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Kevin Hamilton

Abstract

A study of the zonal wind quasi-biennial oscillation (QBO) between 1972 and 1981 and at the 50 and 30 mb levels was performed using a total of over 5000 monthly mean observations from 79 stations between 20°S and 20°N. At each level and for each month a continuous representation of the zonally averaged zonal wind as a function of latitude, ū(θ), was constructed using a simple interpolation procedure. The evolution of ū through the QBO cycle was then examined. A noteworthy feature wen in each cycle was a strong concentration of westerly acceleration within a few degrees of the equator at the initial onset of the transition away from the extreme easterly phase. Arguments are presented which show that these westerly accelerations are much narrower than those that would be produced by the direct absorption of a vertically propagating Kelvin wave (at least if the wave satisfies the usual WKB scaling). It is suggested that the initial westerly mean wind acceleration in the QBO may be produced in part by the downward transport of mean flow momentum from higher levels. Such transport might result from mean flow diabatic effects in the manner discussed by Plumb and Bell. Within a month or two the strong equatorial westerly acceleration produces a highly inflected mean wind profile with regions on either side of the equator in which β – uyy, is large and negative.

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Kevin Hamilton

Abstract

The effects of the Southern Oscillation on the December-February mean circulation in the Northern Hemisphere stratosphere were investigated using 34 years of data. No evidence for a significant relation between the Southern Oscillation (SO) and the zonally averaged flow is found for any region poleward of 20°N. The effects of the tropical quasi-biennial oscillation (QBO) on the zonal mean flow are much stronger, and this complicates the detection of 50 effects. Some more suggestive results are evident when hemispheric maps of height anomalies at 50 or 30 mb are composited for the warm extremes of the 50. The present findings are broadly consistent with earlier suggestions that, on average, the Aleutian high is intensified during the warm extremes of the Southern Oscillation. Even using the 34 years of data now available, however, the statistical significance of this relationship cannot be demonstrated unequivocally. Once again the separation of SO effects from QBO influences in the limited data available is a serious problem.

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Kevin Hamilton

Abstract

A numerical model of the interaction of the mean flow with a two-dimensional standing internal gravity wave was constructed. This model was similar to the quasi-biennial oscillation “analogue” model of Plumb (1977), except that it did not employ the WKB approximation and it included thermal excitation of the waves near the lower boundary (the wave forcing region extended over a region meant to correspond to the tropical troposphere). Calculations of the mean flow evolution in the model were performed with values of the wave parameters comparable to those appropriate for the equatorial waves which are believed to force the quasi-biennial oscillation in the real atmosphere. It was found that, when a realistic profile of mean flow dissipation was included, the results of these calculations in the portion of the model corresponding to the stratosphere were strikingly similar to those obtained with the original version of the Plumb model together with an artificial no-slip boundary condition imposed at the level of the tropopause. These calculations thus support the validity of the no-slip lower boundary condition which is used in current theoretical models of the quasi-biennial oscillation.

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Kevin Hamilton

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Kevin Hamilton

Abstract

The mean flow accelerations induced by solar tides in the Martian atmosphere have been calculated using separable tidal theory together with the thermal excitations of Leovy and Zurek (1979). The calculated accelerations are generally small in the dust-free Martian atmosphere, although they may be important in a small region near the surface. During global dust storms the tidally-induced mean flow accelerations are much larger and the tides probably play an important role in the general circulation.

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Kevin Hamilton

Abstract

A zonal-mean climatology of the temperature, geostrophic zonal winds and the beat and momentum transports associated with the geostrophic winds has been constructed on the basis of almost four years of routine weekly analyses from the National Meteorological Center (NMC). The region considered extends from 10 to 85°N and from 100 to 0.4 mb. The complete set of tables describing this climatology is available in an NCAR technical report (Hamilton, 1982). In the present paper the rationale for using the NMC analyses is given and a few of the highlights of the results are displayed.

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Kevin Hamilton

Abstract

A 21-year record of monthly mean determinations of the solar semidiurnal surface pressure oscillation [S 2(p) at Batavia (6.2°S) was analyzed to detect long-period variability. When the S 2(p) determination were resolved into components which peak at local midnight (and noon) and 0900 (and 2100) local solar time, considerable evidence was found for a quasi-biennial variation in the 0900 component (but not in the midnight component). It is shown that this is consistent with the expected response of S 2(p) to the familiar quasi-biennial oscillation of the tropical stratosphere.

