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

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.

Green Mountains and White Plains: The Effect of Northern Hemisphere Ice Sheets on the Global Energy Budget

“…This is sufficiently small compared to the average incoming radiation flux of 341.4 W m 22 that we can consider that the model is at equilibrium and the majority of the adjustment has taken place. We note that the TOA imbalance in the PMIP2 simulations, which have been used to study the energetics of the atmosphere in glacial times (Donohoe et al 2013), has a range from 0.2 to 1.6 W m 22 . Our simulations are therefore no further from equilibrium than other similar modeling studies.…”

“…This has two components: one due to reflections from the surface and one due to reflections from the atmosphere. In the global mean this latter term is the largest (Donohoe and Battisti 2011). Following Donohoe and Battisti (2011) we decompose the planetary albedo into these two components.…”

“…In the global mean this latter term is the largest (Donohoe and Battisti 2011). Following Donohoe and Battisti (2011) we decompose the planetary albedo into these two components. This method assumes that of the incident solar radiation S, a fraction A is absorbed by the atmosphere and a fraction R is reflected.…”

“…Finally, from A and R we can obtain expressions for the surface and atmospheric albedo. Full mathematical detail is contained within Donohoe and Battisti (2011).…”

“…While the local effects of the ice sheet on the midlatitude circulation have long been considered (e.g., Cook and Held 1988;Kutzbach and Wright 1985), their global impact has been somewhat ignored and never examined with a fully coupled oceanatmosphere climate model (Manabe and Broccoli 1985). There has been much written recently about the effect that changing the contrast between the temperature and energy balance in the Northern and Southern Hemispheres can have on the position of tropical rain belts and transports of energy across the equator (e.g., Yoshimori and Broccoli 2008;Kang et al 2008;Chiang and Friedman 2012;Donohoe et al 2013). This framework has proven itself quite useful for understanding changes in the large-scale circulation.…”