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Effects of increased CO2 levels on monsoons

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Abstract

Increased atmospheric carbon dioxide concentration provided warmer atmospheric temperature and higher atmospheric water vapor content, but not necessarily more precipitation. A set of experiments performed with a state-of-the-art coupled general circulation model forced with increased atmospheric CO2 concentration (2, 4 and 16 times the present-day mean value) were analyzed and compared with a control experiment to evaluate the effect of increased CO2 levels on monsoons. Generally, the monsoon precipitation responses to CO2 forcing are largest if extreme concentrations of carbon dioxide are used, but they are not necessarly proportional to the forcing applied. In fact, despite a common response in terms of an atmospheric water vapor increase to the atmospheric warming, two out of the six monsoons studied simulate less or equal summer mean precipitation in the 16×CO2 experiment compared to the intermediate sensitivity experiments. The precipitation differences between CO2 sensitivity experiments and CTRL have been investigated specifying the contribution of thermodynamic and purely dynamic processes. As a general rule, the differences depending on the atmospheric moisture content changes (thermodynamic component) are large and positive, and they tend to be damped by the dynamic component associated with the changes in the vertical velocity. However, differences are observed among monsoons in terms of the role played by other terms (like moisture advection and evaporation) in shaping the precipitation changes in warmer climates. The precipitation increase, even if weak, occurs despite a weakening of the mean circulation in the monsoon regions (“precipitation-wind paradox”). In particular, the tropical east-west Walker circulation is reduced, as found from velocity potential analysis. The meridional component of the monsoon circulation is changed as well, with larger (smaller) meridional (vertical) scales.

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Acknowledgments

This work was done in the “Numerical Application and Scenarios” division of CMCC in the framework of the activity “Understanding of the climate system.” We are grateful to the anonymous reviewers whose comments greatly improved the analysis and to C. Cagnazzo for her advice related to radiative issues. A. Cherchi thanks G.A. Vecchi and H. Annamalai for useful discussion and suggestions in the preliminary part of the study.

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Authors and Affiliations

  1. Centro Euro-Mediterraneo per i Cambiamenti Climatici and Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy

    Annalisa Cherchi, Simona Masina & Antonio Navarra

  2. Centro Euro-Mediterraneo per i Cambiamenti Climatici, Bologna, Italy

    Andrea Alessandri

Authors
  1. Annalisa Cherchi
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  2. Andrea Alessandri
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  3. Simona Masina
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  4. Antonio Navarra
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Correspondence to Annalisa Cherchi.

Appendix: Computation of the relative humidity

Appendix: Computation of the relative humidity

The relative humidity as a measure of the balance between the water-holding capacity of the atmosphere and the tropospheric water vapor content has been computed by means of the formula:

$$ \hbox{RH} = \frac{q}{q_{\rm s}} \times 100 $$
(3)

where q is the specific humidity (in g kg−1) and q s is the saturation-specific humidity. The saturation-specific humidity may be approximated as a function of the saturation vapor pressure e s and of the atmospheric pressure p through

$$ q_{\rm s} \simeq 0.622 \frac{e_{\rm s}}{p} $$
(4)

where 0.622 is the ratio of the gas constant of dry air and of water vapor (R d/R v). The saturation vapor pressure can be expressed as a function of temperature only by means of empirical formulae, e.g.:

$$ e_{\rm s} \simeq 6.11 \times 10^{\frac{7.5 T}{237.7 + T}} $$
(5)

where T is the temperature expressed in °C (Bolton 1980). RH has been computed from the surface up to 200 mb and then averaged in the tropospheric column.

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Cherchi, A., Alessandri, A., Masina, S. et al. Effects of increased CO2 levels on monsoons. Clim Dyn 37, 83–101 (2011). https://doi.org/10.1007/s00382-010-0801-7

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