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The relationship anywhere between ITCZ Venue and you can Cross-Equatorial Atmospheric Heat Transport: Throughout the Seasonal Duration to your Past Glacial Maximum

The relationship anywhere between ITCZ Venue and you can Cross-Equatorial Atmospheric Heat Transport: Throughout the Seasonal Duration to your Past Glacial Maximum

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.dos°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.

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.

step one. Inclusion

The atmospheric meridional overturning circulation in the tropics (the Hadley cell; Hadley 1735) controls the spatial distribution of both the tropical precipitation and the meridional heat transport in the atmosphere (AHT); the tropical precipitation maximum-the intertropical convergence zone (ITCZ)-is collocated with the ascending branch of the Hadley cell. The associated meridional AHT is in the direction of motion in the upper branch of the Hadley cell (Held 2001) and therefore is away from the location of ascending motion (see Fig. 1 for a schematic). Provided that the strength of the mass overturning circulation in the Hadley cell increases with meridional distance from the location of ascent, the magnitude of the atmospheric heat transport across the equator (AHTEQ) is proportional to the location of the ITCZ relative to the equator, with a northern location corresponding to southward atmospheric heat transport across the equator (see Fig. 1). As a consequence, meridional shifts in the ITCZ location are expected to be accompanied by changes in AHTEQ across a myriad of time scales and climate states such as 1) the seasonal cycle, 2) paleoclimate states, 3) idealized climate simulations (Kang et al. 2008), and 4) changes due to increasing greenhouse gas concentrations (Frierson and Hwang 2012). The value of AHTEQ, in turn, is a consequence of the hemispheric difference in energy input to the atmosphere and is influenced by radiation, clouds, aerosols, ocean heat transport, and surface processes at all latitudes (Yoshimori and Broccoli 2008). As a result, forcing and feedbacks in the extratropics can remotely influence the location of the ITCZ (Chiang and Bitz 2005) by way of the mutual connection among the Hadley circulation, AHTEQ, and the ITCZ location.

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