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Since 1966, two types of wave motions have been discovered in the tropical stratosphere. These have been identified with the two gravest modes of a family of equatorial waves. These waves are characterized by downward phase propagation, which renders them important in the vertical transport of energy and zonal momentum. In the tropical lower troposphere there exists a separate class containing wave modes which do not propagate vertically, one of these being the familiar easterly wave.
The role of these two classes of waves in the tropical general circulation is discussed and the possible energy sources for the waves are enumerated.
Since 1966, two types of wave motions have been discovered in the tropical stratosphere. These have been identified with the two gravest modes of a family of equatorial waves. These waves are characterized by downward phase propagation, which renders them important in the vertical transport of energy and zonal momentum. In the tropical lower troposphere there exists a separate class containing wave modes which do not propagate vertically, one of these being the familiar easterly wave.
The role of these two classes of waves in the tropical general circulation is discussed and the possible energy sources for the waves are enumerated.
Abstract
Rawinsonde data from the western Pacific region for the summer of 1967 are expanded in terms of complex empirical orthogonal functions and the results are compared with those of previous investigations based on the same data set. The results and conclusions are consistent with those based on spectrum analysis and compositing, except in a few cases where it has been possible to resolve ambiguities in the earlier work. The new method has been particularly helpful in clarifying the relationship between the mixed Rossby-gravity waves of the upper troposphere and the synoptic-scale, westward propagating waves of the lower troposphere. The new results indicate that these two disturbance types are so strongly coupled that it is not possible to separate them.
Abstract
Rawinsonde data from the western Pacific region for the summer of 1967 are expanded in terms of complex empirical orthogonal functions and the results are compared with those of previous investigations based on the same data set. The results and conclusions are consistent with those based on spectrum analysis and compositing, except in a few cases where it has been possible to resolve ambiguities in the earlier work. The new method has been particularly helpful in clarifying the relationship between the mixed Rossby-gravity waves of the upper troposphere and the synoptic-scale, westward propagating waves of the lower troposphere. The new results indicate that these two disturbance types are so strongly coupled that it is not possible to separate them.
Abstract
In large-scale wave disturbances in the lower stratosphere, the poleward and upward velocity components are positively correlated so that typical air trajectories, when projected onto the meridional plane, slope upward toward the pole. The slope of the air trajectories can be reconciled with the observed poleward, countergradient eddy heat flux at these levels if one takes into account the poleward acceleration of warm air in the wave troughs and the equatorward acceleration of cold air in the ridges. These temperature anomalies are produced by subsidence in the wave troughs and ascent in the ridges. The same processes are capable of producing poleward and downward eddy fluxes of potential vorticity, ozone, and other tracers whose values or concentrations increase rapidly with height.
Abstract
In large-scale wave disturbances in the lower stratosphere, the poleward and upward velocity components are positively correlated so that typical air trajectories, when projected onto the meridional plane, slope upward toward the pole. The slope of the air trajectories can be reconciled with the observed poleward, countergradient eddy heat flux at these levels if one takes into account the poleward acceleration of warm air in the wave troughs and the equatorward acceleration of cold air in the ridges. These temperature anomalies are produced by subsidence in the wave troughs and ascent in the ridges. The same processes are capable of producing poleward and downward eddy fluxes of potential vorticity, ozone, and other tracers whose values or concentrations increase rapidly with height.
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Abstract
Low-frequency variability in wintertime 500 mb height is examined, with emphasis on its structure, geographical distribution, and frequency dependence. A 39-year record of 500 mb geopotential height fields from the NMC analyses is time filtered to partition the fluctuations into frequency bands corresponding to periods of 10–60 days, 60–180 days and > 180 days. Winter is defined as the six month period November through April. Variance, teleconnectivity, and anisotropy fields, and selected loading vectors derived from orthogonal and oblique rotations of the eigenvectors of the temporal correlation matrix for each band are shown and discussed.
