Five-Year Climatology of Midtroposphere Dry Air Layers in Warm Tropical Ocean Regions as Viewed by AIRS/Aqua

Many studies have commented on the presence of midtroposphere dry air layers in normally moist areas of the warm-pool region in the tropical western Pacific Ocean. In this study, 5 yr of relative humidity (RH) observations from the Atmospheric Infrared Sounder (AIRS) instrument aboard the Aqua satellite are analyzed to identify areas of anomalously dry air between 600 and 400 hPa over deep convective regions of the tropical oceans. A dry air layer is defined when midlevel RH is ,20%, accounting for the lowest 10% of RH observations. Dry air layers appear to be more frequent over the Indian and Pacific Oceans than over the Atlantic Ocean. Large seasonal differences in the locations of dry air layers are apparent in each ocean basin. Large variations are also noted across the Pacific, suggesting limits on the applicability of case-study trends and observations of dry air layers to the Pacific as a whole. Back trajectories are then calculated for each observed parcel. The origin, or location of dehydration, is identified as the point along each trajectory at which the RH of the parcel is $100%. An analysis of the time between dehydration and dry air observation by AIRS suggests that dry air layers in June–August tend to last 1–2 days longer than those observed in other seasons. Although more dry air layers are observed to come from each hemisphere in its respective winter, most sources of dry air layers are subtropical and contribute anomalously dry air year-round. Other meteorological features are noted in the back trajectories, such as the eastward/westward wind transition from the subtropics to the tropics and the effects of the Indian monsoon on dry air distribution paths.