The quasi‐biennial oscillation (QBO) impacts the Madden‐Julian Oscillation (MJO) activity with a stronger MJO in QBO easterly (QBOE) than QBO westerly (QBOW) winters. However, this relationship is poorly represented in the current generation climate models. For the first time, this paper applies a stratospheric zonal‐mean nudging in a subseasonal prediction system to capture it. Two strong MJO cases in a QBO‐neutral winter are investigated. The QBO temperature and zonal wind anomalies are added separately as well as together to the stratosphere using nudging in MJO case hindcast. Only by nudging the QBO temperature anomalies while leaving the zonal wind free, can the prediction system capture the observed QBO‐MJO connection. The tropopause instability is found positively correlated to the MJO amplitude, but it cannot fully explain the captured connection. The free‐evolving zonal wind anomalies in the stratosphere due to the nudged QBO temperature are crucial for the captured connection.
Analyzing Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations from 2003 to 2018, the interannual variability of 2–5d eastward propagating planetary waves is found to correlate positively with zonal‐mean zonal winds averaged over 67.5°±10°S but negatively with the quasi‐biennial oscillation (QBO) index in austral winter. The composite‐mean wave amplitudes are ~20% larger in QBOe than in QBOw. On statistical average, the poleward flank strengthening and the equatorward flank weakening of polar night jet (PNJ) during QBOe form a dipole‐cell pattern. In contrast, only a single negative cell is seen in the Northern Hemisphere zonal‐mean zonal winds (January) previously explained by the Holton‐Tan theory. Such difference implies an interhemispheric asymmetry and other processes needed to explain the additional positive cell in Antarctica. Mechanistic modeling illustrates that the stronger PNJ generates eastward propagating planetary waves with larger growth rates (stronger waves) in QBOe than QBOw, explaining the QBO‐like signal in the Antarctic planetary waves.
more » « less- Award ID(s):
- 1753214
- NSF-PAR ID:
- 10448174
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 22
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 13526-13534
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Recent observations have indicated significant modulation of the Madden–Julian oscillation (MJO) by the phase of the stratospheric quasi-biennial oscillation (QBO) during boreal winter. Composites of the MJO show that upper-tropospheric ice cloud fraction and water vapor anomalies are generally collocated, and that an eastward tilt with height in cloud fraction exists. Through radiative transfer calculations, it is shown that ice clouds have a stronger tropospheric radiative forcing than do water vapor anomalies, highlighting the importance of incorporating upper-tropospheric–lower-stratospheric processes into simple models of the MJO. The coupled troposphere–stratosphere linear model previously developed by the authors is extended by including a mean wind in the stratosphere and a prognostic equation for cirrus clouds, which are forced dynamically and allowed to modulate tropospheric radiative cooling, similar to the effect of tropospheric water vapor in previous formulations. Under these modifications, the model still produces a slow, eastward-propagating mode that resembles the MJO. The sign of zonal mean wind in the stratosphere is shown to control both the upward wave propagation and tropospheric vertical structure of the mode. Under varying stratospheric wind and interactive cirrus cloud radiation, the MJO-like mode has weaker growth rates under stratospheric westerlies than easterlies, consistent with the observed MJO–QBO relationship. These results are directly attributable to an enhanced barotropic mode under QBO easterlies. It is also shown that differential zonal advection of cirrus clouds leads to weaker growth rates under stratospheric westerlies than easterlies. Implications and limitations of the linear theory are discussed.
Significance Statement Recent observations have shown that the strength of the Madden–Julian oscillation (MJO), a global-scale envelope of wind and rain that slowly moves eastward in the tropics and dominates global-weather variations on time scales of around a month, is strongly influenced by the direction of the winds in the lower stratosphere, the layer of the atmosphere that lies above where weather occurs. So far, modeling studies have been unable to reproduce this connection in global climate models. The purpose of this study is to investigate the mechanisms through which the stratosphere can modulate the MJO, by using simple theoretical models. In particular, we point to the role that ice clouds high in the atmosphere play in influencing the MJO.
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Abstract The quasi‐biennial oscillation (QBO), a ubiquitous feature of the zonal mean zonal winds in the equatorial lower stratosphere, is forced by selective dissipation of atmospheric waves that range in periods from days to hours. However, QBO circulations in numerical models tend to be weak compared with observations, probably because of limited vertical resolution that cannot adequately resolve gravity waves and the height range over which they dissipate. Observations are required to help quantify wave effects. The passage of a superpressure balloon (SPB) near a radiosonde launch site in the equatorial Western Pacific during the transition from the eastward to westward phase of the QBO at 20 km permits a coordinated study of the intrinsic frequencies and vertical structures of two inertia‐gravity wave packets with periods near 1 day and 3 days, respectively. Both waves have large horizontal wavelengths of about 970 and 5,500 km. The complementary nature of the observations provided information on their momentum fluxes and the evolution of the waves in the vertical. The near 1 day westward propagating wave has a critical level near 20 km, while the eastward propagating 3‐day wave is able to propagate through to heights near 30 km before dissipation. Estimates of the forcing provided by the momentum flux convergence, taking into account the duration and scale of the forcing, suggests zonal force of about 0.3–0.4 m s−1 day−1for the 1‐day wave and about 0.4–0.6 m s−1 day−1for the 3‐day wave, which acts for several days.
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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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Abstract This study examines the relationship between the Madden‐Julian oscillation (MJO) and stratospheric quasi‐biennial oscillation (QBO) in a state‐of‐the‐art global numerical weather forecast model. A set of 61‐day model integrations, with 15 ensemble members, is performed across 84 start dates during December–February of 1989–2016. For 28 of those dates—every 1 January—the stratosphere is initialized from observation, and the model simulates stronger MJO events during observed easterly QBO phases (QBOE) than westerly QBO phases (QBOW). However, in these “control experiments,” the QBO's impact on the MJO is already present in the initial conditions, and the direct influence of the model stratosphere during the simulation is unclear. To explore this more directly, the model was rerun with an artificially imposed QBOE and QBOW state, replacing the existing stratospheric initial condition above 150 hPa while leaving the troposphere unaltered. Though the imposed QBO states weaken faster in the model than in observations, their persistence is comparable to the control simulations. The MJO is stronger during imposed‐QBOE experiments than imposed‐QBOW, and differences are statistically significant by several metrics, though magnitude of the differences is smaller than in observations. Analysis suggests that the strength of the MJO response to the QBO increases for simulations with stronger upper‐tropospheric temperature differences and for simulations in which the MJO at the initialization time is strong and active over the Maritime Continent. However, tropospheric conditions still appear to have a dominant effect in explaining the apparent QBO influence in this model.
