Abstract
AbstractClimate models have substantial biases in the climatological latitude of the Southern Hemisphere eddy‐driven jet and the time scale of annular mode variability and disagree on the jet response to climate change. Zonally symmetric dry dynamical cores are often used for idealized modeling of the jet response to forcing and its sensitivity to model setup changes. The limits to which these models represent the key mechanisms that control the jet in complex models or the real world have not been systematically investigated. Here we show that substantial intermodel differences in jet latitude and strength can arise from differences in dynamical cores and resolved topography. Including topography and a more realistic surface drag in a dry model substantially alters the jet response to changes in drag strength. Using real‐world maps, enhanced drag over land shifts the jet poleward, whereas enhanced drag over the ocean leads to an equatorward shift. No universal relationship between annular mode time scale and forced response emerges in the dry model with topography. These results suggest that zonally symmetric models with Rayleigh drag lack important mechanisms that control the behavior of the midlatitude jet in coupled climate models. A dry model with topography and quadratic surface drag can fill this gap in the model hierarchy.
Highlights
Understanding what controls the strength, latitude, and variability of the midlatitude eddy-driven jet is a fundamental research question with substantial implications for society
Jet Latitude as a Function of Dynamical Core, Forcing, and Surface Drag Differences in jet characteristics between climate models can be caused by physical processes, namely, the temperature gradients generated by diabatic processes such as radiation or cloud formation (Ceppi et al, 2012), surface fluxes of heat and moisture (Polichtchouk & Shepherd, 2016), or drag that is represented in a model by resolved orography and parameterized drag processes (Pithan et al, 2016)
Using the same Held-Suarez forcing in both ICON and CAM with topography and quadratic drag results in different zonal wind distributions and a difference in jet latitudes of about 5◦ (Figure 2, black line for ICON and gray line for CAM5)
Summary
Understanding what controls the strength, latitude, and variability of the midlatitude eddy-driven jet is a fundamental research question with substantial implications for society. It would be highly beneficial to accurately represent eddy-driven jet configuration in numerical weather and climate prediction models and to confidently predict the midlatitude jet response to climate change. These jet characteristics correlate with the jet response to climate change in Coupled Model Intercomparison Project Phase 5 (CMIP5) models (Kidston & Gerber, 2010). Models with longer time scales of jet variability in the present-day climate have larger equatorward jet biases and tend to shift the jet further poleward in future climates, compared to models with shorter time scales of jet variability.
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