The variability in all frequency bands exhibits substantial anistropy, with meridionally elongated features arranged as zonally oriented wave trains prevailing over the continents and zonally elongated features organized in the form of north–south oriented dipole patterns prevailing over the oceanic sectors of the hemisphere. The wave trains are most pronounced in the 10–60 day variability, while the dipoles are most pronounced at lower frequencies. Eastward energy dispersion is apparent in the wave trains, but there is no evidence of phase propagation.
Most of the “teleconnection patterns” identified in previous studies appear among the more prominent loading vectors. However, in most cases the loading vectors occur in pairs, in which the two patterns are in spatial quadrature with one another and account for comparable fractions of the hemispherically integrated variance. It is argued that such patterns should be interpreted as basis functions that can be linearly combined to form a continuum of anisotropic structures. Evidence of the existence of discrete “modal structures” is found only in the interannual (> 180-day period) variability, where two patterns stand out clearly above the background continuum: the Pacific–North American (PNA) pattern and the North Atlantic Oscillation (NAO). These patterns leave clear imprints upon the climatological mean variance of the 500 mb height field and the anisotropy tensor of the 500 mb wine field. The western Atlantic (WA) pattern stands out somewhat above the background continuum in the month-to-month (60–180 day period) variability.
Abstract
Low-frequency variability in wintertime 500 mb height is examined, with emphasis on its structure, geographical distribution, and frequency dependence. A 39-year record of 500 mb geopotential height fields from the NMC analyses is time filtered to partition the fluctuations into frequency bands corresponding to periods of 10–60 days, 60–180 days and > 180 days. Winter is defined as the six month period November through April. Variance, teleconnectivity, and anisotropy fields, and selected loading vectors derived from orthogonal and oblique rotations of the eigenvectors of the temporal correlation matrix for each band are shown and discussed.
The variability in all frequency bands exhibits substantial anistropy, with meridionally elongated features arranged as zonally oriented wave trains prevailing over the continents and zonally elongated features organized in the form of north–south oriented dipole patterns prevailing over the oceanic sectors of the hemisphere. The wave trains are most pronounced in the 10–60 day variability, while the dipoles are most pronounced at lower frequencies. Eastward energy dispersion is apparent in the wave trains, but there is no evidence of phase propagation.
Most of the “teleconnection patterns” identified in previous studies appear among the more prominent loading vectors. However, in most cases the loading vectors occur in pairs, in which the two patterns are in spatial quadrature with one another and account for comparable fractions of the hemispherically integrated variance. It is argued that such patterns should be interpreted as basis functions that can be linearly combined to form a continuum of anisotropic structures. Evidence of the existence of discrete “modal structures” is found only in the interannual (> 180-day period) variability, where two patterns stand out clearly above the background continuum: the Pacific–North American (PNA) pattern and the North Atlantic Oscillation (NAO). These patterns leave clear imprints upon the climatological mean variance of the 500 mb height field and the anisotropy tensor of the 500 mb wine field. The western Atlantic (WA) pattern stands out somewhat above the background continuum in the month-to-month (60–180 day period) variability.
Abstract
Nonseasonal behavior of the North American Monsoon System (NAMS) during the summer months (June–September) is investigated, based on monthly mean satellite data and the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses for the period 1979–97. The leading principal component of the monthly mean precipitation over the domain 5°–35°N, 80°–125°W is used as a reference time series. The associated variations in fields other than the rainfall are identified based on linear regression analysis.
