Atmos. Chem. Phys. Discuss., 8, 1041-1067, 2008
www.atmos-chem-phys-discuss.net/8/1041/2008/
doi:10.5194/acpd-8-1041-2008
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Variability of subtropical upper tropospheric humidity
J.-M. Ryoo1, D. W. Waugh1, and A. Gettelman2
1Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA
2National Center for Atmospheric Research, Boulder, CO, USA

Abstract. Analysis of Atmospheric Infrared Sounder (AIRS) measurements for five northern winters shows significant longitudinal variations in subtropical upper tropospheric relative humidity (RH), not only in the climatological mean values but also in the local distributions and temporal variability. The largest climatological mean values in the northern subtropics occur over the eastern Pacific and Atlantic oceans, where there is also large day-to-day variability. In contrast, there are smaller mean values, and smaller variability that occurs at lower frequency, over the Indian and western Pacific oceans. These differences in the distribution and variability of subtropical RH are related to differences in the key transport processes in the different sectors. The large variability and intermittent high and low RH over the Eastern Pacific and Atlantic oceans, and to a smaller extent over the Indian ocean, are due to intrusions of high potential vorticity air into the subtropics. Intrusions seldom occur over the eastern Indian and western Pacific oceans, and here the subtropical RH is more closely linked to the location and strength of subtropical anticyclones. In this region there are eastward propagating features in the subtropical RH that are out of phase with the tropical RH, and are caused by modulation of the subtropical anticyclones by the Madden-Julian Oscillation.

Citation: Ryoo, J.-M., Waugh, D. W., and Gettelman, A.: Variability of subtropical upper tropospheric humidity, Atmos. Chem. Phys. Discuss., 8, 1041-1067, doi:10.5194/acpd-8-1041-2008, 2008.
 
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