A Comparison between the Kuroshio Extension and Pineapple Express Atmospheric Rivers Affecting the West Coast of North America
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AbstractAtmospheric rivers (ARs) originating near the Kuroshio Extension and the Hawaiian Islands and making landfall onto the west coast of North America in the North Pacific in the boreal winter season (OctoberMarch) are detected and tracked using ERA5 reanalysis (200019), and are classified as Kuroshio Extension (KE) and Pineapple Express (PE) ARs, respectively. Compared with KE ARs, PE ARs are longer and wider with higher intensity and shorter duration. Although the total occurrence of PE ARs is lower, the occurrence of extreme ARs is substantially higher than KE ARs. PE (KE) ARs are oriented more meridionally (zonally) with more equatorward (poleward) landfalling positions and associated precipitation. The genesis, development, and decay of KE and PE ARs and their relationships with extratropical cyclones (ECs) are investigated. Along- and cross-section analyses show that PE ARs are associated with stronger, deeper low pressure systems with closer tropical connections. Compared with KE ARs, PE ARs originate from well-developed ECs with stronger southward intrusion of cold fronts, forming closer to the ECs centers along the sharp temperature/pressure gradient zone. They are accompanied by enhanced and deeper vertical motion and stronger low-level wind. The intensity difference between KE and PE ARs is largely determined by the orientation and the strengths of temperature/pressure gradients of associated ECs rather than the intensity of associated ECs themselves. Furthermore, the evolution of ARs and ECs is not always synchronized, suggesting complicated AR and EC interactions that require further investigations.