The reference time series is highly correlated with summer rainfall averaged over this broadly defined “NAMS domain” and is therefore referred to as the (spatially) “integrated precipitation index” (IPI). It is also correlated with hurricane activity over the tropical and subtropical northeast Pacific. In its positive polarity, the IPI is characterized by an intensification and northward expansion of the ITCZ toward the Mexican coast, with enhanced rainfall throughout Mexico, but particularly in the south and (in a relative sense) in the semiarid northwest. It has a well-defined sea level pressure and surface wind signature, with negative sea level pressure anomalies to the northwest of the region of enhanced rainfall and westerly wind anomalies converging into the region from the west along 10°–20°N. These features extend upward to the 500-hPa level and are overlaid by weak anomalies of opposing sign in the upper troposphere. There are indications of an associated deep barotropic planetary-wave signature over the United States, but the geopotential anomalies are only weakly correlated with the IPI. The IPI is only weakly correlated with summer rainfall over the southwestern United States, with ENSO, and with SST anomalies within the NAMS domain.
Abstract
Nonseasonal behavior of the North American Monsoon System (NAMS) during the summer months (June–September) is investigated, based on monthly mean satellite data and the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses for the period 1979–97. The leading principal component of the monthly mean precipitation over the domain 5°–35°N, 80°–125°W is used as a reference time series. The associated variations in fields other than the rainfall are identified based on linear regression analysis.
The reference time series is highly correlated with summer rainfall averaged over this broadly defined “NAMS domain” and is therefore referred to as the (spatially) “integrated precipitation index” (IPI). It is also correlated with hurricane activity over the tropical and subtropical northeast Pacific. In its positive polarity, the IPI is characterized by an intensification and northward expansion of the ITCZ toward the Mexican coast, with enhanced rainfall throughout Mexico, but particularly in the south and (in a relative sense) in the semiarid northwest. It has a well-defined sea level pressure and surface wind signature, with negative sea level pressure anomalies to the northwest of the region of enhanced rainfall and westerly wind anomalies converging into the region from the west along 10°–20°N. These features extend upward to the 500-hPa level and are overlaid by weak anomalies of opposing sign in the upper troposphere. There are indications of an associated deep barotropic planetary-wave signature over the United States, but the geopotential anomalies are only weakly correlated with the IPI. The IPI is only weakly correlated with summer rainfall over the southwestern United States, with ENSO, and with SST anomalies within the NAMS domain.
Abstract
A framework for interpreting the Pacific decadal oscillation (PDO) and ENSO indices is presented. The two leading principal components (PCs) of sea surface temperature [SST; strictly speaking, the departure from globally averaged SST (SST*)] over the entire Pacific basin comprise a two-dimensional phase space. A linear combination of these pan-Pacific PCs corresponding to a +45° rotation (designated by P) is nearly identical to the PDO, the leading PC of Pacific SST* poleward of 20°N. Both P and the PDO index exhibit apparent “regime shifts” on the interdecadal time scale. The orthogonal axis (rotated by −45° and designated by T′) is highly correlated with conventional ENSO indices, but its spatial regression pattern is more equatorially focused. SST variability along these two rotated axes exhibits sharply contrasting power spectra, the former (i.e., P) suggestive of “red noise” on time scales longer than a decade and the latter (i.e., T′) exhibiting a prominent spectral peak around 3–5 years. Hence, orthogonal indices representative of the ENSO cycle and ENSO-like decadal variability can be generated without resorting to filtering in the time domain. The methodology used here is the same as that used by Takahashi et al. to quantify the diversity of equatorial SST patterns in ENSO; they rotated the two leading EOFs of tropical Pacific SST, whereas the two leading EOFs of pan-Pacific SST* are rotated here.
