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A. O'Neill and C. E. Youngblut

Abstract

Terms in the transformed Eulerian mean equations are computed for the stratospheric warmings of December and January, 1976–77, together with cross sections showing the directions of the Eliassen-Palm (EP) fluxes and residual mean meridional circulations. The picture of warmings that emerges from them transformed diagnostics is compared and contrasted with that presented for the same events by O'Neill and Taylor (1979), whose analysis was based on the traditional momentum and heat budgets. The transformed equations lead to a simpler interpretation of warmings mainly because one term, the convergence of the EP flux, embodies to good approximation the total effect of the waves in forcing the mean flow. Zonal mean temperature changes occur essentially as an adiabatic response to the wave-induced, residual mean meridional circulation. Some evidence that critical lines may act to absorb rather than to reflect planetary waves an a short time-scale is presented.

One aspect of these and other warmings of dynamical importance is the switching of the EP fluxes which occurs from an upward and equatorward direction to an upward and poleward direction. It is proposed that this represents a feedback effect on wave propagation of an evolving mean flow. Some support for this idea comes from an analysis of ray paths in the meridional plane. They are computed for different mean wind fields which arise during a warming, and give the direction of wave propagation according to a linear theory based on the WKB approximation. Within the approximations of the theory, the mean state determines how the planetary waves propagate meridionally and vertically through a quantity Q which may be termed a refractive index. Rays are refracted poleward or equatorward according to whether Q increases poleward or equatorward. After a minor warming when the stratospheric jet has been replaced by weak westerlies, a local minimum in Q occurs in the stratosphere and rays reaching upper levels are refracted equatorward. They are refracted poleward when a strong jet is established at high latitudes, which implies that subsequent wave disturbances are focused into the polar cap, leading there to a deceleration of the mean wind. The ray paths are compared with the directions of wave propagation as given by the observed EP fluxes. Good qualitative agreement is found.

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Peter R. Gent, Kathleen O'Neill, and Mark A. Cane

Abstract

Luyten and Roemmich have shown a strong semiannual signal in zonal velocity in the upper, western part of the equatorial Indian Ocean. Their observations are modeled by assuming that they are directly forced by the observed semiannual component of zonal wind stress, which is relatively large in the equatorial Indian Ocean. The model is linear, periodic, has linear damping, uses the long-wave approximation, and can be solved analytically. A good comparison with the observations is obtained for the phase of the oscillation across the array. The predicted magnitude is somewhat smaller than in the observations. The model sensitivity to friction and the spatial distribution of the wind stress is explored. Some additional model simplifications are discussed, but it is concluded that they all detract substantially from the comparison. The main conclusion is that the observations can be accounted for as a directly forced response to the semiannual component of the near-equatorial zonal winds.

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A. D. J. O'Neill and H. L. Ferguson

Abstract

Spectral investigation of horizontal moisture flux data at a number of levels above Bedford, Mass., yields estimates of rms aliasing errors corresponding to various sampling intervals. An attempt is made to relate the analyses to the problem of the design of a sampling system for an atmospheric water balance study over Lake Ontario proposed for the International Field Year on the Great Lakes in 1972. Results suggest that errors in estimates of vertically integrated, total horizontal moisture flux may be lowered by 50% if the sampling interval is reduced from 12 to 2 hr.

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Morgan E O’Neill, Kerry A. Emanuel, and Glenn R. Flierl

Abstract

Giant planet tropospheres lack a solid, frictional bottom boundary. The troposphere instead smoothly transitions to a denser fluid interior below. However, Saturn exhibits a hot, symmetric cyclone centered directly on each pole, bearing many similarities to terrestrial hurricanes. Transient cyclonic features are observed at Neptune’s South Pole as well. The wind-induced surface heat exchange mechanism for tropical cyclones on Earth requires energy flux from a surface, so another mechanism must be responsible for the polar accumulation of cyclonic vorticity on giant planets. Here it is argued that the vortical hot tower mechanism, claimed by Montgomery et al. and others to be essential for tropical cyclone formation, is the key ingredient responsible for Saturn’s polar vortices. A 2.5-layer polar shallow-water model, introduced by O’Neill et al., is employed and described in detail. The authors first explore freely evolving behavior and then forced-dissipative behavior. It is demonstrated that local, intense vertical mass fluxes, representing baroclinic moist convective thunderstorms, can become vertically aligned and accumulate cyclonic vorticity at the pole. A scaling is found for the energy density of the model as a function of control parameters. Here it is shown that, for a fixed planetary radius and deformation radius, total energy density is the primary predictor of whether a strong polar vortex forms. Further, multiple very weak jets are formed in simulations that are not conducive to polar cyclones.

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M. J. Bailey, A. O'Neill, and V. D. Pope

Abstract

Since late 1978, the U.K. Meteorological Office (UKMO) has processed accurate radiance measurements of the stratosphere from a set of three radiometers (Statospheric Sounding Unit, Microwave Sounding Unit, High-Resolution Infrared Sounder) flown on satellites in the TIROS series. The radiometers scan both sides of the subsatellite track giving almost global coverage. UKMO uses its own retrieval algorithm to derive thickness from radiances. The retrieval is a linear, physiral–statistical method that utilizes a reference set of temperature profiles as a priori data. Thickness are added to a contemporaneous field of geopotential height at 100 hPa to give geopotential heights at standard pressure levels in the stratosphere up to 1 hPa. Global synoptic fields for 1200 UTC are produced every day on a regular latitude–longitude grid by linear interpolation in space and time. The paper describes the retrieval method and objective analysis in detail. It outlines the availability of data in the 14-year archive and summarizes the main findings of several studies on data quality. Finally, it tells prospective users how to get the data and what cheeks they should make before trying to interpret them.

