AMO-Forced Regional Processes Affecting Summertime Precipitation Variations in the Central United States

Michael C. Veres Department of Earth and Atmospheric Sciences, University of Nebraska at Lincoln, Lincoln, Nebraska

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Qi Hu Department of Earth and Atmospheric Sciences, and School of Natural Resources, University of Nebraska at Lincoln, Lincoln, Nebraska

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Abstract

Numerous previous studies have provided insight into the influence of the Atlantic multidecadal oscillation (AMO) on North American precipitation. However, these studies focused on large-scale processes, and additional studies are needed to gain understanding of local and regional processes that develop in different phases of the AMO and substantiate its influences on precipitation. In this study, the Weather Research and Forecasting (WRF) regional model is used to examine AMO-forced local and regional processes and how they have affected summertime precipitation variation in the central United States.

While moisture transport and convergence by the Great Plains low-level jet have been recognized as necessary conditions for summer precipitation, model simulations show similar low-level moisture flux convergence in the central United States between the cold and warm phases of the AMO. However, there was a strong moistening in the lower troposphere during the AMO cold phase, making the atmosphere more unstable for convection and precipitation. The source of the moisture was found to be a strong positive surface evaporation–precipitation feedback initiated and sustained by increased relative vorticity along a frontal zone. Along the frontal zone, isentropic stretching of the upper-level atmosphere and cyclonic circulation anomalies increased the relative vorticity during the AMO cold phase, providing the dynamic support needed to release the low-level moist instability and produce the increased precipitation. These results indicate that the dynamics of the circulation in the AMO cold phase played key roles to organize regional vorticity processes that further sustained a coupling of precipitation and the surface evaporation and perpetuated the precipitation.

Corresponding author address: Dr. Qi Hu, 707 Hardin Hall, University of Nebraska at Lincoln, Lincoln, NE 68583-0987. E-mail: qhu2@unl.edu

Abstract

Numerous previous studies have provided insight into the influence of the Atlantic multidecadal oscillation (AMO) on North American precipitation. However, these studies focused on large-scale processes, and additional studies are needed to gain understanding of local and regional processes that develop in different phases of the AMO and substantiate its influences on precipitation. In this study, the Weather Research and Forecasting (WRF) regional model is used to examine AMO-forced local and regional processes and how they have affected summertime precipitation variation in the central United States.

While moisture transport and convergence by the Great Plains low-level jet have been recognized as necessary conditions for summer precipitation, model simulations show similar low-level moisture flux convergence in the central United States between the cold and warm phases of the AMO. However, there was a strong moistening in the lower troposphere during the AMO cold phase, making the atmosphere more unstable for convection and precipitation. The source of the moisture was found to be a strong positive surface evaporation–precipitation feedback initiated and sustained by increased relative vorticity along a frontal zone. Along the frontal zone, isentropic stretching of the upper-level atmosphere and cyclonic circulation anomalies increased the relative vorticity during the AMO cold phase, providing the dynamic support needed to release the low-level moist instability and produce the increased precipitation. These results indicate that the dynamics of the circulation in the AMO cold phase played key roles to organize regional vorticity processes that further sustained a coupling of precipitation and the surface evaporation and perpetuated the precipitation.

Corresponding author address: Dr. Qi Hu, 707 Hardin Hall, University of Nebraska at Lincoln, Lincoln, NE 68583-0987. E-mail: qhu2@unl.edu
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