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- Author or Editor: Harry Van Loon x
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Abstract
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Abstract
An evaluation is made of the synoptic features and characteristic pressure patterns of the Southern Hemisphere as they relate to the seasonal circulations. Examples of both strong and weak polar outbreaks are presented, and their effect on the zonal circulation is discussed. The implication of the strong zonal circulation and its short-term as well as seasonal variations are examined.
Abstract
An evaluation is made of the synoptic features and characteristic pressure patterns of the Southern Hemisphere as they relate to the seasonal circulations. Examples of both strong and weak polar outbreaks are presented, and their effect on the zonal circulation is discussed. The implication of the strong zonal circulation and its short-term as well as seasonal variations are examined.
Abstract
The spatial variability of mean sea level pressure (SLP) and 500 mb height anomalies over the Southern Hemisphere during summer (DJF) and winter (JJA) is determined using eigenvector analysis based on daily synoptic maps from 1972 to 1979. The patterns of spatial distribution of pressure and height anomalies are further verified and examined by means of station data, and the eigenvectors are compared between the seasons and to those found for the Northern Hemisphere.
The first eigenvector shows that midlatitude anomalies of SLP and 500 mb height are of an opposite sign to those found over and around Antarctica. The pattern is highly barotropic and suggests strengthening and weakening of the zonal wind in alternating latitude belts. The 500 mb height differences are calculated for five midlatitude to Antarctic station pairs using data from the late 1950's onward. These latitudinal height differences are also used to describe the association between the hemispheric westerlies and the Southern Oscillation. They also suggest that the summertime westerlies strengthen and weaken simultaneously around the hemisphere, whereas in winter this zonal symmetry is interrupted in the South America–Antarctic Peninsula region. In this region, wintertime variations in the westerlies are associated with the second eigenvector, and large interannual variations in the SLP and latitudinal variations in storm tracks occur over the Drake Passage. The second eigenvector is shown to be associated with the large standard deviations of winter mean temperature along the Antarctic Peninsula, to variations in sea ice duration at Laurie Island in the South Orkneys, and to the Trans-Polar Index of Pittock, which describes the tendency for the surface polar vortex to be displaced either toward Tasmania or the Falkland-South Sandwich Islands region.
It is concluded that temperature and circulation teleconnections, similar to those of the Northern Hemisphere, also occur in the Southern Hemisphere, and are associated with standing waves in the atmosphere The small amount of land south of 35°S probably accounts for the uniform variations in the westerlies across the Southern Hemisphere, whereas, in the Northern Hemisphere, variations in the westerlies shown by the eigenvectors are largely confined to the oceans.
Abstract
The spatial variability of mean sea level pressure (SLP) and 500 mb height anomalies over the Southern Hemisphere during summer (DJF) and winter (JJA) is determined using eigenvector analysis based on daily synoptic maps from 1972 to 1979. The patterns of spatial distribution of pressure and height anomalies are further verified and examined by means of station data, and the eigenvectors are compared between the seasons and to those found for the Northern Hemisphere.
The first eigenvector shows that midlatitude anomalies of SLP and 500 mb height are of an opposite sign to those found over and around Antarctica. The pattern is highly barotropic and suggests strengthening and weakening of the zonal wind in alternating latitude belts. The 500 mb height differences are calculated for five midlatitude to Antarctic station pairs using data from the late 1950's onward. These latitudinal height differences are also used to describe the association between the hemispheric westerlies and the Southern Oscillation. They also suggest that the summertime westerlies strengthen and weaken simultaneously around the hemisphere, whereas in winter this zonal symmetry is interrupted in the South America–Antarctic Peninsula region. In this region, wintertime variations in the westerlies are associated with the second eigenvector, and large interannual variations in the SLP and latitudinal variations in storm tracks occur over the Drake Passage. The second eigenvector is shown to be associated with the large standard deviations of winter mean temperature along the Antarctic Peninsula, to variations in sea ice duration at Laurie Island in the South Orkneys, and to the Trans-Polar Index of Pittock, which describes the tendency for the surface polar vortex to be displaced either toward Tasmania or the Falkland-South Sandwich Islands region.