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Morgan E O’Neill, Diamilet Perez-Betancourt, and Allison A. Wing

Abstract

A recent observational analysis has reported significant repeating diurnal signals propagating outward at cloud-top height from tropical cyclone centers. Modeling studies suggest that the visible upper-level impacts reflect a diurnal cycle through the depth of the troposphere. In this study, the possibility of wavelike diurnal responses in tropical cyclones is characterized using 3D cloud-resolving numerical simulations with and without a diurnal cycle. Diurnal waves can only begin to propagate well beyond the storm core, and the outflow region is most receptive to near-core diurnal propagation because of its anticyclonic flow. The tropical cyclone structure appears particularly hostile to diurnal wave propagation during periods when the eyewall experiences a temporary breakdown similar to an eyewall replacement cycle.

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P. O. Canziani, A. O'Neill, R. Schofield, M. Raphael, G. J. Marshall, and G. Redaelli
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Kimberly A. Hoogewind, Daniel R. Chavas, Benjamin A. Schenkel, and Morgan E O’Neill

Abstract

Globally, on the order of 100 tropical cyclones (TCs) occur annually, yet the processes that control this number remain unknown. Here we test a simple hypothesis that this number is limited by the geography of thermodynamic environments favorable for TC formation and maintenance. First, climatologies of TC potential intensity and environmental ventilation are created from reanalyses and are used in conjunction with historical TC data to define the spatiotemporal geography of favorable environments. Based on a range of predefined separation distances, the geographic domain of environmental favorability is populated with randomly placed TCs assuming a fixed minimum separation distance to achieve a maximum daily packing density of storms. Inclusion of a fixed storm duration yields an annual “maximum potential genesis” (MPG) rate, which is found to be an order of magnitude larger than the observed rate on Earth. The mean daily packing density captures the seasonal cycle reasonably well for both the Northern and Southern Hemispheres, though it substantially overestimates TC counts outside of each hemisphere’s active seasons. Interannual variability in MPG is relatively small and is poorly correlated with annual storm count globally and across basins, though modest positive correlations are found in the North Atlantic and east Pacific basins. Overall, the spatiotemporal distribution of favorable environmental conditions appears to strongly modulate the seasonal cycle of TCs, which certainly strongly influences the TC climatology, though it does not explicitly constrain the global annual TC count. Our methodology provides the first estimate of an upper bound for annual TC frequency and outlines a framework for assessing how local and large-scale factors may act to limit global TC count below the maximum potential values found here.

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G. L. Manney, R. W. Zurek, L. Froidevaux, J. W. Waters, A. O'Neill, and R. Swinbank

Abstract

Trajectory calculations are used to examine ozone transport in the polar winter stratosphere during periods of the Upper Atmosphere Research Satellite (UARS) observations. The value of these calculations for determining mass transport was demonstrated previously using UARS observations of long-lived tracers. In the middle stratosphere, the overall ozone behavior observed by the Microwave Limb Sounder in the polar vortex is reproduced by this purely dynamical model. Calculations show the evolution of ozone in the lower stratosphere during early winter to be dominated by dynamics in December 1992 in the Arctic. Calculations for June 1992 in the Antarctic show evidence of chemical ozone destruction and indicate that ≈ 50% of the chemical destruction may be masked by dynamical effects, mainly diabatic descent, which bring higher ozone into the lower-stratospheric vortex. Estimating differences between calculated and observed fields suggests that dynamical changes masked ≈20%–35% of chemical ozone loss during late February and early March 1993 in the Arctic. In the Antarctic late winter, in late August and early September 1992, below ≈520 K, the evolution of vortex-averaged ozone is entirely dominated by chemical effects; above this level, however, chemical ozone depiction can be partially or completely masked by dynamical effects. Our calculations for 1992 showed that chemical loss was nearly completely compensated by increases due to diabatic descent at 655 K.

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G. L. Manney, R. W. Zurek, A. O'Neill, and R. Swinbank

Abstract

Trajectory calculations using horizontal winds from the U.K. Meteorological Office data assimilation system and vertical velocities from a radiation calculation are used to simulate the three-dimensional motion of air through the stratospheric polar vortex for Northern Hemisphere (NH) and Southern Hemisphere (SH) winters since the launch of the Upper Atmosphere Research Satellite. Throughout the winter, air from the upper stratosphere moves poleward and descends into the middle stratosphere. In the SH lower to middle stratosphere, strongest descent occurs near the edge of the polar vortex, with that edge defined by mixing characteristics. The NH shows a similar pattern in late winter, but in early winter strongest descent is near the center of the vortex, except when wave activity is particularly strong. Strong barriers to latitudinal mixing exist above about 420 K throughout the winter. Below this, the polar night jet is weak in early winter, so air descending below that level mixes between polar and middle latitudes. In late winter, parcels descend less and the polar night jet moves downward, so there is less latitudinal mixing. The degree of mixing in the lower stratosphere thus depends strongly on the position and evolution of the polar night jet and on the amount of descent experienced by the air parcels; these characteristics show considerable interannual variability in both hemispheres.

The computed trajectories provide a three-dimensional picture of air motion during the final warming. Large tongues of air are drawn off the vortex and stretched into increasingly long and narrow tongues extending into low latitudes. This vortex erosion process proceeds more rapidly in the NH than in the SH. In the lower stratosphere, the majority of air parcels remain confined within a lingering region of strong potential vorticity gradients into December in the SH and April in the NH, well after the vortex breaks up in the midstratosphere.

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