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- Author or Editor: JOHN E. KUTZBACH x
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Abstract
The combined representation of fields of three climatic variables with empirical orthogonal functions, herein referred to as eigenvectors, is discussed. The eigenvectors are derived from measurements of monthly mean sea-level pressure, surface temperature and precipitation at 23 points in North America for 25 Januarys.
Selected eigenvectors of the individual climatic variables are presented; however, the major part of the paper is devoted to the presentation of eigenvectors consisting of combinations of three climatic variables. Empirical eigenvectors derived from fields of two or more meteorological variables have been used in statistical prediction models, but none of the studies to date displayed examples of these eigenvectors or discussed the internal consistency of the combined representations. In this paper it is shown that the structure of the covariances between the three climatic variables,as portrayed by the combined representations, is consistent with synoptic experience. This result illustrates that eigenvector representations derived from fields of several variables can be of considerable descriptive or diagnostic value.
Abstract
The combined representation of fields of three climatic variables with empirical orthogonal functions, herein referred to as eigenvectors, is discussed. The eigenvectors are derived from measurements of monthly mean sea-level pressure, surface temperature and precipitation at 23 points in North America for 25 Januarys.
Selected eigenvectors of the individual climatic variables are presented; however, the major part of the paper is devoted to the presentation of eigenvectors consisting of combinations of three climatic variables. Empirical eigenvectors derived from fields of two or more meteorological variables have been used in statistical prediction models, but none of the studies to date displayed examples of these eigenvectors or discussed the internal consistency of the combined representations. In this paper it is shown that the structure of the covariances between the three climatic variables,as portrayed by the combined representations, is consistent with synoptic experience. This result illustrates that eigenvector representations derived from fields of several variables can be of considerable descriptive or diagnostic value.
Abstract
Spatial patterns of circulation variability over the Northern Hemisphere and their changes during the past 70 yr (1899–1969) are examined using eigenvector analyses of mean January and July sea-level pressure maps. The first several eigenvectors display variability associated with the major centers of action (the subpolar oceanic Lows, the subtropical oceanic Highs, the winter Siberian High, and the summer Asiatic Low). The pattern of the first eigenvector of January suggests that the intensity and latitudinal position of the major circulation features over the North Atlantic are associated with the intensity and position of the Aleutian Low over the North Pacific.
The time series of the coefficients of the hemispheric eigenvectors are used to identify intervals of change in the hemispheric circulation associated with features on the scale of thc major centers of action. These time series provide a more general description of circulation change than that obtained from local or regional indices; but at the same time, they provide more detailed information than time series of hemispherically averaged indices. Two intervals of change in the large-scale hemispheric circulation are identified: the early to mid-1920s and the early to mid-1950s. Of the three periods separated by these two intervals of change, maximum contrast is noted between the first and third. For January, the strongest circulation features are found in the North Atlantic and European sectors in the first (earliest) period and in the North Pacific and Asian sectors in the third (latest) period.
Abstract
Spatial patterns of circulation variability over the Northern Hemisphere and their changes during the past 70 yr (1899–1969) are examined using eigenvector analyses of mean January and July sea-level pressure maps. The first several eigenvectors display variability associated with the major centers of action (the subpolar oceanic Lows, the subtropical oceanic Highs, the winter Siberian High, and the summer Asiatic Low). The pattern of the first eigenvector of January suggests that the intensity and latitudinal position of the major circulation features over the North Atlantic are associated with the intensity and position of the Aleutian Low over the North Pacific.
The time series of the coefficients of the hemispheric eigenvectors are used to identify intervals of change in the hemispheric circulation associated with features on the scale of thc major centers of action. These time series provide a more general description of circulation change than that obtained from local or regional indices; but at the same time, they provide more detailed information than time series of hemispherically averaged indices. Two intervals of change in the large-scale hemispheric circulation are identified: the early to mid-1920s and the early to mid-1950s. Of the three periods separated by these two intervals of change, maximum contrast is noted between the first and third. For January, the strongest circulation features are found in the North Atlantic and European sectors in the first (earliest) period and in the North Pacific and Asian sectors in the third (latest) period.
