Tropical Rainfall Intensifies While The Doldrums Narrow

Scientists show long-term changes in the Intertropical Convergence Zone's location, extent, and rainfall intensity. This satellite image shows a textbook Intertropical Convergence Zone (ITCZ) on 24 November 2010 that stretches all the way across the Pacific Ocean. New research improves understanding of ITCZ and shows that rainfall in the zone is intensifying. Courtesy: NASA GOES Project ScienceScientists show long-term changes in the Intertropical Convergence Zone's location, extent, and rainfall intensity. This satellite image shows a textbook Intertropical Convergence Zone (ITCZ) on 24 November 2010 that stretches all the way across the Pacific Ocean. New research improves understanding of ITCZ and shows that rainfall in the zone is intensifying. Courtesy: NASA GOES Project Science

Scientists show long-term changes in the Intertropical Convergence Zone’s location, extent, and tropical rainfall intensity. The data also show that tropical rainfall has intensified, especially toward the center of the zone. 

by Sarah Stanley, 

Silence is not golden around Earth’s equator—when trade winds settle down, it spells disaster for sailing ships. In the Northern Hemisphere, trade winds blow southwest toward the equator, whereas Southern Hemisphere trade winds blow northwest. These winds meet at the Intertropical Convergence Zone (ITCZ), a ring around Earth traditionally known as the doldrums for its periodically calm winds that once trapped sail-driven seafarers.ITCZ is a major tropical atmospheric feature. In this zone, hot air rises through the troposphere as it follows the Hadley cell, causing frequent thunderstorms and heavy rainfall. Seasonal shifts of ITCZ toward and away from the equator give rise to the intensely wet and dry seasons experienced by residents of the tropics. Forecasts of future global weather patterns depend heavily on accurately modeling the characteristics of ITCZ, but scientists have struggled to objectively identify ITCZ using existing measurements.
Now, in a new paper, Wodzicki and Rapp have improved upon previous efforts to identify ITCZ’s central location, its northern and southern boundaries, and the monthly intensity of the rainfall it causes.The scientists modified an algorithm to identify and characterize ITCZ. They applied it to satellite observations from the NASA Tropical Rainfall Measuring Mission’s Microwave Imager (TMI), rainfall data from NASA’s Global Precipitation Climatology Project (GPCP), and the European Centre for Medium-Range Weather Forecasts’s ERA-Interim data set, which provides detailed climate data for every year since 1979.The ERA-Interim data place the center of the Pacific ITCZ at approximately 8°N. Although the center has remained relatively constant over the past 36 years, TMI and GPCP data show that the southern and northern boundaries have shifted toward the center, effectively narrowing the ITCZ band in the Pacific. ITCZ’s boundaries vary, but its width was more variable prior to 1998, when a large El Niño event occurred and the Pacific Decadal Oscillation changed phase.The data also show that rainfall in ITCZ has intensified, especially toward the center of the zone. Further research is needed to pinpoint what drives ITCZ’s shifting characteristics and whether the increased rainfall in ITCZ is linked to its narrowing width. Meanwhile, the authors’ ITCZ algorithm could provide a quantitative benchmark for how well ITCZ characteristics are represented in our climate models.

Abstract

An objective, automated Intertropical Convergence Zone (ITCZ) identification and characterization algorithm is developed and applied to European Centre for Medium-Range Weather Forecasts Reanalysis Interim (ERA-Interim) variables, and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Global Precipitation Climatology Project (GPCP) rain rates (RRs) to create 15 and 36 year climatologies, respectively, in the tropical Pacific ocean. Characteristics derived from the algorithm include ITCZ latitude, northern and southern ITCZ convection boundaries and extent, and precipitation intensity. The climatological location of the ITCZ was found near 8°N, consistent with previous studies, with a preferred southern boundary location of 4°N. The northern ITCZ boundary did not exhibit a preferred boundary with locations between 7° and 15°N having similar frequencies of occurrence. The northern and southern extents of the ITCZ were symmetric in the central Pacific and asymmetric in the east Pacific. Long-term trends in the ITCZ width and precipitation intensity showed significant narrowing and intensifying. Separation of characteristics by season and location (i.e., central and eastern Pacific) revealed negative trends in ITCZ width in both domains, with trends in the east Pacific weaker than those in the central Pacific. Trends in precipitation intensity near the center of the ITCZ indicate a strong intensification, with slightly stronger trends in the central Pacific than in the east Pacific. These findings show that while the location of the ITCZ has not changed significantly over the past three decades, the ITCZ has narrowed and convection has intensified.

Citation

K.R. Wodzicki and A.D. Rapp; Long-term characterization of the Pacific ITCZ using TRMM, GPCP, and ERA-Interim; Journal of Geophysical Research: Atmospheres, doi:10.1002/2015JD024458, 2016.

Source

American Geophysical Union’s EOS.

 

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