Summertime Rossby Wave Breaking in the Eastern North Pacific: Links to Extreme Weather in the North American Monsoon Region

The majority of North American Monsoon (NAM) literature has focused on the Tier I region of the North American Monsoon Experiment (NAME), spanning central-western Mexico, southern Arizona and New Mexico. Previous work has demonstrated the ability of both easterly (inverted troughs, tropical easterly waves) and westerly disturbances (short/long wave troughs) in facilitating extreme precipitation during boreal summer in the North American Monsoon (NAM) region. However, recent work has demonstrated the importance of anticyclonic Rossby wave breaking (RWB) over the US west coast in driving extreme summertime precipitation at the northern periphery of the NAME Tier I region. Motivated by the importance of RWB for extreme summertime precipitation in the southwestern United States, this work develops a climatological context for these dynamical features. An automated detection algorithm is employed to identify RWB events over the northern Pacific Ocean and western North America. To better understand the atmospheric conditions conducive to RWB over western North America, longitudinal bins are used to subset individual events for composite analysis. Further, RWB events occurring in the eastern north Pacific and western US are partitioned into 2 groups: a) those which facilitate extreme weather over the NAM region, and b) those which do not, to gain insight into the necessary conditions for RWB events to drive extreme precipitation in the NAM region. Lastly, the relationship between RWB events and ENSO and the MJO is examined.