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In this paper, we attempt to quantify the relationship between the location of the ITCZ and AHTEQ in models and observations. We demonstrate that this relationship is robust whether considering the seasonal migration of the ITCZ, the ITCZ shift due to anthropogenic forcing, or the ITCZ shift in past climates including the Last Glacial Maximum. We also study the relationship between tropical SST gradients and the ITCZ location. Our paper is organized as follows. In section 2 we analyze the seasonal cycle of the ITCZ location, AHTEQ, and tropical SST gradients in both the observations (section 2a) and coupled climate models (section 2b). We also analyze the seasonal cycle in an ensemble of slab ocean aquaplanet simulations with ocean varying mixed layer depth (section 2c). In section 3, we focus on the annual mean shift in the ITCZ, AHTEQ, and tropical SST gradients in model simulations of CO2 doubling, the Last Glacial Maximum, and 6000 years before present. We conclude with a summary and discussion in section 4.
In this section, we analyze the relationship between ITCZ location and AHTEQ over the seasonal cycle. In the boreal summer, the Northern Hemisphere (NH) receives excess insolation relative to the annual mean, leading to atmospheric heating (Donohoe and Battisti 2013). In contrast, the Southern Hemisphere (SH) receives a deficit of insolation relative to the annual mean, leading to atmospheric cooling. The hemispheric asymmetry of atmospheric energy input is largely balanced by atmospheric energy transport from the source of atmospheric heating to the cooling, resulting in southward atmospheric heat transport across the equator (Fasullo and Trenberth 2008). The Hadley cell and ITCZ shift northward toward the warmer SSTs, which positions the southern branch of the Hadley cell over the equator, resulting in southward AHTEQ in the thermally direct Hadley cell. Similarly, in the austral summer, AHTEQ is northward and the ITCZ is in the Southern Hemisphere. We analyze the seasonal cycle of the observations in section 2a, coupled models in section 2b, and slab ocean aquaplanet simulations with varying mixed layer depths in section 2c.
1) Study sources and methods
Here we describe the data sources and calculation methods for analyzing the relationship among the ITCZ location, the tropical SST gradient, and AHTEQ in the observations.
(i) Tropical rain and you can ITCZ venue
We use the precipitation climatology from the National Oceanographic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) merged analysis (Xie and Arkin 1996), a gridded data product that combines gauge measurements, satellite observations, and numerical models. The climatology is composed of data from 1981 to 2010. We use the precipitation centroid (PPenny) defined by Frierson and Hwang (2012) as a metric for the location of the ITCZ–tropical precipitation maximum. There, the precipitation centroid was defined as the median of the zonal average precipitation from 20°S to 20°N. The precipitation is interpolated to a 0.1° grid over the tropics to allow the precipitation centroid to vary at increments smaller than the grid spacing.