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James J. Gray

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K. Coughlin and L. J. Gray

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The k-means cluster technique is used to examine 43 yr of daily winter Northern Hemisphere (NH) polar stratospheric data from the 40-yr ECMWF Re-Analysis (ERA-40). The results show that the NH winter stratosphere exists in two natural well-separated states. In total, 10% of the analyzed days exhibit a warm disturbed state that is typical of sudden stratospheric warming events. The remaining 90% of the days are in a state typical of a colder undisturbed vortex. These states are determined objectively, with no preconceived notion of the groups. The two stratospheric states are described and compared with alternative indicators of the polar winter flow, such as the northern annular mode. It is shown that the zonally averaged zonal winds in the polar upper stratosphere at ∼7 hPa can best distinguish between the two states, using a threshold value of ∼4 m s−1, which is remarkably close to the standard WMO criterion for major warming events. The analysis also determines that there are no further divisions within the warm state, indicating that there is no well-designated threshold between major and minor warmings, nor between split and displaced vortex events. These different manifestations are simply members of a continuum of warming events.

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Lesley J. Gray and Sarah Ruth

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A simulation of precise years of the quasi-biennial oscillation (QBO) is achieved in a two-dimensional model by relaxing the modeled equatorial winds in the lower stratosphere toward radiosonde observations. The model has been run for the period 1971–90. A QBO signal in column ozone is produced in the model that agrees reasonably well with observational data from the BUV, TOMS, and SAGE II satellite datasets. The model results confirm previous indications of the importance of the interaction of the QBO with the annual cycle in the determination of the subtropical ozone anomaly. The low-frequency modulation of the subtropical ozone anomaly is now particularly clear.

The low-frequency modulation of the subtropical ozone anomaly in the model arises as a result of the interaction of the QBO with the annual cycle in the vertical advection by the Hadley circulation. The possibility of a further, similar modulation arising from the interaction of the equatorial wind QBO and the annual cycle in midlatitude eddy activity is discussed, with particular emphasis on the implications for the eddy transfer of ozone to high latitudes and on the ability to predict the severity of the Antarctic ozone hole. A link is proposed between the QBO signal in the severity of the Antarctic ozone hole and the amount of ozone observed in the subtropical/midlatitude springtime maximum in the Southern Hemisphere. On the basis of this relationship, the reliability of the model as a predictor of the severity of the ozone hole is explored. A conclusion of the study is that a reliable predictor of the severity of the ozone hole must take into account the timing of the descent of the equatorial wind QBO at the equator with respect to the annual cycle and that the use, as in previous studies, of a single parameter, such as the sign of the 50-mb equatorial wind, will not be entirely reliable because it cannot do this.

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J. M. N. T. Gray

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Local contact interactions between sea ice floes can be modeled on the large scale by treating the pack as a two-dimensional continuum with granular properties. One such model, which has gained prominence, is the viscous plastic constitutive rheology, using an elliptical yield curve and normal flow law. It has been used extensively in ice and coupled ice–ocean studies over the past two decades. It is shown that in uniaxial flow this model reduces to a system of three quasi-linear first-order partial differential equations, which are hyperbolic in convergent flow and have mixed elliptic/hyperbolic behavior in divergence with two imaginary wave speeds. A linear stability analysis shows that the change in type causes the equations to be unstable and ill posed in uniaxial divergence. The root cause is a positive feedback mechanism that becomes stronger and stronger with smaller wavelengths. Numerical computations are used to demonstrate that fingers form and break the ice into discrete blocks. The frequency and growth rate of the fingers increase as the numerical resolution is increased, which implies that the model does not converge to a solution as the grid is refined. Two new models are proposed that are well posed. The first retains the positive feedback mechanism and introduces higher-order derivatives to suppress the unbounded growth rate of the instability. The second eliminates the positive feedback mechanism, and the instability, by repositioning the elliptical yield curve in principal stress space. Numerical simulations show that this model diverges without becoming unstable.

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L. J. Gray and J. A. Pyle

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The stratospheric quasi-biennial oscillation (QBO) in zonal wind, temperature and column ozone has been successfully modeled in a two-dimensional dynamical/chemical model by the introduction of a parameterization scheme to model the transfer of momentum to the zonal flow associated with the damping of vertically propagating Kelvin and Rossby-gravity waves. The largest amplitudes of the observed QBO in column ozone are found in high latitudes and this must be taken into account in any explanation of the increased depletion of ozone in the southern polar spring during the 1980s. A strong QBO signal in column ozone is evident in the model at all latitudes. The largest anomalies of approximately 20 DU are present at high latitudes. The equatorial ozone QBO is out of phase with the mid- and high-latitude ozone QBO. A positive (negative) ozone anomaly at the equator coincides with the presence of equatorial westerlies (easterlies) at 50 mb, in good agreement with observations. The modeled zonal wind at the equator varies from +20 m s−1 to −18 m s−1 at 25 km. The period of the modeled QBO is just over 2 yr throughout the model run except for one event when the period extends to almost 3 yr. This anomalously long period is explained in terms of the strong interaction between the modeled QBO and the seasonal cycle; in particular, the timing of the westerly phase of the QBO is influenced by the presence of the modeled semiannual oscillation (SAO). In view of this model behavior a mechanism is proposed to explain the large variability in the period of the observed QBO.

