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Abstract
The observed wintertime intraseasonal variability of the Northern Hemisphere midtropospheric circulation is analyzed within the framework of an equivalent barotropic model. The analysis centers on the wave domain empirical orthogonal functions (E0Fs) of the 500 mb streamfunction anomalies. The projection of the dynamical model onto the EOFs leads to a system of quadratically nonlinear equations involving the EOF coefficients.
A major result of this study is the identification of the barotropically unstable wintertime mean flow as a potentially important energy source for some of the dominant low frequency EOFS. These EOFS are associated with such hemispheric scale variations as an index cycle, the Pacific/North American pattern, and the North Atlantic Oscillation.
The dominant EOFs are most strongly influenced by nonlinear interactions and this decreases as one goes to the higher order modes. In contrast, the beta and mean flow-EOF interaction terms (which are highly negatively correlated) have a relatively weak influence on the first few EOFs while the strongest influence is on the intermediate EOFs (15–25). The parameterized terms (orography, friction and long-wave correction) have a secondary effect on the EOFs when compared to the advection terms.
Generally, the dominant EOFs represent more unstable flow regimes when compared with the time mean flow. The negative instance of EOF 9, which resembles North Atlantic blocking, is particularly unstable and an inspection of the time mean EOF interactions supports the idea that interactions with the dominant low-frequency modes act to destroy this pattern. In the present model, blocking is most likely to occur as a quasi-linear response to the inhomogeneous forcing which enters into the model as a balance requirement of the time-averaged horizontal flow.
Abstract
The observed wintertime intraseasonal variability of the Northern Hemisphere midtropospheric circulation is analyzed within the framework of an equivalent barotropic model. The analysis centers on the wave domain empirical orthogonal functions (E0Fs) of the 500 mb streamfunction anomalies. The projection of the dynamical model onto the EOFs leads to a system of quadratically nonlinear equations involving the EOF coefficients.
A major result of this study is the identification of the barotropically unstable wintertime mean flow as a potentially important energy source for some of the dominant low frequency EOFS. These EOFS are associated with such hemispheric scale variations as an index cycle, the Pacific/North American pattern, and the North Atlantic Oscillation.
The dominant EOFs are most strongly influenced by nonlinear interactions and this decreases as one goes to the higher order modes. In contrast, the beta and mean flow-EOF interaction terms (which are highly negatively correlated) have a relatively weak influence on the first few EOFs while the strongest influence is on the intermediate EOFs (15–25). The parameterized terms (orography, friction and long-wave correction) have a secondary effect on the EOFs when compared to the advection terms.
Generally, the dominant EOFs represent more unstable flow regimes when compared with the time mean flow. The negative instance of EOF 9, which resembles North Atlantic blocking, is particularly unstable and an inspection of the time mean EOF interactions supports the idea that interactions with the dominant low-frequency modes act to destroy this pattern. In the present model, blocking is most likely to occur as a quasi-linear response to the inhomogeneous forcing which enters into the model as a balance requirement of the time-averaged horizontal flow.
Abstract
The winter-time circulation of the Northern Hemisphere is examined for the following time-scale classes. A) greater than 45 days, B)20–45 days C) 10–20 days D) 6–10 days and E) 2.5&ndash 6 days. The spatial structure of variability within each of these frequency buds is determined by an empirical orthogonal function expansion of the coupled vertical mean and difference streamfunction fields. In general, the dominant low-frequency modes (clan or filter A) exhibit zonally elongted patterns with an equivalent barotropic structure. The intermediate time scale modes (classes B and C) show a tendency for more circular anomaly patterns accompanied by a more baroclinic (westward tilt with height) structure for the class C modes. The dominant frequency modes (classes D and E) are characterized by a strong baroclinic component. The latter exhibit meridionally elongated patterns in the storm track regions together with an upstream tilt to the north and south of the jet exits. In general, lag cross correlations between dominant low-frequency modes tend to be small and/ or symmetric about zero. In contrast, the highest frequency modes exhibit highly asymmetric sinusoidal crow correlation functions reminiscent of travelling waves, while some intermediate and short-time scale modes exhibit correlations suggesting an association with the decaying phases of blocking in the Pacific.
