The Role of Stationary Rossby Waves in Springtime Hydroclimate over the Central United States

Abstract:

Rapidly evolving flash droughts, such as the 2012 Great Plains drought, are among the most devastating natural hazards in North America due to their severe impacts on agricultural productions. Spring (April-June) precipitation is key to the central United States agricultural production, the failure of which can lead to rapid soil moisture declines and crop stress. During the spring transition season, diabatic heating over the tropical Indian Ocean to the western Pacific could trigger downstream Rossby wave propagations, imposing impacts on the central US. However, the potential predictability provided from these Rossby waves to the central US could be very sensitive to the evolution of the basic state from April to June. This study addresses the potential subseasonal predictability of large-scale atmospheric circulations affecting the central US hydroclimate during spring.

Our preliminary results based on atmospheric reanalyses suggest that the springtime variability in the central US rainfall can be partially driven by Rossby wave propagations across the North Pacific. However, the details of the wave-trains, such as length scale, are sensitive to changes in the Pacific waveguide that are driven by the rapid evolution of the zonal wind field during the transition season. To further examine the sources of these Rossby waves, we use an empirical diagnostic tool called the influence function, an inverse Green’s function constructed from ERA-Interim atmospheric circulations and NOAA Interpolated satellite outgoing longwave radiation (OLR) data during 1979 to 2013, to detect the sensitivity of anomalies within a target area to remote forcing. Springtime circulation anomalies over the central US are sensitive to forcing by OLR anomalies over such regions as the western tropical Indian Ocean, India, and the subtropical western Pacific, but the most sensitive areas of this forcing vary monthly. Our analyses also show that the responses of wave-trains to a similar heating in these regions can be of opposite sign from one month to the next, imposing different impacts on the central US. Idealized model experiments using a baroclinic stationary wave model will be used to further examine the sensitivity of Rossby wave to diabatic heating over the Indian Ocean to the western Pacific in the transition season.