85th AMS Annual Meeting

Monday, 10 January 2005: 11:45 AM
Simulated tropical weather and climate: internal variability and the response to increased atmospheric CO2
Thomas R. Knutson, NOAA/GFDL, Princeton, NJ
An overview of simulations of tropical climate variability and the response of tropical weather and climate to increased atmospheric CO2 is presented, with an emphasis on results obtained from the Manabe Climate Model (MCM).

The MCM has been useful for assessing potential mechanisms of natural or internal climate variability in the tropics as well as for climate change detection. The model simulates considerable internal variability in the tropical Pacific on decadal timescales, which bears a resemblance to ENSO, though with a longer timescale and broader meridional scale. From a climate change detection perspective, observed sea surface temperatures in a number of tropical regions indicate significant warming trends over the past half century (i.e., trends outside the expected range due to internal climate variability).

The tropical response to increased CO2 has sometimes been termed "dynamically uninteresting", which is to say that the large-scale time-averaged circulation features of the present day tropical atmosphere undergo only relatively minor changes in climate change simulations with increased CO2. Some consequences of such a response are discussed, along with some possible exceptions to this paradigm. Some potential "dynamically interesting" exceptions include CO2-induced El Nino-like or La Nina-like changes in the mean state of the tropical Pacific; changes in ENSO variability itself; or CO2-induced changes in aspects of tropical cyclone activity. The MCM has played an important role in assessments in each of these areas. MCM simulations have also indicated other significant CO2-induced climate effects in the tropics, such as large increases in the heat-related stress indices. While such thermal effects are perhaps not "dynamically interesting", they portend important negative impacts of future CO2-induced climate change on human health and ecosystems in the tropics and humid subtropical regions.

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