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The large-scale extratropical circulation in the Southern Hemisphere is much more zonally symmetric than that of the Northern Hemisphere, but its zonal departures, albeit highly relevant for regional impacts, have been less studied. In this study we analyse the joint variability of the zonally asymmetric springtime stratospheric and tropospheric circulation using Complex Empirical Orthogonal Functions (cEOF) to characterise planetary waves of varying amplitude and phase. The leading cEOF represents variability of a zonal wave 1 in the stratosphere that correlates slightly with the Symmetric Southern Annular Mode (S-SAM). The second cEOF (cEOF2) is an alternative representation of the Pacific-South American modes. One phase of this cEOF is also very highly correlated with the Asymmetric SAM (A-SAM) in the troposphere. Springs with an active ENSO tend to lock the cEOF2 to a specific phase, but have no consistent impact on its magnitude. Furthermore, we find indications that the location of Pacific Sea Surface Temperature anomalies affect the phase of the cEOF2. As a result, the methodology proposed in this study provides a deeper understanding of the zonally asymmetric springtime extratropical SH circulation. [ABSTRACT FROM AUTHOR]

Based on ERA-Interim and JRA-55 daily reanalysis datasets, connections among the variations in the Brewer–Dobson circulation (BDC) intensity, stratospheric air mass, surface pressure, and the tropospheric meridional circulation during the period from 1979 to 2015 are analyzed. The results show that the variations in the surface pressure, particularly at middle and high latitudes, have a close correlation with the variations in the stratospheric BDC intensity. When the upwelling at 450 K intensifies, the surface pressure increases in the high latitudes (poleward of 60°) and decreases in the adjacent mid-latitudes in both hemispheres. And these correlations are most significant during boreal winter in the Northern Hemisphere and during austral autumn in the Southern Hemisphere. It is found that the high latitude surface pressure changes follow the anomalous BDC by about 30 days. The increase in polar surface pressure associated with the stronger BDC is due to an increase in stratospheric air mass, while the decrease in mid-latitude surface pressure is related to a decrease in tropospheric air mass. These air mass changes are caused by the anomalous meridional transport of air mass by the residual mean circulation in the stratosphere and troposphere. In addition, we found a significant connection between the intensity of the BDC and the Ferrel cell, i.e., when the BDC strengthens (weakens), there is a weakened (strengthened) Ferrel cell with its position shifting equatorward (poleward). [ABSTRACT FROM AUTHOR]

We revisit the various stratosphere–troposphere coupling relation from the perspective of the meridional mass circulation. We constructed 10-hPa northern annular mode (NAM) phase composites to show the typical spatiotemporal evolution of circulation anomalies during the NAM's life cycle. Our results indicate that there is large case-to-case difference in the temporal evolution and vertical profile of polar temperature anomalies during NAM events, which shows no strong dependence on the intensity and duration of NAM events, but agrees well with the variations of the three branches of mass circulation at 60°N: the stratospheric poleward warm air branch (ST), the poleward warm air branch in the upper troposphere (WB), and the equatorward cold air branch in the lower troposphere (CB). Such correspondence is due to the dynamic heating and cooling anomalies associated with the redistribution of air masses by the anomalous meridional mass circulation in different isentropic layers. The various relationship among the three mass circulation branches is attributed to anomalous wave activities. The amplitude and westward tilt of waves are always stronger (weaker) throughout the stratosphere before (after) the peak time of negative NAM events, leading to a stronger (weaker) ST before (after) the peak time. Variations in WB and CB are mostly dependent on wave variabilities in the mid- to lower troposphere, leading to variations in the timing of in- or out-of-phase coupling of the ST with the WB and CB, and thus various thermostructure during NAM events. At a later stage of the negative NAM events when the polar temperature becomes colder and the polar jet recovers, the weakened baroclinic instability in the lower stratosphere provides favorable conditions for the strengthening of the WB and CB if wave activities strengthen in the troposphere during that period. [ABSTRACT FROM AUTHOR]

