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Gloria L. Manney

Abstract

The observed characteristics of the 4-day eastward-moving wave 1 in the Southern Hemisphere polar winter stratosphere are surveyed using ten years of National Meteorological Center (NMC) geopotential height data. The 4-day wave is shown to be a ubiquitous feature in the Southern Hemisphere polar winter stratosphere and is usually prominent during July and August. Growth of the 4-day wave is characterized by two types of structures. Some episodes exhibit NW to SE phase tilts and a single high-latitude maximum. Others show NE to SW phase tilts during growth and a high-latitude maximum out of phase with a secondary lower-latitude maximum. Stability analyses show that all characteristics of the first type of episode are consistent with barotropic instability of the stratospheric polar night jet. Analyses of climatological fields suggest that characteristics of the second type of episode may be consistent with barotropic instability of the double-peaked mesospheric jet.

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Gloria L. Manney and Terrence R. Nathan

Abstract

The stability of a basic state composed of a westward-moving wave and a zonal mean jet is examined in a linearized barotropic nondivergent model on a sphere. The sensitivity of the stability to the strength and structure of the zonal jet is emphasized. For certain westward-moving basic state waves, inclusion of a very weak jet in the basic state can dramatically alter the stability of the flow. Examination of the energetics shows that some unstable disturbances depend almost entirely on zonal variations in the basic state for their existence. In cases where meridional variations of the basic state dominate the energy transfer, examination of basic state meridional vorticity gradients is useful in understanding the stability characteristics. At subcritical basic state wave amplitudes, addition of a weak jet, which by itself is stable, can change the meridional absolute vorticity gradient to resemble that for a supercritical basic state wave alone. Unstable disturbances then occur that have spatial structures and propagation characteristics similar to those for the supercritical wave alone.

For a basic state wave resembling the observed “2-day” wave, inclusion of an easterly (summer) jet in the basic state has a strong stabilizing influence. Unstable disturbances occur when a strong easterly jet is included that have structures similar to waves observed concurrently with the “2-day” wave.

Evidence is shown for seasonal dependence in the stability of several westward-moving basic state waves. Implications of these results on the observation of westward-moving waves in the stratosphere are briefly discussed.

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Gloria L. Manney and Michaela I. Hegglin

Abstract

Long-term changes in upper-tropospheric jet latitude, altitude, and strength are assessed for 1980–2014 using five modern reanalyses: MERRA, MERRA-2, ERA-Interim, JRA-55, and NCEP CFSR. Changes are computed from jet locations evaluated daily at each longitude to analyze regional and seasonal variations. The changes in subtropical and polar (eddy driven) jets are evaluated separately. Good agreement among the reanalyses in many regions and seasons provides confidence in the robustness of the diagnosed trends. Jet shifts show strong regional and seasonal variations, resulting in changes that are not robust in zonal or annual means. Robust changes in the subtropical jet indicate tropical widening over Africa except during Northern Hemisphere (NH) spring, and tropical narrowing over the eastern Pacific in NH winter. The Southern Hemisphere (SH) polar jet shows a robust poleward shift, while the NH polar jet shifts equatorward in most regions/seasons. Both subtropical and polar jet altitudes typically increase; these changes are more robust in the NH than in the SH. Subtropical jet wind speeds have generally increased in winter and decreased in summer, whereas polar jet wind speeds have weakened (strengthened) over Africa and eastern Asia (elsewhere) during winter in both hemispheres. The Asian monsoon has increased in area and appears to have shifted slightly westward toward Africa. The results herein highlight the importance of understanding regional and seasonal variations when quantifying long-term changes in jet locations, the mechanisms for those changes, and their potential human impacts. Comparison of multiple reanalyses is a valuable tool for assessing the robustness of jet changes.