Abstract
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General circulation model experiments at 3000-year intervals for the past 18 000 years were made to estimate the magnitude, timing, and pattern of the climatic response to prescribed changes of orbital parameters (date of perihelion, axial tilt, eccentricity) and glacial-age lower boundary conditions (ice sheets, land albedo, sea ice and sea surface temperature). The experiments used the Community Climate Model (CCM) of the National Center for Atmospheric Research (NCAR). The response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial-age boundary conditions. The continental interior of Eurasia was 2–4 K warmer in summer, and summer monsoon precipitation of North Africa-South Asia was increased by 10–20% between 12 000 and 6000 yr BP (before present) when perihelion occurred during northern summer (rather than in winter as now) and the earth's axial tilt was larger than now. Southern Hemisphere summer monsoons were weaker during the same period. In northern midlatitudes, glacial-age features such as the North American ice shed exerted a strong influence on the climate until 9000 yr BP. Much of the climatic change of the period 12 000 to 6000 yr BP can be described as an amplified (weakened) seasonal cycle in response to the larger (smaller) seasonal radiation extremes of the Northern (Southern) Hemisphere. Summers were warmer and winters colder in Northern Hemisphere lands, but there was little change in annual average temperature. However, because of the nonlinear relationship between saturation vapor pressure and temperature, the sensitivity of the hydrologic cycle to orbital parameter changes was larger in summer than in winter (and in the tropics rather than high latitudes); in the northern tropics, this led to a net increase in estimated annual average precipitation and precipitation minus evaporation. Many features of the results are in agreement with geologic evidence.
Abstract
General circulation model experiments at 3000-year intervals for the past 18 000 years were made to estimate the magnitude, timing, and pattern of the climatic response to prescribed changes of orbital parameters (date of perihelion, axial tilt, eccentricity) and glacial-age lower boundary conditions (ice sheets, land albedo, sea ice and sea surface temperature). The experiments used the Community Climate Model (CCM) of the National Center for Atmospheric Research (NCAR). The response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial-age boundary conditions. The continental interior of Eurasia was 2–4 K warmer in summer, and summer monsoon precipitation of North Africa-South Asia was increased by 10–20% between 12 000 and 6000 yr BP (before present) when perihelion occurred during northern summer (rather than in winter as now) and the earth's axial tilt was larger than now. Southern Hemisphere summer monsoons were weaker during the same period. In northern midlatitudes, glacial-age features such as the North American ice shed exerted a strong influence on the climate until 9000 yr BP. Much of the climatic change of the period 12 000 to 6000 yr BP can be described as an amplified (weakened) seasonal cycle in response to the larger (smaller) seasonal radiation extremes of the Northern (Southern) Hemisphere. Summers were warmer and winters colder in Northern Hemisphere lands, but there was little change in annual average temperature. However, because of the nonlinear relationship between saturation vapor pressure and temperature, the sensitivity of the hydrologic cycle to orbital parameter changes was larger in summer than in winter (and in the tropics rather than high latitudes); in the northern tropics, this led to a net increase in estimated annual average precipitation and precipitation minus evaporation. Many features of the results are in agreement with geologic evidence.
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Abstract
The sensitivity of a low resolution, spectral general circulation model (GCM) to specification of physical processes is examined using a new version of the model with refined parameterizations. Specific refinements in parameterization include: 1) smoothing the original orography to greatly diminish undesirable topographic “ripples” occurring near high mountain ranges; 2) adding snowcover on the Tibetan plateau and representing winter snowcover in middle latitudes more realistically; 3) decreasing the land ground wetness and adjusting the drag coefficient and parameters governing condensation-moist convective adjustment.
