ENSO-Induced Teleconnection: Process-Oriented Diagnostics to Assess Rossby Wave Sources and Ambient Flow Properties in Climate Models

Abstract

Abstract Climate model fidelity in representing ENSO-induced teleconnection is assessed with process-oriented diagnostics that examine a chain of processes, from equatorial Pacific precipitation to the midlatitude circulation pattern over the Pacific–North American regions. Such processes are rarely addressed during model development. Using an upper-tropospheric divergent level, local vorticity gradient of the ambient zonal flow ( ) and a restoring force for Rossby waves ( ) are estimated, the equivalent barotropic vorticity equation is solved, and an anomalous Rossby wave source (RWS′) quantified. The analysis is applied to AMIP5 and AMIP6 simulations. For a realistic circulation response representation, the hypothesis that models accurately represent the strength and location of RWS′, and spatial variations in is tested. Compared to AMIP5, in AMIP6 there are clear improvements in representing RWS′ and . To validate the hypothesis, the analysis identifies two metrics: spatially coherent RWS′ in the subtropical North Pacific, and longitudes of negative over the western-central North Pacific. By projecting these metrics in two and three-dimensional views, improvements or degradations in model versions are apparent. If a model’s fidelity in representing and RWS′ are compromised, then radiated Rossby waves are reflected more equatorward, resulting in zonally elongated circulation anomalies over the central North Pacific. Thus, during climate model development, applying this analysis frequently will keep a regular check on the fidelity of the modeled response to anomalous El Niño convection in conjunction with changing model ambient flow dependencies. This analysis is intended to form a process-oriented diagnostics package, a community contribution to the NOAA Model Diagnostics Task Force. Significance Statement The seasonal changes in tropical Pacific sea surface temperatures associated with El Niño events can have a significant impact in the atmospheric circulation through the North Pacific and on the annual climate variations over North America. Our skill in predicting these impacts depends critically on the ability of climate models to represent these global-scale connections accurately. We show a number of metrics that describe critical processes along this North Pacific pathway that can be used to examine the progress in climate model skill. In the future, these models could benefit significantly from using these metrics with the end goal of much improved predictions of El Niño–related variability.