Abstract
Over the past few decades, Rossby wave breaking (RWB) in the upper troposphere/lower stratosphere has been extensively studied due to its wide ranging impacts on regional and large scale weather phenomena. Recent studies have shown that in years with increased anticyclonic RWB frequency, North Atlantic tropical cyclone activity is suppressed due to the persistent intrusion of enhanced vertical wind shear, subsidence, and dryness into the tropical environment by the RWB, particularly in years when RWB occurs more frequently in the western North Atlantic. Furthermore, a link between RWB and United States (US) West Coast atmospheric river (AR) dynamics has been uncovered that suggests RWB plays a role in modulating the trajectories and intensities of ARs, as a majority of US West Coast landfalling ARs are found to be collocated with eastern Pacific anticyclonic RWB events. These findings stress the seemingly unbounded influence of RWB and lay the framework for the overarching goal of this dissertation: to investigate how these RWB dominated relationships fit into and are modulated by the greater global climate system.
In order to fill the knowledge gap left by a lack of literature on summertime RWB, a climatology of summertime North Atlantic anticyclonic RWB is established using 58 years of NCEP-NCAR reanalysis data. Consistent with previous studies, composite analysis suggests that anticyclonic RWB introduces anomalously dry, stable extratropical air into the tropical environment, subsequently inhibiting convection there. To address the idea that the frequency and location of anticyclonic RWB events modulates tropical cyclone activity, an intra-basin climatology is developed in which RWB events are categorized based on whether they occur in the eastern (EB) or western (WB) half of the North Atlantic basin. A multidecadal pattern of intra-basin RWB frequency is observed such that anticyclonic RWB events occur more frequently in the EB (WB) during positive (negative) Pacific decadal oscillation (PDO) regimes. Analysis of the large scale dynamics suggest this variability is driven by changing distributions of convectively forced Rossby wave trains (RWTs) over the North Pacific in response to PDO related positive sea surface temperature (SST) anomalies, such that during PDO+(-) regimes RWTs are forced more frequently in the eastern (western) Pacific, increasing the number of RWTs that complete their great circle routes and consequently break in the EB (WB).
To diagnose the role the PDO plays in modulating the statistical variability of summertime North Atlantic anticyclonic RWB, an idealized modeling study is performed using the Community Atmosphere Model Version 5.0 (CAM5) atmospheric component of the NCAR Community Earth System Model (CESM) Version 1.2.2. Several 15-year simulations are run with the model, each using a unique set of prescribed sea surface temperatures (SSTs) corresponding to different phases and configurations of the PDO. Consistently, results suggest that when the PDO is in its positive (negative) phase, a greater number of anticyclonic RWB events are recorded in the EB (WB). Analysis of the large scale circulation and synoptic environment changes imposed by the SST anomalies of each simulation reveals different pathways for RWT development that, in turn, affect North Atlantic RWB statistics. When the PDO signals are divided into different components, the largest changes in RWB statistics are shown to occur whenever positive SST anomalies are present in the North Pacific, as these serve as fuel for high frequency RWT development. Furthermore, the role of anomalous latent heating driven atmospheric preconditioning to RWB is explored and uncovered to considerably affect RWB statistics, such that, all things being equal, without a sufficient amount of atmospheric preconditioning RWB is less likely to occur.
Investigation into the modulation of ARs by RWB has yet to be expanded to other parts of the globe where landfalling ARs are known to occur, despite proving to be a significant feature of ARs impacting the US West Coast. Given the strikingly similar characteristics of the large scale environment among US West Coast and western European ARs as well as results from case studies that suggest extratropical PV patterns reminiscent of RWB can help drive AR-like moisture transport to western Europe, it is reasonable to hypothesize that RWB also plays a role in the modulation and development of European landfalling ARs. Utilizing 38 years of the Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) reanalysis dataset, a climatology of landfalling ARs from 1980-2017 is developed using a combination of integrated vapor transport (IVT) calculations and a detection algorithm. Results indicate that 73% of the identified ARs are related to anticyclonic RWB, a dynamic feature shown to play a role in AR strength and structure. Additionally, AR variability is found to be closely tied to jet stream latitude modulation by the NAO, such that during a positive (negative) NAO the North Atlantic jet is shifted North (South), creating an environment more favorable for anticyclonic RWB and AR landfalls over northern (southern) Europe, a connection that can be useful for predicting where an AR may make landfall.