Results of comparative 5-year integrations show that better parameterization in the low resolution model produces significant improvement in simulation without resorting to the use of higher horizontal or vertical resolution. The combined changes in ground wetness, drag coefficient and condensation-moist convective parameters produce more realistic zonal banding of precipitation belts and a better representation of continental precipitation relative to the ocean. In addition, mass is more nearly conserved and mean sea level pressure and temperature patterns are in better agreement with observations than in the previous model. Major deficiencies in simulation that are not improved include zonal jet and stratospheric temperature structures. Overall, the improvements in simulation suggest a wider applicability of the low resolution model for use in climate sensitivity studies.
Analysis of sensitivity experiments assessing specific effects of parameterization indicate that decreased ground
Abstract
The sensitivity of a low resolution, spectral general circulation model (GCM) to specification of physical processes is examined using a new version of the model with refined parameterizations. Specific refinements in parameterization include: 1) smoothing the original orography to greatly diminish undesirable topographic “ripples” occurring near high mountain ranges; 2) adding snowcover on the Tibetan plateau and representing winter snowcover in middle latitudes more realistically; 3) decreasing the land ground wetness and adjusting the drag coefficient and parameters governing condensation-moist convective adjustment.
Results of comparative 5-year integrations show that better parameterization in the low resolution model produces significant improvement in simulation without resorting to the use of higher horizontal or vertical resolution. The combined changes in ground wetness, drag coefficient and condensation-moist convective parameters produce more realistic zonal banding of precipitation belts and a better representation of continental precipitation relative to the ocean. In addition, mass is more nearly conserved and mean sea level pressure and temperature patterns are in better agreement with observations than in the previous model. Major deficiencies in simulation that are not improved include zonal jet and stratospheric temperature structures. Overall, the improvements in simulation suggest a wider applicability of the low resolution model for use in climate sensitivity studies.
Analysis of sensitivity experiments assessing specific effects of parameterization indicate that decreased ground
Abstract
Sea temperature anomalies which departed from the December climatic mean by approximately 2C off the coast of Newfoundland were inserted into the NCAR six-layer, 5° mesh, general circulation model of the atmosphere in order to test the model's response to small perturbations in sea surface temperature. The response of the model to the anomalies was analyzed with respect to pressure patterns, heat flux, and cyclone frequency, path and intensity. This response was compared with a statistical analysis of the response of the atmosphere to similar sea temperature anomalies based on approximately 80 years of observations as described by Ratcliffe and Murray.
Analyses of the anomaly experiments are preceded by an analysis of the basic (control) statistics for both model and atmosphere. The most pronounced discrepancies between the two were noted in cyclone statistics. A calculation with double horizontal resolution greatly improved the model features. Detailed comparison was complicated by the fact that the model failed to achieve statistical stationarity.
The extensive verification data of Ratcliffe and Murray proved valuable in distinguishing meaningful anomaly responses from those that could be attributed to the many limitations in the model, including a pronounced natural variability. Both warm and cold anomaly cases were tested. Best agreement with observed data was obtained for the case of the warm anomaly; this agreement was most evident during the middle portions of the integrations and then only in the North Atlantic sector. The response in the case with a cold anomaly was not as satisfactory although there were clear distinctions between the warm and cold anomaly cases.
Abstract
Sea temperature anomalies which departed from the December climatic mean by approximately 2C off the coast of Newfoundland were inserted into the NCAR six-layer, 5° mesh, general circulation model of the atmosphere in order to test the model's response to small perturbations in sea surface temperature. The response of the model to the anomalies was analyzed with respect to pressure patterns, heat flux, and cyclone frequency, path and intensity. This response was compared with a statistical analysis of the response of the atmosphere to similar sea temperature anomalies based on approximately 80 years of observations as described by Ratcliffe and Murray.
Analyses of the anomaly experiments are preceded by an analysis of the basic (control) statistics for both model and atmosphere. The most pronounced discrepancies between the two were noted in cyclone statistics. A calculation with double horizontal resolution greatly improved the model features. Detailed comparison was complicated by the fact that the model failed to achieve statistical stationarity.
