Formation mechanisms of latitudinal CO₂ gradients in the upper troposphere over the subtropics and tropics

Abstract

[1] Aircraft observations and numerical simulations using an atmospheric transport model exhibit large latitudinal gradients in the carbon dioxide (CO 2 ) mixing ratio around the subtropics and equator throughout the troposphere. The formation mechanisms of the latitudinal CO 2 gradients are investigated at aircraft flight altitudes (350-260 hPa) by considering the influences of atmospheric transport and carbon fluxes at the earth's surface. A meridional transport analysis demonstrates how various transport processes create the latitudinal CO 2 gradients in the upper troposphere. The analysis result shows that around the northern subtropics, the suppression of meridional mixing sharpens the CO 2 gradient during boreal winter and spring. In other words, extratropical cyclonic activity effectively flattens the mixing ratio along isentropic surfaces at midlatitudes and induces gradients around the subtropics. The southern subtropical CO 2 gradient is also induced by the subtropical mixing barrier and convergence of the cross-equatorial eddy flux. Different from the subtropical gradients, a CO 2 gradient in the tropical upper troposphere is not created by the suppression of meridional transport but is created by the uplifting of low-level air during boreal winter and spring. The latitudinal CO 2 gradient in the tropical upper troposphere decreases due to interhemispheric transport. The seasonal migration of the mean Hadley circulation yields efficient interhemispheric transport and reduces the tropical CO 2 gradient.

Analysis of <sup>13</sup>C and <sup>18</sup>O isotope data of CO<sub>2</sub> in CARIBIC aircraft samples as tracers of upper troposphere/lower stratosphere mixing and the global carbon cycle

“…Here lower CO 2 is expected due to uplift of strongly depleted continental air in summer. Though the model of Miyazaki et al (2008Miyazaki et al ( , 2009 results by Shia et al (2006) show that cross-tropopause exchange is pronounced in certain seasons, but the results do not allow evaluating this contribution correctly. A recent study by Boenisch et al (2008) aimed to model CO 2 and SF 6 in the extratropical UT/LMS, including STE flux, has demonstrated little gradient at the CARIBIC flight altitudes in the NH latitudes (14 to 55 • N), also with an increased gradient in summer.…”

“…5 in Miyazaki et al, 2008 and also Fig. 3 in Miyazaki et al, 2009). Here lower CO 2 is expected due to uplift of strongly depleted continental air in summer.…”

“…8). To understand the latitudinal distribution of the CONTRAIL aircraft CO 2 data obtained at similar flight altitudes, Miyazaki et al (2008Miyazaki et al ( , 2009) applied a global atmospheric transport model. They concluded: "In the NH subtropical upper troposphere, the formation of the latitudinal CO 2 gradient is mostly attributable to the upward and poleward (adiabatic) transports of CO 2 ..." Based on their model results, the CO 2 concentration gradient at flight altitudes of 300 to 200 hPa is very limited or, in some months negligible.…”

Greenhouse gas analysis of air samples collected onboard the CARIBIC passenger aircraft

“…A study based on flask samples from biweekly flights between Japan and Australia 2000-2003 again found strong latitudinal gradients in the CO 2 mixing ratio in the upper troposphere between 30 • N and 30 • S (Miyazaki et al, 2009). A comparison with ground based measurements and model simulations showed larger latitudinal gradients at the surface than in the upper troposphere.…”

Sources of variations in total column carbon dioxide

“…Studies suggest that gradients in free tropospheric CO 2 concentrations can differ substantially from gradients at the surface (Miyazaki et al, 2008(Miyazaki et al, , 2009). Here, we examine how variations in the total CO 2 column, CO 2 , arise.…”