INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 27: 697–713 (2007) Published online 14 November 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1430 Seasonality of the northern hemisphere circumpolar vortex Kalyn M. Wrona a, * and Robert V. Rohli a a Department of Geography and Anthropology, Louisiana State University, Louisiana, USA Abstract: In previous research, Rohli et al. (2005) identified long-term features of the northern hemispheric circumpolar vortex (NHCPV) in January. This research provides a seasonal analysis using December and February to augment the previously analyzed January data in representing winter, along with April, July, and October data to represent spring, summer, and autumn, respectively. A representative 500 hPa geopotential height contour was selected to delineate the NHCPV in each of the five months. The area, shape, and centroid of the monthly December, February, April, July, and October NHCPV are computed for 1959–2001 to supplement the previously identified January properties. These geometrical features of the NHCPV reveal relationships between hemispheric-scale circulation and temperature anomalies throughout the year. A circularity ratio (Rohli et al., 2005) is used to characterize the shape of the hemispheric-scale circulation. Results suggest that only October exhibit long-term trends in either area or circularity, with July being the most variable month in area and October being the most variable month in circularity. Centroids tend to be skewed toward the Pacific basin, except in spring, but few systematic temporal shifts in centroid position were noted for any month. The NHCPV is correlated with atmospheric teleconnection patterns in several months. For example, as was the case for January (Rohli et al., 2005), the Arctic Oscillation (AO) is associated with the area of the December, February, and April NHCPV, while in December the circularity is positively correlated to the AO Index. Also, the Pacific-North American index is correlated with the area of the December and February NHCPV and with the shape of the December and October NHCPV. Copyright  2006 Royal Meteorological Society KEY WORDS circumpolar vortex; atmospheric circulation variability; atmospheric teleconnections; climate variability and change Received 5 April 2006; Revised 25 August 2006; Accepted 25 August 2006 INTRODUCTION Variation in the hemispheric-scale, upper-level, prevail- ing atmospheric circulation results from variations in the input of energy across the earth’s surface. One indi- cator of this prevailing circulation is the circumpolar vortex (CPV), a cyclonic circulation in the middle- to upper-troposphere circumnavigating the poles in each hemisphere. Because the CPV exists over the region of strongest thermal gradients, variation in area, shape, and position of the CPV characterizes the impacts of tem- perature changes on circulation. Furthermore, the CPV also may explain variation in other circulation-dependent atmospheric features, such as humidity, precipitation, and pollution advection. Rohli et al. (2005) suggested that in Januaries from 1959 to 2001, the Northern Hemisphere’s CPV (NHCPV) exhibited no linear temporal trends in area or shape, and that the mean centroid of the NHCPV is displaced from the North Pole in the direction of the Pacific Ocean. Fur- thermore, Rohli et al. (2005) linked the Arctic Oscillation (AO) to variability in the area of the January NHCPV, and the Pacific-North American (PNA) teleconnection to ∗ Correspondence to: Kalyn M. Wrona, E-mail: kwrona@hotmail.com the shape (but not area) of the January NHCPV over the same period. This research expands on previous research by exam- ining variability and changes in the area, shape, and position of the NHCPV, and linkages to atmospheric teleconnections, throughout the year. Because the win- ter season is the time of strongest pressure gradients, and therefore, the most meaningful and identifiable NHCPV, December and February analyzes were added to corrob- orate the previous exploratory January results. Because of the labor-intensive nature of representing the NHCPV and calculating geometric features of it, April, July, and October were selected to represent the spring, summer, and autumn, respectively. PURPOSE The purpose of this research is to provide a more com- plete history and better understanding of the NHCPV and atmospheric circulation. The most fundamental question addressed in both this research and in Rohli et al. (2005) is whether the NHCPV exhibits long-term trends in geo- metrical and spatial features over the 1959–2001 period. Therefore, this research includes several objectives to characterize the change in the NHCPV including: Copyright  2006 Royal Meteorological Society