the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jan 2025
QBOi El Niño Southern Oscillation experiments: Assessing relationships between ENSO, MJO, and QBO
Abstract. This study uses an ensemble of climate model experiments coordinated by the Quasi-Biennial Oscillation initiative (QBOi) to analyze the Madden-Julian Oscillation (MJO) in the presence of either perpetual El Niño or La Niña sea surface temperatures during boreal winter. In addition to the prescribed El Niño Southern Oscillation (ENSO) conditions, the nine models internally generate QBOs, meaning each may influence the MJO. The diagnostics used include wavenumber-frequency spectra of tropical convective and dynamical fields, measures of MJO lifetime, an evaluation of MJO diversity and visualizations of MJO vertical structure, as well as an assessment of QBO morphology and the QBO’s impact on tropical convection. Kelvin wave spectral power increases in the El Niño simulations whereas equatorial Rossby waves power is stronger in the La Niña simulations. Consistent with the reported relationship between these waves and the MJO, all models simulate faster MJO propagation under El Niño conditions. This change in speed is corroborated by the MJO diversity analysis, which reveals that models better reproduce the observed “fast propagating” and “standing” MJO archetypes given perpetual El Niño and La Niña, respectively. Regardless of ENSO, QBO descent into the lower stratosphere is underestimated and we detect little QBO influence on tropical tropopause stability and MJO activity. With little influence from the QBO on the MJO activity in these runs, we can be confident that the aforementioned changes in the MJO indeed arise from the different ENSO boundary conditions.
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RC1: 'Comment on egusphere-2024-3950', Anonymous Referee #1, 14 Feb 2025
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Review of “QBOi El Niño Southern Oscillation experiments: Assessing relationships between ENSO, MJO, and QBO” by Elsbury et al., submitted to EGUsphere
This study investigates the extended winter MJO in an ensemble of QBOi climate models forced separately by perpetual El Niño and La Niña SST conditions. The results indicate that the simulated MJOs are largely insensitive to the stratospheric QBO, as the models fail to capture the observed lower stratospheric QBO-related changes that are critical for MJO modulation. However, the authors find that the simulated MJO and other convectively coupled equatorial wave activities differ markedly between the positive and negative phases of ENSO, with some deviations from previous studies.
Overall, the manuscript is well-written, and the methodologies are clearly presented. However, I have major concerns regarding the SST forcing used in the models. Given that the study’s conclusions heavily rely on these model experiments, I recommend that the authors reassess their experimental design and verify whether their results remain robust in light of previous literature before considering the manuscript for publication.
Major comment:
- To enhance the atmospheric response to ENSO in their simulations, the authors scale the annual cycle of SST by two factors. Notably, the multiplicative factor of 1.8 for the positive ENSO phase is quite large, producing conditions similar to those observed during extreme El Niño events. This scaling also amplifies ENSO asymmetry by a factor of two, as illustrated in Fig. 1. Therefore, the conclusion that MJO activity is stronger during El Niño compared to La Niña is likely driven by these amplified SST perturbations in the models. Previous studies (e.g., Hendon et al. 1999) have reported that exceptionally warm Pacific SSTs can influence MJO activity by allowing the eastward propagation farther to the east. Similarly, the record-breaking strong MJO event in March 2015 (Marshall et al. 2016) was linked to warm equatorial central Pacific SSTs. Given the strong SST forcing in these experiments, enhanced MJO events are not unexpected but rather could be a reflection of selective amplification. Therefore, I recommend using composite SST anomalies without artificial amplification. This approach would provide a more representative assessment of ENSO’s overall impact on MJO activity rather than capturing only extreme conditions. Such a revision would ensure that the study’s conclusions are more robust and broadly applicable.
- L167: Since the models are forced with artificially amplified SSTs, comparing their results directly with observed/reanalysis El Niño/La Niña composites is not entirely justified, as the simulations represent exaggerated ENSO conditions rather than typical observed events.
