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A climatological Study of the Atmospheric Circulation in the Southern Hemisphere during the IGY, Part I: 1 July 1957–31 March 1958

Harry van LoonNational Center for Atmospheric Research, Boulder, Colo.

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Abstract

Time sections of monthly average zonal geostrophic wind at sea level and 500 mb in the Southern Hemisphere confirm that the strongest zonal westerlies usually are found in summer, but since the westerlies expand over a bigger area in the colder part of the year their average relative angular momentum is greater in winter than in summer. For the earth as a whole the westerly relative angular momentum at sea level and 500 mb is greater in January than in July.

The highest frequency of surface fronts in the Southern Hemisphere forms a belt round the hemisphere in middle latitude in summer and coincides with the strongest zonal wind at sea level and 500 mb, and with the steepest temperature gradient in the lower half of the troposphere. In winter there are two separate zones of highest frequency of fronts.

The frequencies of sea level highs, lows and cyclogenesis plotted as a function of latitude have their peaks in three different latitudes zones so arranged that the peak frequency of highs is farthest equatorward and that of the lows farthest poleward.

The meridional geostrophic flow at 500 mb between the 30th and 60th parallels, weighted according to area, was used as a relative measure of the mass flux across the given latitude belt. It was found that the flux is considerably greater during the southern than during the northern summer, but little greater during the northern than during the southern winter. The shape of the meridional profiles of mass exchange is the same in the. northern and southern summers and in the northern and southern winters.

An analysis of the 500-mb data on Hovmöller diagrams shows that horizontal dispersion of energy, seen as the downstream intensification of alternating troughs and ridges, took place on several occasions during the period investigated.

Abstract

Time sections of monthly average zonal geostrophic wind at sea level and 500 mb in the Southern Hemisphere confirm that the strongest zonal westerlies usually are found in summer, but since the westerlies expand over a bigger area in the colder part of the year their average relative angular momentum is greater in winter than in summer. For the earth as a whole the westerly relative angular momentum at sea level and 500 mb is greater in January than in July.

The highest frequency of surface fronts in the Southern Hemisphere forms a belt round the hemisphere in middle latitude in summer and coincides with the strongest zonal wind at sea level and 500 mb, and with the steepest temperature gradient in the lower half of the troposphere. In winter there are two separate zones of highest frequency of fronts.

The frequencies of sea level highs, lows and cyclogenesis plotted as a function of latitude have their peaks in three different latitudes zones so arranged that the peak frequency of highs is farthest equatorward and that of the lows farthest poleward.

The meridional geostrophic flow at 500 mb between the 30th and 60th parallels, weighted according to area, was used as a relative measure of the mass flux across the given latitude belt. It was found that the flux is considerably greater during the southern than during the northern summer, but little greater during the northern than during the southern winter. The shape of the meridional profiles of mass exchange is the same in the. northern and southern summers and in the northern and southern winters.

An analysis of the 500-mb data on Hovmöller diagrams shows that horizontal dispersion of energy, seen as the downstream intensification of alternating troughs and ridges, took place on several occasions during the period investigated.

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