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

Abstract

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.

Tropical and Extra-Tropical Influences on the Distribution of Free Tropospheric Humidity Over the Intertropical Belt

“…The extra-tropical influence on the PDF of relative humidity is not confined to the subtropical regions nor is the importance of dry free tropospheric layers. In the deep tropics, dry air also has some connections to mid-latitudes (Yoneyama and Parsons 1999;Roca et al 2005;Casey et al 2009). There, dry air is important to the development of deep convection as surveyed in details in Del Genio (2011 this issue).…”

Increasing Frequency of Midtropospheric Dry Layers in the Pacific Intertropical Convergence Zone

“…The methodology for midtropospheric dry layer identification is based on Casey et al (). They used the outgoing longwave radiation (OLR) seasonal climatology (1968–1996) from the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Prediction to locate areas of oceanic deep convection where monthly mean OLR was less than 240 W/m 2 for at least one calendar month.…”

“…Since previous studies suggest that the ITCZ has narrowed over time with increased precipitation in the center, our study aims to relate the dry layer climatology using ERA‐Interim reanalysis to ITCZ trends and characteristics. To validate that ERA‐Interim water vapor profiles reproduce the dry layer climatology, the Casey et al () dry layer identification is applied for the same time period (2002–2007) and latitude region (20°N to 20°S) as their original study, and the results are compared.…”

Tropospheric dry layers in the tropical western Pacific: comparisons of GPS radio occultation with multiple data sets

“…Regarding dry layer occurrence, Casey et al (2009) created a 5-year climatology on dry layers between 600 and 400 hPa over deep convective regions of the tropical oceans using AIRS (Atmospheric Infrared Sounder) data. Their results show large spatial and seasonal variability for different ocean basins, pointing out the limits of applying case study trends to the whole basin.…”