The winter average barotropic and baroclinic energy sources and sinks associated with the time-mean winter flow am examined in the context of a two-layer quasi-geostrophic model. The mean flow kinetic energy (KE) conversion terms reflect the geometry of the anomalies such that the dominant low-frequency modes gain energy, the intermediate frequencies are approximately neutral and the higher frequency modes low energy to the mean flow. The magnitude of the KE conversion is largest in the upper troposphere and is dominated by the lowest-frequency modes. The magnitude conversion from the mean flow to the anomalies is positive for the dominant modes in each band. However, the highest frequency modes associated with traveling waves in the North Atlantic and North Pacific are most efficient in the conversion process. For a typical Rossby radius of deformation, it is found that the net barotropic and baroclinic conversions are of comparable magnitude for periods greater than 10 days while the baroclinic conversion dominates for shorter periods. The majority of the conversions are accomplished by just a few (∼ five) of the dominant EOFs in each frequency band.
Abstract
The winter-time circulation of the Northern Hemisphere is examined for the following time-scale classes. A) greater than 45 days, B)20–45 days C) 10–20 days D) 6–10 days and E) 2.5&ndash 6 days. The spatial structure of variability within each of these frequency buds is determined by an empirical orthogonal function expansion of the coupled vertical mean and difference streamfunction fields. In general, the dominant low-frequency modes (clan or filter A) exhibit zonally elongted patterns with an equivalent barotropic structure. The intermediate time scale modes (classes B and C) show a tendency for more circular anomaly patterns accompanied by a more baroclinic (westward tilt with height) structure for the class C modes. The dominant frequency modes (classes D and E) are characterized by a strong baroclinic component. The latter exhibit meridionally elongated patterns in the storm track regions together with an upstream tilt to the north and south of the jet exits. In general, lag cross correlations between dominant low-frequency modes tend to be small and/ or symmetric about zero. In contrast, the highest frequency modes exhibit highly asymmetric sinusoidal crow correlation functions reminiscent of travelling waves, while some intermediate and short-time scale modes exhibit correlations suggesting an association with the decaying phases of blocking in the Pacific.
The winter average barotropic and baroclinic energy sources and sinks associated with the time-mean winter flow am examined in the context of a two-layer quasi-geostrophic model. The mean flow kinetic energy (KE) conversion terms reflect the geometry of the anomalies such that the dominant low-frequency modes gain energy, the intermediate frequencies are approximately neutral and the higher frequency modes low energy to the mean flow. The magnitude of the KE conversion is largest in the upper troposphere and is dominated by the lowest-frequency modes. The magnitude conversion from the mean flow to the anomalies is positive for the dominant modes in each band. However, the highest frequency modes associated with traveling waves in the North Atlantic and North Pacific are most efficient in the conversion process. For a typical Rossby radius of deformation, it is found that the net barotropic and baroclinic conversions are of comparable magnitude for periods greater than 10 days while the baroclinic conversion dominates for shorter periods. The majority of the conversions are accomplished by just a few (∼ five) of the dominant EOFs in each frequency band.
Abstract
The relationship between total ozone and tropopause pressure is analyzed using 4 years (1979–82) of Nimbus-7 total ozone data and NMC global analyses of tropopause on a 5° by 5° grid. The fields are separated into medium (synoptic) and large spatial scales via a spherical harmonic expansion. The global distribution of variability and correlation are presented for each season. The large-scale analysis is based primarily on data from 1979 due to pronounced temporal inhomogeneities in the tropical tropopause data.
The synoptic scales show strong correlations (>0.6) in the middle latitudes of both hemispheres with a rapid equatorward drop and a more gradual poleward decline: a similar dependence on latitude is found using tropopause values derived directly from station data. Within a season, the areas of highest correlation tend to be associated with the regions of maximum variance of the storm track regions. In contrast, the seasonal dependence is such that the summer hemispheres tend to have the most extensive regions of high correlation while the more energetic winter seasons have the smallest. A frequency analysis (limited to time scales longer than 3 days) of selected regions indicates that in middle latitudes synoptic-scale fluctuations of total ozone and tropopause pressure exhibit generally similar distributions in power and no significant phase differences: equatorward the coherence drops rapidly at all frequencies.
Nonseasonal fluctuations of the large-scale fields generally show weak correlations (<0.6) everywhere. A major exception is the springtime middle latitude South Pacific. The strongest correspondence between large-scale ozone and tropopause pressure fields involves long period (seasonal) fluctuations in high latitudes. Over Antarctica the coupling is strongest in middle and late spring in association with the spring warming while the decrease in total ozone in early spring shows no apparent relation to tropopause variations.
