A climatology of the tropospheric thermal stratification using saturation potential vorticity

We assess the condition of slantwise convective neutrality in the extratropical troposphere by calculating the saturation potential vorticity, P^*, from reanalysis data. Regions of the atmosphere in which saturation entropy is constant along isosurfaces of absolute angular momentum, a state indicative of slantwise convective neutrality, have values of P^* equal to zero. In a global reanalysis dataset spanning the years 1970-2004, we identify tropospheric regions in which P^* is nearly zero, implying that vertical convection or slantwise convection may be important in determining the local thermal stratification. Convectively neutral air masses are common not only in the Tropics but also in higher latitudes, for example, over midlatitude continents during summer and in storm tracks over oceans during winter. Large-scale eddies appear to stabilize parts of the lower troposphere, particularly during winter.