On horizontal wind gradient variability from the stratosphere to the lower troposphere in the Arctic

Previous studies have demonstrated changes in vertical wind shear in recent decades, with implications for upward‐propagating planetary waves, the stratospheric polar vortex, and tracer transport. Changes in vertical wind shear combined with horizontal gradients have also been shown to contribute to an increase in vertical gradients in tracer fields, with important implications for vertical transport. In order to explore the extent to which changes in vertical wind shear are reflected in changes to horizontal stirring, we examine horizontal wind gradient fields from the stratosphere to the lower troposphere from 1979 to 2006. Studied in particular are the spatial and temporal properties and trends for relative vorticity, divergence, strain, and the Okubo‐Weiss criterion (which monitors the competition between strain‐ and vorticity‐dominated fields) to illustrate how horizontal wind gradients have changed over the last several decades. The results from this investigation show negative vorticity and strain anomalies during winter following 1998 associated with an increase in sudden stratospheric warmings and perturbations to the polar vortex noted in recent years. Spatial distributions for relative vorticity and strain fields exhibit coherent structures near the tropopause and middle stratosphere during fall and winter, which are captured by the Okubo‐Weiss fields. Trend analyses highlight statistically significant trends in relative vorticity and Okubo‐Weiss fields during winter. The results from this investigation demonstrate that statistically significant trends exist from the stratosphere to the lower troposphere. It is hypothesized that the coupling from the lower troposphere to the upper atmosphere will be important in the development of our understanding of the response of sea ice concentrations to climate variability and change.