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Abstract
Mesurements of electron densities in the mesosphere at 43S (Christchurch, N. Z.) have shown that relatively constant values exist in summer, but large variations occur in winter. During the period 21–29 June 1963, isopleths of electron density descended and ascended by approximately 10 km, while stratospheric temperatures rose and fell by 10C. Both effects are ascribed to atmospheric vertical motion. This instance of “anomalous” ionospheric absorption shows the influence of circulation changes in redistributing photochemical constituents through the mesosphere.
Abstract
Mesurements of electron densities in the mesosphere at 43S (Christchurch, N. Z.) have shown that relatively constant values exist in summer, but large variations occur in winter. During the period 21–29 June 1963, isopleths of electron density descended and ascended by approximately 10 km, while stratospheric temperatures rose and fell by 10C. Both effects are ascribed to atmospheric vertical motion. This instance of “anomalous” ionospheric absorption shows the influence of circulation changes in redistributing photochemical constituents through the mesosphere.
Abstract
The spatial spreading of infinitesimal disturbances superposed on a turbulent baroclinic jet is explored. This configuration is representative of analysis errors in an idealized midlatitude storm track and the insight gained may be helpful to understand the spreading of forecast errors in numerical weather prediction models.
This problem is explored through numerical experiments of a turbulent baroclinic jet that is perturbed with spatially localized disturbances. Solutions from a quasigeostrophic model for the disturbance fields are compared with those for a passive tracer to determine whether disturbances propagate faster than the basic-state flow. Results show that the disturbance spreading rate is sensitive to the structure of the initial disturbance. Disturbances that are localized in potential vorticity (PV) have far-field winds that allow the disturbance to travel downstream faster than disturbances that are initially localized in geopotential, which have no far-field wind. Near the jet, the spread of the disturbance field is observed to exceed the tracer field for PV-localized disturbances, but not for the geopotential-localized disturbances. Spreading rates faster than the flow for geopotential-localized disturbances are found to occur only for disturbances located off the jet axis.
These results are compared with those for zonal and time-independent jets to qualitatively assess the effects of transience and nonlinearity. This comparison suggests that the average properties of localized perturbations to the turbulent jet can be decomposed into a superposition of dynamics associated with a time-independent parallel flow plus a “diffusion” process.
Abstract
The spatial spreading of infinitesimal disturbances superposed on a turbulent baroclinic jet is explored. This configuration is representative of analysis errors in an idealized midlatitude storm track and the insight gained may be helpful to understand the spreading of forecast errors in numerical weather prediction models.
This problem is explored through numerical experiments of a turbulent baroclinic jet that is perturbed with spatially localized disturbances. Solutions from a quasigeostrophic model for the disturbance fields are compared with those for a passive tracer to determine whether disturbances propagate faster than the basic-state flow. Results show that the disturbance spreading rate is sensitive to the structure of the initial disturbance. Disturbances that are localized in potential vorticity (PV) have far-field winds that allow the disturbance to travel downstream faster than disturbances that are initially localized in geopotential, which have no far-field wind. Near the jet, the spread of the disturbance field is observed to exceed the tracer field for PV-localized disturbances, but not for the geopotential-localized disturbances. Spreading rates faster than the flow for geopotential-localized disturbances are found to occur only for disturbances located off the jet axis.
These results are compared with those for zonal and time-independent jets to qualitatively assess the effects of transience and nonlinearity. This comparison suggests that the average properties of localized perturbations to the turbulent jet can be decomposed into a superposition of dynamics associated with a time-independent parallel flow plus a “diffusion” process.
Abstract
Spaced-antenna drift measurements between 60 and 100 km, from radiowave partial reflections, are presented in the form of zonal and meridional profiles for May and June 1969, and as a time cross section of zonal winds, from February–June 1969, for 52N, 107W. ROCOB data from 54N, 110W, up to 60 km, are used to complete the profiles. The profiles show satisfactory agreement with profiles derived from a recent model of zonal and meridional winds.
Abstract
Spaced-antenna drift measurements between 60 and 100 km, from radiowave partial reflections, are presented in the form of zonal and meridional profiles for May and June 1969, and as a time cross section of zonal winds, from February–June 1969, for 52N, 107W. ROCOB data from 54N, 110W, up to 60 km, are used to complete the profiles. The profiles show satisfactory agreement with profiles derived from a recent model of zonal and meridional winds.
