Publication Abstracts

Pandolfo 1993

Pandolfo, L., 1993: Observational aspects of the low-frequency intraseasonal variability of the atmosphere in middle latitudes. Adv. Geophys., 34, 93-174, doi:10.1016/S0065-2687(08)60435-5.

Rigorously, there exists at any time in the atmosphere only one state, characterized in space and time by the usual hydrodynamic field variables (velocity, temperature, pressure, and density in the Eulerian description), which obeys the classical equations (momentum, thermodynamics, continuity, and state) governing fluid dynamics. It is a fact of observation that the succession of instantaneous states constitutes more than just plain, isotropic, three-dimensional, white turbulence. It is better described as a mixture of coherent structures and dynamically structured turbulence unfolding on their own appropriate spatial and temporal scales. Hence, it seems indispensable to consider the movement of the atmospheric fluid as a superposition of motions occurring on different scales in order to render the study and depiction of atmospheric dynamics accessible to our understanding. The role of the atmospheric scientist is therefore to transcend conceptual biases and to capture a phenomenon by making meaningful scale resolutions. In mathematical terms this translates into the construction of a model, through relevant approximations to the system of equations of fluid dynamics, appropriate for the scales under consideration. This process of simplification, called filtering, guided by observational studies, is at the heart of meteorological research. Unfortunately, the distinction between motions unfolding on neighboring scales is rather vague and the evolution of the fictitious atmosphere of the model will not necessarily replicate exactly the phenomenon or class of phenomena being studied. In the end, however, it is hoped that investigating the cause of these differences will lead to a better understanding of the mechanisms underlying the physical phenomenon in question.

The atmosphere exhibits variability on all scales dynamically permitted by the physical constraints of the terrestrial system. Spatially, it encompasses the tiny twisters as well as the global atmospheric tides. Temporally, weather and climate are used to label and delimit the different domains of the spectrum of atmospheric fluctuations. Even though the boundary between these will always be blurry, we can define weather as the immediate and transitory response of the atmosphere to given forcings or instabilities. In middle latitudes it would correspond to the never-ending chaotic succession of cyclones and anticyclones. Climate, on the other hand, is felt through averages, which are used to cancel out the effects of weather fluctuations. Clearly, one has to understand that the evolutive responses encompassing weather and climate are slow processes that develop in a quasi-geostrophic manner and are not to be confused with the process of geostrophic adjustment.

During the past 50 years, a large part of the research conducted in dynamical meteorology has been to understand the controls on the weather of the middle latitudes. Baroclinic instability has become the main contender for explaining the emergence and initial growth of cyclonic perturbations. Studies of the life cycle of eddies have helped describe the later, nonlinear stages of their life course. However, the quantitative picture of their interaction with the large-scale planetary flow is not entirely satisfying. Quasi-geostrophic turbulence theory has shed some light on the maintenance of the zonal and barotropic properties of the large-scale flow by the synoptic eddies. Radiation of Rossby waves from middle latitudes to the subtropics has been used to describe the production of poleward momentum fluxes required to maintain the zonal flow of middle latitudes. Nevertheless, the elaboration of a complete theory, on a spherical domain, that could explain the sequence of interactions allowing the midlatitude perturbations to strengthen the general zonal circulation is rather difficult. This is one of the reasons why investigations of the general circulation have dealt with modeling the equilibrated state, in an averaged sense, of the atmosphere (for a review, see Saltzman, 1978).

As mentioned before, the terrestrial atmosphere is a physically constrained system. It corresponds to a very thin (in respect to the Earth's radius) layer of gaseous fluid, held by gravity on a spherically shaped, rotating planet that orbits around the sun. It is activated by a sparse set of energy sources. These constraints force the system into developing certain types of motions. Organized structures of planetary scale might form in space and time at the expense of the energy flowing into the system from the outside. That this is possible in the atmosphere is determined by the number of independent integral invariants (constants of motion). In familiar physical systems where energy is the only conserved quadratic quantity, entropy must increase. However, if one or more other integral invariants exist, the system can in principle form large-scale ordered structures in one physical quantity (energy for the Earth's atmosphere) by letting other physical quantities (e.g., enstrophy) take care of the required increase of entropy. Then it becomes a plausible conjecture that variables representing certain characteristics of the organized, planetary-scale, structures might become predictable on a temporal span longer than the actual limit of theoretical predictability for the atmosphere. Even though the detailed predictability of quantities such as rainfall and temperature, at a precise point in space and time, is lost on a scale of the lifetime of a synoptic system, the variables representing certain global-scale, ordered structures might remain predictable well past that range. Isolating their dynamics would bring us closer to an understanding of the evolutive aspects of the general circulation and its influence on the climate.

Our aim, in the following pages, is to develop an integrated description of the planetary-scale structures that emerge as organized entities on intraseasonal temporal scales in the middle latitudes. Intraseasonal is defined here as ranging from approximately 1 week (lifetime of a cyclonic system) to 3 months (a season). More appropriately, this temporal span is called the low-frequency end of the intraseasonal range. In the next section, we begin our description of low-frequency, intraseasonal variability by specifying the spatial scales of atmospheric motions involved in the generation of variability in this temporal range. Because of the importance of standing oscillations in the dynamics of low-frequency anomalies, we give, in Section 3, an overview of the three-dimensional structure of the seasonally averaged eddies on which the intraseasonal fluctuations are superposed. Then, in Section 4, we discuss the generation and maintenance mechanisms possibly responsible for the existence of the seasonal, quasi-stationary disturbances. After a digression (Section 5) on the different conceptual approaches to studying low-frequency variability (LFV), we present, in Sections, 6 and 7, the various attempts at observationally characterizing atmospheric LFV. One way of coming to an understanding of the mechanisms implicated in the dynamics of LFV is through the use of general circulation models. The latter provide a controlled environment in which cause-and-effect relationships can be investigated. Clearly, it becomes imperative to assess the statistical characteristics and describe the spatial structures of the low-frequency variability appearing in these numerical models. Section 8 gives a brief overview of the diagnostic evidence for the presence of LFV in atmospheric, global circulation models. We conclude our review of the observational evidence for the existence of structured dynamics on intraseasonal time scales by summarizing and discussing, in Section 9, the main results presented in the previous sections. A comprehensive theory, encompassing all aspects of LFV, still has to be developed. Therefore, it is pertinent to explore some of the paths that have been laid down to arrive at such a unified understanding. We end with a discussion, in Section 10, of some of the leading models proposed for theoretical considerations on low-frequency variability.

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BibTeX Citation

@article{pa00500y,
  author={Pandolfo, L.},
  title={Observational aspects of the low-frequency intraseasonal variability of the atmosphere in middle latitudes},
  year={1993},
  journal={Adv. Geophys.},
  volume={34},
  pages={93--174},
  doi={10.1016/S0065-2687(08)60435-5},
}

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RIS Citation

TY  - JOUR
ID  - pa00500y
AU  - Pandolfo, L.
PY  - 1993
TI  - Observational aspects of the low-frequency intraseasonal variability of the atmosphere in middle latitudes
JA  - Adv. Geophys.
VL  - 34
SP  - 93
EP  - 174
DO  - 10.1016/S0065-2687(08)60435-5
ER  -

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