An Objective Procedure for Delineating the Circumpolar Vortex

Abstract

The broad‐scale, steering atmospheric circulation in the Northern Hemisphere, represented by the tropospheric circumpolar vortex (CPV), is an important driver of environmental processes. The area and circularity of the CPV are analyzed hereby delineating the leading edge of the CPV at the steepest 500‐hPa geopotential height gradient globally. The daily CPV area and circularity were aggregated to monthly averages for contrast with measurements identified in previous research for the overlapping period of record (1979–2001). Accuracy of representation of the CPV is assessed through correlations to air‐sea teleconnections known to be associated with broad‐scale, extratropical steering circulation. Correlation to monthly teleconnection indices suggests that the new method allows for improvements in the calculation of area and circularity of the 500‐hPa manifestation of the CPV. These improvements justify extension of the calculation of the standardized CPV area and circularity for the 1979–2017 period of record. Results largely mirror those for the shorter time series, with the Arctic Oscillation, North Atlantic Oscillation, and Pacific‐North American teleconnection showing stronger links to CPV area and circularity than El Niño–Southern Oscillation and Pacific Decadal Oscillation. Collectively, these results suggest that the use of a singular indicator isohypse and/or monthly averaged data to represent the CPV may oversimplify analyses, especially for identifying past and future longwave ridges and troughs. This finding is important because the amplitudes and positions of the undulations in the broad‐scale flow exert the most important impacts on variability at both low‐ and high‐frequency time periods.

Spatiotemporal Trends and Variability in the Centroid of the Northern Hemisphere's Circumpolar Vortex

“…As described more fully in Bushra and Rohli (2019), gridded 500‐hPa geopotential heights from the National Centers for Environmental Prediction/U.S. Dept.…”

“…The shape of the NHCPV may have also changed under the recent warming, as it becomes more or less intertwined with areas of known air-sea interactions in the form of teleconnections (Bushra & Rohli, 2019). Recent research (Bushra & Rohli, 2019) has found that the NHCPV has become wavier over time and is positively correlated most closely with the indices of the Arctic Oscillation (AO; Thompson & Wallace, 1998) and North Atlantic Oscillation (NAO; Lamb & Peppler, 1987), and negatively with Pacific/North American (PNA; Wallace & Gutzler, 1981) teleconnection pattern.…”

“…The authors are grateful for funding from Louisiana State University's Economic Development Assistantship and Dissertation Year Fellowship programs, and for the thoughtful suggestions of three anonymous reviewers. Methods associated with the generation of the data used in this research, which characterize the NHCPV properties (i.e., CPV centroid location, area, and circularity, are described in more detail in Bushra and Rohli (2019), from the gridded data set obtained from the National Centers for Environmental Prediction (NCEP)/Department of Energy (DOE) Reanalysis 2 project. The data are available at https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.pressure.html.…”

“…This research uses an objective method for identifying the centroid of the NHCPV defined in Bushra and Rohli (2019) via geospatial techniques. The centroid position is then examined for both temporal (at both high and low frequencies) and spatial (distribution and frequencies over places) changes.…”

“…Previous research (Bushra & Rohli, 2019) established the “sharpest gradient” approach to defining the CPV and correlated the area and circularity of the Northern Hemisphere's CPV (NHCPV) to important atmospheric‐oceanic teleconnections. Using this definition, a library of daily NHCPV area and circularity has been constructed, based on 500‐hPa geopotential heights, facilitating comparisons to previous research and maximizing the likelihood that the PFJ rather than the subtropical jet stream is being depicted.…”