CGD's Dr. Jeffrey Kiehl

Climate sensitivity of tropical and subtropical marine low cloud amount to ENSO and global warming due to doubled CO2

Ping Zhu, Florida International University, Miami, Florida, USA
James J. Hack, National Center for Atmospheric Research, Boulder, Colorado, USA
Jeffrey T. Kiehl, National Center for Atmospheric Research, Boulder, Colorado, USA
Christopher S. Bretherton, Department of Atmospheric Sciences, University of Washington, USA

Description

In this study, we systematically analyzed the sensitivity of tropical and subtropical marine low cloud amount to the short-term climate anomaly associated with the 1997-1998 El Niño and the long-term climate change caused by doubled CO2 using the International Satellite Cloud Climatology Project (ISCCP) cloud measurements, European Centre for Medium-Range Weather Forecasting (ECMWF) reanalyses, and the sea surface temperature (SST) forced and coupled simulations performed by the latest version of the National Center for Atmospheric Research (NCAR) and Geophysical Fluid Dynamics Laboratory (GFDL) climate models. It is found that the changes in low cloud amount associated with the 1997-1998 El Niño and the doubled CO2 induced climate change have different characteristics and are controlled by different physical processes. Most reduction in low cloud amount related to the 1997-1998 El Niño occurs in the eastern tropical Pacific associated with an upward large-scale motion and a weak atmospheric stability measured by the 500 hPa vertical velocity and the potential temperature difference between 700 hPa and the surface, and is negatively correlated to the local SST anomaly. In addition to the other mechanisms suggested by the previous studies, our analyses based on the ISCCP observations indicate that the change in atmospheric convective activities in these regions is one of the reasons responsible for the change in low cloud amount. In contrast, most increase in low cloud amount due to doubled CO2 simulated by the NCAR and GFDL models occurs in the subtropical subsidence regimes associated with a strong atmospheric stability, and is closely related to the spatial change pattern of SST consistent with previous studies.

Figure caption: Binned change in low cloud amount based on the SST change after removing the mean SST change over the ocean basin between 40°S and 40°N associated with the 1997-1998 El Niño (observations) and doubled CO2 simulated by the NCAR and GFDL GCMs. Solid lines scaled to the right indicate the PDF of each temperature bin. CAM3-SOM and AM2-SOM represent the standard NCAR and GFDL atmospheric models coupled with a slab ocean model. CAM3-UW-SOM is a branch version of CAM3-SOM that uses a new moist turbulent mixing scheme and shallow convection scheme.

Support: NOAA/GFDL; DOE Climate Change Prediction Program

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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L02801, doi:10.1029/2006GL028384, 2007.

Role of hydrogen sulfide in a Permian-Triassic boundary ozone collapse

J.-F. Lamarque, Earth and Sun Systems Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA
Jeffrey T. Kiehl, Earth and Sun Systems Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA
J. J. Orlando, Earth and Sun Systems Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA

Description

IUsing a three-dimensional chemistry-climate model of the troposphere and stratosphere, we find that hydrogen sulfide alone is unlikely to directly affect stratospheric ozone, even for hydrogen sulfide emission rates as large as 5000 Tg(S) per year. However, we also find that large quantities of hydrogen sulfide create a significant decrease in tropospheric hydroxyl radical, leading to a commensurate increase in atmospheric methane. Therefore a large methane flux (possibly from methane clathrate destabilization, Siberian traps or hydrothermal vent complexes) combined with a large hydrogen sulfide oceanic flux is much more likely to lead to an ozone collapse than methane or hydrogen sulfide alone with implications to the Permian-Triassic boundary extinction 250 million years ago.

Figure caption: Annual and zonal average of the volume mixing ratio of H2S (top row, parts per billion), OH (second row, parts per trillion, scaled by 103) and ozone (third row, parts per million) and sulfate aerosol surface area density (bottom row, cm2/cm3, scaled by 109). The left column is for the low H2S emission case (2 Tg(S)/year) and the right column is for the high emission case (5000 Tg(S)/year).

Support: DOE SciDAC program