Abstract
A framework for interpreting the Pacific decadal oscillation (PDO) and ENSO indices is presented. The two leading principal components (PCs) of sea surface temperature [SST; strictly speaking, the departure from globally averaged SST (SST*)] over the entire Pacific basin comprise a two-dimensional phase space. A linear combination of these pan-Pacific PCs corresponding to a +45° rotation (designated by P) is nearly identical to the PDO, the leading PC of Pacific SST* poleward of 20°N. Both P and the PDO index exhibit apparent “regime shifts” on the interdecadal time scale. The orthogonal axis (rotated by −45° and designated by T′) is highly correlated with conventional ENSO indices, but its spatial regression pattern is more equatorially focused. SST variability along these two rotated axes exhibits sharply contrasting power spectra, the former (i.e., P) suggestive of “red noise” on time scales longer than a decade and the latter (i.e., T′) exhibiting a prominent spectral peak around 3–5 years. Hence, orthogonal indices representative of the ENSO cycle and ENSO-like decadal variability can be generated without resorting to filtering in the time domain. The methodology used here is the same as that used by Takahashi et al. to quantify the diversity of equatorial SST patterns in ENSO; they rotated the two leading EOFs of tropical Pacific SST, whereas the two leading EOFs of pan-Pacific SST* are rotated here.
Abstract
Global temperature anomalies associated with ENSO are investigated, making use of a 13-year record of gridded temperature and precipitation data from the microwave sounding unit (MSU). The warm phase of the ENSO cycle during this period was characterized by an overall warming of the tropical troposphere, superimposed upon a distinctive equatorially symmetric dumbbell-shaped pattern straddling the equator near 140°W, accompanied by negative anomalies along the equator over the western Pacific. By means of singular value decomposition (SVD) analysis it is shown that this pattern fluctuated in phase with the displacements of convective activity over the equatorial Pacific, as reflected in the anomalies in outgoing longwave radiation (OLR) and MSU precipitation fields. Fluctuations in mean tropical tropospheric temperature lagged the OLR anomalies and the related temperature pattern by about 3 months. The same dumbbell-shaped pattern was evident, with reversed polarity, in the lower stratosphere, together with the zonally symmetric signature of the quasi-biennial oscillation.
The dumbbell-shaped temperature pattern is related to the off-equatorial upper-tropospheric gyres that have been identified in previous studies. It can be interpreted as the dynamical response to shifts in the distribution of diabatic heating in the equatorial belt. It resembles the linear response to an equatorial heat source, but its major centers of action are shifted slightly eastward. It is detectable in SVD analysis for each season, but appears to be best organized around March, the season in which the equatorial cold tongue is weakest and precipitation anomalies associated with the ENSO cycle impact the equatorial dry zone most strongly.
The fluctuations in mean tropical tropospheric temperature that occur in association with the ENSO cycle are highly coherent with the fluctuations in surface air temperature over the tropical landmasses and sea surface temperatures over the tropical Indian and North Atlantic Oceans. It is argued that these fluctuations represent a thermodynamic response to the perturbations in the surface heat balance induced by the ENSO cycle in the eastern equatorial Pacific. They can be simulated by means of a simple thermodynamic model with a linear damping, an empirically determined heat capacity, and forcing proportional to the observed sea surface temperature anomalies in the cold tongue region of the equatorial eastern Pacific.
The warming of the tropical troposphere is accompanied by a strengthening of the zonally averaged jet stream in both hemispheres induced by an intensified Hadley circulation.
Abstract
Global temperature anomalies associated with ENSO are investigated, making use of a 13-year record of gridded temperature and precipitation data from the microwave sounding unit (MSU). The warm phase of the ENSO cycle during this period was characterized by an overall warming of the tropical troposphere, superimposed upon a distinctive equatorially symmetric dumbbell-shaped pattern straddling the equator near 140°W, accompanied by negative anomalies along the equator over the western Pacific. By means of singular value decomposition (SVD) analysis it is shown that this pattern fluctuated in phase with the displacements of convective activity over the equatorial Pacific, as reflected in the anomalies in outgoing longwave radiation (OLR) and MSU precipitation fields. Fluctuations in mean tropical tropospheric temperature lagged the OLR anomalies and the related temperature pattern by about 3 months. The same dumbbell-shaped pattern was evident, with reversed polarity, in the lower stratosphere, together with the zonally symmetric signature of the quasi-biennial oscillation.
