Variability and Predictability of the Atlantic Warm Pool and Its Impacts on Extreme Events in North America

Our current/previous NOAA/CPO-funded research has pointed out the importance of the Atlantic Warm Pool (AWP) for summer climate and extreme events in the Western Hemisphere. AWP variability occurs on seasonal, interannual, multidecadal, and secular (global warming) timescales, with large AWPs being almost three times larger than small ones. The effect of the AWP is to weaken the North Atlantic subtropical high (NASH) and strengthen the summer continental low over the North American monsoon region. A large AWP also weakens the southerly Great Plains low-level jet, which results in reduced northward moisture transport from the AWP to the central U. S. and thus decreases the summer rainfall over the central United States. A large AWP increases the number of Atlantic hurricanes by reducing vertical wind shear and increasing the moist static instability of the troposphere, and influences the hurricane steering flow changes that become unfavorable for hurricanes to make landfall in the United States. Our research also suggests that the AWP serves as a link between the Atlantic Multidecadal Oscillation (AMO) and climate and hurricane activity. Despite its importance, almost of all stateof-the-art coupled models exhibit serious biases in the AWP region, which limit the seasonal prediction of AWP-related climate and extreme events.

We propose to continue our investigation of the AWP using fully coupled climate models. Two specific areas of proposed work are (1) diagnosing the CMIP5 outputs to assess model biases near the AWP region and to understand their skill in simulating the mechanisms and climate impacts of AWP variability, and (2) performing coupled model experiments using CESM1 (also called CCSM4) and analyzing the Climate Forecast System version 2 (CFSv2) reforecasts to assess and improve predictability of the AWP and its impacts on climate and extreme events such as hurricanes, flood and drought in North America. The diagnostic analyses will mainly focus on variability of the AWP, and its impacts on the NASH, the Caribbean low level jet and its moisture transport, and the Great Plains low-level jet and its moisture transport. Other areas of the focus in the diagnostic analyses include the relationships of rainfall in the U.S. with the AWP, the external influences (such as ENSO, the AMO, and the NAO) versus local ocean-atmosphere processes on AWP variability, and the relationships among environmental factors contributing to hurricane activity. We will perform CESM1 model simulations with and without realistic initialization of the AWP to explore the impact of AWP initialization on seasonal forecasts. We will also examine the influences of model resolution and deep convective parameterizations in CESM1 on AWP SST and rainfall biases. One of the tasks is to analyze the CFSv2 reforecasts to explore its skill for seasonal predication of the AWP and AWP-related climate and extreme events. The CESM1 experiments and the analysis of CFSv2 reforecasts are designed to identify the sources that contribute to the model biases, thus provide a basis for improving model simulations and predictions. In collaboration with scientists at NOAA/CPC, we will attempt to transition research results to operations at NOAA/CPC. The proposed work directly contributes to all of four topics listed in the NOAA/CPO MAPP FY12 Priority Area 3 of “modeling of IAS climate processes associated with extremes over North America”. It is hoped that over a longer time frame, this work will result in the regional implementation of data- and model-based outlooks for flood/drought in the United States, hurricanes and climate variability, when successfully combined with land-based models.