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L. J. Gray and J. M. Russell Jr.

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Measurements of water vapor and methane from the Halogen Occultation Experiment instrument on board the Upper Atmosphere Research Satellite are used to study the interannual variability of trace gas distributions in the atmosphere. Particular attention is paid to the mechanisms influencing trace gas distributions in the subtropics. The study highlights the quasi-biennial oscillation (QBO) dependence of subtropical tracer distributions more clearly than in previous studies. There is a strong correlation between the equatorial wind QBO and the slope of the tracer isolines in the Northern Hemisphere subtropics, with steeper subtropical isoline slopes in the easterly phase compared with the westerly phase. This is particularly so in the lower stratosphere. Two possible mechanisms for the QBO signal in subtropical isoline slopes are identified: advection by the mean circulation and isentropic mixing. A comparison between the QBO signal in the slope of the tracer isolines and the isentropic tracer gradients is proposed as a method of determining which process is dominant. The authors suggest that the behavior of these two data diagnostics provides a stringent constraint on computer models of the atmosphere. On the basis of these diagnostics three height regions of the subtropical atmosphere are identified. 1) Below 450–500 K isentropic mixing associated with tropospheric disturbances penetrating the lower stratosphere is dominant. 2) In the region 500–750 K the data suggest that advection by the mean meridional circulation is important and that the role of isentropic mixing by eddies is relatively small. 3) Above 750 K isentropic mixing becomes increasingly important with height, and both advection and mixing are influential in determining the subtropical tracer distributions.

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Lesley J. Gray and Timothy J. Dunkerton

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Satellite and station data have shown that the quasi-biennial oscillation (QBO) in total column ozone is asymmetric about the equator, unlike the zonal wind oscillation. There is an asymmetry in phase, as subtropical ozone anomalies maximize in the winter–spring season in both hemispheres, showing strong synchronization with the seasonal cycle irrespective of the phase of the equatorial QBO. There is also an asymmetry in amplitude, which we suggest is due to the timing of the equatorial QBO relative to the seasonal cycle and possible seasonal variation of the Hadley circulation. These asymmetries change with time as the phase relationship between the equatorial QBO and seasonal cycle changes, producing a slow modulation of the subtropical ozone QBO.

Numerical simulations of the ozone QBO with a two-dimensional radiative–dynamical–photochemical model successfully reproduce these features of the ozone QBO and show that mean motions near the base of the equatorial stratosphere are largely responsible for the asymmetry of the oscillation. The column oscillation is a complex superposition of number densities at various levels due to phase descent of the dynamical QBO and strong spatial gradients in the strength and direction of the Hadley circulation. The role of ozone photochemistry is also discussed, and comparison is made to the simulated quasi-biennial oscillation of NOy.

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Simon A. Crooks and Lesley J. Gray

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A multiple linear regression analysis of the ERA-40 dataset for the period 1979–2001 has been used to study the influence of the 11-yr solar cycle on atmospheric temperature and zonal winds. Volcanic, North Atlantic Oscillation (NAO), ENSO, and quasi-biennial oscillation (QBO) signatures are also presented. The solar signal is shown to be readily distinguishable from the volcanic signal. The main solar signal is a statistically significant positive response (i.e., warmer in solar maximum) of 1.75 K over the equator with peak values at 43 km and a reversed signal of similar magnitude at high latitudes that is seasonally dependent. Consistent with this is a statistically significant zonal wind response of up to 6 m s−1 in the subtropical upper stratosphere/lower mesosphere that is also seasonally dependent. The wind anomalies are westerly/easterly in solar maximum/minimum. In addition, there is a statistically significant temperature response in the subtropical lower stratosphere that shows similarity in spatial structure to the QBO response, suggesting a possible interaction between the solar and QBO signals in this region. The solar response in tropospheric zonal winds is small but significant, confirming previous studies that indicate a possible modulation of the Hadley circulation.

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David J. Novlan and William M. Gray

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William M. Gray and Dennis J. Shea

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This is the second (Paper II) of two papers on the characteristics of the hurricane’s inner core region as revealed by the research flight data of the National Hurricane Research Laboratory. This paper presents information on the thermal stability and the dynamic characteristics of the hurricane’s inner core region from information derived from Paper I. Discussion is given on the hurricane’s inner core vertical stability, divergence, vertical motion, heating mechanism, wind-pressure acceleration, thermal wind balances, and other features.

It is shown that large vertical moist instability is present in the eye-wall cloud. Large super-gradient winds are present at the radius of maximum winds. Substantial mixing occurs between eye and eye wall and the average hurricane eye ventilates itself by about half of its mass during the time it takes to move the distance of its eye diameter. Maximum heating does not occur at the radius of maximum updraft. Inner core heating comes from the sinking motion within the eye and not from heat diffusion from the cumulus updraft. Other features are discussed.

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