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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 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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Kevin Hamilton

This paper discusses observations of the winds in the tropical stratosphere taken before the advent of regular operational balloon soundings in this region. These observations are at least broadly consistent with modern measurements, in the sense that they show that the winds in the tropical stratosphere have been undergoing some strong interannual variations over the last century. However, the available data appear to be too sparse to construct a detailed chronology of the quasi-biennial oscillation before about 1950.

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Kevin Hamilton

The problem of formulating optimal-regulation strategies for commercial fisheries is complicated by the large interannual fluctuations often observed in the numbers and locations of various fish populations. Much of the interannual variance seen in particular cases can be attributed to the effects of environmental variability. The article reviews three examples of research showing that environmental variations can have important systematic effects on fish stocks. The three examples are all from North America and have been chosen to illustrate the biological significance of meteorological and oceanographic phenomena on a wide range of space and time scales.

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Kevin Hamilton

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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 search was conducted for the principal lunar diurnal tide (O1) in an 18½ year time series of twice-daily digitized sea level pressure analyses covering the region 20–90°N. At 20, 25, 30 and possibly at 35°N there is evidence for a systematic variation of the zonal wavenumber one harmonic of the pressure as a function of the phase of the O1 tidal potential. This variation is clearly dominated by a westward traveling component (i.e., one that follows the tidal potential around the earth each day). The computed amplitudes are very small (less than 0.01 mb), and north of 35°N the random meteorological noise cbscures the O1 tidal oscillation to the point where it cannot be detected from analysis of the present data.

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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

The length of the westerly phase of the quasi-biennial oscillation (QBO) of zonal wind in the equatorial stratosphere is examined using the longest available record (1950–2001). An earlier finding by Salby and Callagan of a systematic quasi-decadal modulation of the QBO period is confirmed, although the earlier suggestion of a strong connection with the solar cycle is less clear in the extended record.

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