On the role of Rossby wave breaking in quasi‐biennial odulation of the stratospheric polar vortex

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 intra‐seasonal variation and its relation to Rossby wave breaking (RWB). Reanalyses in isentropic coordinates from the National Center 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 PRWs of zonal wavenumber 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 20‐40°N, 350‐650 K. In late winter, RWB leads to finite amplitude growth, which hinders upward propagating PRWs of zonal wavenumber 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 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

On the role of Rossby wave breaking in the quasi-biennial modulation of the stratospheric polar vortex during boreal winter

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 intra‐seasonal variation and its relation to Rossby wave breaking (RWB). Reanalyses in isentropic coordinates from the National Center 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 PRWs of zonal wavenumber 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 20‐40°N, 350‐650 K. In late winter, RWB leads to finite amplitude growth, which hinders upward propagating PRWs of zonal wavenumber 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 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.</br

An observational history of the direct influence of the stratospheric quasi-biennial oscillation on the tropical and subtropical upper troposphere and lower stratosphere

The history of observational studies regarding the influence of the stratospheric quasi-biennial oscillation (QBO) on the tropical and subtropical upper troposphere and lower stratosphere (UTLS) is reviewed. QBO phases of westerly (QBO W) and easterly (QBO E) winds are defined in the lower stratosphere. During 1960 – 1978, radio-sonde data revealed QBO modulation of the UTLS with a warm anomaly during QBO W in the tropics and cool anomalies near 30°S and 30°N. This pattern agreed with theory of the QBO mean meridional circulation (MMC), which predicted a coherent, antiphased response between the tropics and subtropics. During 1978 – 1994, satellite observations of aerosol and temperature confirmed the existence of the QBO MMC. During 1994 – 2001, global data sets enabled analysis of zonal mean QBO variations in tropopause temperature. In 2001, National Centers for Environmental Prediction reanalyses for the 42-yr period 1958 – 2000 revealed seasonal and geographical variations in QBO W–E tropopause temperature, pressure, and zonal wind, which are presented here. An update using the 38-yr Modern-Era Retrospective Analysis for Research and Applications, Version 2, and the 40-yr Euro-pean Centre for Medium Range Weather Forecasts Reanalysis (ERA-Interim) data sets provides a more complete view of seasonal and geographical variation. The QBO range in tropical tropopause values is ~ 0.5 – 2 K, ~ 100 – 300 m, and ~ 1 – 3 hPa, being colder and higher during QBO E, especially during boreal winter and spring. The QBO temperature signal tends to be larger near regions where deep convection is common. The QBO signal in the southern subtropics is enhanced during austral winter. During QBO W, the subtropical westerly jet is enhanced, while the Walker circulation is weaker, especially during boreal spring. A new climatology of the QBO MMC is presented. QBO E may enhance convection by reducing both static stability and wind shear in the UTLS

Stratospheric aerosol particles and solar-radiation management

The deliberate injection of particles into the stratosphere has been suggested as a possible geoengineering scheme to mitigate the global warming aspect of climate change. Injected particles scatter solar radiation back to space and thus reduce the radiative balance of Earth. Previous studies investigating this scheme have focused primarily on sulphuric acid particles to mimic volcanic injections of stratospheric aerosol. However, the composition and size of volcanic sulphuric acid particles are far from optimal for scattering solar radiation. We show that aerosols with other compositions, such as minerals, could be used to dramatically increase the amount of light scatter achieved on a per mass basis, thereby reducing the particle mass required for injection. The chemical consequences of injecting such particles into the stratosphere are discussed with regard to the fate of the ozone layer. Research questions are identified with which to assess the feasibility of such geoengineering schemes. © 2012 Macmillan Publishers Limited. All rights reserved