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 1997) 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 rather than global in nature, this is similar to the effect proposed by Lindzen [1997] that could lead to a negative water vapor feedback. These observations provide a useful test of climate models to see how well they model currently observed interannual changes in the water vapor field. Water vapor profiles were retrieved from SSM/T-2 observations by a physical algorithm for all ocean areas between 40 degreesN and 40 degreesS. These data reveal moistening (drying) in regions of equatorial warming (cooling). They also reveal drying (moistening) of the upper troposphere in subsidence zones to the 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 explained by local SST or rainfall and dynamically induced changes indicates that the changes in subtropical upper tropospheric humidity are caused, in part, by large-scale dynamic effects. Numerical values of area averages and plots of zonally averaged quantities are also presented, although analysis is complicated by compensating processes.