The extensive verification data of Ratcliffe and Murray proved valuable in distinguishing meaningful anomaly responses from those that could be attributed to the many limitations in the model, including a pronounced natural variability. Both warm and cold anomaly cases were tested. Best agreement with observed data was obtained for the case of the warm anomaly; this agreement was most evident during the middle portions of the integrations and then only in the North Atlantic sector. The response in the case with a cold anomaly was not as satisfactory although there were clear distinctions between the warm and cold anomaly cases.
Abstract
Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both over and downstream from the region depends strongly on the longitude of the prescribed changes. Many features of the response are statistically significant, although generally not over the continental United States. The amplitude and phase of the mid-tropospheric long waves (zonal wavenumbers 1–4) are also affected. The prescribed change response is largest and of greatest statistical significance when the prescribed change is very large (12°C maximum amplitude) but is also frequently detectable when the prescribed change is one-third as large (4°C maximum amplitude). A comparable experiment involving a prescribed North Atlantic OST change produces a similar mid-tropospheric response.
Abstract
Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both over and downstream from the region depends strongly on the longitude of the prescribed changes. Many features of the response are statistically significant, although generally not over the continental United States. The amplitude and phase of the mid-tropospheric long waves (zonal wavenumbers 1–4) are also affected. The prescribed change response is largest and of greatest statistical significance when the prescribed change is very large (12°C maximum amplitude) but is also frequently detectable when the prescribed change is one-third as large (4°C maximum amplitude). A comparable experiment involving a prescribed North Atlantic OST change produces a similar mid-tropospheric response.
Abstract
Intraseasonal fluctuations of satellite-based observations of outgoing longwave radiation (OLR) and NMC analyses of 250 mb zonal wind (U250) are described based on global data from nine Northern Hemisphere summers (May–October). Cross-spectral analysis of the 28–72 day spectral band is used to establish statistically significant relationships for the entire data period. Hovmöller diagrams are used to examine individual events and to estimate the oscillation's time scale and propagation characteristics.
Intraseasonal OLR fluctuations have their greatest amplitude in the Indian monsoon region and north of the equator in the western tropical Pacific. These two regions have out-of-phase fluctuations and appear to be linked by OLR anomalies propagating eastward (at 3–6 m s−1) along the equator between 50° and 160°E. The OLR oscillation has a preferred time scale of 30–60 days during May–October, based on a sample of more than 30 events. The initiation near the equator of northward-propagating (1–2 m s−1) OLR anomalies in the Indian monsoon region is also associated with the eastward-propagating equatorial OLR anomalies.
The U250 intraseasonal fluctuations have a prominent zonal wavenumber-one structure throughout the tropics with the exception of the Northern Hemisphere tropics over the Atlantic, Africa, and the Indian monsoon region. The U250 anomalies propagate eastward along 0°–10°S at about 6 m s−1 from 40° to 160°E and at about 15 m s−1 from 160°E to 0°W. These longitudinal changes in the oscillation's ground speed may be due in part to longitudinal changes in the zonal wind basic state. The 28–72 day U250 anomalies along 30°S (50°S) are out of phase (in phase) with the tropical U250 anomalies over most of the Pacific and Indian Ocean sectors.
The phase relationships between tropical OLR and U250 anomalies seem dynamically consistent, generally showing 250 mb u-component divergence flanking regions of convection. Although the eastward propagation of OLR anomalies along 5°N–5°S is not continuous around the globe, areas of significant coherence scattered throughout the tropics exhibit a zonal wavenumber-one phase structure. In these remote regions, OLR anomalies may be dynamically linked by an eastward-propagating tropical circulation feature.
Abstract
Intraseasonal fluctuations of satellite-based observations of outgoing longwave radiation (OLR) and NMC analyses of 250 mb zonal wind (U250) are described based on global data from nine Northern Hemisphere summers (May–October). Cross-spectral analysis of the 28–72 day spectral band is used to establish statistically significant relationships for the entire data period. Hovmöller diagrams are used to examine individual events and to estimate the oscillation's time scale and propagation characteristics.
