Variation in the Holton–Tan effect by longitude

Dillon Elsbury,Gudrun Magnusdottir,Yannick Peings

doi:10.1002/qj.3993

Abstract

AbstractThe teleconnection between the Quasi‐Biennial Oscillation (QBO) and the boreal winter polar vortex, the Holton–Tan effect, is analyzed in the Whole Atmosphere Community Climate Model (WACCM) with a focus on how stationary wave propagation varies by QBO phase. These signals are difficult to isolate in reanalyses because of large internal variability in short observational records, especially when decomposing the data by QBO phase. A 1,500‐year ensemble is leveraged by defining the QBO index at five different isobars between 10 and 70 hPa. The Holton–Tan effect is a robust part of the atmospheric response to the QBO in WACCM with warming of the polar stratosphere during easterly QBO (QBOE). A nudging technique is used to reduce polar stratospheric variability in one simulation. This enables isolation of the impact of the QBO on the atmosphere in the absence of a polar stratospheric response to the QBO: referred to as the “direct effect” and the polar stratospheric response, “indirect effect.” This simulation reveals that the polar stratospheric warming during QBOE pushes the tropospheric jet equatorward, opposing the poleward shift of the jet by the QBOE, especially over the North Pacific. The Holton–Tan effect varies over longitude. The QBO induces stronger planetary wave forcing to the mean flow in the extratropical lower stratosphere between Indonesia and Alaska. The North Pacific polar stratosphere responds to this before other longitudes. What follows is a shift in the position of the polar vortex toward Eurasia (North America) during easterly (westerly) QBO. This initiates downstream planetary wave responses over North America, the North Atlantic, and Siberia. This spatiotemporal evolution is found in transient simulations in which QBO nudging is “switched on.” The North Pacific lower stratosphere seems more intrinsically linked to the QBO while other longitudes appear more dependent on the mutual interaction between the QBO and polar stratosphere.

Highlights

Alternating easterly and westerly winds descend through the tropical stratosphere with an averaged periodicity of roughly 28 months, while both longer (35 months) and shorter (22 months) periods have been observed (Bushell et al, 2020)
We focus on three key aspects of the problem: 1. How does the extratropical response evolve with the Quasi-Biennial Oscillation (QBO) cycle? Instead of defining the QBO at one specific pressure level in the atmosphere, we will define it at five levels, examining how the extratropical atmosphere responds to the QBO during each phase of its cycle
Coming back to the key questions identified in the introduction, here is a summary of our findings: 4.1 How does the extratropical response evolve with the QBO cycle?

Summary

Introduction

Alternating easterly and westerly winds descend through the tropical stratosphere with an averaged periodicity of roughly 28 months, while both longer (35 months) and shorter (22 months) periods have been observed (Bushell et al, 2020). This variability is called the Quasi-Biennial Oscillation (QBO). The circumpolar westerlies making up the polar vortex are stronger when QBO westerlies are present in the tropical lower stratosphere (QBOW) Our knowledge on this teleconnection continues to progress (Holton and Tan, 1980; Lu et al, 2020). This work aims to close this gap in our knowledge

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