Collaborative Research: Interhemispheric and Zonal Asymmetries of the ITCZ

Lead PI: Dr. Richard Seager , Honghai Zhang

Unit Affiliation: Ocean and Climate Physics, Lamont-Doherty Earth Observatory (LDEO)

February 2020 - January 2023
Project Type: Research

DESCRIPTION: Satellites views of the earth typically show a narrow band of deep precipitating clouds over the tropical oceans where the trade winds converge. This band, known as the inter-tropical convergence zone, or ITCZ, is generally found north of the equator with a mean latitude of about 6N. Many aspects of the ITCZ including its existence, its role in global atmospheric circulation, and its preference for the Northern Hemisphere can be understood by thinking of the ITCZ as a uniform strip of convection with the same width, central latitude, and rainfall intensity at all longitudes. But this longitudinally averaged view of the ITCZ overlooks substantial variations in ITCZ latitude around the tropics. In particular the ITCZ is found south of the equator in the Indian Ocean and it has a southwest to northeast slope across both the Pacific and Atlantic oceans. The latitude of the ITCZ can thus be anywhere from from 4S to 10N depending on the ocean basin, the longitude within that basin, and the time of year.

This project uses combination of observational analysis and carefully designed model simulations to study regional variations in ITCZ latitude and other properties. For instance the northward displacement of the ITCZ in the eastern Pacific has been attributed to the geography of the North and South American coastlines, an idea which can be tested through simulations in which the west coasts of the Americas are mirror images of each other. The importance of oceanic processes is tested using simplified ocean configurations which allow some mechanisms but disable others. Ocean model configurations include a simple motionless "slab" ocean, a modified slab ocean which captures near-surface heat transport driven by surface winds, and a "shallow water" ocean model which allows surface cooling due to the upwelling of cold water from the deep ocean. In addition, the project develops a version of the Community Earth System Model (CESM) in which flux corrections are applied to improve the representation of the ITCZ.

The work is of practical interest given the importance of the ITCZ for tropical rainfall as well as its outsized effects on the climate and atmospheric circulation of the earth. Despite its importance the ITCZ is not well simulated in models used for weather prediction and climate change projections, thus a better understanding of ITCZ dynamics could be quite useful for model improvement. The work also has value for the broader research enterprise, as model configurations developed for the project will be openly available for adoption by the worldwide research community. In addition, the award provides support and training for two graduate students, thereby contributing to scientific workforce development.