Changes in Tropospheric Stability and the Frequency of Moist Convection in the Northern Hemisphere Extratropics

Warming in the recent decades has led to the poleward expansion of the Hadley Circulation. While moist convection establishes and maintains lapse rates in the troposphere of the tropics, in the middle and polar latitudes they are stable on average. Considering the expansion of the properties of the tropical atmosphere, this study examines if there are corresponding changes in the frequency of both upright and slantwise moist convection in the Northern Hemisphere extratropics through calculating Saturation Potential Vorticity (P^*) from NCAR/NCEP reanalysis data. P^* and its component terms vary with the local lapse rate; the former is approximately zero when the latter is moist adiabatic. This study focuses on changes in the frequency and annual cycle of stable conditions from 1948 to 2014. The spatial coverage and frequency of stable conditions has declined in the Northern Hemisphere extratropics. This reduction in frequency is more pronounced over land in the autumn, winter and spring.