It is concluded that temperature and circulation teleconnections, similar to those of the Northern Hemisphere, also occur in the Southern Hemisphere, and are associated with standing waves in the atmosphere The small amount of land south of 35°S probably accounts for the uniform variations in the westerlies across the Southern Hemisphere, whereas, in the Northern Hemisphere, variations in the westerlies shown by the eigenvectors are largely confined to the oceans.
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.
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.
Abstract
This note deals with the standard deviations of 24-hr changes in 10-mb temperatures and heights. The standard deviations are differently distributed in disturbed and in quiet winter months. In the disturbed months their largest values form a coherent area at high latitudes; in the quiet months they surround the polar region as a ring with its center on the Atlantic side.
Abstract
This note deals with the standard deviations of 24-hr changes in 10-mb temperatures and heights. The standard deviations are differently distributed in disturbed and in quiet winter months. In the disturbed months their largest values form a coherent area at high latitudes; in the quiet months they surround the polar region as a ring with its center on the Atlantic side.
Abstract
During the 1970s and 1980s, elements of the atmospheric circulation over the Southern Hemisphere changed markedly from their state in the two previous decades. The change was evident in the zonal asymmetry, especially in the zonal harmonic wave 3 at higher latitudes, and in the semiannual wave in pressure and wind. The semiannual wave changed in the same sense in all three southern oceans, but by the largest amount in the South Pacific Ocean. The second harmonic in a 12-month series, by which we describe the semiannual wave, dominates the shape of the long-term mean annual curve of sea level pressure in mid- and high latitudes; this harmonic weakened during the period, and its midlatitude peak disappeared in the South Pacific Ocean.
As part of the low-frequency changes during the 18 years from 1972 to 1988, the central pressure in the subantarctic trough fell, the trough moved northward, and the subtropical ridge moved south. The meridional movement of the trough and the ridge, and the concurrent trend in their central pressure, weakened the semiannual wave in midlatitudes.
Abstract
During the 1970s and 1980s, elements of the atmospheric circulation over the Southern Hemisphere changed markedly from their state in the two previous decades. The change was evident in the zonal asymmetry, especially in the zonal harmonic wave 3 at higher latitudes, and in the semiannual wave in pressure and wind. The semiannual wave changed in the same sense in all three southern oceans, but by the largest amount in the South Pacific Ocean. The second harmonic in a 12-month series, by which we describe the semiannual wave, dominates the shape of the long-term mean annual curve of sea level pressure in mid- and high latitudes; this harmonic weakened during the period, and its midlatitude peak disappeared in the South Pacific Ocean.
As part of the low-frequency changes during the 18 years from 1972 to 1988, the central pressure in the subantarctic trough fell, the trough moved northward, and the subtropical ridge moved south. The meridional movement of the trough and the ridge, and the concurrent trend in their central pressure, weakened the semiannual wave in midlatitudes.
Abstract
We show by means of a general circulation model experiment that the atmospheric circulation over the South Pacific Ocean is sensitive to sea surface temperature anomalies in the tropical and subtropical regions of the South Pacific convergence zone. The possible implications for understanding the life cycle of an extreme event in the Southern Oscillation are discussed.
Abstract
We show by means of a general circulation model experiment that the atmospheric circulation over the South Pacific Ocean is sensitive to sea surface temperature anomalies in the tropical and subtropical regions of the South Pacific convergence zone. The possible implications for understanding the life cycle of an extreme event in the Southern Oscillation are discussed.
Abstract
Two patterns dominate changes of monthly mean temperature and pressure-height in the stratosphere. In the one, the middle latitudes vary oppositely to low and high latitudes, and in the other the changes at higher latitudes are out of phase with those at lower latitudes.