Open access
Gloria L. Manney and Michaela I. Hegglin
Open access
Gloria L. Manney, Terrence R. Nathan, and John L. Stanford

Abstract

The stability of realistic jets is examined in a linearized barotropic model on a sphere. Approximately non-dispersive modes associated with a region of negative basic state absolute vorticity gradient on the poleward side of the jet are examined in detail. As in previous studies, broader jets and those which peak at higher latitudes produce poleward modes that are less dispersive. Jet profiles derived from observational data at 10, 5, and 2 mb for three Southern Hemisphere winter months are used in the model, and the results are compared with quasi-nondispersive features (QNDF) which have been observed in satellite data in the Southern Hemisphere winter stratosphere. Characteristics of the barotropically unstable modes compare remarkably well with those of the observed modes. The barotropic model results for a month in which these features are not observed indicate the presence of equatorward modes at wavenumbers 3 and 4 which grow considerably faster than the quasi-nondispersive poleward modes. We also note the appearance of westward moving modes in the summer hemisphere during June, and in analytical profiles with a realistic global structure.

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Gloria L. Manney, Terrence R. Nathan, and John L. Stanford

Abstract

The stability of basic states consisting of a jet similar to the stratospheric polar night jet and a traveling wave with a single zonal wavenumber is examined in a linearized nondivergent barotropic model on a sphere. Basic state waves are chosen to resemble observed traveling and stationary features in the winter stratosphere. Results are presented for disturbance growth rates, propagation characteristics, and energy conversion as a function of the basic state wave amplitude. The effects of small amplitude basic state waves on unstable disturbances arising from a zonally symmetric jet are discussed; results are shown where a small amplitude basic state wave dramatically affects the stability characteristics. Evidence is shown that the presence of a traveling wave may favor the appearance of disturbances that include other zonal wavenumbers which move with the basic state wave; this result is discussed in relation to the origin of observed quasi-nondispersive features in the polar winter stratosphere. Results for a stationary wavenumber 1 basic state wave suggest that a distorted polar vortex may be unstable to disturbances that would lead to further distortion. An unstable disturbance for a basic state with an eastward moving wavenumber 2 has components which resemble, in period and location, traveling waves that are observed in the winter stratosphere.

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Zachary D. Lawrence and Gloria L. Manney

Abstract

Characteristics of the Arctic stratospheric polar vortex are examined using reanalysis data with dynamic time warping (DTW) and a clustering technique to determine whether the polar vortex exhibits canonical signs of preconditioning prior to sudden stratospheric warmings (SSWs). The DTW and clustering technique is used to locate time series motifs in vortex area, vortex edge-averaged PV gradients, and vortex edge-averaged wind speeds. Composites of the motifs reveal that prior to roughly 75% of SSWs, in the middle to upper stratosphere, PV gradients and wind speeds in the vortex edge region increase, and vortex area decreases. These signs agree with prior studies that discuss potential signals of preconditioning of the vortex. However, similar motifs are also found in a majority of years without SSWs. While such non-SSW motifs are strongly associated with minor warming signals apparent only in the middle and upper stratosphere, only roughly half of these can be associated with later “significant disturbances” (SDs) that do not quite meet the threshold for major SSWs. The median lead time for sharpening vortex edge PV gradients represented in the motifs prior to SSWs and SDs is ~25 days, while the median lead time for the vortex area and edge wind speeds is ~10 days. Overall, canonical signs of preconditioning do appear to exist prior to SSWs, but their existence in years without SSWs implies that preconditioning of the vortex may be an insufficient condition for the occurrence of SSWs.

Open access
Peter Hitchcock, Theodore G. Shepherd, and Gloria L. Manney

Abstract

A novel diagnostic tool is presented, based on polar-cap temperature anomalies, for visualizing daily variability of the Arctic stratospheric polar vortex over multiple decades. This visualization illustrates the ubiquity of extended-time-scale recoveries from stratospheric sudden warmings, termed here polar-night jet oscillation (PJO) events. These are characterized by an anomalously warm polar lower stratosphere that persists for several months. Following the initial warming, a cold anomaly forms in the middle stratosphere, as does an anomalously high stratopause, both of which descend while the lower-stratospheric anomaly persists. These events are characterized in four datasets: Microwave Limb Sounder (MLS) temperature observations; the 40-yr ECMWF Re-Analysis (ERA-40) and Modern Era Retrospective Analysis for Research and Applications (MERRA) reanalyses; and an ensemble of three 150-yr simulations from the Canadian Middle Atmosphere Model. The statistics of PJO events in the model are found to agree very closely with those of the observations and reanalyses.

