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- Author or Editor: John C. Marshall x
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Abstract
Using twice daily synoptic charts, objectively analyzed at the National Meteorological Centre, horizontal eddy fluxes of temperature and quasi-geostrophic potential vorticity are computed for the month Of July 1976, when a blocking anticyclone was centered over western Europe. The local time-averaged eddy variance equations are used to provide a dynamical basis for interpreting the spatial pattern of eddy fluxes, and their relation to mean gradients. It is shown that a rotational non-divergent flux can be identified, the cross-gradient component of which balances the mean flow advection of eddy variance. The remaining flux is the dynamically significant one which helps maintain the block and can be understood in terms of a response to sources and sinks of eddy variance.
Abstract
Using twice daily synoptic charts, objectively analyzed at the National Meteorological Centre, horizontal eddy fluxes of temperature and quasi-geostrophic potential vorticity are computed for the month Of July 1976, when a blocking anticyclone was centered over western Europe. The local time-averaged eddy variance equations are used to provide a dynamical basis for interpreting the spatial pattern of eddy fluxes, and their relation to mean gradients. It is shown that a rotational non-divergent flux can be identified, the cross-gradient component of which balances the mean flow advection of eddy variance. The remaining flux is the dynamically significant one which helps maintain the block and can be understood in terms of a response to sources and sinks of eddy variance.
Abstract
The authors explore the use of the “neutral vectors” of a linearized version of a global quasigeostrophic atmospheric model with realistic mean flow in the study of the nonlinear model's low-frequency variability. Neutral vectors are the (right) singular vectors of the linearized model's tendency matrix that have the smallest eigenvalues; they are also the patterns that exhibit the largest response to forcing perturbations in the linear model. A striking similarity is found between neutral vectors and the dominant patterns of variability (EOFs) observed in both the full nonlinear model and in the real world. The authors discuss the physical and mathematical connection between neutral vectors and EOFs.
Investigation of the “optimal forcing patterns”—the left singular vectors—proves to be less fruitful. The neutral modes have equivalent barotropic vertical structure, but their optimal forcing patterns are baroclinic and seem to be associated with low-level heating. But the horizontal patterns of the forcing patterns are not robust and are sensitive to the form of the inner product used in the singular vector decomposition analysis. Additionally, applying “optimal” forcing patterns as perturbations to the full nonlinear model does not generate the response suggested by the linear model.
Abstract
The authors explore the use of the “neutral vectors” of a linearized version of a global quasigeostrophic atmospheric model with realistic mean flow in the study of the nonlinear model's low-frequency variability. Neutral vectors are the (right) singular vectors of the linearized model's tendency matrix that have the smallest eigenvalues; they are also the patterns that exhibit the largest response to forcing perturbations in the linear model. A striking similarity is found between neutral vectors and the dominant patterns of variability (EOFs) observed in both the full nonlinear model and in the real world. The authors discuss the physical and mathematical connection between neutral vectors and EOFs.
Investigation of the “optimal forcing patterns”—the left singular vectors—proves to be less fruitful. The neutral modes have equivalent barotropic vertical structure, but their optimal forcing patterns are baroclinic and seem to be associated with low-level heating. But the horizontal patterns of the forcing patterns are not robust and are sensitive to the form of the inner product used in the singular vector decomposition analysis. Additionally, applying “optimal” forcing patterns as perturbations to the full nonlinear model does not generate the response suggested by the linear model.
Abstract
Summer near-surface temperatures over the northeast coast of the Antarctic Peninsula have increased by more than 2°C over the past 40 years, a temperature increase 3 times greater than that on the northwest coast. Recent analysis has shown a strong correlation between this striking warming trend and significant change in the summer Southern Hemisphere annular mode (SAM), which has resulted in greatly increased summer westerlies across the northern peninsula. It has been proposed that the strengthening westerlies have resulted in increased vertical deflection of relatively warm maritime air over the northern peninsula, contributing significantly to the observed warming and the recent collapse of northern sections of the Larsen Ice Shelf. In this study, laboratory and numerical modeling of airflow incident to the peninsula are employed to further understand this mechanism. It is shown that the effect of the strengthening westerlies has led to a distinct transition from a “blocked” regime to a “flow-over” regime, that is, confirmation of the proposed warming mechanism. The blocked regime is dominated by flow stagnation upstream (i.e., little vertical deflection) and consequent lateral deflection of flow along the western side of the peninsula. The flow-over regime is dominated by vertical deflection of mid/upper-level air over the peninsula, with strong downslope winds following closely to the leeward slope transporting this air (which warms adiabatically as it descends) to the near-surface of the northeast peninsula. The strong rotation typical of high latitudes considerably increases the flow over the peninsula, particularly strengthening it over the southern side (verified by aircraft measurements), suggesting that the warming trend is not solely confined to the northeast. Globally, flow regime transitions such as this may be responsible for other local climate variations.
Abstract
Summer near-surface temperatures over the northeast coast of the Antarctic Peninsula have increased by more than 2°C over the past 40 years, a temperature increase 3 times greater than that on the northwest coast. Recent analysis has shown a strong correlation between this striking warming trend and significant change in the summer Southern Hemisphere annular mode (SAM), which has resulted in greatly increased summer westerlies across the northern peninsula. It has been proposed that the strengthening westerlies have resulted in increased vertical deflection of relatively warm maritime air over the northern peninsula, contributing significantly to the observed warming and the recent collapse of northern sections of the Larsen Ice Shelf. In this study, laboratory and numerical modeling of airflow incident to the peninsula are employed to further understand this mechanism. It is shown that the effect of the strengthening westerlies has led to a distinct transition from a “blocked” regime to a “flow-over” regime, that is, confirmation of the proposed warming mechanism. The blocked regime is dominated by flow stagnation upstream (i.e., little vertical deflection) and consequent lateral deflection of flow along the western side of the peninsula. The flow-over regime is dominated by vertical deflection of mid/upper-level air over the peninsula, with strong downslope winds following closely to the leeward slope transporting this air (which warms adiabatically as it descends) to the near-surface of the northeast peninsula. The strong rotation typical of high latitudes considerably increases the flow over the peninsula, particularly strengthening it over the southern side (verified by aircraft measurements), suggesting that the warming trend is not solely confined to the northeast. Globally, flow regime transitions such as this may be responsible for other local climate variations.