Abstract
The results of radiowave partial reflection wind (drift) observations, 62–116 km, for the years 1969–73, at 52°N, 107°W (Saskatoon), are presented, and are compared with current empirical models. Agreement is satisfactory to 85 km; but at higher altitudes, differences exist; notably an annual variation of zonal flow above 100 km, whose direction is eastward in summer and westward in winter. The semi-annual variation of winds is shown to be limited to 85–100 km, and is considered to be due to two out-of-phase annual variations identifiable at higher and lower altitudes. A region of positive (poleward) temperature gradient (high-latitude warming) is identified in the range ∼80 to at least 107 km in winter, and another region (∼75 to ≳107 km) of negative temperature gradient is identified in summer. The relationships of these regions to circulation characteristics are discussed.
Abstract
The results of radiowave partial reflection wind (drift) observations, 62–116 km, for the years 1969–73, at 52°N, 107°W (Saskatoon), are presented, and are compared with current empirical models. Agreement is satisfactory to 85 km; but at higher altitudes, differences exist; notably an annual variation of zonal flow above 100 km, whose direction is eastward in summer and westward in winter. The semi-annual variation of winds is shown to be limited to 85–100 km, and is considered to be due to two out-of-phase annual variations identifiable at higher and lower altitudes. A region of positive (poleward) temperature gradient (high-latitude warming) is identified in the range ∼80 to at least 107 km in winter, and another region (∼75 to ≳107 km) of negative temperature gradient is identified in summer. The relationships of these regions to circulation characteristics are discussed.
Abstract
The behavior of winds at 52°N, 107°W (Saskatoon, Canada) at altitudes to ∼110 km during major stratospheric warmings of 1969–70,1970–-71 and 1972–73 winters and a minor warming in December 1972, is studied. The zonal component of flow above the stratosphere is found to be affected to varying altitudes; in January 1970 the normal eastward flow was reversed to westward at all altitudes from the surface to ∼110 km. The pattern of winds is reasonably consistent with current suggestions that at middle and high latitudes the mesosphere cools as the stratosphere warms, and that the mesosphere warms subsequent to its cooling. Examples of mesospheric wind perturbations either as precursors of stratospheric warmings or independent of warmings, are also given.
Abstract
The behavior of winds at 52°N, 107°W (Saskatoon, Canada) at altitudes to ∼110 km during major stratospheric warmings of 1969–70,1970–-71 and 1972–73 winters and a minor warming in December 1972, is studied. The zonal component of flow above the stratosphere is found to be affected to varying altitudes; in January 1970 the normal eastward flow was reversed to westward at all altitudes from the surface to ∼110 km. The pattern of winds is reasonably consistent with current suggestions that at middle and high latitudes the mesosphere cools as the stratosphere warms, and that the mesosphere warms subsequent to its cooling. Examples of mesospheric wind perturbations either as precursors of stratospheric warmings or independent of warmings, are also given.
Abstract
A time-dependent climate-change experiment with a coupled ocean–atmosphere general circulation model has been used to study changes in the occurrence of drought in summer in southern Europe and central North America. In both regions, precipitation and soil moisture are reduced in a climate of greater atmospheric carbon dioxide. A detailed investigation of the hydrology of the model shows that the drying of the soil comes about through an increase in evaporation in winter and spring, caused by higher temperatures and reduced snow cover, and a decrease in the net input of water in summer. Evaporation is reduced in summer because of the drier soil, but the reduction in precipitation is larger. Three extreme statistics are used to define drought, namely the frequency of low summer precipitation, the occurrence of long dry spells, and the probability of dry soil. The last of these is arguably of the greatest practical importance, but since it is based on soil moisture, of which there are very few observations, the authors’ simulation of it has the least confidence. Furthermore, long time series for daily observed precipitation are not readily available from a sufficient number of stations to enable a thorough evaluation of the model simulation, especially for the frequency of long dry spells, and this increases the systematic uncertainty of the model predictions. All three drought statistics show marked increases owing to the sensitivity of extreme statistics to changes in their distributions. However, the greater likelihood of long dry spells is caused by a tendency in the character of daily rainfall toward fewer events, rather than by the reduction in mean precipitation. The results should not be taken as firm predictions because extreme statistics for small regions cannot be calculated reliably from the output of the current generation of GCMs, but they point to the possibility of large increases in the severity of drought conditions as a consequence of climate change caused by increased CO2.
