ABSTRACT

This is a grant under a Climate Variability and Predictability (CLIVAR) Program pilot project called DRICOMP, for the Drought in Coupled Models Project, which focuses on making initial explorations into the mechanisms of drought as they are represented in the output of global climate models and on attempting to assess the reliability of these models in simulating drought.

Theory, results from atmospheric general circulation models (AGCMs), and observational and proxy data support the hypothesis that changes in the frequency and amplitude of interannual variability in the tropics can modulate the occurrence and duration of droughts over North America. Various mechanisms for the atmospheric teleconnection between the tropical Pacific and North America have been hypothesized to explain the tendency for droughts to occur in the presence of cool sea surface temperature (SST) anomalies, i.e., in La Nina-like conditions. Analyses of instrumental records from the most recent decades, however, reveal that our understanding of these interactions is incomplete.

This project is an exploratory investigation, using newly available model output, of the possible role played by the tropical Pacific during the second half of the 20th century and in future warmer climates in modulating the occurrence and duration of droughts in North America. The investigators will attempt to answer the following questions:
. Which part of the circulation anomalies that create droughts over North America results from tropical SST forcing (ENSO, El Nino-Southern Oscillation, in particular) and which part results from internal modes of the atmosphere?
. Would changes in the ENSO characteristics - variance and mean state in particular - affect the probability of drought occurrence and/or drought duration and extent in the United States?

The first question will be addressed by means of exploratory analyses of a large number of simulations collected as part of the Climate of the 20th Century (C20C) international project. The potential predictability of North American observed rainfall will be assessed by systematically comparing the output of different AGCMs during the period 1950-1999 and separately during the 1950-1976 and 1977-1999 intervals. This may allow the significance of changes in the atmospheric patterns associated with North American precipitation before and after the 1976 climate shift in the Pacific to be assessed. To investigate the second point the principal investigator will analyze a suite of IPCC AR4 (Intergovernmental Panel on Climate Change Fourth Assessment Report) coupled runs to test the role of ENSO variance, direction of propagation of SST anomalies, and persistence of SST and wind anomalies as well as the role of the general warming in the tropics in modulating the occurrence and duration of droughts over North America.

Broader impacts of the work lie in its potential for assessing how the risk of North American drought will evolve over this century.

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