Frequency of Mid-Tropospheric Dry Layers in the Tropical Pacific ITCZ using ERA-Interim Reanalysis

Recent long-term characterization of the Pacific Intertropical Convergence Zone (ITCZ) shows that the ITCZ has narrowed, while convection in the center of the ITCZ has intensified. Many of the hypotheses regarding the mechanisms for these changes suggest that they may be due to the enhanced transport of dry air layers from the subtropics, which have been shown to affect the development of deep convection. While many studies have investigated the origins of dry layers and their relationship to temperature and moisture processes, we aim to understand how the frequency of dry layers in the mid-troposphere is related to the characteristics of the ITCZ. Following an approach similar to those using satellite observations, we use ERA-Interim reanalysis to create a 36-year mid-troposphere dry layer climatology for the tropical Pacific ITCZ. Mid-level (400 to 600 hPa) dry layers are identified using the lowest 10% of the relative humidity (RH) distribution and compared to existing observational climatologies. This climatology is used to investigate its relationship with ITCZ characteristics by applying an ITCZ location mask and relating the dry layer frequency to ITCZ width and precipitation intensity. For the 2002-2007 period, the mid-level dry layer frequency distribution resembles previous satellite climatologies, with lower frequencies near the center and southern side of the ITCZ and peak frequencies on the northern edge of the ITCZ in the eastern central Pacific. For 1979-2014, the frequency distribution displays a similar peak location, but contains more widespread dry layers on the western and southern portions of the ITCZ. Additionally, the dry layer frequency exhibits responses to Southern Oscillation events, with a strong reduction in dry layers during El Niño events. Preliminary time series analysis also suggests a slight increase in the frequency of dry layers that corresponds with the narrowing of the ITCZ over the past 36 years. Further analysis will be conducted to examine the relationship between dry layer frequency and the ITCZ width and precipitation intensity.