Interannual variations and climate change projections of upper tropospheric water vapor in different tropical ocean basins

Tropical upper tropospheric water vapor (UT WV) lies in the core of scientific interests in projecting future climate due to its important yet entangled role in radiation and dynamics. Five years of Aura Microwave Limb Sounder (MLS) data and two climate model simulations (NCAR_CCSM3.0 and GFDL_CM2.1) are examined to study the interannual variations of UT (~200mb) WV in four tropical ocean basins as well as the UT WV changes in 2070-2099 projected by the models for such basins. The four basins are Western Pacific (WP), Eastern Pacific (EP), Atlantic (ATL), and Indian (IND) Ocean. Observed interannual variability of basin-averaged UT WV is strongly correlated with that of the UT temperature and tropical-mean sea surface temperature (SST) for all four basins. However, the correlations with basin-averaged ice water content (IWC), cloud fraction, and precipitation are much higher in the EP and IND than in the WP and ATL. The atmosphere-ocean coupled model simulations for 1980-1999 in the two GCMs approximately agree with the observations on these features. Simulated UT WV variations of each basin are strongly correlated with UT temperature changes of the same basin and tropical-mean SST. The correlations with cloud fraction and precipitation are the largest in the EP than other basins. The differences of simulated UT WV between the 20th and 21st centuries are also examined. The results show that the changes of UT WV scaled by the changes of tropical-mean SST are proportional to the regressions of the UT WV onto the UT temperature and tropical-mean SST in the 20th century for all basins except the EP. For the EP, the UTWV changes in the 21st century do not exhibit a simple relation to the UT temperature or SST changes as those simulated in the 20th century, which might be due to the influence of clouds. The study suggests that the interannual variability of basin-wide UTWV is largely controlled by temperature changes for all ocean basins, but the influence of clouds has regional dependence. In particular, UTWV in the EP seems to be more correlated with cloud changes than other basins.