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Recent long-term characterization of the Pacific Intertropical Convergence Zone (ITCZ) shows that the ITCZ has narrowed, which may be due to the enhanced transport of dry air layers from the subtropics. In this study, we examine the relationship between the frequency of midtropospheric dry layers and ITCZ width. European Centre for Medium-Range Weather Forecasts Reanalysis Interim midtroposphere (400 to 600 hPa) relative humidity is used to create a 36-year (1979-2014) dry layer climatology for the Pacific ITCZ region. Analysis shows an inverse relationship between ITCZ width and dry layer frequency, with ITCZ width decreasing as the frequency of dry layers increases. Moreover, findings show that the region of greatest long-term increase in dry layer frequency occurs in the central Pacific, where the greatest narrowing of the ITCZ is also observed.

Plain Language Summary The Intertropical Convergence Zone (ITCZ) is a large region of clouds and precipitation, which is responsible for much of the tropical rainfall. Both observations and climate models indicate a narrowing of this precipitation zone as surface temperatures increase. Model studies suggest one mechanism that may impact the width of this precipitation zone is the occurrence of dry layers in the middle atmosphere, which can inhibit the development of thunderstorms and suppress precipitation. In this study, we use observations to determine how frequent dry layers are observed in the ITCZ, how they have changed over time, and whether they are related to the width of this zone of precipitation. We found that the ITCZ narrows as dry layers increase, that the frequency of dry layers has increased in recent decades, and that the zone of greatest narrowing of the tropical rain band occurs where dry layer frequencies show the largest increases.