the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2025
QBOi El Niño Southern Oscillation experiments: Teleconnections of the QBO
Abstract. This study examines Quasi-Biennial Oscillation (QBO) teleconnections and their modulation by the El Niño-Southern Oscillation (ENSO), using a multi-model ensemble of the Atmospheric Processes And their Role in Climate (APARC) QBO initiative (QBOi) models. Some difficulties arise in examining observed QBO-ENSO teleconnections from distinguishing the QBO and ENSO influences outside of the QBO region, due to aliasing between the QBO and ENSO over the historical record. To separate the QBO and ENSO signals, simulations are conducted with annually-repeating prescribed sea-surface temperatures corresponding to idealized El Niño or La Nina conditions (QBOi EN and LN experiments, respectively). In the Arctic winter climate, higher frequencies of sudden stratospheric warmings (SSWs) are found in EN than LN. The frequency differences in SSW between QBO westerly (QBO-W) and QBO easterly (QBO-E) are indistinguishable, suggesting that the polar vortex responses to the QBO are much weaker than those to the ENSO in these models. The Asia-Pacific subtropical jet (APJ) shifts significantly equatorward during QBO-W compared to QBO-E in observations, while the APJ-shift is not robust across models, regardless of the ENSO phases. In the tropics, these experiments do not show a robust or coherent QBO influence on precipitation. The sign and spatial pattern of the precipitation response vary widely across models and experiments, indicating that any potential QBO signal is strongly modulated by the prevailing phases of the ENSO. The QBO teleconnection to the Walker circulation around boreal summer/autumn is investigated to identify the strongest signal in each model. It is found that the upper-level westerly and lower-level easterly anomalies in the equatorial troposphere over the Indian Ocean and Western Pacific are detected in the observations and most models in the La Nina year. Overall, the QBO can modulate the zonal circulation over the tropical Indian-Pacific oceans, with its impact varying depending on the ENSO phase.
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Status: open (until 07 May 2025)
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RC1: 'Comment on egusphere-2025-1148', Anonymous Referee #1, 21 Apr 2025
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This study uses ERA5 data and a multi-model ensemble of APARC QBOi models to investigate how QBO teleconnections are modulated by ENSO. To separate the QBO and ENSO signals, simulations were conducted with annually-repeating prescribed SSTs corresponding to idealized El Niño or La Niña conditions. Models are unable to represent the observed (ERA5) enhanced Holton-Tan effect during La Nina, where QBO W favors a stronger NH winter polar vortex. Models are also unable to represent the observed increase in SSWs during El Nino. Overall, the polar vortex responses to the QBO are much weaker than to ENSO in the models. In addition, the equatorward shift of the boreal winter Pacific subtropical jet (APJ) observed during QBO W in not seen in the models. In the tropics, the model experiments do not show a robust
or coherent QBO influence on precipitation. It was further found that QBO effects on the Walker circulation exhibit a complex dependence on season, longitude, and phase of ENSO. They that suggested that weakness of the QBO polar vortex coupling in the models might arise from systematically weak QBO amplitudes at lower levels in the equatorial stratosphere, polar vortex biases in winter, and inadequate representation of stratospheric-troposphere coupling, while an inadequate representation of QBO effects in the tropical troposphere might arise from the systematically weak QBO amplitudes at lower levels, precipitation bias, and inadequate representation of the Walker circulation in these models. This paper documents the results of a considerable effort in the QBOi community, with well-organization presentation and choice of figures. The narrative provides an authoritative interpretation of the detail and status of observed and modeled QBO/ENSO influences on the extratropics. I recommend publishing with minor revision.
- Idealized time mean La Nina and El Nino states. Would the model results be noticeably different for a time-varying ENSO (then binned by ENSO phase), versus two perpetual ENSO phases? It seems possible that the two-state method represents an upper bound on possible effects.
- l216-217, Fig. 13: This is a kind of discretized time-height section. It is similar to Reed et al.’s original 1961 figure which shows a time-height section of zonal wind. The Hovmoller diagram was originally defined to be the variation of geopotential height or another quantity near 60N as a function of longitude and time. It was generalized to mean a longitude-time diagram, which is usually used to indicate wave propagation. You have a table with dependence on season and altitude and you are not discussing wave propagation in longitude. Please use the phrase “season-altitude variation” instead of Hovmoller diagram to indicate what you are showing.
- l221-226: “when the QBO phase is not defined by the preferred 70 hPa level” – does this mean that there are other ways to define it or that sometimes the 70 hPa level index isn’t well defined? In this discussion of how multiple indices affect significance calculations, please give a sense of the meaning and outcome. For example, If you use more than one index definition at different levels, perhaps one might ascribe reduced significance to a result, but in your method it appears that alpha is reduced, therefore implying greater significance. A little more information would be helpful for understanding this paragraph.
- Fig.1: It looks like only ECCAM5, WACCM and MRI are reasonably correct for neutral ENSO, but none get El Nino right. Maybe ECCAM5 and MRI get LaNina right (relative to ERA5).
- Fig. 2: Only MRI seems to represent the basic sense of the ERA5 signal.
- Fig. 4 caption: suggest adding information to the effect of “La Nina, CTL, and El Nino, from left to right”, to orient the reader about the order of the triplets, and maybe move to near the beginning of the caption.
- l356: suggest refer to (Fig. 4c). In this paragraph, and at times elsewhere, it might be beneficial to include more in-text references to figure panels being discussed.
8. l387, 150W-150E: How sensitive are results in Figs. 6 and 7 to the choice of longitude band?
Citation: https://doi.org/10.5194/egusphere-2025-1148-RC1
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