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Harry van Loon

Abstract

Meridional profiles of sea-level and 500-mb average zonal winds at mid-season in both hemispheres are presented and described. Over much of the southern hemisphere the mean zonal westerly flow changes little during the year, and in each season it is always stronger for the hemisphere as a whole in the southern than in the northern hemisphere. The southern hemisphere circulation and its seasonal changes are shown to be closely related to the surface temperature distribution.

It is pointed out that despite the seemingly symmetrical distribution of heat and cold sources over the southern hemisphere, regional differences in the strength and distribution of the mean zonal wind do exist, particularly during the colder part of the year. Thus, although on the whole there is a strong control arising from the ocenic dominance of the hemisphere, there are, especially in the Australian sector, large disturbances connected with continental influences on heating and cooling.

The strongest zonal westerly mean circulation in the southern hemisphere at the levels dealt with here is found over the Indian Ocean.

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Harry Van Loon

Abstract

Differences of sea-level pressure between pairs of stations in the South Pacific Ocean are used to examine the trades and the trough in the westerlies during the development of the phase of the Southern Oscillation when pressures fall over the tropical Pacific, equatorial waters warm, and rainfall increases in many otherwise dry places. It is demonstrated that this phase is characterized by an appreciable enhancement of the annual cycle of the trades and the trough compared to the year before. The warm event of 1982 followed this pattern closely.

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Harry Van Loon

Abstract

When the total eddy transport of sensible heat in middle latitudes of the Northern Hemisphere increases in winter, the zonally averaged temperature gradient in the subtropics tends to increase, while the temperature gradient decreases in the same latitude as, and north of, the given total eddy flux. This effect is associated mainly with the flux in the quasi-stationary or mean waves. In agreement with this relationship between temperature gradient and heat transport, the correlation between the total eddy flux divergence and zonally averaged temperature in middle latitudes is strongly negative; and the main contribution to this negative correlation also comes from the quasi-stationary eddies. When the mean-eddy flux increases at middle latitudes, the transient-eddy flux tends to decrease over the region of weaker gradients to the north of the stronger mean-eddy flux and to increase to the south of it; and conversely when the mean-eddy flux weakens. From the association between total eddy transport and temperature gradients it follows that the gradients at lower latitudes are negatively correlated with those at higher latitudes.

In the Southern Hemisphere, where the quasi-stationary eddies in temperate latitudes transport little sensible heat, the relationship between total eddy flux and zonally averaged temperature gradient is determined principally by the transient-eddy flux.

All the associations above refer to seasonal averages.

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Harry Van Loon

Abstract

No abstract available.

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Harry van Loon

Abstract

In mid-tropospheric levels of the Southern Hemisphere, the temperature contrast between middle and high latitudes reaches two maxima near the times of the equinoxes, the strongest contrast being at the autumnal equinox. This variation is dominated by the seasonal changes over the Antarctic Ocean where the second (semiannual) harmonic of the mid-tropospheric meridional temperature gradient has a magnitude exceeding that of the first (annual) harmonic. The existence of the marked semiannual component in this region is shown to be a consequence of the difference between the heating and cooling rates in different latitudes where the mid-tropospheric annual temperature ranges are similar. In latitude 50S cooling in autumn is rapid compared with the warming in spring, the reverse being true at latitude 65S. This behavior is related to the heat budget of the oceanic upper layers.

The semiannual maximum temperature gradients in the middle troposphere over the Antarctic Ocean, through increased cyclonic activity, cause the mean position of the circumpolar trough to be closer to the pole during the transition seasons. In turn, this produces a semiannual oscillation in the pressures and winds over the area affected by the trough.

While the hemispheric mean pressure in middle latitudes falls with the equatorward shift in the mean position of the circumpolar trough from autumn to winter, the pressure rises over Australia, South America, and Africa. The resulting increased longitudinal pressure contrast between the continents and the neighboring sea is expressed as an amplification of the mean wave pattern at the surface, greatest in the half hemisphere centered on Australia. The enhanced poleward transfer of warm air accompanying this amplification apparently accounts for the cessation of rapid temperature falls over Antarctica in early winter, and for actual temperature rises at some stations such as Little America.

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Harry Van Loon

Abstract

Although the numerical, daily operational analyses for the Southern Hemisphere, in particular those made in Australia since 1972, can be used to describe large-scale features in time and space, they are not suited to computations of quantities which rely for their accuracy on the correctness of the daily analysis at individual points. This conclusion is based on the analysis of transfer of sensible heat by the transient eddies during FGGE and the years 1972–77. In addition, the Australian daily operational analyses of the Southern Hemisphere were compared with those of the U.S. National Meteorological Center, and it was found that the former are generally the better ones.

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Harry van Loon

Abstract

The seasonal variations during the IGY of sea-level pressures, 500-mb heights, and 1000-500 mb thickness in the Southern Hemisphere are examined to see if they conform to patterns which are deemed intrinsic to the hemisphere. These are: 1) the arrangement of the annual temperature range in four concentric zones of alternating low and high range; 2) the coldness of the lower and middle troposphere over Australasia in winter compared with South America and Africa; 3) the positive temperature isanomals in middle and high latitudes over the South Pacific Ocean in both summer and winter; and 4) the sea-level pressure and 500-mb height variations of opposite sign in middle and high latitudes which give rise to a second harmonic of large amplitude in temperature and height gradients, and in winds and sea-level pressure.

The speed of movement of lows between 30 and 70S was, on the average, only slightly lower in the IGY summer than in the winter.

A comparison between the standard deviations of daily sea-level pressures and 500-mb heights in the two hemispheres shows that the variability is nearly the same in the northern and southern winters, but that the standard deviations in the latitudes near 50N in summer are only two-thirds of those in the same latitudes in the southern summer.

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Harry van Loon

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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Harry van Loon and Jill Williams

Abstract

There was no single relationship between mean temperature and the variability of temperature in the years 1876–1975 in Europe and North America, nor did the variability of precipitation in North America necessarily increase when the temperature decreased

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Harry Van Loon and Jill Williams

Abstract

This paper continues the description of the connection between transport of sensible heat by eddies in winter and temperature, wind and pressure. The emphasis is on the transport by the quasi-stationary eddies, and we demonstrate that when this transport is strong in the latitudes near 50°N, the west wind tends to be strong in the subtropics at all levels in the troposphere and to he weak at middle and high latitudes, and vice versa when the stationary eddy transport is weak. These associations stem principally from the regions of the two major troughs. We show, in addition, that the stationary-eddy transport in the two troughs is negatively correlated; and we outline the teleconnections between the stationary-eddy flux over eastern Asia and pressure and temperature elsewhere.

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