Abstract
The relationship between total ozone and tropopause pressure is analyzed using 4 years (1979–82) of Nimbus-7 total ozone data and NMC global analyses of tropopause on a 5° by 5° grid. The fields are separated into medium (synoptic) and large spatial scales via a spherical harmonic expansion. The global distribution of variability and correlation are presented for each season. The large-scale analysis is based primarily on data from 1979 due to pronounced temporal inhomogeneities in the tropical tropopause data.
The synoptic scales show strong correlations (>0.6) in the middle latitudes of both hemispheres with a rapid equatorward drop and a more gradual poleward decline: a similar dependence on latitude is found using tropopause values derived directly from station data. Within a season, the areas of highest correlation tend to be associated with the regions of maximum variance of the storm track regions. In contrast, the seasonal dependence is such that the summer hemispheres tend to have the most extensive regions of high correlation while the more energetic winter seasons have the smallest. A frequency analysis (limited to time scales longer than 3 days) of selected regions indicates that in middle latitudes synoptic-scale fluctuations of total ozone and tropopause pressure exhibit generally similar distributions in power and no significant phase differences: equatorward the coherence drops rapidly at all frequencies.
Nonseasonal fluctuations of the large-scale fields generally show weak correlations (<0.6) everywhere. A major exception is the springtime middle latitude South Pacific. The strongest correspondence between large-scale ozone and tropopause pressure fields involves long period (seasonal) fluctuations in high latitudes. Over Antarctica the coupling is strongest in middle and late spring in association with the spring warming while the decrease in total ozone in early spring shows no apparent relation to tropopause variations.
Abstract
Numerous studies suggest that local feedback of surface evaporation on precipitation, known recycling, is a significant source of water for precipitation. Quantitative results on the exact amount of recycling have been difficult to obtain in view of the inherent limitations of diagnostic recycling calculations. The current study describes a calculation of the amount of local and remote geographic sources of surface evaporation for precipitation, based on the implementation of three-dimensional constituent tracers of regional water vapor sources [termed “water vapor tracers” (WVTs)] in a general circulation model. The major limitation on the accuracy of the recycling estimates is the veracity of the numerically simulated hydrological cycle, though it is noted that this approach also can be implemented within the context of a data assimilation system. In the WVT approach, each tracer is associated with an evaporative source region for a prognostic three-dimensional variable that represents a partial amount of the total atmospheric water vapor. The physical processes that act on a WVT are determined in proportion to those that act on the model's prognostic water vapor. In this way, the local and remote sources of water for precipitation can be predicted within the model simulation and validated against the model's prognostic water vapor. As a demonstration of the method, the regional hydrologic cycles for North America and India are evaluated for six summers (June, July, and August) of model simulation. More than 50% of the precipitation in the midwestern United States came from continental regional sources, and the local source was the largest of the regional tracers (14%). The Gulf of Mexico and Atlantic regions contributed 18% of the water for midwestern precipitation, but further analysis suggests that the greater region of the tropical Atlantic Ocean may also contribute significantly. In most North American continental regions, the local source of precipitation is correlated with total precipitation. There is a general positive correlation between local evaporation and local precipitation, but it can be weaker because large evaporation can occur when precipitation is inhibited. In India, the local source of precipitation is a small percentage of the precipitation, owing to the dominance of the atmospheric transport of oceanic water. The southern Indian Ocean provides a key source of water for both the Indian continent and the Sahelian region.
Abstract
Numerous studies suggest that local feedback of surface evaporation on precipitation, known recycling, is a significant source of water for precipitation. Quantitative results on the exact amount of recycling have been difficult to obtain in view of the inherent limitations of diagnostic recycling calculations. The current study describes a calculation of the amount of local and remote geographic sources of surface evaporation for precipitation, based on the implementation of three-dimensional constituent tracers of regional water vapor sources [termed “water vapor tracers” (WVTs)] in a general circulation model. The major limitation on the accuracy of the recycling estimates is the veracity of the numerically simulated hydrological cycle, though it is noted that this approach also can be implemented within the context of a data assimilation system. In the WVT approach, each tracer is associated with an evaporative source region for a prognostic three-dimensional variable that represents a partial amount of the total atmospheric water vapor. The physical processes that act on a WVT are determined in proportion to those that act on the model's prognostic water vapor. In this way, the local and remote sources of water for precipitation can be predicted within the model simulation and validated against the model's prognostic water vapor. As a demonstration of the method, the regional hydrologic cycles for North America and India are evaluated for six summers (June, July, and August) of model simulation. More than 50% of the precipitation in the midwestern United States came from continental regional sources, and the local source was the largest of the regional tracers (14%). The Gulf of Mexico and Atlantic regions contributed 18% of the water for midwestern precipitation, but further analysis suggests that the greater region of the tropical Atlantic Ocean may also contribute significantly. In most North American continental regions, the local source of precipitation is correlated with total precipitation. There is a general positive correlation between local evaporation and local precipitation, but it can be weaker because large evaporation can occur when precipitation is inhibited. In India, the local source of precipitation is a small percentage of the precipitation, owing to the dominance of the atmospheric transport of oceanic water. The southern Indian Ocean provides a key source of water for both the Indian continent and the Sahelian region.
