Relative Role of Convection and Large-Scale Flow in Controlling Upper Tropospheric Humidity

The controls on the subtropical Upper Tropospheric Humidity (UTH) are examined using satellite measurements of UTH from the Atmospheric Infrared Sounder (AIRS), meteorological analyses, and trajectory-based water vapor simulations. The subtropics is much drier than the tropics through most of the troposphere, but there are significant zonal variations that are related to regional variability in the processes that determine subtropical UTH. Both observations and trajectory model calculations show that there are intermittent high and low UTH values that are due to intrusions of high PV air into the subtropical 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, which appear to be linked to the intraseasonal tropical convection variability (e.g. Madden Julian Oscillation). Clustering analysis shows that coherent variations of trajectory clusters with longitude are also consistent with these transient intraseasonal convections. The dominant trajectory patterns differ between convective and non-convective regions, indicating that the transport processes are different in these regions. Furthermore, the variability of relative humidity (RH) can also be partially explained by the mean RH of the trajectory clusters. The trajectory-based water vapor simulations and clustering analysis are used to understand the relative contribution of convection, subsidence and horizontal transport of the exchanges of air between the tropics and subtropics. In addition to helping to explain the variability of RH associated with different physical processes, our analysis also helps to quantify the difference in transport pathways and origins of moisture.