Abstract
A time-dependent climate-change experiment with a coupled ocean–atmosphere general circulation model has been used to study changes in the occurrence of drought in summer in southern Europe and central North America. In both regions, precipitation and soil moisture are reduced in a climate of greater atmospheric carbon dioxide. A detailed investigation of the hydrology of the model shows that the drying of the soil comes about through an increase in evaporation in winter and spring, caused by higher temperatures and reduced snow cover, and a decrease in the net input of water in summer. Evaporation is reduced in summer because of the drier soil, but the reduction in precipitation is larger. Three extreme statistics are used to define drought, namely the frequency of low summer precipitation, the occurrence of long dry spells, and the probability of dry soil. The last of these is arguably of the greatest practical importance, but since it is based on soil moisture, of which there are very few observations, the authors’ simulation of it has the least confidence. Furthermore, long time series for daily observed precipitation are not readily available from a sufficient number of stations to enable a thorough evaluation of the model simulation, especially for the frequency of long dry spells, and this increases the systematic uncertainty of the model predictions. All three drought statistics show marked increases owing to the sensitivity of extreme statistics to changes in their distributions. However, the greater likelihood of long dry spells is caused by a tendency in the character of daily rainfall toward fewer events, rather than by the reduction in mean precipitation. The results should not be taken as firm predictions because extreme statistics for small regions cannot be calculated reliably from the output of the current generation of GCMs, but they point to the possibility of large increases in the severity of drought conditions as a consequence of climate change caused by increased CO2.
Abstract
Simulations of the last 500 yr carried out using the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3) with anthropogenic and natural (solar and volcanic) forcings have been analyzed. Global-mean surface temperature change during the twentieth century is well reproduced. Simulated contributions to global-mean sea level rise during recent decades due to thermal expansion (the largest term) and to mass loss from glaciers and ice caps agree within uncertainties with observational estimates of these terms, but their sum falls short of the observed rate of sea level rise. This discrepancy has been discussed by previous authors; a completely satisfactory explanation of twentieth-century sea level rise is lacking. The model suggests that the apparent onset of sea level rise and glacier retreat during the first part of the nineteenth century was due to natural forcing. The rate of sea level rise was larger during the twentieth century than during the previous centuries because of anthropogenic forcing, but decreasing natural forcing during the second half of the twentieth century tended to offset the anthropogenic acceleration in the rate. Volcanic eruptions cause rapid falls in sea level, followed by recovery over several decades. The model shows substantially less decadal variability in sea level and its thermal expansion component than twentieth-century observations indicate, either because it does not generate sufficient ocean internal variability, or because the observational analyses overestimate the variability.
Abstract
Simulations of the last 500 yr carried out using the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3) with anthropogenic and natural (solar and volcanic) forcings have been analyzed. Global-mean surface temperature change during the twentieth century is well reproduced. Simulated contributions to global-mean sea level rise during recent decades due to thermal expansion (the largest term) and to mass loss from glaciers and ice caps agree within uncertainties with observational estimates of these terms, but their sum falls short of the observed rate of sea level rise. This discrepancy has been discussed by previous authors; a completely satisfactory explanation of twentieth-century sea level rise is lacking. The model suggests that the apparent onset of sea level rise and glacier retreat during the first part of the nineteenth century was due to natural forcing. The rate of sea level rise was larger during the twentieth century than during the previous centuries because of anthropogenic forcing, but decreasing natural forcing during the second half of the twentieth century tended to offset the anthropogenic acceleration in the rate. Volcanic eruptions cause rapid falls in sea level, followed by recovery over several decades. The model shows substantially less decadal variability in sea level and its thermal expansion component than twentieth-century observations indicate, either because it does not generate sufficient ocean internal variability, or because the observational analyses overestimate the variability.
Abstract
Teleconnections between the midlatitudes of the Northern and Southern Hemispheres are diagnosed in National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data and separately in European Centre for Medium-Range Weather Forecasts reanalysis data. The teleconnections are manifested as a small but significant tendency for blocking to occur simultaneously in the two hemispheres, though at different longitudes and different relative latitudes, during boreal winters over the period 1979–94 in both datasets.