Abstract
A method is demonstrated for evaluating global and zonally averaged heat balance statistics based on a four-dimensional assimilation with an atmospheric general circulation model (GCM). The procedure, which provides observationally constrained model diagnostics, uses the GCM of NASA's Goddard Laboratory for Atmospheric Sciences to evaluate the atmospheric heat balance for the February 1976 Data Systems Test period. The global distribution of the adiabatic and diabatic components of the heat balance are obtained by sampling the continuous GCM assimilation shortly after the insertion of conventional synoptic observations. Sampling times of 6 and 9 h after data insertion were chosen to provide adequate damping of high-frequency oscillations in the vertical velocity field caused by the data insertion.
Salient features of the February 1976 analysis include the following: Maximum rising motion in the mean vertical velocity field at 500 mb over South America, south-central Africa, Australia and the Indonesian archipelago. These regions also were characterized by large values of diabatic heating due to convective latent heat release. The cyclogenetically active regions over the North Atlantic and North Pacific oceans were characterized by maxima in latent heat release due to supersaturation cloud formation, and also maxima in the upward and northward transient eddy heat fluxes. In contrast, the continental west coasts showed a tendency for large downward and southward transient eddy beat fluxes.
Some differences are obtained between the heating rates calculated with the model parameterizations and through a residual method. Other shortcomings of the procedure include data deficiencies in the Southern Hemisphere, which cause the results to be comparatively more model dependent in the high southern latitudes.
The potential applicability of this method of analysis to the recently acquired FGGE data is noted.
Abstract
A method is demonstrated for evaluating global and zonally averaged heat balance statistics based on a four-dimensional assimilation with an atmospheric general circulation model (GCM). The procedure, which provides observationally constrained model diagnostics, uses the GCM of NASA's Goddard Laboratory for Atmospheric Sciences to evaluate the atmospheric heat balance for the February 1976 Data Systems Test period. The global distribution of the adiabatic and diabatic components of the heat balance are obtained by sampling the continuous GCM assimilation shortly after the insertion of conventional synoptic observations. Sampling times of 6 and 9 h after data insertion were chosen to provide adequate damping of high-frequency oscillations in the vertical velocity field caused by the data insertion.
Salient features of the February 1976 analysis include the following: Maximum rising motion in the mean vertical velocity field at 500 mb over South America, south-central Africa, Australia and the Indonesian archipelago. These regions also were characterized by large values of diabatic heating due to convective latent heat release. The cyclogenetically active regions over the North Atlantic and North Pacific oceans were characterized by maxima in latent heat release due to supersaturation cloud formation, and also maxima in the upward and northward transient eddy heat fluxes. In contrast, the continental west coasts showed a tendency for large downward and southward transient eddy beat fluxes.
Some differences are obtained between the heating rates calculated with the model parameterizations and through a residual method. Other shortcomings of the procedure include data deficiencies in the Southern Hemisphere, which cause the results to be comparatively more model dependent in the high southern latitudes.
The potential applicability of this method of analysis to the recently acquired FGGE data is noted.
Abstract
Intraseasonal (20–70 day) variability is examined in the Atlantic region during Northern Hemisphere winter using ECMWF analyses and NOAA outgoing longwave radiation (OLR). It is found that the dominant 200-mb zonal-wind fluctuation over the tropical Atlantic, A1, is related to global-scale circulation anomalies with their origins in the Pacific. Compositing techniques are used to investigate the nature of the Pacific-Atlantic teleconnections and related changes in the tropical OLR and moisture convergence.
