Exploring uncertainty of trends in the lower-tropospheric North Pacific Jet position

It has been difficult to establish trends in the observed jet streams, despite modelling studies suggesting they will move polewards in a warming world. While this is partly due to biases between the models and observations, we propose that another uncertainty is rooted in the choice of statistic used to determine the ‘jet latitude’ — one measure used to quantify the jet position. We use seven different jet latitude statistics, four climate reanalysis products, and CMIP6 simulations to assess the relative importance of different uncertainties associated with North Pacific Jet (NPJ) trends. Our results show a statistically significant poleward trend in the observed winter NPJ across all reanalyses and using all jet latitude statistics. The magnitude of this trend is most sensitive to the choice of statistic. Furthermore, we find that the NPJ shifts poleward in Autumn under high emission scenarios, which is robust to the choice of jet statistic.