- The objective of this study is not clearly discussed. While I recognize that ongoing research involves perpetual ENSO experiments in QBOi models, the specific aim of this work should be explicitly stated at the end of the introduction to provide clarity and context for the reader.
Minor comments:
- Please include the horizontal resolution and the number of vertical levels for each model in Table 1 to provide a clearer representation of their configurations.
- L146: The figure references for El Niño and La Niña SSTs appear to be reversed.
- L240: How does ENSO influence the periodicity of the QBO? A brief explanation would be helpful.
- Instead of showing wavenumber-frequency spectra for three different parameters in Figs.2-4, I suggest considering any usual variable like OLR/precipitation. However, a more detailed analysis of eastward and westward propagating modes could provide valuable insights. For example, comparing the ratio of eastward to westward modes in the low-frequency range between ERA5 and simulations might offer an interesting perspective.
- 6: The ERA5 tropospheric temperature anomalies appear slightly stronger during El Niño compared to La Niña, potentially leading to stronger cold caps in the UTLS and modulating MJO convection. Are the differences shown in the right column statistically significant?
- L467: How does the precipitation-based WF spectra capture the UTLS K-wave signal, which is most likely the dry K-wave?
- Abhik et al. (2019) and Sakaeda et al. (2020) highlighted that many DJF EQBO events tend to coincide with La Niña conditions, whereas Niño 3.4 values are more uniformly distributed during WQBO. Did the authors find a similar nonlinear relationship between ENSO and QBO in their model simulations? A brief discussion on how well the models capture this observed relationship would be valuable.
- Is the QBO Fourier amplitude calculation based on the zonal wind? Given that QBO-related temperature changes typically occur above and below zonal wind anomalies, cooler (warmer) temperature anomalies are expected to be located below the lowest level of QBO descent in EQBO (WQBO). Including the zonal-mean temperature profile from both the simulations and ERA5 would provide better clarity. The authors may consider following the approach of Son et al. (2017).
- L562: In WACCM, the QBO descends to the lower stratosphere (~90 hPa), which could contribute to the weak sensitivity of MJO activity to the QBO. In MIROC, the weak positive MJO variability difference appears to be off-equatorial. Discuss these differences.
References:
Abhik, S., Hendon, H. H., & Wheeler, M. C. (2019). On the sensitivity of convectively coupled equatorial waves to the quasi-biennial oscillation. Journal of Climate, 32(18), 5833-5847.
Hendon, H. H., Zhang, C., & Glick, J. D. (1999). Interannual variation of the Madden–Julian oscillation during austral summer. Journal of Climate, 12(8), 2538-2550.
Marshall, A. G., Hendon, H. H., & Wang, G. (2016). On the role of anomalous ocean surface temperatures for promoting the record Madden‐Julian Oscillation in March 2015. Geophysical Research Letters, 43(1), 472-481.
Sakaeda, N., Dias, J., & Kiladis, G. N. (2020). The unique characteristics and potential mechanisms of the MJO‐QBO relationship. Journal of Geophysical Research: Atmospheres, 125(17), e2020JD033196.
Son, S. W., Lim, Y., Yoo, C., Hendon, H. H., & Kim, J. (2017). Stratospheric control of the Madden–Julian oscillation. Journal of Climate, 30(6), 1909-1922.
Citation: https://doi.org/10.5194/egusphere-2024-3950-RC1 -
CC1: 'Comment on egusphere-2024-3950 -- ENSO leads MJO', Paul Pukite, 15 Feb 2025
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My comment is is on the clear finding that ENSO leads MJO by 21 days. This is a chart of a lag-shifted MJO high-resolution (pentad=5-day) time-series at 140 longitude against SOI. The idea is that MJO is a traveling wave offshoot of the ENSO standing wave, so a lag delay is inherent depending on the longitude.
https://imagizer.imageshack.com/img921/7305/bXNFwm.png (this is a rather primitive commenting interface and the chart image shows up below but link is provided to the left)
Citation: https://doi.org/10.5194/egusphere-2024-3950-CC1
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