The OLR anomalies associated with A1 are characterized by eastward propagation over the Indian Ocean and the western Pacific and a standing oscillation over the tropical Atlantic; the latter extends from Northeast Brazil to West Africa and is the dominant component of the Atlantic OLR variability on these time scales. An analysis of the velocity potential and moisture convergence fields suggests that the fluctuations in convection are coupled between the western Pacific and Atlantic via large-scale (zonal wavenumber 1), equatorially trapped, eastward-propagating waves associated with the Madden-Julian oscillation.
The zonal-wind fluctuation, A1, is also related to extratropical waves propagating into the tropics from both the Northern and Southern hemispheres. The Southern Hemispheric wave train, which makes up the dominant contribution to the A1 circulation pattern, appears to emanate from the western South Pacific and amplifies near the west coast of South America. The Northern Hemispheric wave train resembles the Pacific/North American pattern and emanates from the central North Pacific near the East Asian jet exit region.
These results suggest that a major component of the 20–70-day variability over the Atlantic region is remotely forced. The forcing occurs via the Madden-Julian oscillation, which is strongly coupled with eastward-migrating heating anomalies in the western Pacific and Rossby wave trains, which appear to have their origins in the middle latitudes of the Pacific. The Northern Hemispheric wave train appears to be maintained by energy exchange with the East Asian jet, while the nature of the Southern Hemispheric branch is unclear.
Abstract
Intraseasonal (20–70 day) variability is examined in the Atlantic region during Northern Hemisphere winter using ECMWF analyses and NOAA outgoing longwave radiation (OLR). It is found that the dominant 200-mb zonal-wind fluctuation over the tropical Atlantic, A1, is related to global-scale circulation anomalies with their origins in the Pacific. Compositing techniques are used to investigate the nature of the Pacific-Atlantic teleconnections and related changes in the tropical OLR and moisture convergence.
The OLR anomalies associated with A1 are characterized by eastward propagation over the Indian Ocean and the western Pacific and a standing oscillation over the tropical Atlantic; the latter extends from Northeast Brazil to West Africa and is the dominant component of the Atlantic OLR variability on these time scales. An analysis of the velocity potential and moisture convergence fields suggests that the fluctuations in convection are coupled between the western Pacific and Atlantic via large-scale (zonal wavenumber 1), equatorially trapped, eastward-propagating waves associated with the Madden-Julian oscillation.
The zonal-wind fluctuation, A1, is also related to extratropical waves propagating into the tropics from both the Northern and Southern hemispheres. The Southern Hemispheric wave train, which makes up the dominant contribution to the A1 circulation pattern, appears to emanate from the western South Pacific and amplifies near the west coast of South America. The Northern Hemispheric wave train resembles the Pacific/North American pattern and emanates from the central North Pacific near the East Asian jet exit region.
These results suggest that a major component of the 20–70-day variability over the Atlantic region is remotely forced. The forcing occurs via the Madden-Julian oscillation, which is strongly coupled with eastward-migrating heating anomalies in the western Pacific and Rossby wave trains, which appear to have their origins in the middle latitudes of the Pacific. The Northern Hemispheric wave train appears to be maintained by energy exchange with the East Asian jet, while the nature of the Southern Hemispheric branch is unclear.
Abstract
Average predictability and error growth are studied in a realistic two-level general circulation model of the atmosphere via a series of Monte Carlo experiments for fixed external forcing (perpetual winter in the Northern Hemisphere). For realistic initial errors, the dependence of the limit of dynamic predictability on total wave number is similar to that found for the ECMWF model for 1980/81 winter conditions, with the lowest wavenumbers showing significant skill for forecast ranges of more than 1 month. For very small amplitude error (1.2 m rms height at 500 mb) distributed according to the climate spectrum, the total error growth is superexponential, reaching a maximum growth rate (2-day doubling time) in about 1 week.
A simple empirical model of error variance involving two broad wavenumber bands (large scales: n < 10 and small scales: 10 ≤ n ≤ 15), provides an excellent fit of the GCM's error growth behavior. The interpretation of the empirical model, based on an analogy with the stochastic dynamic equations developed by Epstein, suggests that the initial rapid increase in the growth rate of errors in the large scales is primarily due to interactions with the small-scale error. These interactions have preferred geographical locations associated with the position of the climate mean jet streams. However, the error growth of the small scales is large unaffected by the presence of the large-scale error. The initial strong growth rate (2-day doubling time) of the small scales is attributed to the model's high level of eddy activity.