This study evaluates the prediction skill for the stratospheric mass circulation variability in winter (November–March) in the Climate Forecast System, version 2 (CFSv2), from 2011 to 2018. Three stratospheric mass circulation indices measuring meridional mass transport into the polar stratosphere by the total flow (ST60N), wavenumber-1 (ST60N_W1), and wavenumber-2 waves (ST60N_W2) are considered. The variability of ST60N is mainly contributed by ST60N_W1 and ST60N_W2, and these indices are good indicators of the timing and locations of the continental-scale cold-air outbreaks (CAOs) at mid-latitudes. Thus, their potentially useful prediction skill can be utilized to make sub-seasonal forecasts of CAOs in a dynamical and statistical hybrid paradigm. Systematic forecast bias is found in both the 7-year averaged winter mean and seasonal cycle, which is tied to the overestimation of damping in amplitude and westward tilting variations of total waves and difficulties in forecasting the exact contributions from different spatial scales of waves. The intraseasonal variations of stratospheric mass circulation indices, with the systematic forecast bias corrected, can be modestly predicted at a forecast lead time of about 20 days, in terms of both the anomaly value and timing of negative and positive peak events. The sub-seasonal predictability of the ST60N value mainly comes from the ST60N_W1, while the predictability of the timing of positive and negative peaks comes from the ST60N_W2. The 20-day prediction limit of the stratospheric mass circulation indices is mainly due to the 2-week limit of the CFSv2 model in predicting the variability of anomalous wave tilt angle, whereas the prediction limit of the wave amplitude anomaly can exceed 50 days. [ABSTRACT FROM AUTHOR]

The present study for the first time reports the global gravity wave activity in terms of their potential energy derived from TIMED/SABER observations right from the stratosphere to the mesosphere lower thermosphere (MLT) region. The potential energy profiles obtained from SABER temperature are validated by comparing them with ground based LIDAR observations over a low latitude site, Gadanki (13.5° N, 79.2° E). The stratospheric and mesospheric global maps of gravity wave energy showed pronounced maxima over high and polar latitudes of the winter hemisphere. The interannual variability of the stratospheric gravity wave activity exhibited prominent annual oscillation over mid-latitudes. The equatorial gravity wave activity exhibited quasi-biennial oscillation in the lower stratosphere and semi-annual oscillation in the upper stratosphere. The MLT region maps revealed summer hemispheric maxima over polar latitudes and secondary maxima over the equatorial region. The results are discussed in the light of present understanding of global gravity wave observations. The significance of the present study lies in emphasizing the importance of satellite measurements in elucidating gravity waves, which is envisaged to have profound impact on parameterizing these waves. [ABSTRACT FROM AUTHOR]

The Lagrangian scheme ATTILA is used to calculate age spectra and the mean age of air in the general circulation model ECHAM4. The advantage of the Lagrangian method is that temporal variation in transport is taken into account and that beyond transport times the actual transport pathways can be investigated. We found a strong seasonal cycle in mean age and age spectra, especially at high latitudes. When plotting polar age spectra against time, it can clearly be seen that the edge of the polar vortex acts as an efficient transport barrier and that exchange with extra-polar air takes place only for a short period of approximately two months after the polar vortex has broken down. Compared to observations the mean age is reproduced satisfactorily below approximately 20 km. Above that level however, the mean age is underestimated, especially at high latitudes. Furthermore, the observed sharp meridional gradient is located too far polewards in the model, which indicates that the subtropical transport barrier is too weak. There is a distinct variation in the shape of the age spectra with latitude. At low latitudes the age spectra consist of one single peak, whereas at higher latitudes secondary peaks appear, which are one year apart and whose positions in the spectrum are independent of the location. At polar latitudes there are even several peaks of approximately equal size. We explain these peaks with two superposing processes. First, the seasonal cycle of the upward mass flux at the tropical tropopause produces a single peak age distribution. And second, at polar latitudes, the temporal evolution of the polar vortex allows mixing of polar and subtropical air only once a year, which results in a superposition of these single peak age distributions. A final investigation of the transport pathways gave indications for predominant routes from the tropics to high latitudes resulting in altitude dependent meridional transport, however, more detailed studies of 3D trajectory data will be needed to clarify this issue. [ABSTRACT FROM AUTHOR]