Diagnosis of subtropical humidity dynamics using tracers of last saturation

We present a technique for diagnosing the mechanisms that control tropospheric water vapor distribution, given data sets for the winds, temperature, and humidity. The technique involves defining a large number of tracers, each of which represents air which has last been saturated in a particular region of the atmosphere. The time-mean, zonal-mean tracer fields show the typical pathways that air parcels take between one occurrence of saturation and the next. Because saturation vapor pressure is a function only of temperature, and because mixing ratio is conserved for unsaturated parcels, these tracer fields can be used together with the temperature field to reconstruct the water vapor field. We apply the technique to NCEP/NCAR Reanalysis data for DJF 2001-2002 using the offline tracer transport model MATCH. The results show that the dryness of the subtropical troposphere is primarily controlled by isentropic transport of very dry air by midlatitude eddies, and that diabatic descent from the tropical upper troposphere plays a secondary role in controlling the dryness of the subtropics.