The United States experienced record-breaking droughts in recent years and the physical mechanisms and the predictability of such severe and persistent droughts are not entirely understood. While the tropical Pacific plays a key role in U.S. precipitation fluctuations on a year-to-year basis through the impact of ENSO-related sea surface temperature (SST) anomalies, particularly during the winter season, the persistence and severity of U.S. droughts cannot be explained entirely by the tropical Pacific SST fluctuations. Many previous studies have shown the important role played by the North Atlantic SST in causing U.S. rainfall and temperature anomalies as well as the combined effect of the Pacific and the Atlantic SST on the U.S. hydroclimate variability. However, there is no systematic examination of the impacts of North Atlantic conditions on U.S. drought predictability.
The frequency and severity of droughts across North America has been modulated by the phase of the Atlantic Multidecadal Variability (AMV) over the historical period. The decadal oscillations in U.S. West hydroclimate (associated with ENSO) reach extreme severity during the warm and neutral phases of AMV, such as in the 1930s and the 1950s when the U.S. Great Plains and the Southwest experienced the extremely dry conditions of the Dust Bowl and the persistent Texas drought, respectively. While when AMV was in its cold phase in the early 1900s and from 1965 to 1995 droughts were less frequent or severe. The recent increases in drought severity in the Central U.S. concurs with the cold phase of the Pacific Decadal Oscillation but also with the current warm phase of AMV.
The proposed work includes two major components: (1) Mechanisms of North Atlantic SST impact on U.S. drought: We will conduct atmospheric GCM experiments using the NCAR CAM5 as well as IRI’s operational seasonal forecast models (currently running ECHAM4.5), with prescribed tropical and subpolar North Atlantic SST to understand the dynamical linkages between SST and U.S. precipitation and temperature. (2) Predictability of U.S. precipitation given AMV phases: Given the longer time scale of the AMV and the advancements in the field of decadal prediction carried out as part of the CMIP5, we will examine in detail how predictability may be improved by knowing the state of the AMV while the rest of the oceans are varying seasonally and interannually.
This proposal is submitted to NOAA MAPP program and address MAPP’s research priority “To advance understanding, monitoring, and prediction of drought” and the focus area 1) understanding predictability of past droughts over North America. It will also help with the focus area 2) advancing the development of a national drought monitoring and prediction system, through improving IRI’s seasonal forecasts. The proposed work will build on recent NOAA supported research of AMOC and the AMV done by the Lamont PIs and the expertise of the IRI collaborators in the areas of seasonal and interannual climate predictions.