The dumbbell-shaped temperature pattern is related to the off-equatorial upper-tropospheric gyres that have been identified in previous studies. It can be interpreted as the dynamical response to shifts in the distribution of diabatic heating in the equatorial belt. It resembles the linear response to an equatorial heat source, but its major centers of action are shifted slightly eastward. It is detectable in SVD analysis for each season, but appears to be best organized around March, the season in which the equatorial cold tongue is weakest and precipitation anomalies associated with the ENSO cycle impact the equatorial dry zone most strongly.
The fluctuations in mean tropical tropospheric temperature that occur in association with the ENSO cycle are highly coherent with the fluctuations in surface air temperature over the tropical landmasses and sea surface temperatures over the tropical Indian and North Atlantic Oceans. It is argued that these fluctuations represent a thermodynamic response to the perturbations in the surface heat balance induced by the ENSO cycle in the eastern equatorial Pacific. They can be simulated by means of a simple thermodynamic model with a linear damping, an empirically determined heat capacity, and forcing proportional to the observed sea surface temperature anomalies in the cold tongue region of the equatorial eastern Pacific.
The warming of the tropical troposphere is accompanied by a strengthening of the zonally averaged jet stream in both hemispheres induced by an intensified Hadley circulation.
Abstract
Clusters in the Northern Hemisphere wintertime, 10-day low-pass-filtered 500-hPa height field are identified using the method of self-organizing maps (SOMs). Results are based on 1) a 57-winter record of ERA and 2) a 93-winter record of the NOAA Twentieth-Century Reanalysis (20CR). The clusters derived from SOMs appear to be more robust and more linearly independent than their counterparts derived from Ward’s method, and clusters with comparable numbers of member days are more distinctive in terms of the standardized Euclidean distances of their centroids from the centroid of the dataset. The reproducible SOM clusters in the hemispheric domain are 1) the negative polarity of the North Atlantic Oscillation (NAO), 2) a pattern suggestive of Alaska blocking with a downstream wave train extending over North America and the North Atlantic, 3) an enhancement of the climatological-mean stationary wave pattern in the Western Hemisphere that projects positively upon the Pacific–North America (PNA) pattern, and 4) a pattern that projects upon the negative polarity of the PNA pattern. The first three patterns have important impacts on the wintertime climate in North America and Europe. In particular, they are helpful in interpreting prevailing flow patterns during the exceptional winters of 1930–31, 2009–10, and 2013–14. Because of the very limited number of independent samples in a single winter, the number of days per winter in which the circulation resides within individual clusters varies erratically from winter to winter, rendering attribution difficult.
Abstract
Clusters in the Northern Hemisphere wintertime, 10-day low-pass-filtered 500-hPa height field are identified using the method of self-organizing maps (SOMs). Results are based on 1) a 57-winter record of ERA and 2) a 93-winter record of the NOAA Twentieth-Century Reanalysis (20CR). The clusters derived from SOMs appear to be more robust and more linearly independent than their counterparts derived from Ward’s method, and clusters with comparable numbers of member days are more distinctive in terms of the standardized Euclidean distances of their centroids from the centroid of the dataset. The reproducible SOM clusters in the hemispheric domain are 1) the negative polarity of the North Atlantic Oscillation (NAO), 2) a pattern suggestive of Alaska blocking with a downstream wave train extending over North America and the North Atlantic, 3) an enhancement of the climatological-mean stationary wave pattern in the Western Hemisphere that projects positively upon the Pacific–North America (PNA) pattern, and 4) a pattern that projects upon the negative polarity of the PNA pattern. The first three patterns have important impacts on the wintertime climate in North America and Europe. In particular, they are helpful in interpreting prevailing flow patterns during the exceptional winters of 1930–31, 2009–10, and 2013–14. Because of the very limited number of independent samples in a single winter, the number of days per winter in which the circulation resides within individual clusters varies erratically from winter to winter, rendering attribution difficult.
