  The possibility of constructing Lorenz's concept of available potential
energy (APE) from a local principle has been known for some time, but has
received very little attention so far. Yet, the local APE framework offers the
advantage of providing a positive definite local form of potential energy,
which like kinetic energy can be transported, converted, and created/dissipated
locally. In contrast to Lorenz's definition, which relies on the exact from of
potential energy, the local APE theory uses the particular form of potential
energy appropriate to the approximations considered. In this paper, this idea
is illustrated for the dry hydrostatic primitive equations, whose relevant form
of potential energy is the specific enthalpy. The local APE density is
non-quadratic in general, but can nevertheless be partitioned exactly into mean
and eddy components regardless of the Reynolds averaging operator used.
  This paper introduces a new form of the local APE that is easily computable
from atmospheric datasets. The advantages of using the local APE over the
classical Lorenz APE are highlighted. The paper also presents the first
calculation of the three-dimensional local APE in observation-based atmospheric
data. Finally, it illustrates how the eddy and mean components of the local APE
can be used to study regional and temporal variability in the large-scale
circulation. It is revealed that advection from high latitudes is necessary to
supply APE into the storm track regions, and that Greenland and Ross Sea, which
have suffered from rapid land ice and sea ice loss in recent decades, are
particularly susceptible to APE variability.
