Atmospheric Mixed Rossby Gravity Waves over Tropical Pacific during the Austral Summer

Braga, Hugo Alves; Magaña, Victor

doi:https://doi.org/10.5194/egusphere-2024-3317

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https://doi.org/10.5194/egusphere-2024-3317

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05 Nov 2024

| 05 Nov 2024

Status: this preprint is open for discussion and under review for Weather and Climate Dynamics (WCD).

Atmospheric Mixed Rossby Gravity Waves over Tropical Pacific during the Austral Summer

Hugo Alves Braga and Victor Magaña

Abstract. Atmospheric Mixed Rossby-Gravity Wave (MRGW) activity during the austral summer months (Dec-Jan-Feb) is examined by means of observational analyses for the 1991–2020 period. The main objective of the study is to explore the relationship between tropical circulations at upper and lower tropospheric levels and tropical convective activity. Using an Empirical Orthogonal Function (EOF) analysis of the high-frequency meridional component anomalies of the wind at 200 hPa, for zonal wavenumber 5–6, episodes of intense MRGW activity are detected. Composite analyses based on an EOF analysis show a quadrature phase over the central-eastern equatorial Pacific between the MRGW structure in the upper and lower troposphere. Lagged correlations between the first two EOFs principal components, and the wind field and OLR, show that MRGWs are laterally forced at upper tropospheric levels over the westerly duct region and later propagate westward and downward. Once the MRGW reaches the lower tropospheric levels, it induces zones of moisture convergence that modulates convective activity. Tropical convection develops in the divergent region of the MRGW at 200 hPa and in the MRGW moisture convergence region at 700 hPa. Since the MRGW phase tilts eastward with height, moisture convergence at lower tropospheric levels tends to coincide with divergence at upper levels favoring intense convective activity which results in the antisymmetric outgoing longwave radiation anomalies off the equator near the MRGW. Therefore, the occurrence of MRGWs over the eastern Pacific, is a form of tropical – extratropical interaction that generates tropical convection anomalies by means of induced lower tropospheric moisture convergence and divergence anomalies.

Received: 23 Oct 2024 – Discussion started: 05 Nov 2024

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Hugo Alves Braga and Victor Magaña

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CC1: 'Comment on egusphere-2024-3317', Paul Roundy, 20 Nov 2024 reply

The authors analyze atmospheric Mixed Rossby Gravity waves over the central tropical Pacific Ocean. They identify the waves in a straightforward way by filtering meridional wind data near the equator for 2-6 days and for wavenumbers 5-6 followed by EOF analysis. They confirm the already well-known development of the waves by lateral forcing of middle latitude Rossby waves, and by analysis of data following a range of time lags and vertical moisture fluxes, they show the wave phase planes propagating downward as they move westward over the Pacific Ocean. The waves ultimately disturb the lower troposphere farther west, where they can develop tropical depressions. The vertical propagation is shown in greater detail in this manuscript than in previous works, but the connections to the extratropics are not new (the authors have cited well the relevant previous works). I found the work interesting, and I think the ultimate conclusions are robust to my concerns. However, I think the details could be substantially improved by responding to two major comments:
1. The zonal spatial filtering to wavenumbers 5-6 is very restrictive. Although the results appear to provide clear and robust connections to real signals over the Central Pacific Ocean, the Gibbs phenomenon would extended the filtered signal substantially east and west of the central point. In the real world, the MRG wave signals have a broad spectral footprint, with a peak in the power spectrum extending over planetary to small synoptic scales. Many previous works have shown substantial variance at the target band the authors are using. Yet individual disturbances have their zonal scales evolve across their lifetimes. For example, a disturbance moving westward across the Dateline near wavenumber 4 or 5 might arrive over the west Pacific, slowing down as it moves, ultimately projecting more strongly to narrower wavelengths, better characterized by wavenumbers 6 or 7. Figure 1 suggests that the authors' data are overfiltered, thus masking the scale change evident in previous works as the waves move westward. See Figure 12 of Kiladis et al. (2009) for an example. Although the central results of the authors over the middle of the Pacific basin conform well to previous works, Figure 1 does not allow for the disturbance to evolve in its zonal scale, because the wavenumber is over prescribed. It is unclear how this issue will impact the timing and other characteristics of the downward propagation of the disturbance that they diagnose. The authors should broaden their wavenumber filter and repeat their analysis to assess the extent of the difference associated with the narrower scales that are evidently important as the disturbances move to the West Pacific. The filtering is likely not the only way the algorithm constrains results zonally. Even using a broader wavenumber filter, the EOF analysis will constrain the results to a particular range of zonal scales, but it will allow the zonal widths of the anomalies to vary geographically. Data filtered for a broad band along the MRG spectral peak ultimately expresses in several EOF pairs, each higher EOF pair explaining progressively smaller zonal scales. This means that one pair of EOFs is not sufficient to describe the whole population of waves. There is nothing wrong with the authors emphasizing a particular range of these scales through selecting a single pair of EOFs, but they should acknowledge that MRG energy also occurs at longer and narrower wavelengths than those that they show here. EOFs based on data filtered for a broader band of wavenumbers will still have leading modes concentrate at wavenumbers 4, 5, or 6 over the central Pacific, but the individual modes will associate with signals at narrower scales as the disturbances move westward.
2. The MRG wave exhibits eastward group velocity, not westward. The manuscript appears to state that the group velocity is westward. The wavenumber filtering and the EOF analysis selects for wave scale in a particularly narrow way, which will mask the development of the group velocity in their results. If the authors filtered for a broader wavenumber band, a pair of EOFs would still select for a particular narrow range of wavenumbers (even though the patterns would allow the same disturbance to be characterized by different wavelengths in different regions). In that case, analysis of multiple pairs of EOFs of MRG filtered data retained together would reveal the group velocity as the interference pattern that emerges from including wave signals propagating at different phase speeds over a range of zonal wavenumbers.

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Citation: https://doi.org/10.5194/egusphere-2024-3317-CC1

Hugo Alves Braga and Victor Magaña

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Short summary

This study presents a comprehensive dynamic and statistical analysis of mixed Rossby-gravity waves during the austral summer. It highlights the significant influence of upper-level lateral forcing on wave dynamics and the critical role of lower-tropospheric humidity in modulating asymmetric convection, providing new insights into the behavior of equatorial waves and their impact on regional weather patterns.


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