Controls on subtropical upper tropospheric humidity

The controls on subtropical Upper Tropospheric Humidity (UTH) are examined using measurements from the Atmospheric Infrared Sounder (AIRS) satellite instrument together with meteorological analyses. 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. Over the eastern Pacific and Atlantic Ocean there are intermittent high and low values that are due to intrusions of high PV air into subtropics. There is very dry air (relative humidity (RH) less than 20%) behind and within the intrusions and moist (RH around 70%) air ahead of it. 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.