SSM/T-2 measurements of regional changes in three-dimensional water vapor fields during ENSO events

Observations of water vapor fields are presented from the SSM/T-2 (Special Sensor Microwave Temperature-2) radiometer during both El Nino (November 19 97) and La Nina (November 1995). Increased SST associated with the El Nino Southern Oscillation is found to cause enhanced circulation with drying in the upper troposphere of subtropical subsidence zones. Although regional ra ther than global in nature, this is similar to the effect proposed by Lindz en [1997] that could lead to a negative water vapor feedback. These observa tions provide a useful test of climate models to see how well they model cu rrently observed interannual changes in the water vapor field. Water vapor profiles were retrieved from SSM/T-2 observations by a physical algorithm f or all ocean areas between 40 degreesN and 40 degreesS. These data reveal m oistening (drying) in regions of equatorial warming (cooling). They also re veal drying (moistening) of the upper troposphere in subsidence zones to th e north and south. Patterns of computed and observed clear-sky OLR changes match changes in upper tropospheric humidity closely, with increased OLR in regions of drying. Partitioning of the water vapor field into changes expl ained by local SST or rainfall and dynamically induced changes indicates th at the changes in subtropical upper tropospheric humidity are caused, in pa rt, by large-scale dynamic effects. Numerical values of area averages and p lots of zonally averaged quantities are also presented, although analysis i s complicated by compensating processes.