Abstract
Hierarchical cluster analysis based on the method of Ward is performed on the Northern Hemisphere wintertime 10-day low-pass-filtered 500-hPa height field, using the NMC operational analyses for the period 1946–85. Input data are gridded fields at 5-day intervals, a total of 702 maps, each with 445 grid points. The measure of similarity between maps is the squared height difference, averaged over all grid points; that is, the squared “distance” between the maps in multidimensional phase space. The closest two of the 702 maps are merged to form a cluster that, in subsequent calculations, replaces the maps from which it was formed. This procedure (modified slightly, to deal with the differing numbers of maps in the clusters) is repeated 701 times until all the maps have been merged to form a single cluster whose centroid corresponds to the climatological mean map. The two clusters involved in the final merger, the pair of smaller clusters that merged to form each of them, and so on, are represented in terms of a “family tree” that is traced back to the point where the clusters become too small to be of practical interest. The reproducibility of the larger clusters is compared by seeing how well various ones are replicated when the analysis is repeated on randomly chosen halves of the dataset in an ensemble of 50 runs.
The three most reproducible clusters, which together account for ∼⅓ of the 702 maps in the dataset, can be reconstructed remarkably well from linear combinations of the two leading EOFs of the covariance matrix. They are related to features of the probability density function (PDF) in a two-dimensional phase space defined by the expansion coefficients of these EOFs. One is marked by a closed anticyclone over the southern tip of Greenland, one by a ridge over the Gulf of Alaska, and one by a ridge over the Rockies. In comparison to other clusters of comparable size, their centroids are conspicuously far from the climatological mean map. Positive 500-hPa height anomalies in excess of 200 m are observed in association with the first two clusters, over regions of large variance and strong positive skewness of the 500-hPa height field. Occurrences of these two clusters have often been marked by extreme cold over parts of North America. Similar clusters are obtained when the analysis is performed on the Pacific/North American and Atlantic/European sectors of the hemisphere. The results are compared with those obtained in other studies, based on a variety of analysis techniques.
Abstract
Hierarchical cluster analysis based on the method of Ward is performed on the Northern Hemisphere wintertime 10-day low-pass-filtered 500-hPa height field, using the NMC operational analyses for the period 1946–85. Input data are gridded fields at 5-day intervals, a total of 702 maps, each with 445 grid points. The measure of similarity between maps is the squared height difference, averaged over all grid points; that is, the squared “distance” between the maps in multidimensional phase space. The closest two of the 702 maps are merged to form a cluster that, in subsequent calculations, replaces the maps from which it was formed. This procedure (modified slightly, to deal with the differing numbers of maps in the clusters) is repeated 701 times until all the maps have been merged to form a single cluster whose centroid corresponds to the climatological mean map. The two clusters involved in the final merger, the pair of smaller clusters that merged to form each of them, and so on, are represented in terms of a “family tree” that is traced back to the point where the clusters become too small to be of practical interest. The reproducibility of the larger clusters is compared by seeing how well various ones are replicated when the analysis is repeated on randomly chosen halves of the dataset in an ensemble of 50 runs.
The three most reproducible clusters, which together account for ∼⅓ of the 702 maps in the dataset, can be reconstructed remarkably well from linear combinations of the two leading EOFs of the covariance matrix. They are related to features of the probability density function (PDF) in a two-dimensional phase space defined by the expansion coefficients of these EOFs. One is marked by a closed anticyclone over the southern tip of Greenland, one by a ridge over the Gulf of Alaska, and one by a ridge over the Rockies. In comparison to other clusters of comparable size, their centroids are conspicuously far from the climatological mean map. Positive 500-hPa height anomalies in excess of 200 m are observed in association with the first two clusters, over regions of large variance and strong positive skewness of the 500-hPa height field. Occurrences of these two clusters have often been marked by extreme cold over parts of North America. Similar clusters are obtained when the analysis is performed on the Pacific/North American and Atlantic/European sectors of the hemisphere. The results are compared with those obtained in other studies, based on a variety of analysis techniques.
