Ten Years of Measurements of Tropical Upper-Tropospheric Water Vapor by the MOZAIC: Climatology, Variability, Transport and Relation to Deep Convection

Ten years of measurements of tropical upper-tropospheric water vapor by the MOZAIC (Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft) are studied over three selected tropical regions (tropical Atlantic, tropical Africa and the Asian Monsoon region) in the following aspects: 1) characterization of UTH climatology and interannual/decadal variability; 2) comparison with the ECMWF analysis; and 3) examination of the influences on UTH of large-scale transport and deep convection.

The UTH annual cycle keeps pace with the corresponding seasonal migration of the ITCZ. Some regional differences are found and they are closely related to the differences in the distribution of deep convection frequency. In the deep tropics, the moisture level frequently reaches ice supersaturation. UTH generally increases with height from 300 to 200 hPa. The distribution of tropical UTH is bimodal. The two modes stay rather constant at around 20% and 100%; differences in the mean value are largely due to variations in the proportion of the two modes as opposed to changes in modes themselves. A noticeable interannual variability of UTH is associated with the 1997/1998 ENSO. A surprising moistening trend is found over tropical Africa from 2000 to 2002 (and maybe beyond); the reason for the trend is unknown and requires more study.

Comparison between MOZAIC and the ECMWF analysis shows that the ECMWF constantly predicts a drier upper troposphere by 10 – 20 %. Moreover, the bimodality distribution of UTH and ice supersaturation are hardly seen in the ECMWF. Although differences in temporal/spatial resolutions can explain some of these discrepancies, others are related to the deficiencies of the assimilation system.

Meridional and zonal moisture fluxes as measured onboard of the aircraft are studied. Zonal flux is found to dominate the meridional flux. Decomposition of the total flux into contributions from mean circulation and transient motion shows that the former outweighs the latter, suggesting that the tropical upper-tropospheric circulation is, to a large extent, quasi-stationary on the seasonal timescale. Transient flux seems to be more significant between very dry and very moist regions serving as an important agent linking the two. Finally, we study the influence on UTH of transport and deep convection from a global perspective using reanalysis wind and satellite-observed convection.