The Relationship between ITCZ Location and Cross-Equatorial Atmospheric Heat Transport: From the Seasonal Cycle to the Last Glacial Maximum

Aaron Donohoe Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts

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John Marshall Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts

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David Ferreira Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts

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David Mcgee Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts

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Abstract

The authors quantify the relationship between the location of the intertropical convergence zone (ITCZ) and the atmospheric heat transport across the equator (AHTEQ) in climate models and in observations. The observed zonal mean ITCZ location varies from 5.3°S in the boreal winter to 7.2°N in the boreal summer with an annual mean position of 1.65°N while the AHTEQ varies from 2.1 PW northward in the boreal winter to 2.3 PW southward in the boreal summer with an annual mean of 0.1 PW southward. Seasonal variations in the ITCZ location and AHTEQ are highly anticorrelated in the observations and in a suite of state-of-the-art coupled climate models with regression coefficients of −2.7° and −2.4° PW−1 respectively. It is also found that seasonal variations in ITCZ location and AHTEQ are well correlated in a suite of slab ocean aquaplanet simulations with varying ocean mixed layer depths. However, the regression coefficient between ITCZ location and AHTEQ decreases with decreasing mixed layer depth as a consequence of the asymmetry that develops between the winter and summer Hadley cells as the ITCZ moves farther off the equator.

The authors go on to analyze the annual mean change in ITCZ location and AHTEQ in an ensemble of climate perturbation experiments including the response to CO2 doubling, simulations of the Last Glacial Maximum, and simulations of the mid-Holocene. The shift in the annual average ITCZ location is also strongly anticorrelated with the change in annual mean AHTEQ with a regression coefficient of −3.2° PW−1, similar to that found over the seasonal cycle.

Corresponding author address: Aaron Donohoe, Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Room 54-918, 77 Massachusetts Avenue, Cambridge, MA 02139-4307. E-mail: thedhoe@mit.edu

Abstract

The authors quantify the relationship between the location of the intertropical convergence zone (ITCZ) and the atmospheric heat transport across the equator (AHTEQ) in climate models and in observations. The observed zonal mean ITCZ location varies from 5.3°S in the boreal winter to 7.2°N in the boreal summer with an annual mean position of 1.65°N while the AHTEQ varies from 2.1 PW northward in the boreal winter to 2.3 PW southward in the boreal summer with an annual mean of 0.1 PW southward. Seasonal variations in the ITCZ location and AHTEQ are highly anticorrelated in the observations and in a suite of state-of-the-art coupled climate models with regression coefficients of −2.7° and −2.4° PW−1 respectively. It is also found that seasonal variations in ITCZ location and AHTEQ are well correlated in a suite of slab ocean aquaplanet simulations with varying ocean mixed layer depths. However, the regression coefficient between ITCZ location and AHTEQ decreases with decreasing mixed layer depth as a consequence of the asymmetry that develops between the winter and summer Hadley cells as the ITCZ moves farther off the equator.

The authors go on to analyze the annual mean change in ITCZ location and AHTEQ in an ensemble of climate perturbation experiments including the response to CO2 doubling, simulations of the Last Glacial Maximum, and simulations of the mid-Holocene. The shift in the annual average ITCZ location is also strongly anticorrelated with the change in annual mean AHTEQ with a regression coefficient of −3.2° PW−1, similar to that found over the seasonal cycle.

Corresponding author address: Aaron Donohoe, Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Room 54-918, 77 Massachusetts Avenue, Cambridge, MA 02139-4307. E-mail: thedhoe@mit.edu
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