Abstract This study tested a numerical and theoretical prediction that the stratosphere and troposphere are coupled through the effect of stratospheric vertical wind shear on baroclinic waves. Wavelengths, phase speeds, and background quasigeostrophic potential vorticity gradients were analyzed over the Pacific and Atlantic during strong and weak stratospheric polar vortex events and interpreted in terms of the counterpropagating Rossby wave perspective on baroclinic instability. Effects of zonal variations in the background flow were included in the analysis of phase speeds. Observed changes in wave packet average wavelength and phase speed support the vertical shear hypothesis for stratosphere–troposphere coupling; however, changes in the intrinsic phase speed contradict the hypothesis. This inconsistency was resolved by considering the change in zonal wind speed in the lower stratosphere, which accounts for most of the change in phase speeds during strong and weak vortex events. Changes in the average wavelengths and meridional wave activity flux are also consistent with this modified hypothesis involving the stratospheric zonal wind. The results demonstrate that a simple mechanism for stratosphere–troposphere coupling can be found in the observational record.
The horizontal and temporal variations of static stability prior to blocking onset are characterized through composite analysis of twenty blocking events in the Southern Hemisphere. It is found that, along with a low potential vorticity (PV) anomaly formation, a local minimum of static stability in the upper troposphere and on the tropopause is achieved over the block-onset region when blocking onset takes place. By partitioning the isentropic PV into the absolute vorticity and static stability contributions, it is found that they account for roughly 70 % and 30 % of low-PV anomaly formation over the block-onset region, respectively. A static stability budget analysis revealed that the decrease of static stability in the upper troposphere and on the tropopuase prior to blocking onset is attributable to horizontal advection of low static stability from subtropics to midlatitude as well as the stretching effect associated with upper-level convergence over the block-onset region, with the horizontal advection forcing being the primary contributor. On the other hand, the vertical advection of static stability tends to oppose the decreasing static stability through advecting more stable air downward such that it stabilizes the local air over the block-onset region. Furthermore, the direct effect of diabatic heating is negligible as its magnitude is generally an order of magnitude smaller than other effects in the static stability budget. Nevertheless, the indirect effect of diabatic heating, manifested as the advection of low static stability by diabatically forced upper-tropospheric outflow, greatly favors blocking onsets by destabilizing the air upstream block-onset region.
Researchers have been exploring methods to facilitate the prediction of rapidly intensifying surface cyclones. Recognizing that synoptic-scale systems, such as these cyclones, are less predictable at medium range and beyond than are planetary-scale circulations, researchers propose that the planetary-scale environment for explosive cyclogenesis could be better predicted than the cyclones themselves. Researchers have therefore constructed a planetary-scale climatology of explosive cyclogenesis by compositing together filtered 500 mb height fields (retaining planetary waves only) corresponding to a large sample of rapidly intensifying surface cyclones, stratified geographically and according to the direction of 500 mb geostrophic flow (southwesterly, northwesterly or westerly) over the cyclone center. The composites are calculated from five days preceding to five days following each rapid cyclogenesis event, and have climatology subtracted so that the evolution of planetary-scale anomalies before and after cyclogenesis can be followed. Whether the anomalies are distinct from background variability and thus provide predictive value is now being evaluated. Following explosive cyclogenesis over which the filtered 500 mb flow is southwesterly, there appear in the composites large positive 500 mb height anomalies downstream. In some cases, these anomalies are associated with blocking patterns. Whether the objectively-defined blocking patterns in the data set are preceded by upstream intense surface cyclone activity is being investigated. Finally, the contribution of synoptic-scale processes, notably warm air advection, to planetary-scale height rises during a block formation following an explosive cyclogenesis event is being diagnosed. Researchers hope to eventually evaluate the impact of satellite derived latent heat release upon the warm air advection in this case.
The structure and evolution of planetary-scale waves in the 500 mb geopotential height field over the Northern Hemisphere during December 1976 and January 1977 are studied with the techniques of one- and two-dimensional Fourier analysis and spherical harmonic analysis. The incipient stages of “split-flow” blocking episodes in this period are associated with an increase in amplitude in two-dimensional wave components having zonal wavenumbers 1 and 2 and meridional wavenumber 1. At the time of observed split-flow blocking, the wave component with zonal wavenumber 3 and meridional wavenumber 2 achieves peak amplitude, while the amplitudes of the other wave components decline. In addition, the height field reconstructed from the sum of these three components is found to simulate well the observed 500 mb height pattern during this episode. It is suggested that these observations may be explained by nonlinear resonant wave interaction theory. A streamline configuration resembling the observed split-flow blocking pattern is found to be generated in the two-layer quasi-geostrophic channel model through the mechanism of resonant triad interactions. The favorable comparison between observations and theory in this particular case study suggests that resonant interactions among planetary-scale waves may be an important physical mechanism at work during the evolution of atmospheric blocking.