The time scale for the recovery of the polar vortex following sudden warmings correlates strongly with the depth to which the warming initially descends. PJO events occur following roughly half of all major sudden warmings and are associated with an extended period of suppressed wave-activity fluxes entering the polar vortex. They follow vortex splits more frequently than they do vortex displacements. They are also related to weak vortex events as identified by the northern annular mode; in particular, those weak vortex events followed by a PJO event show a stronger tropospheric response. The long time scales, predominantly radiative dynamics, and tropospheric influence of PJO events suggest that they represent an important source of conditional skill in seasonal forecasting.

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Gloria L. Manney, Michaela I. Hegglin, and Zachary D. Lawrence

Abstract

The relationship of upper tropospheric jet variability to El Niño–Southern Oscillation (ENSO) in reanalysis datasets is analyzed for 1979–2018, revealing robust regional and seasonal variability. Tropical jets associated with monsoons and the Walker circulation are weaker and the zonal mean subtropical jet shifts equatorward in both hemispheres during El Niño, consistent with previous findings. Regional and seasonal variations are analyzed separately for subtropical and polar jets. The subtropical jet shifts poleward during El Niño over the Northern Hemisphere (NH) eastern Pacific Ocean in December–February (DJF) and in some Southern Hemisphere (SH) regions in March–May and September–November (SON). Subtropical jet altitudes increase during El Niño, with significant changes in the zonal mean in the NH and during summer/autumn in the SH. Although zonal mean polar jet correlations with ENSO are rarely significant, robust regional/seasonal changes occur: The SH polar jet shifts equatorward during El Niño over Asia and the western Pacific in DJF and significantly poleward over the eastern Pacific in June–August and SON. During El Niño, polar jets are weaker in the Western Hemisphere and stronger in the Eastern Hemisphere, especially in the SH; conversely, subtropical jets are stronger in the Western Hemisphere and weaker in the Eastern Hemisphere during El Niño in winter and spring. These opposing changes, along with an anticorrelation between subtropical and polar jet wind speeds, reinforce subtropical–polar jet strength differences during El Niño and suggest ENSO-related covariability of the jets. ENSO-related jet latitude, altitude, and wind speed changes can reach 4°, 0.6 km, and 6 m s−1, respectively, for the subtropical jets and 3°, 0.3 km, and 3 m s−1, respectively, for the polar jets.

Open access
Gloria L. Manney, Michelle L. Santee, Zachary D. Lawrence, Krzysztof Wargan, and Michael J. Schwartz

Abstract

A comprehensive investigation of the climatology of and interannual variability and trends in the Asian summer monsoon anticyclone (ASMA) is presented, based on a novel area and moments analysis. Moments include centroid location, aspect ratio, angle, and “excess kurtosis” (measuring how far the shape is from elliptical) for an equivalent ellipse with the same area as the ASMA. Key results are robust among the three modern reanalyses studied. The climatological ASMA is nearly elliptical, with its major axis aligned along its centroid latitude and a typical aspect ratio of ~5–8. The ASMA centroid shifts northward with height, northward and westward during development, and in the opposite direction as it weakens. New evidence finding no obvious climatological bimodality in the ASMA reinforces similar suggestions from previous studies using modern reanalyses. Most trends in ASMA moments are not statistically significant. ASMA area and duration, however, increased significantly during 1979–2018; the 1958–2018 record analyzed for one reanalysis suggests that these trends may have accelerated in recent decades. ASMA centroid latitude is significantly positively (negatively) correlated with subtropical jet-core latitude (altitude), and significantly negatively correlated with concurrent ENSO; these results are consistent with and extend previous work relating monsoon intensity, ENSO, and jet shifts. ASMA area is significantly positively correlated with the multivariate ENSO index 2 months previously. These results improve our understanding of the ASMA using consistently defined diagnostics of its size, geometry, interannual variability, and trends that have not previously been analyzed.

Open access