Abstract
Average predictability and error growth are studied in a realistic two-level general circulation model of the atmosphere via a series of Monte Carlo experiments for fixed external forcing (perpetual winter in the Northern Hemisphere). For realistic initial errors, the dependence of the limit of dynamic predictability on total wave number is similar to that found for the ECMWF model for 1980/81 winter conditions, with the lowest wavenumbers showing significant skill for forecast ranges of more than 1 month. For very small amplitude error (1.2 m rms height at 500 mb) distributed according to the climate spectrum, the total error growth is superexponential, reaching a maximum growth rate (2-day doubling time) in about 1 week.
A simple empirical model of error variance involving two broad wavenumber bands (large scales: n < 10 and small scales: 10 ≤ n ≤ 15), provides an excellent fit of the GCM's error growth behavior. The interpretation of the empirical model, based on an analogy with the stochastic dynamic equations developed by Epstein, suggests that the initial rapid increase in the growth rate of errors in the large scales is primarily due to interactions with the small-scale error. These interactions have preferred geographical locations associated with the position of the climate mean jet streams. However, the error growth of the small scales is large unaffected by the presence of the large-scale error. The initial strong growth rate (2-day doubling time) of the small scales is attributed to the model's high level of eddy activity.
Abstract
Low-frequency (20–70 day) variability is examined during Northern Hemisphere (NH) winter based on seven Years (1981–87) of European Centre for Medium Range Weather Forecasts initialized analyses. The dominant 200 mb zonal wind fluctuations in the Pacific sector, determined from an empirical orthogonal function (EOF) analysis, provide the baseline modes of atmospheric variability, which are related to fluctuations in other circulation parameters and outgoing longwave radiation (OLR). The composite circulation associated with the extreme phases of the zonal wind modes are examined for differences in forcing, wave propagation characteristics and stability.
The dominant upper level zonal wind fluctuation (EOF Z1) is associated with an expanded (contracted) region of easterlies in the tropical western Pacific and changes in the shape and intensity of the subtropical jets. The anomalous (difference between composites) eddy streamfunction at 200 mb shows an enhanced pair of anticyclones (cyclones) straddling the equator. These fluctuations are strongly coupled with eastward traveling tropical convection in the western Pacific with a time scale of about 40 days. The composite circulations show marked differences in the propagation of wave activity in the NH at 200 mb. The low phase (reduced easterlies) shows strong propagation away from the dominant source region over East Asia into the tropical western Pacific in conjunction with what appears to be significant reflection from the equatorward flank of the subtropical jet. In contrast, the high composite (enhanced easterlies) shows much weaker equatorward propagation together with reduced vertical propagation over East Asia and the western North Pacific.
The second zonal wind EOF (Z2) displays a more asymmetric structure with respect to the equator, describing a simultaneous decrease (increase) in the easterly extent of the East Asian jet and increase (decrease) in the strength of the jet over southern Australia. The anomalous eddy stream function at 200 mb shows wave trains apparently emanating from the tropical central Pacific extending into both hemispheres: in the winter hemisphere this resembles the Pacific-North American (PNA) pattern. These fluctuations show some coupling with preceding tropical convection anomalies in the western and central Pacific. Stability calculations show that the PNA pattern is maintained through barotropic energy exchanges with the mean flow. For the low composite, an enhanced source of stationary wave activity in the Gulf of Alaska is associated with an increase in synoptic-scale eddy activity.
These results suggest that tropical convection in the western Pacific has a strong modifying influence on (extratropically forced) middle latitude low-frequency variability. The influence is primarily indirect via zonal wind changes which influence the propagation of waves originating in middle latitudes. The zonal wind changes include those associated with the strength and extent of the tropical easterlies as well as more subtle (but important) changes which effect the curvature of the East Asian jet leading in some instances to turning points for middle latitude waves. The PNA also appears to have its main energy source in middle latitudes and in this case the link with the tropics appears to be more tied to phase locking with anomalies forced by tropical convection in the western and central Pacific.
Abstract
Low-frequency (20–70 day) variability is examined during Northern Hemisphere (NH) winter based on seven Years (1981–87) of European Centre for Medium Range Weather Forecasts initialized analyses. The dominant 200 mb zonal wind fluctuations in the Pacific sector, determined from an empirical orthogonal function (EOF) analysis, provide the baseline modes of atmospheric variability, which are related to fluctuations in other circulation parameters and outgoing longwave radiation (OLR). The composite circulation associated with the extreme phases of the zonal wind modes are examined for differences in forcing, wave propagation characteristics and stability.
