The Ellipse‐Fitting Detection of Winter North Pacific Jet and the Associated Air Temperature Variations in the Northern Hemisphere.

In the mid‐latitudes, the strongly meandering jet stream has contributed to worldwide weather patterns and climate variations. However, the shape variations of the jet and its climatic impacts remain unclear. In this study, we introduced an ellipse‐fitting technique to fit the high‐value wind speeds and identified the location, shape, and strength variations of the winter‐averaged North Pacific jet (NPJ). Particularly, this method has the advantage of objectively detecting the shape variations of the NPJ, including the area and orientation. Focused on the NPJ shape, the area and orientation variations are generally out‐of‐phase, leading to a large‐straight jet and a small‐tilted jet. The shape variation of the NPJ is highly linked to the variations in both mean and extreme temperatures in winter over East Asia and North America. Associated with the small‐tilted NPJ, northern East Asia and northern North America frequently suffer from warm and cold events in winter, respectively, while associated with the large‐straight NPJ, the patterns are almost opposite but with an asymmetric distribution to the small‐tilted NPJ. The distribution of the 300 hPa wind speed anomalies shows a spatial‐asymmetric feature at the exit and entrance of the NPJ. Large positive (negative) anomalies and the poleward (equatorward) shift at the NPJ exit and locations downstream (170°E−140°W) act in combination on the formation and maintenance of the large‐straight (small‐tilted) jet, which may be strongly connected with the sea surface temperature variations and synoptic‐scale transient eddy activities there. Our findings would be helpful in providing potential indicators of temperature variations. Plain Language Summary: The shape of the upper‐level jet stream is closely associated with weather and climate variations, which is not clearly understood yet. In this study, we introduced an ellipse‐fitting technique to fit high‐value wind speeds and identify the location, shape, and strength variations of the North Pacific jet (NPJ). This method has the advantage of objectively detecting characteristics of the jet stream, particularly for shape variations. The shape variations of the NPJ are closely linked to the variations both in winter mean and extreme temperature, particularly over East Asia and North America. Associated with the small‐tilted NPJ, warm events would more frequently occur in the northern part of East Asia, while cold events would easily affect northern North America. Sea surface temperature anomalies and the increased presence of surface high and low‐pressure systems around the jet exit region may have a strong connection with the different structures of the NPJ. Our findings would be helpful in providing a potential indicator of temperature variations. Key Points: An ellipse‐fitting technique has been introduced to detect the jet stream automatically and objectivelyPrimary combinations of the North Pacific jet's (NPJ's) shape include a large‐straight and small‐tilted jetVariations in the NPJ's shape are highly linked to out‐of‐phase variations in temperature extremes over East Asia and North America [ABSTRACT FROM AUTHOR]