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Sensitivity of Intertropical Convergence Zone Movement to the Latitudinal Position of Thermal Forcing

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  • 1 School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan, South Korea
  • | 2 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

A variety of recent studies have shown that extratropical heating anomalies can be remarkably effective at causing meridional shifts in the intertropical convergence zone (ITCZ). But what latitudinal location of forcing is most effective at shifting the ITCZ? In a series of aquaplanet simulations with the GFDL Atmospheric Model, version 2 (AM2), coupled to a slab mixed layer ocean, it is shown that high-latitude forcing actually causes a larger shift in the ITCZ than when equivalent surface forcing is applied in the tropics. Equivalent simulations are run with an idealized general circulation model (GCM) without cloud and water vapor feedbacks, also coupled to an aquaplanet slab ocean, where the ITCZ response instead becomes weaker the farther the forcing is from the equator, indicating that radiative feedbacks must be important in AM2.

In the absence of radiative feedbacks, the tendency for anomalies to decrease in importance the farther away they are from the equator is due to the quasi-diffusive nature of energy transports. Cloud shortwave responses in AM2 act to strengthen the ITCZ response to extratropical forcing, amplifying the response as it propagates toward the equator. These results emphasize the great importance of the extratropics in determining the position of the ITCZ.

Corresponding author address: Sarah M. Kang, School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, 100 Banyeon-ri, Eonyang-eup, Ulsan 689-798, South Korea. E-mail: skang@unist.ac.kr

Abstract

A variety of recent studies have shown that extratropical heating anomalies can be remarkably effective at causing meridional shifts in the intertropical convergence zone (ITCZ). But what latitudinal location of forcing is most effective at shifting the ITCZ? In a series of aquaplanet simulations with the GFDL Atmospheric Model, version 2 (AM2), coupled to a slab mixed layer ocean, it is shown that high-latitude forcing actually causes a larger shift in the ITCZ than when equivalent surface forcing is applied in the tropics. Equivalent simulations are run with an idealized general circulation model (GCM) without cloud and water vapor feedbacks, also coupled to an aquaplanet slab ocean, where the ITCZ response instead becomes weaker the farther the forcing is from the equator, indicating that radiative feedbacks must be important in AM2.

In the absence of radiative feedbacks, the tendency for anomalies to decrease in importance the farther away they are from the equator is due to the quasi-diffusive nature of energy transports. Cloud shortwave responses in AM2 act to strengthen the ITCZ response to extratropical forcing, amplifying the response as it propagates toward the equator. These results emphasize the great importance of the extratropics in determining the position of the ITCZ.

Corresponding author address: Sarah M. Kang, School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, 100 Banyeon-ri, Eonyang-eup, Ulsan 689-798, South Korea. E-mail: skang@unist.ac.kr
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