the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Nov 2024
Atmospheric Mixed Rossby Gravity Waves over Tropical Pacific during the Austral Summer
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.
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RC1: 'Comment on egusphere-2024-3317', Anonymous Referee #1, 21 Nov 2024
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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.
Citation: https://doi.org/10.5194/egusphere-2024-3317-RC1 -
RC3: 'Reply on RC1', George Kiladis, 29 Nov 2024
reply
In looking at Rev. 1's comment, it is clear that they had a similar concern regarding the restrictive wavenumber filtering used in this study. While the morphing of MRGs into TD-like disturbances should be less of an issue during DJF, the scales of MRGs during that season in past studies are still somewhat larger than what is obtained here, so testing the filtering does appear to be a good idea. Signed, George Kiladis
Citation: https://doi.org/10.5194/egusphere-2024-3317-RC3
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RC3: 'Reply on RC1', George Kiladis, 29 Nov 2024
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RC2: 'Comment on egusphere-2024-3317', George Kiladis, 29 Nov 2024
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This is an interesting study of the statistical structure and behavior of tropospheric mixed
Rossby-gravity waves over the eastern equatorial Pacific during northern winter. Using
EOF analysis, the authors have obtained some nice results related to the extratropical
forcing of MRG waves within the westerly duct, however I think that the zonal
wavenumber 5-6 filtering they used is unnecessarily narrow, and that this has the
potential to distort the actual scales of MRG waves compared to what has been shown in
past studies (see comments below). In addition, the descriptions of the methodology used
are incomplete, and while I was finally able to back out what they are actually using in
their approach, this should be made more obvious to the reader at the outset in Section
3.1. In addition, a lot of relevant literature has not been cited, and I think the authors need
to compare their results to those from these past studies. I recommend revisions of
this manuscript while taking into account the comments by line number below:
25: The term “WMRG” has also been used to identify westward propagating MRGs by
Yang et al. starting in 2003, to distinguish from the eastward propagating EIGs of
Matsuno’s n=0 meridional mode. MRG-E has also been used to describe the eastward
propagating side by Knippertz et al. (2022), for example. If you are going to use MRGW
instead of simply MRG then I think it would be important to make this point in order to
avoid confusion.
29: I think it’s important to distinguish convectively coupled MRGs in the troposphere
from the free MRGs in the stratosphere. The early studies focused on MRGs in the
stratosphere, and these have decidedly different scales from those in the stratosphere (e.g.
Wheeler et al. 2000 their Fig. 12, Yang references below, Kiladis et al. 2016).
33: MGWs => MRGWs
33: Kiladis et al. 2009 did not document lateral forcing but mentioned previous studies
that did. More recent examples include Yang and Hoskins (2016), Yang et al. (2018),
Kiladis et al. (2016) and Suhas et al. (2020). Suggest citing these here for completeness,
as these will also become relevant below.
89: Examples of different methods employed are discussed in detail in Knippertz et al.
(2022).
90: as was done in Kiladis et al. 2016.
Based on what is stated on line 107 it appears that you are using a correlation matrix and
not a covariance matrix for the EOF analysis. This should be stated here.
91: not evident in this statement is the important point that EOF analysis of propagating
disturbances will generally yield two EOFs in spatial and temporal quadrature, which is
why you can use the combined PC1 and PC2 as an activity index.
95: what is the basis for the spatial and temporal filtering? 2-6 days can be justified in the
troposphere, but using only zonal wavenumbers 5 and 6 is very restrictive. In 1982
Hayashi had limited knowledge of the spatial scales of MRGs, which we now know are
localized wavepackets that are comprised of a number of zonal wavenumbers in both the
troposphere and stratosphere. While spectra of meridional wind at 200 hPa do have
power concentrated on wavenumbers 5, the power is broad band and extends especially
to lower wavenumbers (Randel 1992). Indices based on antisymmetric OLR (Kiladis et
al. 2009, 2016) or dynamical based indices (Yang et al. 2003; Knippertz et al. 2022)
generally include wavenumber 1-4 components as well, and the structures of MRGs
obtained in these studies are generally broader in scale than what is obtained here. I think
you need to reconsider your filtering by including a broader zonal wavenumber range,
while testing the sensitivity of your results to these choices. It seems to me that including
wavenumbers 1-4 initially would be a good test. In the 2-6 day period range, it is
probably not necessary to only include westward propagating wavenumbers, but I suggest
testing that approach as well. In other seasons such as northern summer, this broader
band filter would also include tropical depression (TD-type) disturbances that MRGs
often morph into, but that should not be an issue during DJF.
98: “MRGW”
99: was the meridional wind area weighted by cos latitude? This will not make a
difference at the latitudes used but still should be done. Justification for the domain used
should also be given.
101: should point out that PC1 would lead PC2 for a westward propagating disturbance.
