A Study of Frontal-Scale Air-Sea Interaction in Midlatitude Western Boundary Current Regimes

Frontal-scale air-sea interactions during boreal winter season in midlatitude western boundary current (WBC) regimes, including the Kuroshio Extension Region (KER) and Gulf Stream Region (GSR), are investigated using both observational (reanalysis) data and regional climate model simulations. The focus of the study is on the KER in the North Pacific.

Extreme flux events associated with cold air outbreaks (CAOs) in boreal winter in the KER of the Northwestern Pacific and the GSR of the Northwestern Atlantic are analyzed and compared based on different reanalysis datasets. A close relationship between extreme flux events over the KER/GSR and the Pacific Decadal Oscillation (PDO)/East Atlantic Pattern (EAP) is found with more frequent occurrence of extreme flux events during a positive PDO/EAP phase. Furthermore, the PDO/EAP may be explained as the rectified effects of the synoptic winter storms accompanied with the extreme flux events. A lag-composite analysis shows that event-day storms tend to have a preferred southeastward propagation path, potentially contributing to the southward shift of the storm track over the eastern North Pacific/Atlantic basin during positive PDO/EAP phase.

Using an atmosphere-only model (Weather Research Forecasting, WRF) at 27 km, two ensembles of simulations were conducted for boreal winter season in the North Pacific to study the possible local and remote influence of meso-scale oceanic eddies in the KER on the atmosphere. Filtering out meso-scale oceanic eddies results in a deep tropospheric response along and downstream of the KER, including a significant decrease (increase) in winter season mean rainfall along the KER (west coast of US), a reduction of storm growth rate in the KER, and a southward shift of the jet stream and North Pacific storm track in the eastern North Pacific. A mechanism invoking moist baroclinic instability is proposed to link meso-scale oceanic eddies in the KER to largescale atmospheric circulations and weather patterns in the North Pacific.

Using a high-resolution (9 km) coupled regional climate model (CRCM), three sets of experiment were performed to investigate potential feedbacks of ocean-eddy induced atmospheric response to the ocean. Filtering out meso-scale oceanic eddies during coupling causes a surface warming and a weak Kuroshio strength.