gradient, Coriolis and centrifugal forces acting on an air parcel, an imbalance that is traditionally
diagnosed under conditions of anticyclonic absolute vorticity. Although past authors have argued
whether long-lived inertial instability could occur in the midlatitude troposphere, observations have
suggested otherwise. In fact, circulations associated with the release of inertial instability (or even
weak inertial stability) have been proposed to facilitate the intensification of mesoscale convective
systems, tropical cyclones and banded precipitation. However, few studies have investigated the
global distribution and causes of inertial instability in the troposphere.
This talk presents results from a global climatology of tropospheric inertial instability constructed
with the ECMWF ERA-Interim reanalysis for 250, 500 and 850 hPa during 1979–2014. The
climatology is constructed according to two instability criteria. The first criterion is the traditional
criterion of anticyclonic absolute vorticity, and the second criterion, referred to as the gradient
criterion, is the traditional criterion with an added term incorporating flow curvature.
With each criterion, the global distribution shows that, at all levels, instability occurs most
frequently in the winter hemisphere near the equator and decreases toward the poles. Furthermore,
the global distribution also shows many local maxima in the occurrence of instability. The cause of
instability for three of these local maxima is investigated by compositing the large-scale flow
associated with both inertial instability and inertial stability. These composites show that inertial
instability is associated with the cross-equatorial Somali jet in the North Indian Ocean, upper-level
ridging over the Pacific Northwest, and anticyclonic shear and curvature in the North Atlantic jet-
exit region, where large-scale, relatively long-lived instability is found.
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