83rd Annual

Monday, 10 February 2003: 9:45 AM
Modes of Tropical water cycle variability (Invited Presentation)
John J. Bates, NOAA/NESDIS/NCDC, Asheville, NC
Poster PDF (232.7 kB)
In order to separate the natural and anthropogenic effects of climate change, it is important to understand and quantify feedback mechanisms. For example, given the magnitude of a man-made greenhouse gas such as CO2, the relationship between the magnitude of this anthropogenic climate forcing and the magnitude of the climate change response in a general circulation model, perhaps a global warming of 2 C, defines the climate sensitivity. Any process that changes the sensitivity of the climate response to an imposed anthropogenic forcing is called a feedback mechanism. Feedback mechanisms can either increase (a positive feedback) or decrease (a negative feedback) the magnitude of the climate response to forcing agents.

Large uncertainties occur in the feedback processes associated with clouds and water vapor, particularly in the Tropics. This is because in the Tropics there is a very large range in outgoing longwave radiation between cold, high clouds near the Tropical tropopause and the clear, warm and dry atmospheres in the subtropical deserts. Recent papers have documented large variability in outgoing longwave radiation on both short and long time scales. Wielicki and colleagues (2002) find both large temporal (white in frequency space) variability in broadband outgoing longwave radiation OLR and a significant decadal trend. The white frequency variability is on the order of 4 Wm-2 for the tropical strip 20N-20S. The trends they find include a drop of about 2 Wm-2 from the late 1970s to the mid 1980s and a rise of about 4Wm-2 from the late 1980s to the late 1990s. Work by Chen and colleagues (2002) examine possible mechanisms for the OLR trends documented by Wielicki. They find this trend in OLR is consistent changes in cloudiness and upper tropospheric humidity that suggest a decadal-time-scale strengthening of the tropical Hadley-Walker circulation.

Examination of multiple satellite-derived indices over the past two decades suggests that there are important interactions between the different temporal scales due to the unique dynamical wave modes in the tropics. In the Tropics, a dynamical wave duct opens in the northern Winter and Spring seasons such that mid-latitude Rossby waves can propagate deep into the subtropics and, sometimes, between hemispheres. The amount of this wave activity strongly affects the water and energy budget of the Tropics and is a function of ENSO state and mid-latitude transient activity. This discovery indicates that one must examine both changes in the transient Tropical circulation modes, not just changes in the mean Hadley-Walker circulation, when seeking mechanisms to explain the large observed changes in the Tropical water and energy cycle.

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