Intraseasonal OLR fluctuations have their greatest amplitude in the Indian monsoon region and north of the equator in the western tropical Pacific. These two regions have out-of-phase fluctuations and appear to be linked by OLR anomalies propagating eastward (at 3–6 m s−1) along the equator between 50° and 160°E. The OLR oscillation has a preferred time scale of 30–60 days during May–October, based on a sample of more than 30 events. The initiation near the equator of northward-propagating (1–2 m s−1) OLR anomalies in the Indian monsoon region is also associated with the eastward-propagating equatorial OLR anomalies.
The U250 intraseasonal fluctuations have a prominent zonal wavenumber-one structure throughout the tropics with the exception of the Northern Hemisphere tropics over the Atlantic, Africa, and the Indian monsoon region. The U250 anomalies propagate eastward along 0°–10°S at about 6 m s−1 from 40° to 160°E and at about 15 m s−1 from 160°E to 0°W. These longitudinal changes in the oscillation's ground speed may be due in part to longitudinal changes in the zonal wind basic state. The 28–72 day U250 anomalies along 30°S (50°S) are out of phase (in phase) with the tropical U250 anomalies over most of the Pacific and Indian Ocean sectors.
The phase relationships between tropical OLR and U250 anomalies seem dynamically consistent, generally showing 250 mb u-component divergence flanking regions of convection. Although the eastward propagation of OLR anomalies along 5°N–5°S is not continuous around the globe, areas of significant coherence scattered throughout the tropics exhibit a zonal wavenumber-one phase structure. In these remote regions, OLR anomalies may be dynamically linked by an eastward-propagating tropical circulation feature.
Abstract
Ten years of outgoing longwave radiation (OLR) and 250 mb circulation data are used in a statistical study which concentrates on 28–72 day fluctuations during Northern Hemisphere winter. The results of spectral and cross-spectral analyses show that 28–72 day planetary-scale oscillations of OLR and 250 mb circulation are statistically significant features of the entire 10-year dataset.
The strongest OLR fluctuations at 28–72 day periods are located from the equator to 15°S and extend from about 60 to 160°E and in the vicinity of the South Pacific Convergence Zone (SPCZ). The streamfunction variance shows significant 28–72 day fluctuations over the subtropics of both hemispheres and over the extratropical North Atlantic.
The OLR anomalies propagate from west to east between 60 and 160°E at about 5 m s−1. There are statistically significant relationships between the regions of (inferred) equatorial cloudiness and planetary-scale circulation features. Fluctuations in the windfield near the exit regions of the east Asian and North American jets are important components of the life cycle of 28–72 day oscillations during northern winter. The life cycle also includes a prominent wavenumber 1 evolution which manifests itself synoptically as an eccentric circumpolar vortex, expanded in regions of enhanced equatorial cloudiness and contracted in regions of suppressed equatorial cloudiness.
Abstract
Ten years of outgoing longwave radiation (OLR) and 250 mb circulation data are used in a statistical study which concentrates on 28–72 day fluctuations during Northern Hemisphere winter. The results of spectral and cross-spectral analyses show that 28–72 day planetary-scale oscillations of OLR and 250 mb circulation are statistically significant features of the entire 10-year dataset.
The strongest OLR fluctuations at 28–72 day periods are located from the equator to 15°S and extend from about 60 to 160°E and in the vicinity of the South Pacific Convergence Zone (SPCZ). The streamfunction variance shows significant 28–72 day fluctuations over the subtropics of both hemispheres and over the extratropical North Atlantic.
The OLR anomalies propagate from west to east between 60 and 160°E at about 5 m s−1. There are statistically significant relationships between the regions of (inferred) equatorial cloudiness and planetary-scale circulation features. Fluctuations in the windfield near the exit regions of the east Asian and North American jets are important components of the life cycle of 28–72 day oscillations during northern winter. The life cycle also includes a prominent wavenumber 1 evolution which manifests itself synoptically as an eccentric circumpolar vortex, expanded in regions of enhanced equatorial cloudiness and contracted in regions of suppressed equatorial cloudiness.