The dominant upper level zonal wind fluctuation (EOF Z1) is associated with an expanded (contracted) region of easterlies in the tropical western Pacific and changes in the shape and intensity of the subtropical jets. The anomalous (difference between composites) eddy streamfunction at 200 mb shows an enhanced pair of anticyclones (cyclones) straddling the equator. These fluctuations are strongly coupled with eastward traveling tropical convection in the western Pacific with a time scale of about 40 days. The composite circulations show marked differences in the propagation of wave activity in the NH at 200 mb. The low phase (reduced easterlies) shows strong propagation away from the dominant source region over East Asia into the tropical western Pacific in conjunction with what appears to be significant reflection from the equatorward flank of the subtropical jet. In contrast, the high composite (enhanced easterlies) shows much weaker equatorward propagation together with reduced vertical propagation over East Asia and the western North Pacific.
The second zonal wind EOF (Z2) displays a more asymmetric structure with respect to the equator, describing a simultaneous decrease (increase) in the easterly extent of the East Asian jet and increase (decrease) in the strength of the jet over southern Australia. The anomalous eddy stream function at 200 mb shows wave trains apparently emanating from the tropical central Pacific extending into both hemispheres: in the winter hemisphere this resembles the Pacific-North American (PNA) pattern. These fluctuations show some coupling with preceding tropical convection anomalies in the western and central Pacific. Stability calculations show that the PNA pattern is maintained through barotropic energy exchanges with the mean flow. For the low composite, an enhanced source of stationary wave activity in the Gulf of Alaska is associated with an increase in synoptic-scale eddy activity.
These results suggest that tropical convection in the western Pacific has a strong modifying influence on (extratropically forced) middle latitude low-frequency variability. The influence is primarily indirect via zonal wind changes which influence the propagation of waves originating in middle latitudes. The zonal wind changes include those associated with the strength and extent of the tropical easterlies as well as more subtle (but important) changes which effect the curvature of the East Asian jet leading in some instances to turning points for middle latitude waves. The PNA also appears to have its main energy source in middle latitudes and in this case the link with the tropics appears to be more tied to phase locking with anomalies forced by tropical convection in the western and central Pacific.
Abstract
The Great Plains region of the United States is characterized by some of the most frequent and regular occurrences of a nocturnal low-level jet (LLJ). While the LLJ is generally confined to the lowest Kilometer of the atmosphere, it may cover a substantial region of the Great Plains, and typically reaches maximum amplitudes of more than 20 m s−1.
A two-month, springtime simulation with the Goddard Earth Observing System (GEOS-1) atmospheric general circulation model (AGCM) has produced a Great Plains LLJ with a vertical and temporal structure, directionality, and climatological distribution that compare favorably with observations. The diurnal cycle of the low-level flow is dramatic and coherent over a subcontinental area that includes much of the western United States and northern Mexico. This cycle can be interpreted as the nightly intrusion of the anticyclonic, subtropical gyre (associated with the Bermuda high) into the North American continent as surface friction decreases. The AGCM also simulates a pair of northerly LLJ maxima off the California coast, which seem to correspond to observations of a so-called “Baja Jet.” Other apparently related diurnal variations extending well into the upper troposphere are documented and compared with observations.
The time-averaged climatological picture of the low-level flow is dominated over land by the nocturnal phase of the diurnal cycle, in which surface friction is minimal and wind speeds are strongest. This pattern, with its zones of strong convergence, is characteristic of an unsteady, strongly forced flow. Over the open ocean, the mean low-level flow is more reminiscent of a smooth, climatological pattern.
Analysis of the simulated moisture budget for the continental United States reveals a horizontally confined region of strong southerly moisture transport with a strong diurnal cycle in the region of the Great Plains LLJ, as has been found in observations of water vapor transport. The LLJ plays a key role in that budget by transporting almost one-third of all the moisture that enters the continental United States with most of the influx from the LLJ (slightly less than two-thirds of it) entering during the 12 nighttime hours. However, it is the mean flow pattern and not covariances associated with the diurnal cycle that contribute most significantly to the total time-mean moisture transport. Covariances on the synoptic and longer timescales contribute only about one-fifth of the total time-mean transport of moisture in the jet region, and covariances on the diurnal timescale are negative and negligible despite the strong diurnal signal in the wind.
Abstract
The Great Plains region of the United States is characterized by some of the most frequent and regular occurrences of a nocturnal low-level jet (LLJ). While the LLJ is generally confined to the lowest Kilometer of the atmosphere, it may cover a substantial region of the Great Plains, and typically reaches maximum amplitudes of more than 20 m s−1.
