Observed Changes in ITCZ Extent and the Relationship to Midtropospheric Dry Layers

Many recent modeling studies suggest that the region of heavy precipitation associated with the Intertropical Convergence Zone (ITCZ) becomes narrower while precipitation becomes more intense in enhanced greenhouse gas forcing scenarios. Although there have been many observational studies defining the climatological location of the ITCZ and the intensity of general tropical precipitation, there has been less work using observations to determine the extent of the ITCZ. To test whether we observe ITCZ changes in the current climate, satellite observations from the Tropical Rainfall Measuring Mission (TRMM), the Global Precipitation Climatology Project (GPCP), and ERA-Interim reanalysis are combined to develop an automated identification of the ITCZ and metrics describing its characteristics, including ITCZ center location, width, and precipitation intensity. Results from 1979-2015 show that the meridional extent of precipitation associated with the ITCZ has decreased, with a simultaneous increase in precipitation intensity, especially in the center of the ITCZ. Some hypothesized mechanisms for this decrease in extent suggest that it may be due to enhanced transport of dry air layers from the subtropics, which suppresses the development of deep convection along edge of convective zones like the ITCZ. To investigate this hypothesis, a 36-year mid-tropospheric dry layer climatology is developed from ERA-Interim for the tropical Pacific ITCZ. Positive (negative) anomalies in dry layer frequency correspond with a narrower (wider) ITCZ. Time series analysis suggests a slight increase in the frequency of dry layers over this period, with the largest increases on the margins of the ITCZ. The greatest dry layer frequency increases are observed in the central Pacific region, which also corresponds with the region where the largest ITCZ narrowing is observed.