  In contrast to the Earth, where frictional heating is typically negligible,
we show that drag mechanisms could act as an important heat source in the
strongly-forced atmospheres of some exoplanets, with the potential to alter the
circulation. We modify the standard formalism of the atmospheric energy cycle
to explicitly track the loss of kinetic energy and the associated frictional
(re)heating, for application to exoplanets such as the asymmetrically heated
"hot Jupiters" and gas giants on highly eccentric orbits. We establish that an
understanding of the dominant drag mechanisms and their dependence on local
atmospheric conditions is critical for accurate modeling, not just in their
ability to limit wind speeds, but also because they could possibly change the
energetics of the circulation enough to alter the nature of the flow. We
discuss possible sources of drag and estimate the strength necessary to
significantly influence the atmospheric energetics. As we show, the frictional
heating depends on the magnitude of kinetic energy dissipation as well as its
spatial variation, so that the more localized a drag mechanism is, the weaker
it can be and still affect the circulation. We also use the derived formalism
to estimate the rate of numerical loss of kinetic energy in a few previously
published hot Jupiter models with and without magnetic drag and find it to be
surprisingly large, at 5-10% of the incident stellar irradiation.
