A polar low embedded in a blocking high over the Pacific Arctic

A polar low (PL) is a short‐lived phenomenon involving strong winds that occurs over polar oceans. In October 2009, the R/V Mirai encountered a PL with a 600‐km‐wide, comma‐shaped cloud that developed over the Chukchi Sea. A shipboard Doppler radar and radiosondes were used to understand the fine structure of this PL. Analyses of low‐level winds and the thermodynamic structure indicated that the development of the PL was decoupled from sea surface thermal forcing. The PL was likely triggered by an intrusion of a potential vorticity (PV) anomaly at the tropopause. A southerly warm advection associated with a blocking high over Alaska resulted in rapid development of the PL in front of the cold dome induced by the upper‐level PV anomaly. The westerly winds after passage of the PL seemed to modify the upper‐ocean structure dramatically.