Relative role of convection and large-scale flow in controlling upper tropospheric humidity

The controls on Upper Tropospheric Humidity (UTH) are examined using satellite measurements of UTH, meteorological analyses from the Atmospheric Infrared Sounder (AIRS) instrument, and trajectory-based water vapor simulations. The subtropics is much drier than the tropics through most of subtropical troposphere, but there are significant zonal variations that are related regional variability in the processes that determine subtropical UTH. Both observations and trajectory model show that there are intermittent high and low values that are due to intrusions of high PV air into subtropics over the eastern Pacific and Atlantic Ocean. On the other hand, the humidity distribution over the Indian Ocean and western Pacific is more closely linked to the location and strength of subtropical anticyclones. In these regions there are eastward propagating features in the subtropical UTH that are out of phase with tropical UTH and are linked to the Madden Julian Oscillation.