Breanna Zavadoff, RSMAS: Rossby Wave Breaking

COMPASS 2019-11-13: Breanna Zavadoff, RSMAS "North Atlantic Rossby Wave Breaking and Its Relationship to European Atmospheric Rivers" Anticyclonic Rossby wave breaking (RWB) is characterized by the rapid and irreversible deformation of potential vorticity (PV) contours on isentropic surfaces. Despite being the season in which anticyclonic RWB events are most prevalent, no work has focused solely on the variability of these events during the North Atlantic summertime. Using 58 years (1960-2017) of NCEP-NCAR reanalysis data, a climatology of boreal North Atlantic anticyclonic RWB is developed and a multidecadal pattern of intra-basin RWB frequency is observed. Analysis of the large scale dynamics suggest this variability is driven by convectively forced atmospheric responses to Pacific decadal oscillation (PDO) sea surface temperature (SST) anomalies, such that anticyclonic RWB events are favored in the East (West) North Atlantic during PDO+(–). To test this hypothesized relationship, an idealized modeling study is performed using the NCAR Community Earth System Model. Eleven 15-year simulations are run, each using a unique set of prescribed SSTs corresponding to different configurations and phases of the PDO. Preliminary results show that, as hypothesized, more anticyclonic RWB events are recorded in the eastern (western) half of the North Atlantic during PDO+(–). Further analysis is performed to determine how the different PDO+/– signals alter the large scale circulation to drive these frequency changes. Recent studies have found that US West Coast atmospheric rivers (ARs), narrow filaments of high water vapor content that extend thousands of kilometers, are dynamically related to anticyclonic RWB. While ARs occur all over the world, this connection has not been made elsewhere. To determine if this AR-RWB relationship exists over western Europe, 38 years (1980-2017) of MERRA-2 reanalysis data is employed to develop an AR climatology. Of the 578 ARs identified, 73% are related to anticyclonic RWB. Additionally, AR variability is shown to be closely tied to jet stream latitude modulation by the North Atlantic Oscillation (NAO), such that during NAO+(–), the North Atlantic jet is shifted north (south), creating an environment more (less) favorable for anticyclonic RWB and AR landfalls at higher (lower) latitudes.