Prior to the 24-26 March 2015 extreme precipitation event that impacted northern Chile, the scenarios for Pleistocene and Holocene wetter paleoclimate intervals in the hyperarid core of the Atacama Desert had been attributed to eastern or southwestern moisture sources. The March 2015 precipitation event offered the first modern opportunity to evaluate a major regional precipitation event relative to those hypothetical paleoclimate scenarios. It was the first opportunity to determine the 18O and 2H composition of a major precipitation event that might eventually be preserved in geological materials. The driver for the March 2015 event was a synoptic-scale weather system, a cutoff cold upper-level low system that traversed the Pacific Ocean at a time of unusually warm temperatures of Pacific surface water. Ground-based precipitation data, stable isotopes in precipitation and river samples, NCEP/NCAR reanalysis atmospheric data and air mass tracking are utilized to connect the Earth surface processes to atmospheric conditions. The δ18O and δ2H of the precipitation and ephemeral rivers were significantly heavier than the rain, snow and ephemeral rivers fed by more frequent but less voluminous precipitation events registered prior to March 2015. Consistent with the atmospheric analyses, the rain isotopic compositions are typical of a water vapor whose source was at more equatorial latitudes of the Pacific and which moved southward. The late March 2015 system was an unforeseen scenario even for El Niño Pacific ocean conditions. Furthermore, the late summer season warmth led to greater potential for erosion and sediment transport than typical of more common moderate precipitation scenarios which usually include widely distributed snow. A comparison of the March 2015 scenario to the spatial distribution of wetter paleoclimate intervals leads to the hypothesis that the March 2015 scenario likely better fits some parts of the paleoclimate record of the continental interior hyperarid Atacama Desert than do the eastern or southwestern moisture source paleoclimate scenarios deduced previously.
The forecastability of a cold-air outbreak over eastern North America during January 1985 has been studied with ensemble forecasts from the NCAR Community Climate Model version 2 run at T42 horizontal resolution. The cold-air outbreak case was characterized by a pool of very cold air (T < −35°C at 850 mb) that moved southward into the central United States and intensified. The ensemble's 10 member forecasts were initialized at 0000 UTC 15 January 85, a few days before the cold-air pool began its southward movement and reached its peak intensity. The ensemble members predicted the southward passage of the cold air but faster and weaker than analyzed. The predicted weakening of the cold-air pool was consistent with the model's systematic error. Quasi-Lagrangian diagnosis of the 850-mb temperature tendency budget revealed that the analyzed intensification of the cold-air pool was due to residual rather than adiabatic effects. These residual effects could have been diabatic in origin but also attributable to observational errors. Similar diagnoses applied to selected ensemble members indicated that diabatic cooling, specifically longwave radiative cooling, contributed to the forecast cooling of the cold-air pool by one ensemble member but was overwhelmed by adiabatic warming in a weakening cold-air pool predicted by another ensemble member. These results suggest that the forecast details of a cold-air outbreak may depend upon the subtle balance between diabatic and adiabatic processes. Furthermore, forecasts constructed from ensemble predictions must account for model biases as well as information from the ensembles.
The geographical and monthly frequencies of 500 mb cyclones and anticyclones in the National Meteorological Center analyses over the western half of the Northern Hemisphere are investigated for the period 1950–85. These cyclones and anticyclones, defined by the appearance of at least one closed (approximately) 6-dekameter contour around relatively low or high heights in the 500 mb height field, are generally observed less than ten percent of the time in any 10° by 10° latitude-longitude quadrangle, with cyclones being more numerous than anticyclones. The 500 mb cyclones are found primarily at middle and high latitudes, while anticyclones are observed most frequently over the subtropics. Cyclone frequency increases over the northern oceanic regions during summer, while anticyclone frequency increases throughout the subtropics during summer, especially over southwestern North America. Exceptions to these rules are observed; relatively high springtime 500 mb anticyclone frequency is found over the northeastern Atlantic Ocean while relatively high 500 mb cyclone frequency is found over the central subtropics Pacific Ocean and near Alaska during summer, southwestern North America during winter, and near southwestern Europe throughout the year. Abnormally strong diffluent flow over southwestern North America is suggested as an antecedent condition for 500 mb cyclogenesis in this same region. The correlation between 500 mb cyclone frequencies and 300 mb westerly momentum transports is also investigated, revealing that 500 mb cyclones may be associated with the convergence of westerly momentum into the 300 mb westerly jet. Finally, temporal trends in the frequencies indicate that 500 mb cyclone frequencies declined from 1950 through 1970 but increased from 1971 through 1985, while 500 mb anticyclone frequencies declined from 1950 through 1985.
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