Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 1 2008 10.5194/acpd-8-1041-2008 http://www.atmos-chem-phys-discuss.net/8/1041/2008/ http://www.atmos-chem-phys-discuss.net/8/1041/2008/acpd-8-1041-2008.html http://www.atmos-chem-phys-discuss.net/8/1041/2008/acpd-8-1041-2008.pdf 1041 1067 2008-01-22 Variability of subtropical upper tropospheric humidity J.-M. Ryoo D. W. Waugh waugh@jhu.edu A. Gettelman Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA National Center for Atmospheric Research, Boulder, CO, USA 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. 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