A Global Climatology of Tropospheric Inertial Instability

A climatology of tropospheric inertial instability is constructed using the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) at 250, 500, and 850 hPa. For each level, two criteria are used. The first criterion is the traditional criterion of absolute vorticity that is opposite in sign to the local Coriolis parameter. The second criterion, referred to as the gradient criterion, is the traditional criterion with an added term incorporating flow curvature. Both criteria show that instability, on all pressure levels, occurs most frequently in the tropics and decreases toward the poles. Compared to the traditional criterion, the gradient criterion diagnoses instability much more frequently outside the tropics and less frequently near the equator. The global distribution of inertial instability also shows many local maxima in the occurrence of instability. A sample of these local maxima is investigated further by constructing composites of the synoptic-scale flow associated with instability. The composites show that instability occurs in association with cross-equatorial flow in the North Atlantic Ocean, the Somali jet, tip jets off northern Madagascar, the western Pacific subtropical high, gap winds across Central America, upper-level ridging over western North America, and the North Atlantic polar jet. Furthermore, relatively long-lived synoptic-scale regions of instability are found within the midlatitude jet streams.