A two-month, springtime simulation with the Goddard Earth Observing System (GEOS-1) atmospheric general circulation model (AGCM) has produced a Great Plains LLJ with a vertical and temporal structure, directionality, and climatological distribution that compare favorably with observations. The diurnal cycle of the low-level flow is dramatic and coherent over a subcontinental area that includes much of the western United States and northern Mexico. This cycle can be interpreted as the nightly intrusion of the anticyclonic, subtropical gyre (associated with the Bermuda high) into the North American continent as surface friction decreases. The AGCM also simulates a pair of northerly LLJ maxima off the California coast, which seem to correspond to observations of a so-called “Baja Jet.” Other apparently related diurnal variations extending well into the upper troposphere are documented and compared with observations.
The time-averaged climatological picture of the low-level flow is dominated over land by the nocturnal phase of the diurnal cycle, in which surface friction is minimal and wind speeds are strongest. This pattern, with its zones of strong convergence, is characteristic of an unsteady, strongly forced flow. Over the open ocean, the mean low-level flow is more reminiscent of a smooth, climatological pattern.
Analysis of the simulated moisture budget for the continental United States reveals a horizontally confined region of strong southerly moisture transport with a strong diurnal cycle in the region of the Great Plains LLJ, as has been found in observations of water vapor transport. The LLJ plays a key role in that budget by transporting almost one-third of all the moisture that enters the continental United States with most of the influx from the LLJ (slightly less than two-thirds of it) entering during the 12 nighttime hours. However, it is the mean flow pattern and not covariances associated with the diurnal cycle that contribute most significantly to the total time-mean moisture transport. Covariances on the synoptic and longer timescales contribute only about one-fifth of the total time-mean transport of moisture in the jet region, and covariances on the diurnal timescale are negative and negligible despite the strong diurnal signal in the wind.
Abstract
The column budget technique described by Oort and Vonder Haar (1976) is used to assess the physical consistency and accuracy of estimates of the earth-atmosphere energy balance. Regional estimates of the atmospheric budget terms, the net radiation at the top of the atmosphere, and the time tendency and flux divergence of energy are calculated for the Special Observing Periods of the FGGE year. The data are assimilated by the Goddard Laboratory for the Atmospheres (GLA) four-dimensional analysis system. Ocean heat storage is obtained from marine temperature records while the energy flux through the surface and ocean heat flux divergence are computed as residuals.
During winter the midlatitude oceans supply large quantities of energy to the overlying atmosphere which then transports the energy to the continental heat sinks, the energy flows in the opposite direction during summer. The energy exchange between continental and oceanic regions is much stronger in the Northern Hemisphere where land coverage and land-sea differences are greater.
The uncertainties in the energy balance calculations are assessed by examining the errors in the observations, the data assimilation system including the GLA general circulation model, and the energy budget procedures. Sensitivity tests, error analyses and comparison with other studies indicate that the uncertainties in the continental-scale atmospheric energy flux divergence and the surface energy flux are approximately 20 W m−2 and 30 W m−2, respectively. We conclude that at present it is not possible to estimate accurately the ocean heat divergence and transport using the column budget technique.
Abstract
The column budget technique described by Oort and Vonder Haar (1976) is used to assess the physical consistency and accuracy of estimates of the earth-atmosphere energy balance. Regional estimates of the atmospheric budget terms, the net radiation at the top of the atmosphere, and the time tendency and flux divergence of energy are calculated for the Special Observing Periods of the FGGE year. The data are assimilated by the Goddard Laboratory for the Atmospheres (GLA) four-dimensional analysis system. Ocean heat storage is obtained from marine temperature records while the energy flux through the surface and ocean heat flux divergence are computed as residuals.
During winter the midlatitude oceans supply large quantities of energy to the overlying atmosphere which then transports the energy to the continental heat sinks, the energy flows in the opposite direction during summer. The energy exchange between continental and oceanic regions is much stronger in the Northern Hemisphere where land coverage and land-sea differences are greater.
The uncertainties in the energy balance calculations are assessed by examining the errors in the observations, the data assimilation system including the GLA general circulation model, and the energy budget procedures. Sensitivity tests, error analyses and comparison with other studies indicate that the uncertainties in the continental-scale atmospheric energy flux divergence and the surface energy flux are approximately 20 W m−2 and 30 W m−2, respectively. We conclude that at present it is not possible to estimate accurately the ocean heat divergence and transport using the column budget technique.