Also apparent in the record is a very long term trend in S 2(p). It is suggested that this way be an indication of a similar trend in stratospheric ozone, and the possibility of using the surface pressure oscillation in monitoring long-term changes in atmospheric ozone is discussed.

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Kevin Hamilton

Abstract

This paper reports on interannual variability of the Northern Hemisphere winter stratospheric circulation as simulated by the 40-level GFDL “SKYHI” general circulation model. A 31-year control simulation was performed using a climatological annual cycle of sea surface temperatures. The interannual variability of the stratospheric circulation in this model has some realistic features. In particular, the simulated variance of monthly mean, zonal-man temperature and wind in the. extratropical Northern Hemisphere agrees fairly well with observations. The day-to-day variability of the circulation also appears to be rather well simulated, with midwinter warmings of realistic intensity and suddenness appearing in the polar regions. The major deficiency is the absence of a realistic quasi-biennial oscillation (QBO) in the simulated winds in the tropical lower stratosphere. There is also an indication of long period (∼10 year) variability in the winter polar vortex. This appears not to be related to any obvious source of long-term memory in the atmosphere such as surface boundary conditions or the flow in the tropical stratosphere.

The model has also been run through a large number of boreal winter simulations with imposed perturbations. In one set of experiments the Pacific sea surface temperatures have been changed to these appropriate for strong El Niño or La Niña conditions. The model is found to reproduce the observed extratropical stratospheric response to El Niño conditions quite well. Interestingly, the results suggest that including the interannual variations in SST would not greatly enhance the simulated interannual variance of the extratropical stratosphere circulation.

Another set of integrations involved arbitrarily altering the mean flow in the tropical lower stratosphere to be appropriate for different extremes of the QBO. The effect of these modifications on the simulated zonal-mean circulation in the extratropical winter stratosphere is found to be quite modest relative to that seen in comparable observations. The model results do display a clear effect of the imposed tropical lower-stratospheric wind perturbations on the extratropical summer mesospheric circulation. This could reflect the influence of the mean flow variations on the gravity waves forced in the Tropics, propagating upward and poleward and ultimately breaking in the extratropical mesosphere. The model behavior in this regard may be related to reported observations of an extratropical mesospheric QBO.

The equilibration of the stratospheric water vapor field in the long SKYHI control integration is examined. The results suggest that the mean residence time for upper-stratospheric air in the model is about 4 years.

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Kevin Hamilton

Abstract

A 48-yr integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI troposphere–stratosphere–mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly two-thirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonal-mean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model.

The QBO was also found to have a profound influence on stratospheric stationary waves at low latitudes. Near and above 10 mb the NH stationary waves were found to penetrate across the equator during the westerly QBO phase, but to be restricted to latitudes poleward of ∼10°N during the easterly phase. This means that the equatorial QBO in prevailing wind near and above 10 mb has a significant zonally asymmetric component. If this is also true in the real atmosphere, there are important implications for the adequacy of the current observational rawindsonde network near the equator.

Analysis of the zonal-mean zonal momentum budget in the tropical stratosphere reveals that the resolved waves in the model are strong enough to force the realized accelerations through much of the QBO cycle. The exception appears to be the easterly acceleration phase below about 20 mb. The implications of this for the generation of a self-consistent QBO by GCMs will be considered.

The effects of the imposed QBO on the troposphere were found to be very modest. There does appear to be a statistically significant weakening (by ∼1 m s−1) of the high-latitude winter vortex in the middle and upper troposphere. Given the very high predictability of the stratospheric QBO itself, this effect could possibly be used to enhance the skill of seasonal weather forecasts. No significant QBO influence was found in the model precipitation field.

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Kevin Hamilton

Abstract

The quasi-biennial oscillation (QBO) in total column ozone has been examined at several tropical stations. The ozone QBO at Mauna Loa (19.5°N) was found to have a remarkable annual synchronization. Both positive and negative extremes in the deseasonalized ozone time series almost always occur between December and March. The annual cycle-QBO phase locking is much more pronounced in this ozone record than it is for the familiar QBO in the prevailing tropical stratospheric winds. This result is taken as evidence that the dynamical QBO acts to modulate a strong seasonal ozone transport from midlatitudes to the tropics. If this transport is connected with quasi-stationary planetary waves, then this interpretation offers an obvious explanation for the interhemispheric asymmetry in the ozone QBO that has been noted in many earlier studies.

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