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Abstract The joint influence of the stratospheric quasi‐biennial oscillation (QBO) and the El Niño Southern Oscillation (ENSO) on the polar vortex, subtropical westerly jets (STJs), and wave patterns during boreal winter is investigated in 40 years (1979–2018) of monthly mean ERA‐Interim reanalyses. The method of Wallace et al. (1993),https://doi.org/10.1175/15200469(1993)050<1751:ROTESQ>2.0.CO;2is used to conduct a QBO phase angle sweep. QBO westerly (W) and easterly (E) composites are then segregated by the phase of ENSO. Two pathways are described by which the QBO mean meridional circulation (MMC) influences the northern winter hemisphere. The “stratospheric pathway” modulates stratospheric planetary wave absorption via the Holton‐Tan mechanism. The “tropospheric pathway” modulates the tropical and subtropical upper troposphere and lower stratosphere. QBO MMC anomalies exhibit a checkerboard pattern in temperature and arched structures in zonal wind which extend into midlatitudes, and are stronger on the winter side. During QBO W, the polar vortex and STJs are enhanced. QBO signals in the polar vortex are amplified during La Niña. During El Niño and QBO W, the strongest STJs occur, and a warm pole/wave two pattern is found. During El Niño and QBO E, a trough is found over Eurasia and a ridge over the North Atlantic, in a wave one pattern. El Niño diminishes QBO anomalies in the tropical stratosphere and reduces the poleward extent and amplitude of the QBO MMC, thereby influencing the stratospheric pathway. Effects on the boreal winter hemisphere are attributed to the combined influence of the QBO and ENSO via both pathways.
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On the role of Rossby wave breaking in the quasi‐biennial modulation of the stratospheric polar vortex during boreal winter
Abstract The boreal‐winter stratospheric polar vortex is more disturbed when the quasi‐biennial oscillation (QBO) in the lower stratosphere is in its easterly phase (eQBO), and more stable during the westerly phase (wQBO). This so‐called “Holton–Tan effect” (HTE) is known to involve Rossby waves (RWs) but the details remain obscure. This tropical–extratropical connection is re‐examined in an attempt to explain its intraseasonal variation and its relation to Rossby wave breaking (RWB). Reanalyses in isentropic coordinates from the National Centers for Environmental Prediction Climate Forecast System for the 1979–2017 period are used to evaluate the relevant features of RWB in the context of waveguide, wave–mean‐flow interaction, and the QBO‐induced meridional circulation. During eQBO, the net extratropical wave forcing is enhanced in early winter with ∼25% increase in upward propagating planetary‐scale Rossby waves (PRWs) of zonal wave‐number 1 (wave‐1). RWB is also enhanced in the lower stratosphere, characterized by convergent anomalies in the subtropics and at high latitudes and strengthened waveguide in between at 20°N–40°N, 350–650 K. In late winter, RWB leads to finite amplitude growth, which hinders upward propagating PRWs. The effect is most significant for zonal wave‐numbers 2 and 3 (wave‐2‐3). During wQBO, RWB in association with wave‐2‐3 is enhanced in the upper stratosphere. Wave absorption/mixing in the surf zone reinforces a stable polar vortex in early to middle winter. A poleward confinement of the extratropical waveguide in the upper stratosphere forces RWB to extend downward around January. A strengthening of upward propagating wave‐2‐3 follows and the polar‐vortex response switches from reinforcement to disturbance around February, thus a sign reversal of the HTE in late winter. Key Findings• Rossby wave breaking (RWB) is enhanced in the height regions where the zero‐wind line is shifted into the winter hemisphere and where the QBO‐induced meridional circulation is directed toward the winter pole• Polar vortex responses differ in terms of the height location of RWB, zonal wave‐number‐dependent disturbances and seasonal development• Significant increase in wave‐1 occurs when the QBO is in its easterly phase• A cumulative effect of RWB results in enhanced wave forcing of zonal wave‐numbers 2 and 3 during westerly QBO, which manifests in a sign reversal of the Holton–Tan effect in late winter.
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- Award ID(s):
- 1947658
- PAR ID:
- 10457742
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Quarterly Journal of the Royal Meteorological Society
- Volume:
- 146
- Issue:
- 729
- ISSN:
- 0035-9009
- Page Range / eLocation ID:
- p. 1939-1959
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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