The Influences of Mesoscale SSTA on North Pacific Storm Tracks During Different PDO Phases

Abstract

Previous studies have demonstrated that extratropical large-scale and mesoscale sea surface temperature anomalies (SSTA) could impact the storm tracks. However, their combined influence on storm tracks remains to be determined. To answer this question, we defined four phases to represent the different combinations of large-scale SSTA patterns in the North Pacific and mesoscale SSTA in Kuroshio and Oyashio confluence region (KOCR), based on the standardized mesoscale SSTA index and the Pacific decadal oscillation (PDO) index. The storm tracks' response mainly exhibits a significant meridional dipolar structure in all phases, primarily driven by the mesoscale SSTA and consistent with previous studies. However, the details of the reaction differ under the modulation of PDO. Our findings also suggest the role of PDO is more pronounced when the variation of mesoscale SSTA is strong. Additionally, the storm tracks show more elongated and asymmetric anomalies in the warm PDO phase than the cold PDO phase. The key mechanism behind this is as follows. It revealed that the baroclinic processes associated with mesoscale SSTA are important in shaping storm tracks. The variation of mesoscale SSTA in KOCR changes the upward transportation of moisture and heat, which may alter diabatic heating. Consequently, differences emerge in the meridional gradient of air temperature, modifying the baroclinicity of the troposphere. Through baroclinic energy conversion, the influence finally impacts the storm tracks. PDO totally modulates these processes. In conclusion, the development of the North Pacific storm tracks are influenced by the combined effects of large-scale and mesoscale SSTA.