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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
The probable association in the northern winter between the atmosphere and the 11-yr solar cycle extends to the frequency of lows in the North American east coast trough and thus adds a synoptic aspect to the previously described atmospheric variability on the 11-yr time scale. Statistically significant correlations of sea level pressure, 700-mb height, and surface air temperature on the Northern Hemisphere in July–August with the 11-yr solar cycle are found primarily over the oceans. The few years for which data of sea level pressure at grid points are available an the Southern Hemisphere yield coherent correlation patterns in summer and winter which are especially marked in the East years of the QBO. The temperature in the lower stratosphere over the South Pole in spring is well correlated with the solar activity in the East and hardly at all in the West years of the QBO. On the Northern Hemisphere the West years in spring are as strongly correlated with the solar cycle in the stratosphere as they are in winter. The pattern of positive and negative correlations is, however, the opposite of that in winter, which we interpret as being related to the different time of occurrence of the final warming in years with or without major midwinter warmings.
Abstract
The probable association in the northern winter between the atmosphere and the 11-yr solar cycle extends to the frequency of lows in the North American east coast trough and thus adds a synoptic aspect to the previously described atmospheric variability on the 11-yr time scale. Statistically significant correlations of sea level pressure, 700-mb height, and surface air temperature on the Northern Hemisphere in July–August with the 11-yr solar cycle are found primarily over the oceans. The few years for which data of sea level pressure at grid points are available an the Southern Hemisphere yield coherent correlation patterns in summer and winter which are especially marked in the East years of the QBO. The temperature in the lower stratosphere over the South Pole in spring is well correlated with the solar activity in the East and hardly at all in the West years of the QBO. On the Northern Hemisphere the West years in spring are as strongly correlated with the solar cycle in the stratosphere as they are in winter. The pattern of positive and negative correlations is, however, the opposite of that in winter, which we interpret as being related to the different time of occurrence of the final warming in years with or without major midwinter warmings.
Abstract
Sea level pressure, surface air temperature, and 700-mb temperature and geopotential height show a probable association with the 11-year solar cycle which can be observed only if the data are divided according to the phase of the Quasi-Biennial Oscillation. The range of the response is as large as the interannual variability of the given element, and the correlations prove statistically meaningful when tested by Monte Carlo techniques. The sign of the correlations changes over the hemisphere on the spatial scale of extensive teleconnections. The correlations at 700 mb tend to be of opposite sign in the east and west years of the QBO, a result which Labitzke and van Loon also found in an analysis of the stratosphere. The pattern of correlation between the 700-mb heights on the Northern Hemisphere and the solar flux is the same as that of point-to-point correlations (teleconnections) between the 700-mb height at selected points and the heights at all other points. We interpret this similarity as a property of the atmosphere's internal dynamics, a favored resonance evoked within the atmosphere itself or by extraneous effects.
Abstract
Sea level pressure, surface air temperature, and 700-mb temperature and geopotential height show a probable association with the 11-year solar cycle which can be observed only if the data are divided according to the phase of the Quasi-Biennial Oscillation. The range of the response is as large as the interannual variability of the given element, and the correlations prove statistically meaningful when tested by Monte Carlo techniques. The sign of the correlations changes over the hemisphere on the spatial scale of extensive teleconnections. The correlations at 700 mb tend to be of opposite sign in the east and west years of the QBO, a result which Labitzke and van Loon also found in an analysis of the stratosphere. The pattern of correlation between the 700-mb heights on the Northern Hemisphere and the solar flux is the same as that of point-to-point correlations (teleconnections) between the 700-mb height at selected points and the heights at all other points. We interpret this similarity as a property of the atmosphere's internal dynamics, a favored resonance evoked within the atmosphere itself or by extraneous effects.
Abstract
Composite surface pressure, temperature, and precipitation anomalies are mapped over the Indian and Pacific sectors during the various stages of Warm and Cold Events in the Southern Oscillation. In the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial Pacific (Year–1 of a Warm Event), a strong South Pacific High is associated with below normal surface pressure over Australia and the Indian Ocean. This occurs concurrently with a poleward displacement of the Pacific convergence zones, with above normal air temperature and precipitation over the subtropical Pacific, and opposite conditions along the equator. By the next year (Year 0) of the Warm Event, thew anomalies have the opposite sign. The sequence of anomalies during a Cold Event is inverse to that during a Warm Event but otherwise the anomaly patterns are remarkably similar.
It appears that enhanced convection and low surface pressure within the Pacific convergence zones contribute to the observed westerly wind anomalies in the western equatorial Pacific at the end of Year–1, which are in turn tied to the onset of above normal equatorial SST in the following year. The observed reversal in atmospheric anomalies over the Indian and Pacific oceans daring Warm Events is an extreme manifestation of a general biennial tendency in these anomalies, with Cold Events occupying the opposite extreme.
Abstract
Composite surface pressure, temperature, and precipitation anomalies are mapped over the Indian and Pacific sectors during the various stages of Warm and Cold Events in the Southern Oscillation. In the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial Pacific (Year–1 of a Warm Event), a strong South Pacific High is associated with below normal surface pressure over Australia and the Indian Ocean. This occurs concurrently with a poleward displacement of the Pacific convergence zones, with above normal air temperature and precipitation over the subtropical Pacific, and opposite conditions along the equator. By the next year (Year 0) of the Warm Event, thew anomalies have the opposite sign. The sequence of anomalies during a Cold Event is inverse to that during a Warm Event but otherwise the anomaly patterns are remarkably similar.
It appears that enhanced convection and low surface pressure within the Pacific convergence zones contribute to the observed westerly wind anomalies in the western equatorial Pacific at the end of Year–1, which are in turn tied to the onset of above normal equatorial SST in the following year. The observed reversal in atmospheric anomalies over the Indian and Pacific oceans daring Warm Events is an extreme manifestation of a general biennial tendency in these anomalies, with Cold Events occupying the opposite extreme.