A shorter trend consisting of opposite changes at middle and high latitudes is superposed on the above variations which a cross-spectrum analysis shows has a preferred time scale of one to three weeks. The contrast between middle and high latitudes thus undergoes a series of corresponding fluctuations and we show that these are associated with amplitude changes in waves 1 and 2 in that the meridional contrast decreases when the amplitude of one or both waves is large, and vice versa.
Abstract
Two patterns dominate changes of monthly mean temperature and pressure-height in the stratosphere. In the one, the middle latitudes vary oppositely to low and high latitudes, and in the other the changes at higher latitudes are out of phase with those at lower latitudes.
A shorter trend consisting of opposite changes at middle and high latitudes is superposed on the above variations which a cross-spectrum analysis shows has a preferred time scale of one to three weeks. The contrast between middle and high latitudes thus undergoes a series of corresponding fluctuations and we show that these are associated with amplitude changes in waves 1 and 2 in that the meridional contrast decreases when the amplitude of one or both waves is large, and vice versa.
Abstract
The upper-air wind data generated by the Global Data Assimilation System of the National Meteorological Center for 1986–1992 are used to depict the three-dimensional structure of the semiannual oscillation of tropical stationary eddies in terms of the eddy streamfunction. An eddy streamfunction budget analysis was also performed to disclose the cause of this semiannual oscillation. The major findings are: 1) The tropical stationary eddies exhibit a seesaw semiannual oscillation between the eastern and Western Hemisphere with a phase reversal vertically at 400–500 mb and meridionally at the equator. 2) It is inferred from the streamfunction budget analysis that the semiannual oscillation of the tropical stationary eddies is caused by the semiannual cast-west seesaw of the global divergent circulation between the areas of the Asian-Australian (AA) monsoon (60°E–120°W) and the extra-AA monsoon (120°W–60°E). This mechanism is particularly clear in the Southern Hemisphere Tropics but less well established in the Northern Hemisphere Tropics, which may be attributed to the strong effect of the land-sea contrast on the east-west interseasonal phase change of the stationary eddies in the Northern Hemisphere.
Abstract
The upper-air wind data generated by the Global Data Assimilation System of the National Meteorological Center for 1986–1992 are used to depict the three-dimensional structure of the semiannual oscillation of tropical stationary eddies in terms of the eddy streamfunction. An eddy streamfunction budget analysis was also performed to disclose the cause of this semiannual oscillation. The major findings are: 1) The tropical stationary eddies exhibit a seesaw semiannual oscillation between the eastern and Western Hemisphere with a phase reversal vertically at 400–500 mb and meridionally at the equator. 2) It is inferred from the streamfunction budget analysis that the semiannual oscillation of the tropical stationary eddies is caused by the semiannual cast-west seesaw of the global divergent circulation between the areas of the Asian-Australian (AA) monsoon (60°E–120°W) and the extra-AA monsoon (120°W–60°E). This mechanism is particularly clear in the Southern Hemisphere Tropics but less well established in the Northern Hemisphere Tropics, which may be attributed to the strong effect of the land-sea contrast on the east-west interseasonal phase change of the stationary eddies in the Northern Hemisphere.
Abstract
The 11-yr solar cycle [decadal solar oscillation (DSO)] at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of twentieth-century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols, as well as black carbon aerosols in one of the models) and natural (volcano and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11-yr ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Mainly due to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the trade winds, thereby strengthening the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Niña events) occurs. There is a strengthening of the trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure (SLP) anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.
Abstract
The 11-yr solar cycle [decadal solar oscillation (DSO)] at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of twentieth-century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols, as well as black carbon aerosols in one of the models) and natural (volcano and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11-yr ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Mainly due to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the trade winds, thereby strengthening the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Niña events) occurs. There is a strengthening of the trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure (SLP) anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.