One way to explain the correlations between blocking events is as an instance of synchronized chaos, the tendency of some coupled chaotic systems to synchronize, permanently or intermittently, regardless of initial conditions. As the coupling is weakened, the systems no longer synchronize completely, but small correlations between the states of the coupled systems are observed instead. In previous work, such behavior was observed in an idealized coupled-hemisphere model constructed from a midlatitude model due to de Swart, which extended the earlier Charney–DeVore spectral truncation of the barotropic vorticity equation by including a few extra modes. The direct coupling of the two midlatitude systems in the coupled-hemisphere model represented the exchange of Rossby waves through the upper-tropospheric “westerly ducts” in the Tropics.
Significant correlations are found between blocking events, which are chaotically timed in each hemisphere considered singly, even without several of the idealizations used in the previous study. In a model modified to include an extended tropical region, the correlations are little affected by attenuation and phase shift of the Rossby waves that couple the two midlatitude systems. Variations in the relative longitudes of topographic features in the two hemispheres leave significant correlations or anticorrelations. The annual cycle, which imposes directionality on the coupling, since the Northern Hemisphere is more strongly forced than the Southern Hemisphere at the times when the hemispheres are coupled, increases the correlations slightly. A two-hemisphere model constructed from a higher-order (wavenumber 3) truncation of the barotropic vorticity equation exhibits regime transitions between blocked and zonal flow at a more realistic rate in each hemisphere but still exhibits interhemispheric correlations.
Abstract
Teleconnections between the midlatitudes of the Northern and Southern Hemispheres are diagnosed in National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data and separately in European Centre for Medium-Range Weather Forecasts reanalysis data. The teleconnections are manifested as a small but significant tendency for blocking to occur simultaneously in the two hemispheres, though at different longitudes and different relative latitudes, during boreal winters over the period 1979–94 in both datasets.
One way to explain the correlations between blocking events is as an instance of synchronized chaos, the tendency of some coupled chaotic systems to synchronize, permanently or intermittently, regardless of initial conditions. As the coupling is weakened, the systems no longer synchronize completely, but small correlations between the states of the coupled systems are observed instead. In previous work, such behavior was observed in an idealized coupled-hemisphere model constructed from a midlatitude model due to de Swart, which extended the earlier Charney–DeVore spectral truncation of the barotropic vorticity equation by including a few extra modes. The direct coupling of the two midlatitude systems in the coupled-hemisphere model represented the exchange of Rossby waves through the upper-tropospheric “westerly ducts” in the Tropics.
Significant correlations are found between blocking events, which are chaotically timed in each hemisphere considered singly, even without several of the idealizations used in the previous study. In a model modified to include an extended tropical region, the correlations are little affected by attenuation and phase shift of the Rossby waves that couple the two midlatitude systems. Variations in the relative longitudes of topographic features in the two hemispheres leave significant correlations or anticorrelations. The annual cycle, which imposes directionality on the coupling, since the Northern Hemisphere is more strongly forced than the Southern Hemisphere at the times when the hemispheres are coupled, increases the correlations slightly. A two-hemisphere model constructed from a higher-order (wavenumber 3) truncation of the barotropic vorticity equation exhibits regime transitions between blocked and zonal flow at a more realistic rate in each hemisphere but still exhibits interhemispheric correlations.
Abstract
The continuous development of a meso-β-scale boundary layer over sloping terrain upwind of a high mountain barrier was simulated through a complete diurnal cycle using a nonhydrostatic boundary-layer model. The simulation detailed the evolution of a 500–800-m deep nocturnal boundary layer containing 1–3 m s−1 thermal circulations in the region upwind of a high ridge. Shear between the 5 m s−1 gradient level winds above the boundary layer and the mesoscale thermal circulations maintained the turbulent mixing of cold air upward against the stable stratification. The nocturnal boundary layer is replaced the following morning by a growing convective boundary layer containing 3–5 m s−1 warm thermal flows under its base. A multiple layer structure appears during the morning transition with the coexistence of the synoptic, nocturnal and developing daytime wind systems. As the morning progresses, the downwind edge of the stable layer slowly retreats back toward lower elevations while the convective layer grows under its base. By 5 h after sunrise, the morning transition is complete. Comparisons of the model simulation with field data show that the model accurately simulates the diurnal development of the mesoscale boundary layer.
