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The influence of the stratospheric quasi-biennial oscillation (QBO) on the polar night jets (PNJs), subtropical easterly jets (SEJs), and associated Rossby wave breaking (RWB) is investigated using global meteorological analyses spanning 10 recent QBO cycles. The seasonal dependence of the descent of the QBO is shown by using five layered shear indices. It is found that the influence of the QBO is distinctive for each combination of QBO phase, season, and hemisphere (NH or SH). The following QBO westerly (W) minus easterly (E) differences in the PNJs were found to be significant at the 97% level: When a QBO W (E) maximum is in the lower stratosphere (∼500 K or ∼50 hPa), the NH winter PNJ is stronger (weaker), in agreement with previous results (mode A). Mode A does not appear to operate in other seasons in the NH besides DJF or in the SH in any season. When a QBO W (E) maximum is in the middle stratosphere (∼700–800 K or ∼10–20 hPa), the PNJ in the SH spring is stronger (weaker), also in agreement with previous results (mode B). It is found that mode B also operates in the NH spring. A third distinctive mode is found during autumn in both hemispheres: a QBO W (E) maximum in the middle stratosphere coincides with a weaker (stronger) PNJ (mode C). The signs of wind anomalies are the same at low and high latitudes for modes A and B, but are opposite for mode C. This sensitive dependence on QBO phase and season is consistent with the nonlinear nature of the interaction between planetary waves and the shape of the seasonal wind structures. During the solstices the meridional circulation associated with QBO connects primarily with the winter hemisphere, whereas during the equinoxes it is more symmetric about the equator. QBO W enhance the equatorial potential vorticity (PV) gradient maximum, but the time-mean maximum may be related to chronic instabilities in the subtropics. The equatorial PV gradient maximum and flanking RWB tend to be more pronounced in the Eastern Hemisphere in stratospheric analyses. When QBO W are in the middle stratosphere, the flanking PV gradient minima (SEJs) are enhanced and RWB is more frequent and symmetric about the equator. When QBO W are in the upper stratosphere, a strong seasonal asymmetry is seen, with enhanced RWB in the summer SEJ, primarily during boreal winter. This is consistent with an upward increase of summer to winter flow and modulation by a strong “first” and weak “second” semiannual oscillation.

Differential advection in Rossby waves can lead to potential vorticity (PV; P) contours on isentropic surfaces folding over in latitude (Py < 0) in a process called Rossby wave breaking (RWB). Exploring the properties of RWB may shed light on underlying dynamics and enable quantification of irreversible transport. A seasonal climatology of Py and RWB statistics is presented for the 320–850-K layer using NCEP reanalysis data during 1979–2005 and for the 320–2000-K layer using the Met Office (UKMO) data during 1991–2003. A primary goal is to depict the spatial extent and seasonality of RWB maxima. This analysis shows seven distinct RWB regimes: poleward and equatorward of the subtropical westerly jets, poleward and equatorward of the stratospheric polar night jets, flanking the equator in the stratosphere and mesosphere, equatorward of subtropical monsoon anticyclones, and the summertime polar stratosphere. A striking PV gradient maximum exists at the equator throughout the layer 360–2000 K, flanked by subtropical RWB maxima, integral components of the Lagrangian cross-equatorial flow. Strong RWB occurs in the polar night vortex where β is small. Over the summer pole, strong poleward RWB associated with synoptic waves decays into small amplitude motions in the upper stratosphere, where heating gradients cause Py < 0. The seven spatial regimes are linked to three different dynamical causes of reversals: wave breaking associated with westerly jets, a combined barotropic/inertial instability in cross-equatorial flow, and on the periphery of monsoon anticyclones.

The height and amount of tropical deep convection are examined for a correlation with the stratospheric quasi-biennial oscillation (QBO). A new 23-yr record of outgoing longwave radiation (OLR) and a corrected 17-yr record of the highly reflective cloud (HRC) index are used as measures of convection. When binned by phase of the QBO, zonal means and maps of OLR and HRC carry a QBO signal. The spatial patterns of the maps highlight the QBO signal of OLR and HRC in typically convective regions. Spectral analysis of zonal mean OLR and HRC near the equator reveals significant peaks at QBO frequencies. Rotated empirical orthogonal function (REOF) analysis is used to determine if ENSO variations of convection are aliased into the observed QBO signals. Some analyses are repeated using the OLR record after ENSO REOF modes have been removed, yielding very similar results compared to the original analyses. It appears that the QBO signal is distinct from the ENSO signal, although the relative brevity of the ...

Global quasi-biennial variation in the lower stratosphere and tropopause region is studied using 41 years (1958–1998) of reanalyses from the National Centers for Environmental Prediction (NCEP). Horizontal wind, temperature, geopotential height, tropopause temperature and pressure fields are used. A new quasi-biennial oscillation (QBO) indexing method is presented, which is based on the zonal mean zonal wind shear anomaly at the equator and is compared to the Singapore index. A phase difference compositing technique provides “snapshots” of the QBO meridional-vertical structure as it descends, and “composite phases” provide a look at its time progression. Via binning large amounts of data, the first observation-based estimate of the QBO meridional circulation is obtained. High-latitude QBO variability supports previous studies that invoke planetary wave-mean flow interaction as an explanation. The meridional distribution of the range in QBO zonal wind is compared with the stratospheric annual cycle, with the annual cycle dominating poleward of ∼12° latitude but still being significant in the deep tropics. The issues of temporal shear zone asymmetries and phase locking with the annual cycle are critically examined. Subtracting the time mean and annual cycle removes ∼2/3 of the asymmetry in wind (and wind shear) zone descent rate. The NCEP data validate previous findings that both the easterly and westerly QBO anomalous wind regimes in the lower stratosphere change sign preferentially during northern summer. It is noteworthy that the NCEP QBO amplitude and the relationships among the reanalyzed zonal wind, temperature, and meridional circulation undergo a substantial change around 1978.

[1] The National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP) reanalysis exhibits interdecadal changes in the structure of the quasi-biennial oscillation (QBO) and annual mean fields in the tropical stratosphere during 1958–2001. A sudden temperature increase occurred around 1978, with equatorial maxima of ∼3 K at 10 hPa and near the tropopause. When this 1978 jump is removed, a moderate cooling trend is found throughout the tropical lower stratosphere, with an equatorial maximum cooling of 3 K over 40 years near 30 hPa. While the QBO in zonal wind is similar in both the 1958–1978 and 1979–2001 periods, the QBO in temperature is significantly reduced in amplitude and latitudinally contracted after 1978, and the QBO meridional circulation is stronger. Vertical temperature variations in the annual mean and QBO have been underestimated since the introduction of nadir-sounding satellites. A mechanism is suggested for how this may affect meridional circulations in data assimilation models.