102: what is the lagged correlation between PC1 and PC2? I don’t doubt it’s quite large,
but knowing that would further justify using the combined first two EOFs as an index.
107: Suggest pointing out that the standard deviation of PC1 will be equal to one when
using a correlation matrix (or standardized input). I assume you are compositing on local
temporal maxima in PC1? It is stated in the figure caption that the winds are band passed
filtered but is that also true of humidity and OLR? More information on the compositing
technique is needed here.
110: The patterns compare favorably with those obtained by Wheeler et al. (2000) and
Kiladis et al. (2009, 2016) using OLR or brightness temperature as a basis, including the
diagonal tilt of the OLR signals, although the circulation gyres are somewhat smaller
scale, likely due to the wavenumber filtering used here.
124: not sure I see the quadrature relationship referenced in the text in Fig. 3. It seems
more that divergence is out of phase in the vertical without much longitudinal
displacement between moist and dry regions, also reflected in the locations of the OLR
anomalies.
138: I can see from the scale that these really are lag correlations (not lagged regressions)
for OLR, but how are you scaling the wind field using correlations? What does a vector
length of “1” mean, a perfect correlation with both u and v? Much more detail is needed
here. Also, are “anomalies” 2-6 day band pass filtered fields?
144: Kiladis et al. 2009 does not show midlatitude coupling with MRGs but Kiladis et al.
2016 does.
147: In what sense does the omega equation hold? Looks like negative OLR does occur
ahead of troughs, as expected.
148: 10 m/s is quite a bit slower than the 15-25 m/s reported in past studies. This may be
because of the restriction to wavenumber 5-6, which in itself would yield a slower phase
speed just based on the dynamics of the MRGs.
150: I guess you are talking about the weak OLR anomalies off the west coast of South
America in Fig. 3c? This is not a particularly strong signal.
176: “eastward group velocity”
181: I think what you mean to say is that the group velocity causes an MRG to form over
the Atlantic which then has the characteristic antisymmetric specific humidity field
associated with it.
215: I think you need to refer to Fig. 7 here. I don’t understand why you are using PC2
for Fig. 7, since that can’t be directly compared with the circulation Fig. 6c, for instance.
Is there justification for this?
219: A comparable statistical 200 hPa sequence for MRG activity further west during
DJF is also shown in Kiladis et al. 2016, their Fig. 16.
226: Can’t say that I see the humidity and OLR signals over southern Mexico referred to,
if you are talking about either Fig. 6 or 7.
234: Once again, are these anomalies 2-6 day filtered? They should be identified as such.
244: You mean it’s a standing wave?
305: Please see Yang and Hoskins (2017) for a discussion of the eastward tilt in height of
MRGs within the westerly duct during Dec.-Feb.
Signed,
George Kiladis
References:
Kiladis, G. N., J. Dias, and M. Gehne, 2016: The relationship between equatorial mixed
Rossby-gravity and eastward inertio-gravity waves: Part I. J. Atmos. Sci., 73, 2123-2145.
Knippertz, P., M. Gehne, G. N. Kiladis, K. Kikuchi, A. R. Satheesh, P. E. Roundy, G. -Y.
Yang, N. Zagar, J. Dias, A. H. Fink, J. Methven, A. Schlueter, F. Sielmann, and M. C.
Wheeler, 2022: The intricacies of identifying equatorial waves. Quart. J. Roy. Met. Soc.,
148, 2814-2852.
Randel, W. J., 1992: Upper tropospheric equatorial waves in ECMWF reanalysis. Quart.
J. Roy. Met. Soc., 118, 365-394.
Suhas, E. J. M. Neena and X. Jiang, 2020: Exploring the Factors Influencing the Strength
and Variability of Convectively Coupled Mixed Rossby–Gravity Waves. J. Climate, 33,
9705-9719.
Wheeler, M., G. N. Kiladis, and P. J. Webster, 2000: Large-scale dynamical fields
associated with convectively coupled equatorial waves. J. Atmos. Sci., 57, 613-640.
Yang, G., and B. J. Hoskins, 2016: ENSO-related variation of equatorial MRG and
Rossby waves and forcing from higher latitudes. Quart. J. Roy. Met. Soc., 142, 2488-
2504.
Yang, G., and B. J. Hoskins, 2017: The Equivalent Barotropic Structure of Waves in the
Tropical Atmosphere in the Western Hemisphere. J. Atmos. Sci., 74, 1689-1704.
Yang, G., Methven, J., S. Woolnough, K. Hodges and B. J. Hoskins, 2018: Linking
African Easterly Wave Activity with Equatorial Waves and the Influence of Rossby
Waves from the Southern Hemisphere. J. Atmos. Sci., 75, 1783-1809.
.Citation: https://doi.org/10.5194/egusphere-2024-3317-RC2
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