixed Rossby-gravity waves as inferred from CFSR data

Wave properties of the observed mixed Rossby-gravity (MRG) waves in the central-to-western equatorial Pacific during boreal summer were analyzed with the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data for the period from 1984-2009. From the Matsuno solution of the shallow water equations, the v-wind perturbations associated with MRG waves are expected to have a Gaussian meridional structure. Here MRG waves were identified from the 6-hourly 0.5º×0.5º CFSR data whenever the band-pass filtered symmetric 850hPa v-wind has such a meridional structure. These waves were then traced in time, the local wavenumber and instantaneous phase speed determined, and the associated dynamical fields were archived. Over the whole central-to-western equatorial Pacific, the average wavenumber of MRG waves is around 9-10, while the corresponding equivalent height was about 3 to 42m. Consistent with previous studies, the wavenumber (wavelength) averaged over those waves in the western-to-central Pacific (140ºE-180º) tends to be higher (shorter) than that found farther east (180º-140ºW). Moreover, the average equivalent height is around 6-59m in the western part of the domain, while it is 1m or less over the central-to-eastern Pacific – meaning that wave dispersion in the western Pacific is closer to the asymptotic limit of free Rossby wave dispersion. There were also a number of waves in the western Pacific with dispersion characteristics that lie outside the possible range permitted by the MRG wave solution, which could be related to activities of “TD-type disturbances” in the region. Composites maps of the low-level anomalous circulation show that the MRG waves in the eastern Pacific are more equatorially trapped than their counterpart in the western Pacific. Overall, our method provides a way to identify the synoptic-scale 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 over a certain period of time. It is hoped that this method can lead to better understanding of the growth and decay, propagation and dispersion of MRG waves, as well as their interaction with other weather systems in the Tropics.