A satellite-derived upper-tropospheric water vapor transport index for climate studies

A new approach is presented to quantify upper-level moisture transport from geostationary satellite data Daily time sequences of Geostationary Operati onal Environmental Satellite GOES-7 water vapor imagery were used to produc e estimates of winds and water vapor mixing ratio in the cloud-free region of the upper troposphere sensed by the 6.7-mu m water vapor channel. The wi nds and mixing ratio values were gridded and then combined to produce a par ameter called the water vapor transport index (WVTI), which represents the magnitude of the two-dimensional transport of water vapor in the upper trop osphere. Daily grids of WVTI, meridional moisture transport, mixing ratio, pressure, and other associated parameters were averaged to produce monthly fields for June, July, and August (JJA) of 1987 and 1986 over the Americas and surrounding oceanic regions. The WVTI was used to compare upper-troposp heric moisture transport between the summers of 1987 and 1988, contrasting the latter part of the 1986/87 El Nino event and the La Nina period of 1988 . A similar product derived from the National Centers for Environmental Pre diction (NCEP)-National Center for Atmospheric Research (NCAR) 40-Year Rean alysis Project was used to help to validate the index. Although the goal of this research was to describe the formulation and utility of the WVTI, con siderable insight was obtained into the interannual variability of upper-le ver water vapor transport. Both datasets showed large upper-level water vapor transport associated wit h synoptic features over the Americas and with outflow from tropical convec tive systems. Minimal transport occurred over tropical and subtropical high pressure regions where winds were light. Index values from NCEP-NCAR were 2-3 times larger than that determined from GOES. This difference resulted f rom large zonal wind differences and an apparent overestimate of upper-trop ospheric moisture in the reanalysis model. A comparison of the satellite-derived monthly values between the summers of 1987 and 1988 provided some insight into the impact of the ENSO event on u pper-level moisture and its transport during the period. During July 1987, a large portion of the Tropics in the eastern Pacific Ocean and Caribbean S ea was dominated by strong vapor transport in excess of 4.0 g kg(-1) m s(-1 ), with relatively small amounts in the other months. JJA 1988 transport va lues reached similar magnitude and showed similar patterns for all three mo nths. The meridional transport of upper-level water vapor indicated large p oleward transport from the Tropics to the higher latitudes. This transport favored the Southern Hemisphere, with large transport occurring south of th e ITCZ, which extended across the eastern Pacific and northern South Americ a. Zonally averaged monthly transport values were shown to provide a simple way to quantify the monthly and interannual changes in water vapor transpo rt. Zonally averaged WVTI values peaked in the Southern Hemisphere subtropi cs during both austral winters. Ln the Tropics, a single, more-pronounced p eak located over the equator and south latitudes occurred in 1988 as oppose d to a dual peak in 1987. The second peak around 20 degrees N latitude is c onsistent with findings of others in which upper-tropospheric winds were no ted to be stronger in this region during warm ENSO events. Zonally averaged meridional transport was southward for all summer months and was stronger in 1988. The asymmetric nature of the zonally averaged meridional transport (more southerly water vapor transport) was enhanced during JJA 1988, thus indicating a stronger upper-level branch of the Hadley circulation during t his notably strong La Nina period.