Environmental Science and Engineering Seminar

Wednesday January 7, 2015 4:00 PM

Why is the ITCZ in the Northern Hemisphere? And why is there a double ITCZ problem?

Speaker: Dargan Frierson, Department of Atmospheric Sciences, University of Washington
Location: South Mudd 365

The Intertropical Convergence Zone (ITCZ), the narrow band near the equator with some of the heaviest rainfall on Earth, is clearly located in the Northern Hemisphere (NH) for the majority of the year.  We propose a new explanation for this feature, based on a theoretical framework that essentially postulates that the ITCZ shifts towards heating in any part of the globe.  We show that the NH location of the ITCZ is due to the meridional overturning circulation of the ocean, that transports heat northward in the Atlantic.  The ocean circulation warms the North Atlantic, and the warmth spreads into the tropics, bringing the ITCZ into the NH. 

Climate models struggle to simulate the ITCZ, often producing too much rainfall in the Southern Hemisphere.  This aspect of the "double ITCZ problem" has plagued models for decades, and has not improved in recent years.  We show that this is due to the remote response to poor simulation of clouds over the Southern Ocean.  A lack of clouds in the Southern Hemisphere high latitudes makes this region too warm, which again spreads into the tropics, causing the double ITCZ problem. 

The ITCZ shifted southward in the late 20th century, a feature that is simulated by climate models.  Within models, we show that the change is due to increased sulfate aerosols within the NH, which cooled the entire hemisphere and shifted the tropical rains southward.

Series Environmental Science and Engineering Seminar

Contact: Kathy Young at 626-395-8732 katyoung@gps.caltech.edu