Modeling the global bomb tritium transient signal with the AGCM LMDZ‐iso: A method to evaluate aspects of the hydrological cycle

Abstract

Improving the representation of the hydrological cycle in atmospheric general circulation models (AGCMs) is one of the main challenges in modeling the Earth's climate system. One way to evaluate model performance is to simulate the transport of water isotopes. Among those available, tritium is an extremely valuable tracer, because its content in the different reservoirs involved in the water cycle (stratosphere, troposphere, and ocean) varies by order of magnitude. Previous work incorporated natural tritium into Laboratoire de Météorologie Dynamique Zoom (LMDZ)‐iso, a version of the LMDZ general circulation model enhanced by water isotope diagnostics. Here for the first time, the anthropogenic tritium injected by each of the atmospheric nuclear bomb tests between 1945 and 1980 has been first estimated and further implemented in the model; it creates an opportunity to evaluate certain aspects of LDMZ over several decades by following the bomb tritium transient signal through the hydrological cycle. Simulations of tritium in water vapor and precipitation for the period 1950–2008, with both natural and anthropogenic components, are presented in this study. LMDZ‐iso satisfactorily reproduces the general shape of the temporal evolution of tritium. However, LMDZ‐iso simulates too high a bomb tritium peak followed by too strong a decrease of tritium in precipitation. The too diffusive vertical advection in AGCMs crucially affects the residence time of tritium in the stratosphere. This insight into model performance demonstrates that the implementation of tritium in an AGCM provides a new and valuable test of the modeled atmospheric transport, complementing water stable isotope modeling.

Tritium Records to Trace Stratospheric Moisture Inputs in Antarctica

“…This is the reason why we restrict our analysis to the comparison of the relative variations of tritium and stable water isotopes in both snow pit profiles and model outputs. The pertinence of such a data‐model comparison is supported by the validation of stable water isotopes and tritium modeled variability (both temporally and spatially) in Antarctica with the LMDZ‐iso model (Risi et al, ; Cauquoin et al, , ). In addition, even if a straightforward comparison is limited by the chronology issue, the mean tritium level is similar in our data (Figure ) and in the modeled outputs (Figure ).…”

“…The erroneous scheme, so-called "upstream scheme," is too diffusive (Hourdin & Armengaud, 1999) and affects all water isotopes, with as the main consequence a humid bias at the poles and especially in the polar stratosphere (Cauquoin et al, 2016;Cauquoin & Risi, 2017). As in Cauquoin et al (2016), the isotopic outputs shown here are from simulations that were nudged by the horizontal winds from 20CR reanalyzes (Compo et al, 2011). Risi et al (2010) and Cauquoin et al (2015) have shown that LMDZ-iso captures well the relative spatial variability of δ 18 O and tritium in Antarctica with (1) a decrease of δ 18 O toward low-temperature areas because of the distillation of the water mass and (2) an increase of tritium toward the East Antarctic plateau reflecting, on top of the elsewhere observed "continental effect," the strong influence of stratospheric input combined with very low humidity conditions.…”

Importance of the advection scheme for the simulation of water isotopes over Antarctica by general circulation models: a case study with LMDZ-iso (LMDZ5a revision 1750)

“…We use here the isotopic AGCM LMDZ-iso (Risi et al, 2010a) following the model setup from Cauquoin et al (2016) at a standard latitude-longitude grid resolution of 2.5° × 3.75°, and with 39 layers in the vertical spread in a way to ensure a realistic description of the stratosphere and of the Brewer-Dobson circulation (Lott et al, 2005). The model has been validated in polar regions for the simulation of both atmospheric (Hourdin et al, 2006) and isotopic (Risi et al, 2010a) 25 variables.…”

“…15 The goal of this paper is to test if the warm and enriched biases in Antarctica are associated with an excessively diffusive water vapor transport. This paper follows the work of Cauquoin et al (2016) who compare two advection schemes, which differ by their diffusive character.…”

“…The diffusivity of the advection scheme in the vertical has also important consequences on modeling of tracers like tritium, which is related to stratospheric moisture input (Cauquoin et al, 2016). 15 The goal of this paper is to test if the warm and enriched biases in Antarctica are associated with an excessively diffusive water vapor transport.…”