Collaborative Research: The Relationship between the Trade Wind Inversion Layer and the Seasonal Development of the Southeast Pacific Inter-Tropical Convergence Zone (ITCZ)

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For most of the year there is a strong rainfall contrast across the equator in the eastern Pacific, with heavy convective precipitation in the intertropical convergence zone (ITCZ) north of the equator but only cloud cover and drizzle to the south. Several factors contribute to the rainfall contrast: in the ITCZ surface wind convergence provides uplift, low atmospheric stability promotes convective clouds, and high sea surface temperatures (SSTs) supply moisture through evaporation, while opposite conditions hold to the south. But starting around February a second ITCZ develops south of the equator and from February to April the eastern Pacific features a double ITCZ, with convective rainfall around 5 degrees north and south of the equator. The appearance of this secondary ITCZ in the Southern Hemisphere thus provides an opportunity to understand ITCZ genesis, in particular the dynamic and thermodynamic mechanisms that produce the transition from unfavorable to favorable conditions for deep convection.Research conducted here examines the genesis of the secondary ITCZ in the southeast Pacific with a particular focus on the trade wind inversion (TWI), a shallow layer which separates the moist atmospheric boundary layer from the warmer and drier free troposphere above it. The high stability of the TWI directly inhibits the convection necessary for ITCZ formation and instead promotes the stratocumulus (Sc) cloud deck which shades the southeast Pacific, producing cold SSTs which further discourage convection. Cold SSTs also inhibit ITCZ formation by discouraging boundary layer wind convergence, as boundary layer winds generally blow from colder to warmer SST. The project considers the factors which reduce the stability of the TWI in over the southeast Pacific, looking in particular at the transport of boundary layer moist static energy from the Northern Hemisphere ITCZ, as well as the possible reduction of stratus cloud cover due to reduced stability, which could lead to warmer SSTs. Further work uses analysis of momentum budgets to understand the change from surface wind divergence to convergence, one idea being that downward momentum transport across the TWI plays an important role. The work is conducted through analysis of observational datasets and climate model simulations, as well as specialized simulations performed for the project using the Community Atmosphere Model, the atmospheric component model of the Community Earth System Model (CESM).The work has educational broader impacts through the support and training of a graduate student. In addition, the project supports undergraduate students who conduct research during the summers, in part through the NSF-supported Research Experience for Undergraduates (REU) site at Colorado State University and the Significant Opportunities in Atmospheric Research and Science (SOARS) Program run by the University Corporation for Atmospheric Research. Research results are of interest for climate model development as most climate and earth system models suffer from a double ITCZ bias, in which a Southern Hemisphere ITCZ persists for most of the year over much of the Pacific ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.