North American drought is among the costliest extreme climatic events, with impacts to the U.S economy of several billion dollars per year. Droughts are typically amplified during the warm season and can rapidly intensify, as the flash drought of 2012 can attest. Some recent studies suggest that during meteorological summer (JJA) the interannual variability of North American precipitation has been increasing over the last 6 decades. However, upon closer examination it is evident that trends in this variability are more nuanced than previously thought. During spring (AMJ) both the Great Plains and Southeast show an increasing trend in interannual variability of precipitation over 1950-2010, however, during summer (JAS) the changes in variability exhibit a more decadal-like appearance. These regional precipitation variations during the spring and summer are acutely sensitive to North American low-level jet (NALLJ) variability. This dynamical feature of the atmosphere acts as a scale transfer mechanism between the large-scale forcing and regional climate variability.
The main goals of this proposal are to (1) Advance the understanding of the physical mechanisms linking changes in NALLJ fluctuations and regional precipitation variability, (2) Determine the ability of the current generation of global climate models to simulate and predict NALLJ variability and its related precipitation impacts, (3) Examine the roles of natural climate variability and anthropogenic climate change to recent changes in regional precipitation and NALLJ variability. These goals will be achieved by completing the following tasks: (task-1) expand the observational analysis of NALLJ variability and regional warm season precipitation variations to include the entire 20th century; (task-2) diagnose the role of global SST variability on the NALLJ modes and precipitation in observations and multi-model AMIP simulations; (task-3) examine the predictability of NALLJ variability modes in the National Multi-Model Ensemble (NMME) reforecasts; (task-4) Develop an experimental NALLJ prediction system; (task 5) evaluate the role of GHG increases on changes to regional precipitation variability using several thousand seasonal realizations provided by the NMME effort.
The proposed work contributes directly to a high priority topic for the NOAA FY 2014 MAPP funding Priority Area-1, Research to Advance Understanding, Monitoring, and Prediction of Drought: (i) “Understanding predictability of past droughts over North America”. This work will be conducted under the auspices of the NCEP Climate Prediction Center and will enable NOAA to achieve the major objectives of MAPP especially, “improving methodologies for global to regional-scale analysis, predictions, and projections” and “developing integrated assessment and prediction capabilities relevant to decision makers based on climate analyses, predictions, and projections.” The work is also highly relevant to NOAA’s goals as expressed in the NOAA Next generation Strategic Plan, specifically, Weather Ready Nation: Society is prepared for and responds to weather-related events, and Climate Adaptation and Mitigation: An informed society anticipating and responding to climate and its impacts.