Abstract
The continuous development of a meso-β-scale boundary layer over sloping terrain upwind of a high mountain barrier was simulated through a complete diurnal cycle using a nonhydrostatic boundary-layer model. The simulation detailed the evolution of a 500–800-m deep nocturnal boundary layer containing 1–3 m s−1 thermal circulations in the region upwind of a high ridge. Shear between the 5 m s−1 gradient level winds above the boundary layer and the mesoscale thermal circulations maintained the turbulent mixing of cold air upward against the stable stratification. The nocturnal boundary layer is replaced the following morning by a growing convective boundary layer containing 3–5 m s−1 warm thermal flows under its base. A multiple layer structure appears during the morning transition with the coexistence of the synoptic, nocturnal and developing daytime wind systems. As the morning progresses, the downwind edge of the stable layer slowly retreats back toward lower elevations while the convective layer grows under its base. By 5 h after sunrise, the morning transition is complete. Comparisons of the model simulation with field data show that the model accurately simulates the diurnal development of the mesoscale boundary layer.
Abstract
Sea surface temperature (SST) in the southwestern tropical Indian Ocean exerts a significant influence on global climate through its influence on the Indian summer monsoon and Northern Hemisphere atmospheric circulation. In this study, measurements from a long-term moored buoy are used in conjunction with satellite, in situ, and atmospheric reanalysis datasets to analyze the seasonal mixed layer heat balance in the thermocline ridge region of the southwestern tropical Indian Ocean. This region is characterized by a shallow mean thermocline (90 m, as measured by the 20°C isotherm) and pronounced seasonal cycles of Ekman pumping and SST (seasonal ranges of −0.1 to 0.6 m day−1 and 26°–29.5°C, respectively). It is found that surface heat fluxes and horizontal heat advection contribute significantly to the seasonal cycle of mixed layer heat storage. The net surface heat flux tends to warm the mixed layer throughout the year and is strongest during boreal fall and winter, when surface shortwave radiation is highest and latent heat loss is weakest. Horizontal heat advection provides warming during boreal summer and fall, when southwestward surface currents and horizontal SST gradients are strongest, and is close to zero during the remainder of the year. Vertical turbulent mixing, estimated as a residual in the heat balance, also undergoes a significant seasonal cycle. Cooling from this term is strongest in boreal summer, when surface wind and buoyancy forcing are strongest, the thermocline ridge is shallow (<90 m), and the mixed layer is deepening. These empirical results provide a framework for addressing intraseasonal and interannual climate variations, which are dynamically linked to the seasonal cycle, in the southwestern tropical Indian Ocean. They also provide a quantitative basis for assessing the accuracy of numerical ocean model simulations in the region.
Abstract
Sea surface temperature (SST) in the southwestern tropical Indian Ocean exerts a significant influence on global climate through its influence on the Indian summer monsoon and Northern Hemisphere atmospheric circulation. In this study, measurements from a long-term moored buoy are used in conjunction with satellite, in situ, and atmospheric reanalysis datasets to analyze the seasonal mixed layer heat balance in the thermocline ridge region of the southwestern tropical Indian Ocean. This region is characterized by a shallow mean thermocline (90 m, as measured by the 20°C isotherm) and pronounced seasonal cycles of Ekman pumping and SST (seasonal ranges of −0.1 to 0.6 m day−1 and 26°–29.5°C, respectively). It is found that surface heat fluxes and horizontal heat advection contribute significantly to the seasonal cycle of mixed layer heat storage. The net surface heat flux tends to warm the mixed layer throughout the year and is strongest during boreal fall and winter, when surface shortwave radiation is highest and latent heat loss is weakest. Horizontal heat advection provides warming during boreal summer and fall, when southwestward surface currents and horizontal SST gradients are strongest, and is close to zero during the remainder of the year. Vertical turbulent mixing, estimated as a residual in the heat balance, also undergoes a significant seasonal cycle. Cooling from this term is strongest in boreal summer, when surface wind and buoyancy forcing are strongest, the thermocline ridge is shallow (<90 m), and the mixed layer is deepening. These empirical results provide a framework for addressing intraseasonal and interannual climate variations, which are dynamically linked to the seasonal cycle, in the southwestern tropical Indian Ocean. They also provide a quantitative basis for assessing the accuracy of numerical ocean model simulations in the region.
