The "slow manifold" of balanced motions is relevant to a broad range of problems in atmospheric and oceanic dynamics, and carries with it a complementary notion of imbalance comprising a fast manifold of unbalanced disturbances. Imbalance represents (i) a flow configuration into which the balanced flow may evolve spontaneously, e.g., in localized regions of the flow; (ii) forced overturning circulations that attempt to maintain balance, existing within and immediately outside the regions of imbalance; and (iii) a manifold of gravity or inertia-gravity waves excited as a result of the imbalance: waves which may propagate conservatively away from their source region, into the far field, causing interactions at a distance.

There are various sources of gravity waves, such as topography, shear instability, and convectively unstable motions impinging on a stable layer. In addition to these well-known sources is the mechanism of spontaneous imbalance that arises entirely within a fluid, from an initial state of balance, as a result of flow distortion, collapse of flow features to small scales, and generation of large parcel accelerations: that is, abrupt changes in parcel speed or direction. Imbalances resulting from slow motion project most efficiently onto the fast manifold of inertia-gravity waves at the lowest possible frequencies of inertia-gravity wave motion: viz., intrinsic frequencies near the local (effective) Coriolis parameter. Inherent to the fast manifold is that low-frequency motions are characterized by small vertical wavelengths. We expect, therefore, that balanced motions with short vertical scale or small equivalent depth may lead to spontaneous imbalance and gravity-wave radiation most effectively.

The local character of spontaneous imbalance in physical space implies that the mechanism will act in nearly inviscid flows regardless of the initial timescales of motion in relation to the local inertial period, provided that a strong source of disturbance energy (e.g., instability) and well resolved spectral cascade exist. Large-scale oceanic motions, in particular, may lead to spontaneous imbalance almost as readily as large-scale atmospheric motions, despite the disparity of initial timescales. It is therefore appropriate to consider spontaneous imbalance in the context of oceanic as well as atmospheric dynamics.

In roughly the same time frame, theoreticians have begun to explore the mathematical nature of spontaneous imbalance: to estimate its importance and (related to this) to provide some idea of the slow manifold itself. A consistent theme emerging from theoretical studies thus far emphasizes the smallness of the process: a smallness which, in some yet undetermined way, is presumably consistent with the spatio-temporal localization of imbalance noted by modelers and observers. It is important to recognize that small does not imply insignificant . Gravity waves excited by spontaneous imbalance radiate to the far field, where they may attain relatively large amplitude (e.g., via exponential growth of velocity). They may also interact with balanced motions and various physical processes in the source region, leading to positive feedbacks and amplification of unbalanced motion (e.g., through latent heat release).

Several of us working on the problem feel that the time has come for a first Workshop on Spontaneous Imbalance which, owing to the newness of the subject, will seek primarily to gather various perspectives from a broad selection of experts as well as scientists working on closely related problems (such as the role of organized mesoscale motions in tropospheric precipitation or oceanic mixing). Several goals may be envisioned for such a workshop, including:

Each of these broad goals will provide a framework for more specific goals, some of which may be met during the workshop, and others which may crystallize as result of the workshop presentations.