Abstract.The radiative effects induced by including interactive ozone, in particular, the zonally asymmetric part of the ozone field, have been shown to significantly change the temperature of the NH winter polar cap, and correspondingly the strength of the polar vortex. However, there is still a debate on whether this effect is important enough for climate simulations to justify the numerical cost of including chemistry calculations in long climate integrations. In this paper we aim to understand the 5 physical processes by which the radiative effects of including interactive ozone, and in particular the radiative effects of zonally asymmetric ozone anomalies (ozone waves), amplify to significantly influence the winter polar vortex. Using the NCAR Whole Atmosphere Community Climate Model in the natural configuration, in which ozone depleting substances and green house gases are fixed at 1960's levels, we find a significant effect on the winter polar vortex only when examining the QBO phases separately. Specifically, the seasonal evolution of the midlatitude signal of the QBO -the Holton-Tan effect -is 10 delayed by one to two months when radiative ozone wave effects are removed. Since the ozone waves affect the vortex in an opposite manner during the different QBO phases, when we examine the full time series, besides an early fall direct radiative effect, we find no statistically significant winter effect. We start by quantifying the direct radiative effect of ozone waves on temperature waves, and consequently on the zonal mean zonal wind, and show that this effect is most significant during early fall. We then show how the direct radiative effect amplifies by modifying the evolution of individual upward planetary wave 15 pulses and their induced mean flow deceleration during early winter when stratospheric westerlies just form and waves start propagating up to the stratosphere. The resulting mean-flow differences accumulate during fall and early winter, after which they get amplified through wave-mean flow feedbacks. We find that the evolution of these early-winter upward planetary wave pulses and their induced stratospheric zonal mean flow deceleration are qualitatively different between QBO phases, providing a new mechanistic view of the extratropical QBO signal (the Holton-Tan effect). We further show how these differences result 20 in an opposite effect of the radiative ozone wave perturbations on the mean flow deceleration for east and west QBO phases.

Abstract. The radiative effects induced by the zonally asymmetric part of the ozone field have been shown to significantly change the temperature of the NH winter polar cap, and correspondingly the strength of the polar vortex. In this paper, we aim to understand the physical processes behind these effects using the National Center for Atmospheric Research (NCAR)'s Whole Atmosphere Community Climate Model, run with 1960s ozone-depleting substances and greenhouse gases. We find a mid-winter polar vortex influence only when considering the quasi-biennial oscillation (QBO) phases separately, since ozone waves affect the vortex in an opposite manner. Specifically, the emergence of a midlatitude QBO signal is delayed by 1–2 months when radiative ozone-wave effects are removed. The influence of ozone waves on the winter polar vortex, via their modulation of shortwave heating, is not obvious, given that shortwave heating is largest during fall, when planetary stratospheric waves are weakest. Using a novel diagnostic of wave 1 temperature amplitude tendencies and a synoptic analysis of upward planetary wave pulses, we are able to show the chain of events that lead from a direct radiative effect on weak early fall upward-propagating planetary waves to a winter polar vortex modulation. We show that an important stage of this amplification is the modulation of individual wave life cycles, which accumulate during fall and early winter, before being amplified by wave–mean flow feedbacks. We find that the evolution of these early winter upward planetary wave pulses and their induced stratospheric zonal mean flow deceleration is qualitatively different between QBO phases, providing a new mechanistic view of the extratropical QBO signal. We further show how these differences result in opposite radiative ozone-wave effects between east and west QBOs.

Abstract. It is well established that variable wintertime planetary wave forcing in the stratosphere controls the variability of Arctic stratospheric ozone through changes in the strength of the polar vortex and the residual circulation. While previous studies focused on the variations in upward wave flux entering the lower stratosphere, here the impact of downward planetary wave reflection on ozone is investigated for the first time. Utilizing the MERRA2 reanalysis and a fully coupled chemistry–climate simulation with the Community Earth System Model (CESM1(WACCM)) of the National Center for Atmospheric Research (NCAR), we find two downward wave reflection effects on ozone: (1) the direct effect in which the residual circulation is weakened during winter, reducing the typical increase of ozone due to upward planetary wave events and (2) the indirect effect in which the modification of polar temperature during winter affects the amount of ozone destruction in spring. Winter seasons dominated by downward wave reflection events (i.e., reflective winters) are characterized by lower Arctic ozone concentration, while seasons dominated by increased upward wave events (i.e., absorptive winters) are characterized by relatively higher ozone concentration. This behavior is consistent with the cumulative effects of downward and upward planetary wave events on polar stratospheric ozone via the residual circulation and the polar temperature in winter. The results establish a new perspective on dynamical processes controlling stratospheric ozone variability in the Arctic by highlighting the key role of wave reflection.

Abstract. The mistral is a northerly low-level jet blowing through the Rhône valley in southern France and down to the Gulf of Lion. It is co-located with the cold sector of a low-level lee cyclone in the Gulf of Genoa, behind an upper-level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the lower-tropospheric mistral and lee cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life cycle, from broad troughs to thin PV streamers to distinguished cutoffs. Each of these PV patterns exhibits a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident, and transitions between the clusters correspond to different Rossby-wave-breaking processes. This analysis provides a new perspective on the variability of the mistral and of the Genoa lee cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

Superposed epoch analysis of meteorological reanalysis data is used to demonstrate a significant connection between intraseasonal solar variability and temperatures in the stratosphere. Decreasing solar flux leads to a cooling of the tropical upper stratosphere above 7 hPa, while increasing solar flux leads to a warming of the tropical upper stratosphere above 7 hPa, after a lag of approximately 6-10 days. Late winter (February-March) Arctic stratospheric temperatures also change in response to changing incoming solar flux in a manner consistent with that seen on the 11 year timescale: 10-30 days after the start of decreasing solar flux, the polar cap warms during the easterly phase of the quasi-biennial oscillation. In contrast, cooling is present after decreasing solar flux during the westerly phase of the quasi-biennial oscillation (though it is less robust than the warming during the easterly phase). The estimated composite mean changes in Northern Hemisphere upper stratospheric (∼ 5 hPa) polar temperatures exceed 8 K and are potentially a source of intraseasonal predictability for the surface. These changes in polar temperature are consistent with the changes in wave driving entering the stratosphere.